Human Reproduction

Class 12 | Pure Science — Zoology | Chapter 2

Class 12 Unit 2 Pure_Science

Human Reproduction

"In every child who is born, the potentiality of the human race is born again" — James Agee

Chapter Outline

• 2.1 Human reproductive system
• 2.2 Gametogenesis
• 2.3 Menstrual cycle
• 2.4 Menstrual disorders and menstrual hygiene
• 2.5 Fertilization and implantation
• 2.6 Maintenance of pregnancy and embryonic development
• 2.7 Parturition and lactation

Learning Objectives

• Creates an awareness towards a healthy reproductive life in adolescents.
• Understands the structure of the male and female reproductive systems.
• Explains the functions of the structures associated with the male and female reproductive system.
• Compares the process of spermatogenesis and oogenesis.
• Discusses the changes in a female body during and after Fertilization.
• Appraises the role of hormones in the process of reproduction.
• Understands the events in pregnancy and foetal development.

Every organ system in the human body works continuously to maintain homeostasis for the survival of the individual. The human reproductive system is essential for the survival of the species. An individual may live a long healthy life without producing an offspring, but reproduction is inevitable for the existence of a species.

The reproductive system has four main functions:

• To produce the gametes namely sperms and ova
• To transport and sustain these gametes
• To nurture the developing offspring
• To produce hormones

Major Reproductive Events in Human Beings

Gametogenesis

Formation of gametes by spermatogenesis and oogenesis.

Insemination

Transfer of sperms by the male into the female genital tract.

Fertilization

Fusion of male and female gametes to form zygote, called Fertilization.

Cleavage

Rapid mitotic divisions of the zygote which convert the single celled zygote into a multicellular structure called blastocyst.

Implantation

Attachment of blastocyst to the uterine wall.

Placentation

Formation of placenta which is the intimate connection between foetus and uterine wall of the mother for exchange of nutrients.

Gastrulation

Process by which blastocyst is changed into a gastrula with three primary germ layers.

Organogenesis

Formation of specific tissues, organs and organ systems from three germ layers.

Parturition

Expulsion of the foetus from the mother's womb.

These functions are carried out by the primary and accessory reproductive organs. The primary reproductive organs namely the ovary and testis are responsible for producing the ova and sperms respectively. Hormones secreted by the pituitary gland and the gonads help in the development of the secondary sexual characteristics, maturation of the reproductive system and regulation of normal functioning of the reproductive system. The accessory organs help in transport and to sustain the gametes and to nurture the developing offspring.

2.1 Human Reproductive System

The male reproductive system comprises of a pair of testes, accessory ducts, glands and external genitalia (Figure 2.1). Testes are the primary male sex organs. They are a pair of ovoid bodies lying in the scrotum (Figure 2.2a). The scrotum is a sac of skin that hangs outside the abdominal cavity. Since viable sperms cannot be produced at normal body temperature, the scrotum is placed outside the abdominal cavity to provide a temperature 2–3 degree centigrade lower than the normal internal body temperature. Thus, the scrotum acts as a thermoregulator for spermatogenesis.

Each testis is covered by an outermost fibrous tunica albuginea and is divided by septa into about 200 to 250 lobules each containing 2 to 4 highly coiled testicular tubules or seminiferous tubules. These highly convoluted tubules which form 80 percent of the testicular substance are the sites for sperm production.

The stratified epithelium of the seminiferous tubule is made of two types of cells namely sertoli cells or nurse cells and spermatogonic cells or male germ cells. Sertoli cells are elongated and pyramidal and provide nourishment to the sperms till maturation. They also secrete inhibin, a hormone which is involved in the negative feedback control of sperm production. Spermatogonic cells divide meiotically and differentiate to produce spermatozoa.

Interstitial cells or Leydig cells are embedded in the soft connective tissue surrounding the seminiferous tubules. These cells are endocrine in nature and are characteristic features of the testes of mammals. They secrete androgens namely the testosterone hormone which initiates the process of spermatogenesis. Other immunologically competent cells are also present.

Box Item: Cryptorchism

The failure of one or both testes to descend down into the scrotal sacs is known as cryptorchism (crypto means hidden and orchis means testicle). It occurs in 1 to 3 percent of new born males. A surgical correction at a young age can rectify the defect, else these individuals may become sterile and are unable to produce viable sperms.

The accessory ducts associated with the male reproductive system include rete testis, vasa efferentia, epididymis and vas deferens (Figure 2.2b). The seminiferous tubules of each lobule converge to form a tubulus rectus that conveys the sperms into the rete testis. The rete testis is a tubular network on the posterior side of the testis. The sperms leave the rete testis and enter the epididymis through the vasa efferentia. The epididymis is a single highly coiled tube that temporarily stores the spermatozoa and they undergo physiological maturation and acquire increased motility and fertilizing capacity.

The epididymis leads to the vas deferens and joins the duct of the seminal vesicle to form the ejaculatory duct which passes through the prostate and opens into the urethra. The urethra is the terminal portion of the male reproductive system and is used to convey both urine and semen at different times. It originates from the urinary bladder and extends through the penis by an external opening called urethral meatus.

The accessory glands of the male reproductive system include the paired seminal vesicles and bulbourethral glands also called Cowper's gland and a single prostate gland. The seminal vesicles secrete an alkaline fluid called seminal plasma containing fructose sugar, ascorbic acid, prostaglandins and a coagulating enzyme called vesiculase which enhances sperm motility. The bulbourethral glands are inferior to the prostate and their secretions also help in the lubrication of the penis. The prostate encircles the urethra and is just below the urinary bladder and secretes a slightly acidic fluid that contains citrate, several enzymes and prostate specific antigens.

Semen or seminal fluid is a milky white fluid which contains sperms and the seminal plasma (secreted from the seminal vesicles, prostate gland and the bulbourethral glands). The seminal fluid acts as a transport medium, provides nutrients, contains chemicals that protect and activate the sperms and also facilitate their movement.

The penis is the male external genitalia functioning as a copulatory organ. It is made of a special tissue that helps in the erection of penis to facilitate insemination. The enlarged end of the penis called glans penis is covered by a loose fold of skin called foreskin or prepuce.

The female reproductive system is far more complex than the male because in addition to gamete formation, it has to nurture the developing foetus. The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina and the external genitalia located in the pelvic region (Figure 2.3a). These parts along with the mammary glands are integrated structurally and functionally to support the process of ovulation, Fertilization, pregnancy, child birth and child care.

