Human Reproduction

1. Chapter at a Glance

Humans are sexually reproducing and viviparous. The reproductive events in humans include formation of gametes (gametogenesis), i.e. sperms in males and ovum in females; transfer of sperms into the female genital tract (insemination); fusion of male and female gametes (fertilisation) leading to formation of the zygote. This is followed by formation and development of the blastocyst and its attachment to the uterine wall (implantation), embryonic development (gestation) and delivery of the baby (parturition). These reproductive events occur after puberty. There are remarkable differences between the reproductive events in the male and in the female; for example, sperm formation continues even in old men, but formation of ovum ceases in women around the age of fifty years. This chapter covers the male and female reproductive systems, gametogenesis, the menstrual cycle, fertilisation and implantation, pregnancy and embryonic development, parturition and lactation. NEET asks three to four questions every year from this chapter — it is one of the most important chapters of the entire syllabus.

2. The Male Reproductive System

The male reproductive system is located in the pelvis region. It includes a pair of testes along with accessory ducts, glands and the external genitalia.

• The testes are situated outside the abdominal cavity within a pouch called the scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5°C lower than the normal internal body temperature) necessary for spermatogenesis.
• In adults, each testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm. The testis is covered by a dense covering. Each testis has about 250 compartments called testicular lobules.
• Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells. The male germ cells undergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide nutrition to the germ cells.
• The regions outside the seminiferous tubules, called interstitial spaces, contain small blood vessels and interstitial cells or Leydig cells. Leydig cells synthesise and secrete testicular hormones called androgens. Other immunologically competent cells are also present.

Accessory ducts, glands and external genitalia

• The male sex accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens. The seminiferous tubules of the testis open into the vasa efferentia through rete testis. The vasa efferentia leave the testis and open into the epididymis located along the posterior surface of each testis. The epididymis leads to the vas deferens, which ascends to the abdomen and loops over the urinary bladder. It receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct. These ducts store and transport the sperms from the testis to the outside through the urethra.
• The urethra originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus.
• The penis is the male external genitalia. It is made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of the penis called the glans penis is covered by a loose fold of skin called the foreskin.
• The male accessory glands include paired seminal vesicles, a prostate and paired bulbourethral glands. Secretions of these glands constitute the seminal plasma, which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands also help in the lubrication of the penis.

3. The Female Reproductive System

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. These parts of the system, along with a pair of the mammary glands, are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care.

• Ovaries are the primary female sex organs that produce the female gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen. Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The stroma is divided into two zones — a peripheral cortex and an inner medulla.
• The oviducts (fallopian tubes), uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10–12 cm long and extends from the periphery of each ovary to the uterus. The part closer to the ovary is the funnel-shaped infundibulum. The edges of the infundibulum possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider part of the oviduct called ampulla. The last part of the oviduct, isthmus, has a narrow lumen and it joins the uterus.
• The uterus is single and it is also called the womb. The shape of the uterus is like an inverted pear. It is supported by ligaments attached to the pelvic wall. The uterus opens into the vagina through a narrow cervix. The cavity of the cervix is called the cervical canal, which along with the vagina forms the birth canal.
• The wall of the uterus has three layers of tissue. The external thin membranous perimetrium, the middle thick layer of smooth muscle, myometrium, and the inner glandular layer called endometrium that lines the uterine cavity. The endometrium undergoes cyclical changes during the menstrual cycle, while the myometrium exhibits strong contraction during delivery of the baby.
• The female external genitalia include mons pubis, labia majora, labia minora, hymen and clitoris. The opening of the vagina is often covered partially by a membrane called the hymen. The hymen is often torn during the first coitus. However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling etc. In some women the hymen persists even after coitus. In fact, the presence or absence of hymen is not a reliable indicator of virginity or sexual experience. The clitoris is a tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening.

Mammary glands

A functional mammary gland is characteristic of all female mammals. The mammary glands are paired structures (breasts) that contain glandular tissue and variable amounts of fat. The glandular tissue of each breast is divided into 15–20 mammary lobes containing clusters of cells called alveoli. The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli. 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 through which milk is sucked out.

4. Gametogenesis

The primary sex organs — the testis in the males and the ovaries in the females — produce gametes, i.e. sperms and ovum, respectively, by the process called gametogenesis.

Spermatogenesis

• In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis, which begins at puberty. The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes.
• Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each.
• The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. What would be the number of chromosomes in the spermatids? (23.)
• The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called spermiation.

Hormonal control of spermatogenesis

Spermatogenesis starts at the age of puberty due to a significant increase in the secretion of gonadotropin-releasing hormone (GnRH). This is a hypothalamic hormone. The increased levels of GnRH then act at the anterior pituitary gland and stimulate secretion of two gonadotropins — luteinising hormone (LH) and follicle-stimulating hormone (FSH). LH acts at the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

Structure of a sperm

It is a microscopic structure composed of a head, neck, a middle piece and a tail. A plasma membrane envelops the whole body of the sperm. The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, the acrosome. The acrosome is filled with enzymes that help fertilisation of the ovum. The middle piece possesses numerous mitochondria, which produce energy for the movement of the tail that facilitates sperm motility essential for fertilisation. The human male ejaculates about 200 to 300 million sperms during a coitus, of which, for normal fertility, at least 60 per cent of sperms must have normal shape and size and at least 40 per cent of them must show vigorous motility.