Ovaries are the primary female sex organs that produce the female gamete, ovum. The ovaries are located one on each side of the lower abdomen. The ovary is an elliptical structure about 2 to 4 cm long. Each ovary is covered by a thin cuboidal epithelium called the germinal epithelium which encloses the ovarian stroma. The stroma is differentiated as the outer cortex and inner medulla. Below the germinal epithelium is a dense connective tissue, the tunica albuginea. The cortex appears dense and granular due to the presence of ovarian follicles in various stages of development. The medulla is a loose connective tissue with abundant blood vessels, lymphatic vessels and nerve fibres. The ovary remains attached to the pelvic wall and the uterus by an ovarian ligament called mesovarium.

Do You Know?

Female uterus contains one of the strongest muscles of the human body.

The fallopian tubes (uterine tubes or oviducts), uterus and vagina constitute the female accessory organs (Figure 2.3b). Each fallopian tube extends from the periphery of each ovary to the uterus. The proximal part of the fallopian tube bears a funnel shaped infundibulum. The edges of the infundibulum have many fingerlike projections called fimbriae which help in collection of the ovum after ovulation. The infundibulum leads to a wider central portion called ampulla. The last part of the oviduct is the isthmus which is short and thick walled connecting the ampulla and infundibulum to the uterus.

The uterus or womb is a hollow, thick-walled, muscular, highly vascular and inverted pear-shaped structure lying in the pelvic cavity between the urinary bladder and rectum. The major portion of the uterus is the body and the rounded region superior to it, is the fundus. The uterus opens into the vagina through a narrow cervix. The cavity of the cervix called the cervical canal communicates with the vagina through the external orifice and with the uterus through the internal orifice. The cervical canal along with vagina forms the birth canal.

The wall of the uterus has three layers of tissues. The outermost thin membranous serous layer called the perimetrium, the middle thick muscular layer called myometrium and the inner glandular layer called endometrium. The endometrium undergoes cyclic changes during the menstrual cycle while myometrium exhibits strong contractions during parturition.

Vagina is a large fibromuscular tube that extends from the cervix to the exterior. It is the female organ of copulation. The female reproductive structures that lie external to the vagina are called as the external genitalia or vulva comprising of labia majora, labia minora, hymen and clitoris.

The Bartholin's glands (also called greater vestibular glands) are located posterior to the left and right of the opening of the vagina. They secrete mucus to lubricate the vagina and are homologous to the bulbourethral glands of the male. The Skene's glands are located on the anterior wall of the vagina and around the lower end of the urethra. They secrete a lubricating fluid and are homologous to the prostate gland of the males.

The external opening of the vagina is partially closed by a thin ring of tissue called the hymen. The hymen is often torn during the first coitus (physical union). However in some women it remains intact. It can be stretched or torn due to a sudden fall or jolt and also during strenuous physical activities such as cycling, horseback riding, etc., and therefore cannot be considered as an indicator of a woman's virginity.

The mammary glands are modified sweat glands present in both sexes. It is rudimentary in the males and functional in the females. A pair of mammary glands is located in the thoracic region. It contains glandular tissue and variable quantities of fat with a median nipple surrounded by a pigmented area called the areola. Several sebaceous glands called the areolar glands are found on the surface and they reduce cracking of the skin of the nipple.

Internally each mammary gland consists of 2 to 25 lobes, separated by fat and connective tissues (Figure 2.4). Each lobe is made up of lobules which contain acini or alveoli lined by epithelial cells. Cells of the alveoli secrete milk. The alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla which is connected to the lactiferous duct in the nipple. Under the nipple, each lactiferous duct expands to form the lactiferous sinus which serves as a reservoir of milk. Each lactiferous duct opens separately by a minute pore on the surface of the nipple.

Normal development of the breast begins at puberty and progresses with changes during each menstrual cycle. In non-pregnant women, the glandular structure is largely underdeveloped and the breast size is largely due to amount of fat deposits. The size of the breast does not have an influence on the efficiency of lactation.

2.2 Gametogenesis

Gametogenesis is the process of formation of gametes (i.e., sperms and ovum) from the primary sex organs in all sexually reproducing organisms. Meiosis plays the most significant role in the process of gametogenesis (Figure 2.5).

Spermatogenesis

Spermatogenesis is the sequence of events in the seminiferous tubules of the testes that produce the male gametes, the sperms. During development, the primordial germ cells migrate into the testes and become immature germ cells called sperm mother cells or spermatogonia in the inner surfaces of the seminiferous tubules (Figure 2.6a). The spermatogonia begin to undergo mitotic division at puberty and continue throughout life.

In the first stage of spermatogenesis, the spermatogonia migrate among sertoli cells towards the central lumen of the seminiferous tubule and become modified and enlarged to form primary spermatocytes which are diploid with 23 pairs (46 chromosomes). Some of the primary spermatocytes undergo first meiotic division to form two secondary spermatocytes which are haploid with 23 chromosomes each. The secondary spermatocytes undergo second meiotic division to produce four haploid spermatids.

The spermatids are transformed into mature spermatozoa (sperms) by the process called spermiogenesis. Sperms are finally released into the cavity of seminiferous tubules by a process called spermiation. The whole process of spermatogenesis takes about 64 days. At any given time, different regions of the seminiferous tubules contain spermatocytes in different stages of development (Figure 2.6b). The sperm production remains nearly constant at a rate of about 200 million sperms per day.

Spermatogenesis starts at the age of puberty and is initiated due to the increase in the release of Gonadotropin Releasing Hormone by the hypothalamus. GnRH acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins namely Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH). FSH stimulates testicular growth and enhances the production of Androgen Binding Protein by the sertoli cells and helps in the process of spermiogenesis. LH acts on the Leydig cells and stimulates the synthesis of testosterone which in turn stimulates the process of spermatogenesis.

Structure of Human Spermatozoan

The human sperm is a microscopic, flagellated and motile gamete (Figure 2.7). The whole body of the sperm is enveloped by plasma membrane and is composed of a head, neck, middle piece and a tail.

The head comprises of two parts namely acrosome and nucleus. Acrosome is a small cap like pointed structure present at the tip of the nucleus and is formed mainly from the Golgi body of the spermatid. It contains hyaluronidase, a proteolytic enzyme, popularly known as sperm lysin which helps to penetrate the ovum during Fertilization. The nucleus is flat and oval.

The neck is very short and is present between the head and the middle piece. It contains the proximal centriole towards the nucleus which plays a role in the first division of the zygote and the distal centriole gives rise to the axial filament of the sperm.

The middle piece possesses mitochondria spirally twisted around the axial filament called mitochondrial spiral or nebenkern. It produces energy in the form of ATP molecules for the movement of sperms.

The tail is the longest part of the sperm and is slender and tapering. It is formed of a central axial filament or axoneme and an outer protoplasmic sheath. The lashing movements of the tail push the sperm forward.

The human male ejaculates about 200 to 300 million sperms during coitus. It is estimated that around 60 percent of sperms must have normal shape of which at least 40 per cent must show vigorous motility for normal fertility.

Do You Know?

The sperm is the smallest human cell and the ovum or egg is the largest human cell.

Oogenesis

Oogenesis is the process of development of the female gamete or ovum or egg in the ovaries. During foetal development, certain cells in the germinal epithelium of the foetal ovary divide by mitosis and produce millions of egg mother cells or oogonia. No more oogonia are formed or added after birth. The oogonial cells start dividing and enter into Prophase I of meiotic division I to form the primary oocytes which are temporarily arrested at this stage. The primary oocytes then get surrounded by a single layer of granulosa cells to form the primordial or primary follicles (Figure 2.8a).

A large number of follicles degenerate during the period from birth to puberty, so at puberty only 60,000 to 80,000 follicles are left in each ovary.

Do You Know?

Out of the million eggs women possess during birth, only about 300 to 400 will ovulate before menopause. On the other hand, males produce more than 500 billion sperms in their life time.

The primary follicle gets surrounded by many layers of granulosa cells and a new theca layer to form the secondary follicle. A fluid filled space, the antrum develops in the follicle and gets transformed into a tertiary follicle. The theca layer gets organized into an inner theca interna and an outer theca externa. At this time, the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division and forms the secondary oocyte. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a first polar body. The first polar body disintegrates.

During Fertilization, the secondary oocyte undergoes second meiotic division and produces a large cell, the ovum and a second polar body. The second polar body also degenerates. The tertiary follicle eventually becomes a mature follicle or Graafian follicle. If Fertilization does not take place, second meiotic division is never completed and the egg disintegrates. At the end of gametogenesis in females, each primary oocyte gives rise to only one haploid ovum.

Structure of Ovum

Human ovum is non-cleidoic, alecithal and microscopic in nature (Figure 2.8b). Its cytoplasm called ooplasm contains a large nucleus called the germinal vesicle. The ovum is surrounded by three coverings namely an inner thin transparent vitelline membrane, middle thick zona pellucida and outer thick coat of follicular cells called corona radiata. Between the vitelline membrane and zona pellucida is a narrow perivitelline space.

2.3 Menstrual Cycle

The menstrual or ovarian cycle occurs approximately once in every 28 or 29 days during the reproductive life of the female from menarche (puberty) to menopause except during pregnancy. The cycle of events starting from one menstrual period till the next one is called the menstrual cycle during which cyclic changes occurs in the endometrium every month. Cyclic menstruation is an indicator of normal reproductive phase (Figure 2.9).

Menstrual cycle comprises of the following phases:

1. Menstrual phase
2. Follicular or proliferative phase
3. Ovulatory phase
4. Luteal or secretory phase

1. Menstrual Phase

The cycle starts with the menstrual phase when menstrual flow occurs and lasts for 3 to 5 days. Menstrual flow is due to the breakdown of endometrial lining of the uterus, and its blood vessels due to decline in the level of progesterone and oestrogen. Menstruation occurs only if the released ovum is not fertilized. Absence of menstruation may be an indicator of pregnancy. However it could also be due to stress, hormonal disorder and anaemia.

2. Follicular or Proliferative Phase

The follicular phase extends from the fifth day of the cycle until the time of ovulation. During this phase, the primary follicle in the ovary grows to become a fully mature Graafian follicle and simultaneously, the endometrium regenerates through proliferation. These changes in the ovary and the uterus are induced by the secretion of gonadotropins like Follicle Stimulating Hormone and Lutenizing Hormone, which increase gradually during the follicular phase. It stimulates follicular development and secretion of oestrogen by the follicle cells.

3. Ovulatory Phase

Both Follicle Stimulating Hormone and Lutenizing Hormone attain peak level in the middle of the cycle (about the 14th day). Maximum secretion of LH during the mid cycle called Lutenizing Hormone surge induces the rupture of the Graafian follicle and the release of the ovum (secondary oocyte) from the ovary wall into the peritoneal cavity. This process is called ovulation.

4. Luteal or Secretory Phase

During luteal phase, the remaining part of the Graafian follicle is transformed into a transitory endocrine gland called corpus luteum. The corpus luteum secretes large amount of progesterone which is essential for the maintenance of the endometrium. If Fertilization takes place, it paves way for the implantation of the fertilized ovum. The uterine wall secretes nutritious fluid in the uterus for the foetus. So, this phase is also called as secretory phase. During pregnancy all events of menstrual cycle stop and there is no menstruation. In the absence of Fertilization, the corpus luteum degenerates completely and leaves a scar tissue called corpus albicans. It also initiates the disintegration of the endometrium leading to menstruation, marking the next cycle.

Box Item: Polycystic Ovary Syndrome (PCOS)

Polycystic ovary syndrome is a complex endocrine system disorder that affects women in their reproductive years. Polycystic means 'many cysts'. It refers to many partially formed follicles on the ovaries, which contain an egg each. But they do not grow to maturity or produce eggs that can be fertilized. Women with Polycystic ovary syndrome may experience irregular menstrual cycles, increased androgen levels, excessive facial or body hair growth (hirsutism), acne, obesity, reduced fertility and increased risk of diabetes. Treatment for Polycystic ovary syndrome includes a healthy lifestyle, weight loss and targeted hormone therapy.

2.4 Menstrual Disorders

Absence of menstruation is called amenorrhoea. If menarche does not appear till the age of 18, it is called primary amenorrhoea. Absence of menstruation for over three consecutive months is secondary amenorrhoea.

Polymenorrhoea is a term used to describe a menstrual cycle that is shorter than 21 days. It may be due to hyperactivity of the anterior pituitary gland causing frequent ovulation, psychological disturbances and malnutrition. Chronic pelvic inflammation by certain sexually transmitted diseases such as chlamydiasis or gonorrhoea can cause inflammation in the uterus causing polymenorrhoea.

Pain associated with menstruation is called dysmenorrhoea. It is the most commonly reported menstrual disorder. There are two types of dysmenorrhoea viz primary and secondary dysmenorrhoea. Primary dysmenorrhoea is pain or cramps during menstrual period and is caused by secretions of prostaglandin in the uterus. Secondary dysmenorrhoea is caused by a disorder in the reproductive system like endometriosis or uterine fibroids.

Heavy and prolonged menstrual period that disrupts a woman's normal activities is referred to as menorrhagia. Menorrhagia may be due to hormonal imbalance, ovarian dysfunction, uterine fibroids and may also be due to cancer of the ovary, uterus or cervix.

Oligomenorrhoea is a condition with infrequent menstrual periods. It occurs in women of childbearing age. Some variation in menstruation is normal, but a woman who regularly goes more than 35 days without menstruating may be diagnosed with oligomenorrhoea.

Menstrual Hygiene

Menstrual hygiene is vital for good health, well-being, dignity, empowerment and productivity of women. The impact of poor menstrual hygiene on girls is increased stress levels, fear and embarrassment during menstruation. This can keep girls inactive during such periods leading to absenteeism from school.

Clean and safe absorbable clothing materials, sanitary napkins, pads, tampons and menstrual cups have been identified as materials used to manage menstruation. Changing sanitary material 4 to 5 hours as per the requirement, provides comfort, cleanliness and protection from infections. It also helps in enhancing the quality of life of women during this period. Used sanitary napkins should be wrapped in paper and disposed. It should not be thrown in open areas or drain pipe of toilets. Flushing of sanitary napkins in the drain pipes causes choking of the drainage line leading to water pollution.

Box Item: Disposal of Napkins

The ecofriendly way to dispose menstrual waste scientifically and hygienically is to destroy the sanitary napkins using incinerators. Measures are being taken to install incinerators and napkin vending machines in washrooms of schools, colleges and public facilities.

Menopause

Menopause is the phase in a woman's life when ovulation and menstruation stops. The average age of menopause is 45 to 50 years. It indicates the permanent cessation of the primary functions of the ovaries.

2.5 Fertilization and Implantation

Fertilization occurs when a haploid sperm fuses with a haploid ovum to form a fertilized egg or diploid zygote. The sperms deposited in the female reproductive tract undergo capacitation, which is a biochemical event that enables the sperm to penetrate and fertilise the egg. Fertilization occurs only if the ovum and sperms are transported simultaneously to the ampullary isthmic junction of the fallopian tube.

Before a sperm can enter the egg, it must penetrate the multiple layers of granulosa (follicular) cells which are around the ovum forming the corona radiata (Figure 2.10). The follicular cells are held together by an adhesive cementing substance called hyaluronic acid. The acrosomal membrane disintegrates releasing the proteolytic enzyme, hyaluronidase during sperm entry through the corona radiata and zona pellucida. This is called acrosomal reaction. Once Fertilization is accomplished, cortical granules from the cytoplasm of the ovum form a barrier called the Fertilization membrane around the ovum preventing further penetration of other sperms. Thus polyspermy is prevented.

The first cleavage produces two identical cells called blastomeres. These produce 4 cells, then 8 and so on. After 72 hours of Fertilization, a loose collection of cells forms a berry shaped cluster of 16 or more cells called the morula (Figure 2.11). Under the influence of progesterone, smooth muscles of the fallopian tube relax and the dividing embryo takes 4 to 5 days to move through the fallopian tube into the uterine cavity and finally gets implanted in the uterine wall.

At this point the embryo consists of a fluid filled hollow ball of about 100 cells, called the blastocyst. The blastocyst is composed of a single layer of large flattened cells called trophoblast and a small cluster of 20 to 30 rounded cells called the inner cell mass. The inner cell mass of the blastocyst develops into the embryo and becomes embedded in the endometrium of the uterus. This process is called implantation and it results in pregnancy.

If the fertilized ovum is implanted outside the uterus it results in ectopic pregnancy. About 95 percent of ectopic pregnancies occur in the fallopian tube. The growth of the embryo may cause internal bleeding, infection and in some cases even death due to rupture of the fallopian tube.

Box Item: Twins

Twins are two offsprings produced in the same pregnancy.

• Monozygotic (Identical) twins are produced when a single fertilized egg splits into two during the first cleavage. They are of the same sex, look alike and share the same genes.
• Dizygotic (Fraternal) twins are produced when two separate eggs are fertilized by two separate sperms. The twins may be of the same sex or different sex and are non-identical.
• Siamese (United) twins are the conjoined twins who are joined during birth.

2.6 Maintenance of Pregnancy and Embryonic Development

The inner cell mass in the blastula is differentiated into epiblast and hypoblast immediately after implantation. The hypoblast is the embryonic endoderm and the epiblast is the ectoderm. The cells remaining in between the epiblast and the endoderm form the mesoderm. Thus the transformation of the blastocyst into a gastrula with the primary germ layers by the movement of the blastomeres is called gastrulation. Each germ layer gives rise to specific tissues, organs and organ systems during organogenesis.

The extra embryonic membranes namely the amnion, yolk sac, allantois and chorion protect the embryo from dessication, mechanical shock and help in the absorption of nutrients and exchange of gases (Figure 2.12).

The amnion is a double layered translucent membrane filled with the amniotic fluid. It provides a buoyant environment to protect the developing embryo from injury, regulates the temperature of the foetus and provides a medium in which the foetus can move.

The yolk sac forms a part of the gut and is the source of the earliest blood cells and blood vessels.

The allantois forms a small out pocketing of embryonic tissue at the caudal end of the yolk sac. It is the structural base for the umbilical cord that links the embryo to the placenta and ultimately it becomes part of the urinary bladder.

The chorion is the outermost membrane which encloses the embryo and all other membranes and also helps in the formation of the placenta.

The trophoblast cells in the blastocyst send out several fingerlike projections called chorionic villi carrying foetal blood and are surrounded by sinuses that contain maternal blood. The chorionic villi and the uterine tissues form the disc-shaped placenta. Placenta is a temporary endocrine organ formed during pregnancy and it connects the foetus to the uterine wall through the umbilical cord. It is the organ by which the nutritive, respiratory and excretory functions are fulfilled. The embryo's heart develops during the fourth week of pregnancy and circulates blood through the umbilical cord and placenta as well as through its own tissues.

Primary Germ Layers and Their Derivatives

• Ectoderm gives rise to the central nervous system (brain and spinal cord), peripheral nervous system, epidermis and its derivatives and mammary glands.
• Mesoderm: The connective tissue, cartilage and bone, muscles, organs of urinogenital system (kidney, ureter and gonads) arise from the mesoderm.
• Endoderm: The endodermal derivatives are epithelium of gastrointestinal and respiratory tract, liver, pancreas, thyroid and parathyroids.

Human pregnancy lasts for about 280 days or 40 weeks and is called the gestation period. It can be divided for convenience into three trimesters of three months each.

Trimesters of Pregnancy

• First Trimester: The main period of organogenesis; body organs namely the heart, limbs, lungs, liver and external genital organs are well developed.
• Second Trimester: The face is well formed with features, eyelids and eyelashes, eyes blink, body is covered with fine hair, muscle tissue develops and bones become harder.
• Third Trimester: The foetus is fully developed and is ready for delivery by the end of nine months.

During pregnancy, the placenta acts as a temporary endocrine gland and produces large quantities of human Chorionic Gonadotropin (hCG), human Chorionic Somatomammotropin or human Placental Lactogen (hPL), oestrogens and progesterone which are essential for a normal pregnancy. A hormone called relaxin is also secreted during the later phase of pregnancy which helps in relaxation of the pelvic ligaments at the time of parturition. It should be noted that human Chorionic Gonadotropin, human Chorionic Somatomammotropin and relaxin are produced only during pregnancy. In addition, during pregnancy the level of other hormones like oestrogen and progesterone, cortisol, prolactin, thyroxine, etc., is increased several folds in the maternal blood. These hormones are essential for supporting foetal growth.

Do You Know?

A female uterus is normally about 3 inches long and 2 inches wide but can expand 20 times during pregnancy.

2.7 Parturition and Lactation

Parturition is the completion of pregnancy and giving birth to the baby. The series of events that expels the infant from the uterus is collectively called "labour". Throughout pregnancy the uterus undergoes periodic episodes of weak and strong contractions. These contractions called Braxter-Hick's contractions lead to false labour.

As the pregnancy progresses, increase in the oestrogen concentration promotes uterine contractions. These uterine contractions facilitate moulding of the foetus and downward movement of the foetus. The descent of the foetus causes dilation of cervix of the uterus and vaginal canal resulting in a neurohumoral reflex called Foetal ejection reflex or Ferguson reflex. This initiates the secretion of oxytocin from the neurohypophysis which in turn brings about the powerful contraction of the uterine muscles and leads to the expulsion of the baby through the birth canal. This sequence of events is called as parturition or childbirth.

Relaxin is a hormone secreted by the placenta and also found in the corpus luteum. It promotes parturition by relaxing the pelvic joints and by dilatation of the cervix with continued powerful contractions. The amnion ruptures and the amniotic fluid flows out through the vagina, followed by the foetus. The placenta along with the remains of the umbilical cord called "after birth" is expelled out after delivery.

Box Item: Caesarean

When normal vaginal delivery is not possible due to factors like position of the baby and nature of the placenta, the baby is delivered through a surgical incision in the woman's abdomen and uterus. It is also termed as abdominal delivery or Caesarean Section or 'C' Section.

Lactation is the production of milk by mammary glands. The mammary glands show changes during every menstrual cycle, during pregnancy and lactation. Increased level of oestrogens, progesterone and human Placental Lactogen towards the end of pregnancy stimulate the hypothalamus towards prolactin releasing factors. The anterior pituitary responds by secreting prolactin which plays a major role in lactogenesis. Oxytocin causes the "Let-Down" reflex — the actual ejection of milk from the alveoli of the mammary glands. During lactation, oxytocin also stimulates the recently emptied uterus to contract, helping it to return to pre pregnancy size.

Box Item: Colostrum

Colostrum, a nutrient rich fluid produced by the human female immediately after giving birth, is loaded with immune, growth and tissue repair factors. It acts as a natural antimicrobial agent to actively stimulate the maturation of the infant's immune system. No artificial feed can substitute the first milk, with all its natural benefits and therefore should be definitely fed to the baby after birth.

The mammary glands secrete a yellowish fluid called colostrum during the initial few days after parturition. It has less lactose than milk and almost no fat, but it contains more proteins, vitamin A and minerals. Colostrum is also rich in Immunoglobulin A antibodies. This helps to protect the infant's digestive tract against bacterial infection.

Breast milk is the ideal food for infants as it contains all the constituents in suitable concentration and is easily digestible. It is fully sufficient till about 6 months of age and all infants must be breast fed by the mother to ensure the growth of a healthy baby.

Summary

• Reproduction is a process which helps in the continuity and maintenance of a species. Human beings are sexually reproducing and viviparous.
• The reproductive events include gametogenesis, insemination, Fertilization, cleavage, implantation, placentation, gastrulation, organogenesis and parturition.
• The female reproductive system consists of a pair of ovaries, a pair of oviducts, uterus, cervix, vagina and external genitalia.
• The male reproductive system consists of a pair of testes, a pair of duct system, accessory glands and external genitalia called penis.
• The process of formation of gametes in the male is called spermatogenesis and in the female is called oogenesis.
• The reproductive cycle in females is called menstrual cycle and it is initiated at puberty.
• The ovum released during the menstrual cycle is fertilized by the sperm and the zygote is formed. Zygote undergoes repeated mitotic division and the blastocyst is implanted on the walls of the uterus.
• It takes about 280 days or 40 weeks for the entire development of the human foetus and it is delivered out through the process of child birth or parturition.
• The new born baby is breast fed by the mother.

Box Item: World Breastfeeding Week

World Breastfeeding Week is observed during the first week of August. It is organized and promoted worldwide by World Alliance for Breastfeeding Action, World Health Organization and United Nations International Children's Emergency Fund to stress the importance of breast feeding during the first six months of baby's life and a supplemented breast feeding for two years in order to encourage new mothers for the healthy growth and development of their children, to guard them from lethal health problems and diseases including neonatal jaundice, pneumonia, cholera, etc. The Government of Tamil Nadu has also initiated various projects like Mother's Milk Bank, Feeding rooms in bus terminals and also organises awareness campaigns during the first week of August to highlight the importance of breast feeding to infants.

Box Item: Interesting Facts

1. Males are said to be sterile when they fail to produce viable sperms.
2. Azoospermia refers to the failure of spermatogenesis.
3. Enlargement of prostate gland is called prostatitis and can lead to difficulty in urination.
4. Castration or surgical removal of testis is known as orchidectomy.
5. Spermarche is the first ejaculation of the semen.

Case Study: Arunachalam Muruganantham — Inventor and Social Entrepreneur

Arunachalam Muruganantham is the man behind the world's first low cost sanitary napkin making machine. His mission was to provide sanitary napkins at minimal cost to poor women across the country, especially in rural areas. The journey began when he was shocked by the fact that women in India including his wife often used things such as old rags, leaves and even ash during menstruation. Approximately 70 percent of all reproductive diseases in India are caused by poor menstrual hygiene. 23 percent of girls drop out of schools once they attain puberty.

He wished to make a social impact by creating more livelihoods and improving the menstrual hygiene of rural women. Arunachalam initiated his research in 1999 and almost after 5 years, successfully created a low-cost machine for the production of sanitary napkins. He presented his prototype to IIT, Madras for a national innovation competition in 2006 and out of 943 entries, his machine stood first. Arunachalam made 250 machines in 18 months and set out to states in Northern India namely Bihar, Madhya Pradesh, Rajasthan and Uttar Pradesh.

Arunachalam Muruganantham was named one of the Time Magazine's 100 most influential people in 2014. He was awarded the Padma Shri in 2016.

Evaluation

Exercise Multiple Choice Questions

1. The mature sperms are stored in the

a. Seminiferous tubules b. Vas deferens c. Epididymis d. Seminal vesicle

2. The male sex hormone testosterone is secreted from

a. Sertoli cells b. Leydig cell c. Epididymis d. Prostate gland

3. The glandular accessory organ which produces the largest proportion of semen is

a. Seminal vesicle b. Bulbourethral gland c. Prostate gland d. Mucous gland

4. The male homologue of the female clitoris is

a. Scrotum b. Penis c. Urethra d. Testis

5. The site of embryo implantation is the

a. Uterus b. Peritoneal cavity c. Vagina d. Fallopian tube

6. The foetal membrane that forms the basis of the umbilical cord is

a. Allantois b. Amnion c. Chorion d. Yolk sac

7. The most important hormone in initiating and maintaining lactation after birth is

a. Oestrogen b. Follicle stimulating hormone c. Prolactin d. Oxytocin

8. Mammalian egg is

a. Mesolecithal and non cleidoic b. Microlecithal and non cleidoic c. Alecithal and non cleidoic d. Alecithal and cleidoic

9. The process which the sperm undergoes before penetrating the ovum is

a. Spermiation b. Cortical reaction c. Spermiogenesis d. Capacitation

10. The milk secreted by the mammary glands soon after child birth is called

a. Mucous b. Colostrum c. Lactose d. Sucrose

11. Colostrum is rich in

a. Immunoglobulin E b. Immunoglobulin A c. Immunoglobulin D d. Immunoglobulin M

12. The Androgen Binding Protein is produced by

a. Leydig cells b. Hypothalamus c. Sertoli cells d. Pituitary gland

13. Find the wrongly matched pair

a. Bleeding phase – fall in oestrogen and progesterone b. Follicular phase – rise in oestrogen c. Luteal phase – rise in Follicle Stimulating Hormone level d. Ovulatory phase – Lutenizing Hormone surge Exercise Assertion and Reason Questions

Directions: Choose the correct answer from the following options:

• a. Assertion and Reason are true, Reason is the correct explanation of Assertion
• b. Assertion and Reason are true, Reason is not the correct explanation of Assertion
• c. Assertion is true, Reason is false
• d. Both Assertion and Reason are false

14. Assertion: In human male, testes are extra abdominal and lie in scrotal sacs.
Reason: Scrotum acts as thermoregulator and keeps temperature lower by 2°C for normal sperm production.

a. Assertion and Reason are true, Reason is the correct explanation of Assertion b. Assertion and Reason are true, Reason is not the correct explanation of Assertion c. Assertion is true, Reason is false d. Both Assertion and Reason are false

15. Assertion: Ovulation is the release of ovum from the Graafian follicle.
Reason: It occurs during the follicular phase of the menstrual cycle.

a. Assertion and Reason are true, Reason is the correct explanation of Assertion b. Assertion and Reason are true, Reason is not the correct explanation of Assertion c. Assertion is true, Reason is false d. Both Assertion and Reason are false

16. Assertion: Head of the sperm consists of acrosome and mitochondria.
Reason: Acrosome contains spiral rows of mitochondria.

a. Assertion and Reason are true, Reason is the correct explanation of Assertion b. Assertion and Reason are true, Reason is not the correct explanation of Assertion c. Assertion is true, Reason is false d. Both Assertion and Reason are false

17. Painful menstruation is termed as

a. Dysmenorrhoea b. Menorrhagia c. Amenorrhoea d. Oligomenorrhoea

18. Which one of the following menstrual irregularities is correctly matched?

a. Menorrhagia – Excessive menstruation b. Amenorrhoea – Absence of menstruation c. Dysmenorrhoea – Irregularity of menstruation d. Oligomenorrhoea – Painful menstruation Exercise Short Answer Questions

19. Mention the differences between spermiogenesis and spermatogenesis.

20. At what stage of development are the gametes formed in new born male and female?

21. Expand the acronyms:

a. FSH:

b. LH:

c. hCG:

d. hPL:

22. How is polyspermy avoided in humans?

23. What is colostrum? Write its significance.

24. Placenta is an endocrine tissue. Justify.

25. Draw a labeled sketch of a spermatozoan.

26. What is inhibin? State its functions.

27. Mention the importance of the position of the testes in humans.

28. What is the composition of semen?

Exercise Long Answer Questions

29. Explain the process of fertilization and implantation of the fertilized ovum.

30. Define gametogenesis.

31. Describe the structure of the human ovum with a neat labelled diagram.

32. Give a schematic representation of spermatogenesis and oogenesis in humans.

33. Explain the various phases of the menstrual cycle.

34. Explain the role of oxytocin and relaxin in parturition and lactation.

35. Identify the given image and label its parts marked as a, b, c and d.

Label	Part Name
a
b
c
d

36. The following is the illustration of the sequence of ovarian events (a to i) in a human female.

a) Identify the figure that illustrates ovulation and mention the stage of oogenesis it represents.

b) Name the ovarian hormone and the pituitary hormone that have caused the above-mentioned events.

c) Explain the changes that occurs in the uterus simultaneously in anticipation.

d) Write the difference between C and H.

37. List the various menstrual disorders.

ICT Corner: Human Reproduction — Baby's Journey to the World

Procedure

• Step 1: Use the URL or scan the QR Code to launch the "Stages of Development before Birth" activity page.
• Step 2: On the right of the window, click "Video" and view the development of embryo during that particular stage.
• Step 3: Click "Show Features" to know the parts. Click "Heartbeat-Symbol" to hear the heartbeat of the embryo at that particular stage. Click "Weighing Machine" placed below to know the weight of the offspring at that stage.
• Step 4: Repeat the above steps with the different weeks by clicking the respective week tabs placed below.

Human Reproduction URL: http://www.glencoe.com/sites/common_assets/science/virtual_labs/LS26/LS26.html

*Pictures are indicative only. *Allow flash player.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. The mature sperms are stored in the

a) Seminiferous tubules b) Vas deferens c) Epididymis d) Seminal vesicle

Answer: c) Epididymis

Explanation: The epididymis is a single highly coiled tube that temporarily stores the spermatozoa and they undergo physiological maturation and acquire increased motility and fertilizing capacity.

2. The male sex hormone testosterone is secreted from

a) Sertoli cells b) Leydig cell c) Epididymis d) Prostate gland

Answer: b) Leydig cell

Explanation: Interstitial cells or Leydig cells are endocrine in nature and secrete androgens namely the testosterone hormone which initiates the process of spermatogenesis.

3. The glandular accessory organ which produces the largest proportion of semen is

a) Seminal vesicle b) Bulbourethral gland c) Prostate gland d) Mucous gland

Answer: a) Seminal vesicle

Explanation: The seminal vesicles secrete an alkaline fluid called seminal plasma containing fructose sugar, ascorbic acid, prostaglandins and a coagulating enzyme called vesiculase which enhances sperm motility.

4. The male homologue of the female clitoris is

a) Scrotum b) Penis c) Urethra d) Testis

Answer: b) Penis

Explanation: The penis is the male external genitalia functioning as a copulatory organ and is the male homologue of the female clitoris.

5. The site of embryo implantation is the

a) Uterus b) Peritoneal cavity c) Vagina d) Fallopian tube

Answer: a) Uterus

Explanation: The inner cell mass of the blastocyst develops into the embryo and becomes embedded in the endometrium of the uterus — this process is called implantation and it results in pregnancy.

6. The foetal membrane that forms the basis of the umbilical cord is

a) Allantois b) Amnion c) Chorion d) Yolk sac

Answer: a) Allantois

Explanation: The allantois forms a small outpocketing of embryonic tissue at the caudal end of the yolk sac. It is the structural base for the umbilical cord that links the embryo to the placenta.

7. The most important hormone in initiating and maintaining lactation after birth is

a) Oestrogen b) Follicle stimulating hormone c) Prolactin d) Oxytocin

Answer: c) Prolactin

Explanation: The anterior pituitary responds by secreting prolactin which plays a major role in lactogenesis (milk production).

8. Mammalian egg is

a) Mesolecithal and non cleidoic b) Microlecithal and non cleidoic c) Alecithal and non cleidoic d) Alecithal and cleidoic

Answer: c) Alecithal and non cleidoic

Explanation: Human ovum is non cleidoic, alecithal and microscopic in nature, as stated in the description of the structure of the ovum.

9. The process which the sperm undergoes before penetrating the ovum is

a) Spermiation b) Cortical reaction c) Spermiogenesis d) Capacitation

Answer: d) Capacitation

Explanation: The sperms deposited in the female reproductive tract undergo capacitation, which is a biochemical event that enables the sperm to penetrate and fertilise the egg.

10. The milk secreted by the mammary glands soon after child birth is called

a) Mucous b) Colostrum c) Lactose d) Sucrose

Answer: b) Colostrum

Explanation: The mammary glands secrete a yellowish fluid called colostrum during the initial few days after parturition.

11. Colostrum is rich in

a) Immunoglobulin E b) Immunoglobulin A c) Immunoglobulin D d) Immunoglobulin M

Answer: b) Immunoglobulin A

Explanation: Colostrum is also rich in Immunoglobulin A antibodies, which helps to protect the infant's digestive tract against bacterial infection.

12. The Androgen Binding Protein is produced by

a) Leydig cells b) Hypothalamus c) Sertoli cells d) Pituitary gland

Answer: c) Sertoli cells

Explanation: FSH stimulates testicular growth and enhances the production of Androgen Binding Protein by the sertoli cells and helps in the process of spermiogenesis.

13. Find the wrongly matched pair

a) Bleeding phase – fall in oestrogen and progesterone b) Follicular phase – rise in oestrogen c) Luteal phase – rise in FSH level d) Ovulatory phase – LH surge

Answer: c) Luteal phase – rise in Follicle Stimulating Hormone level

Explanation: During the luteal phase, the corpus luteum secretes large amounts of progesterone (not FSH). FSH rises during the follicular phase, not the luteal phase.

14. Assertion: In human male, testes are extra abdominal and lie in scrotal sacs. Reason: Scrotum acts as thermoregulator and keeps temperature lower by 2°C for normal sperm production.

a) Assertion and Reason are true, Reason is the correct explanation of Assertion b) Assertion and Reason are true, Reason is not the correct explanation of Assertion c) Assertion is true, Reason is false d) Both Assertion and Reason are false

Answer: a) Assertion and Reason are true, Reason is the correct explanation of Assertion

Explanation: The scrotum is placed outside the abdominal cavity to provide a temperature 2–3°C lower than the normal internal body temperature, thus acting as a thermoregulator for spermatogenesis.

15. Assertion: Ovulation is the release of ovum from the Graafian follicle. Reason: It occurs during the follicular phase of the menstrual cycle.

a) Assertion and Reason are true, Reason is the correct explanation of Assertion b) Assertion and Reason are true, Reason is not the correct explanation of Assertion c) Assertion is true, Reason is false d) Both Assertion and Reason are false

Answer: c) Assertion is true, Reason is false

Explanation: Ovulation is indeed the release of the ovum from the Graafian follicle, but it occurs during the ovulatory phase (around the 14th day), not the follicular phase.

16. Assertion: Head of the sperm consists of acrosome and mitochondria. Reason: Acrosome contains spiral rows of mitochondria.

a) Assertion and Reason are true, Reason is the correct explanation of Assertion b) Assertion and Reason are true, Reason is not the correct explanation of Assertion c) Assertion is true, Reason is false d) Both Assertion and Reason are false

Answer: d) Both Assertion and Reason are false

Explanation: The head of sperm consists of acrosome and nucleus (not mitochondria). Mitochondria are spirally arranged in the middle piece, not the acrosome. Hence both statements are false.

17. Painful menstruation is termed as

a) Dysmenorrhoea b) Menorrhagia c) Amenorrhoea d) Oligomenorrhoea

Answer: a) Dysmenorrhoea

Explanation: Pain associated with menstruation is called dysmenorrhoea. It is the most commonly reported menstrual disorder.

18. Which one of the following menstrual irregularities is correctly matched?

a) Menorrhagia – Excessive menstruation b) Amenorrhoea – absence of menstruation c) Dysmenorrhoea – irregularity of menstruation d) Oligomenorrhoea – painful menstruation

Answer: b) Amenorrhoea – absence of menstruation

Explanation: Absence of menstruation is called amenorrhoea, as stated in the chapter. Menorrhagia is heavy and prolonged menstrual period; Dysmenorrhoea is painful menstruation; Oligomenorrhoea is infrequent menstrual periods.

IV. Answer Briefly

19. Mention the differences between spermiogenesis and spermatogenesis.

Answer: Spermatogenesis is the entire sequence of events in the seminiferous tubules that produces male gametes (sperms) from spermatogonia through meiotic divisions. Spermiogenesis is specifically the transformation of haploid spermatids into mature spermatozoa. Thus, spermiogenesis is only the final step within the broader process of spermatogenesis.

20. At what stage of development are the gametes formed in new born male and female?

Answer: In new born males, the immature germ cells called spermatogonia are present in the inner surfaces of the seminiferous tubules; spermatogenesis begins only at puberty. In new born females, primary oocytes are formed which are temporarily arrested at Prophase I of meiosis I; these are surrounded by granulosa cells to form primordial follicles present at birth.

21. Expand the acronyms: a. FSH b. LH c. hCG d. hPL

Answer: a. FSH – Follicle Stimulating Hormone
b. LH – Luteinizing Hormone
c. hCG – human Chorionic Gonadotropin
d. hPL – human Placental Lactogen (also called human Chorionic Somatomammotropin)

22. How is polyspermy avoided in humans?

Answer: Once fertilization is accomplished, cortical granules from the cytoplasm of the ovum form a barrier called the fertilization membrane around the ovum. This fertilization membrane prevents the further penetration of other sperms, thus avoiding polyspermy.

23. What is colostrum? Write its significance.

Answer: Colostrum is a nutrient-rich yellowish fluid secreted by the mammary glands during the initial few days after parturition. It has less lactose and almost no fat, but contains more proteins, vitamin A, minerals and Immunoglobulin A antibodies. It acts as a natural antimicrobial agent, stimulates maturation of the infant's immune system and protects the infant's digestive tract against bacterial infection.

24. Placenta is an endocrine tissue. Justify.

Answer: Placenta acts as a temporary endocrine gland during pregnancy. It produces large quantities of human Chorionic Gonadotropin (hCG), human Chorionic Somatomammotropin (hPL), oestrogens, progesterone and relaxin. These hormones are essential for the maintenance of a normal pregnancy and foetal development, justifying placenta as an endocrine tissue.

25. Draw a labeled sketch of a spermatozoan.

Answer: The human sperm consists of the following parts:
Head: Contains acrosome (cap-like structure with hyaluronidase enzyme) and a flat oval nucleus.
Neck: Short region containing proximal centriole (towards nucleus) and distal centriole (gives rise to axial filament).
Middle piece: Contains mitochondria spirally arranged (mitochondrial spiral/nebenkern) around the axial filament; produces ATP for sperm movement.
Tail: Longest part; consists of a central axial filament (axoneme) and outer protoplasmic sheath; lashing movements propel the sperm forward. The entire sperm is enveloped by a plasma membrane.
(Students should draw and label the diagram as described above.)

26. What is inhibin? State its functions.

Answer: Inhibin is a hormone secreted by the Sertoli cells (nurse cells) of the seminiferous tubules in the testes. Its function is to act in the negative feedback control of sperm production, thereby regulating spermatogenesis and maintaining normal sperm output.

27. Mention the importance of the position of the testes in humans.

Answer: The testes are located in the scrotum, a sac of skin that hangs outside the abdominal cavity. Since viable sperms cannot be produced at normal body temperature, the scrotum provides a temperature 2–3°C lower than the normal internal body temperature. Thus, the scrotum acts as a thermoregulator for spermatogenesis, ensuring the production of healthy and viable sperms.

28. What is the composition of semen?

Answer: Semen or seminal fluid is a milky white fluid which contains sperms and seminal plasma. The seminal plasma is secreted from the seminal vesicles (alkaline fluid with fructose, ascorbic acid, prostaglandins and vesiculase), prostate gland (slightly acidic fluid with citrate, enzymes and prostate specific antigens) and bulbourethral glands (lubricating secretions). It acts as a transport medium, provides nutrients and facilitates sperm movement.

V. Answer in Detail

29. Explain the process of fertilization and implantation of the fertilized ovum.

Answer: Fertilization occurs when a haploid sperm fuses with a haploid ovum to form a diploid zygote. Sperms deposited in the female reproductive tract undergo capacitation, a biochemical event enabling the sperm to penetrate and fertilize the egg. Fertilization occurs at the ampullary-isthmic junction of the fallopian tube. The sperm first penetrates the corona radiata by releasing hyaluronidase (acrosomal reaction), then passes through the zona pellucida. Once fertilization is complete, cortical granules form a fertilization membrane preventing polyspermy.

The zygote undergoes rapid mitotic divisions (cleavage) forming a morula (16 or more cells) after 72 hours. The dividing embryo moves through the fallopian tube into the uterine cavity over 4–5 days, forming a blastocyst — a fluid-filled ball of about 100 cells. The blastocyst consists of an outer trophoblast layer and an inner cell mass. The inner cell mass embeds into the endometrium of the uterus — this is implantation — resulting in pregnancy.

30. Define gametogenesis.

Answer: Gametogenesis is the process of formation of gametes (sperms and ova) from the primary sex organs in all sexually reproducing organisms. Meiosis plays the most significant role in the process of gametogenesis. In males it is called spermatogenesis and in females it is called oogenesis.

31. Describe the structure of the human ovum with a neat labelled diagram.

Answer: Human ovum is non cleidoic, alecithal and microscopic in nature. Its cytoplasm is called ooplasm and contains a large nucleus called the germinal vesicle.

The ovum is surrounded by three coverings:
1. Vitelline membrane – the innermost thin transparent layer.
2. Zona pellucida – the middle thick layer.
3. Corona radiata – the outermost thick coat of follicular cells.

Between the vitelline membrane and zona pellucida is a narrow space called the perivitelline space.
(Students should draw and label the ovum showing ooplasm, germinal vesicle, vitelline membrane, zona pellucida, corona radiata, and perivitelline space.)

32. Give a schematic representation of spermatogenesis and oogenesis in humans.

Answer: Spermatogenesis:
Spermatogonia (2n) → [Mitosis] → Primary spermatocytes (2n, 46 chromosomes) → [Meiosis I] → Two Secondary spermatocytes (n, 23 chromosomes each) → [Meiosis II] → Four Spermatids (n) → [Spermiogenesis] → Four Mature Spermatozoa (n)

Oogenesis:
Oogonia (2n) → [Mitosis during foetal life] → Primary oocyte (2n, arrested at Prophase I)

📝 Summary

1. The human reproductive system has four main functions: producing gametes (sperms and ova), transporting and sustaining gametes, nurturing the developing offspring, and producing hormones essential for reproduction.
2. Major reproductive events include gametogenesis, insemination, fertilization, cleavage, implantation, placentation, gastrulation, organogenesis, and parturition — each representing a critical stage from gamete formation to birth.
3. The male reproductive system comprises a pair of testes located in the scrotum, which maintains a temperature 2–3 degrees centigrade lower than normal body temperature to enable viable sperm production (spermatogenesis).
4. Each testis is divided into about 200 to 250 lobules, each containing 2 to 4 seminiferous tubules, which form 80 percent of the testicular substance and serve as the sites for sperm production.
5. Hormones secreted by the pituitary gland and the gonads regulate the development of secondary sexual characteristics, maturation of the reproductive system, and the normal functioning of the overall reproductive process.