Sperms released from the seminiferous tubules are transported by the accessory ducts. Secretions of epididymis, vas deferens, seminal vesicle and prostate are essential for maturation and motility of sperms. The seminal plasma along with the sperms constitute the semen. The functions of male sex accessory ducts and glands are maintained by the testicular hormones (androgens).

Oogenesis

• The process of formation of a mature female gamete is called oogenesis, which is markedly different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary; no more oogonia are formed and added after birth.
• These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000–80,000 primary follicles are left in each ovary.
• The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles. The secondary follicle soon transforms into a tertiary follicle, which is characterised by a fluid-filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa.
• It is important to draw your attention that it is at this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains bulk of the nutrient-rich cytoplasm of the primary oocyte.
• The tertiary follicle further changes into the mature follicle or Graafian follicle. The secondary oocyte forms a new membrane called zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.

5. Menstrual Cycle

The reproductive cycle in the female primates (e.g. monkeys, apes and human beings) is called the menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. One ovum is released (ovulation) during the middle of each menstrual cycle.

• Menstrual phase (days 1–5): the cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3–5 days. The menstrual flow results due to the breakdown of the endometrial lining of the uterus and its blood vessels, which forms liquid that comes out through the vagina. Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other underlying causes like stress, poor health etc.
• Follicular (proliferative) phase (days 6–13): during this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of the uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.
• Ovulatory phase (around day 14): both LH and FSH attain a peak level in the middle of the cycle (about the 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle, called the LH surge, induces rupture of the Graafian follicle and thereby the release of ovum (ovulation).
• Luteal (secretory) phase (days 15–28): the ovulation is followed by the luteal phase, during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone, which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy all events of the menstrual cycle stop and there is no menstruation. In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, marking a new cycle.
• In human beings, menstrual cycles cease around 50 years of age; that is termed as menopause. Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause.

6. Fertilisation and Implantation

• During copulation (coitus), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary–isthmic junction) of the fallopian tube. The ovum released by the ovary is also transported to the ampullary–isthmic junction, where fertilisation takes place. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary–isthmic junction. This is the reason why not all copulations lead to fertilisation and pregnancy.
• The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
• This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.
• The sex of the baby is decided at this stage. As you know, the chromosome pattern in the human female is XX and that in the male is XY. Therefore, all the haploid gametes produced by the female (ova) have the sex chromosome X, whereas in the male gametes (sperms) the sex chromosome could be either X or Y; hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y. After fusion of the male and female gametes, the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male. That is why, scientifically, it is correct to say that the sex of the baby is determined by the father and not by the mother.

Cleavage and implantation

• The mitotic division starts as the zygote moves through the isthmus of the oviduct, called cleavage, towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula.
• The morula continues to divide and transforms into a blastocyst as it moves further into the uterus. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation, and it leads to pregnancy.

7. Pregnancy and Embryonic Development

• After implantation, finger-like projections appear on the trophoblast called chorionic villi, which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between the developing embryo (foetus) and the maternal body called the placenta.
• The placenta facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo. The placenta is connected to the embryo through an umbilical cord, which helps in the transport of substances to and from the embryo.
• The placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens etc. In the later phase of pregnancy, a hormone called relaxin is also secreted by the ovary. Let us remember that hCG, hPL and relaxin are produced in women only during pregnancy. In addition, during pregnancy the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine etc. are increased several folds in the maternal blood. Increased production of these hormones is essential for supporting the foetal growth, metabolic changes in the mother and maintenance of pregnancy.
• Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm. These three layers give rise to all tissues (organs) in adults. It needs to be mentioned here that the inner cell mass contains certain cells called stem cells, which have the potency to give rise to all the tissues and organs.

8. Parturition and Lactation

The average duration of human pregnancy is about 9 months, which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition. Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta, which induce mild uterine contractions called the foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulate further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues, resulting in stronger and stronger contractions. This leads to expulsion of the baby out of the uterus through the birth canal — parturition. Soon after the infant is delivered, the placenta is also expelled out of the uterus.

The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation. This helps the mother in feeding the new-born. The milk produced during the initial few days of lactation is called colostrum, which contains several antibodies absolutely essential to develop resistance for the new-born babies. Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.

9. All Figures at a Glance

10. Master Summary Table

11. Mind Map

12. Tips and Tricks

13. Important One-liners

• Each testis has about 250 testicular lobules.
• Leydig cells secrete androgens.
• Spermiation is the release of sperms from seminiferous tubules.
• Spermiogenesis transforms spermatids into sperms.
• The acrosome contains enzymes for fertilisation.
• No oogonia are formed after birth.
• The antrum is the fluid-filled cavity of a tertiary follicle.
• Zona pellucida changes after sperm contact to block polyspermy.
• The embryo's heart forms after about one month of pregnancy.
• Most major organ systems form by the end of the first trimester.
• Inner cell mass contains stem cells.
• Colostrum provides antibodies to the newborn.

14. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

15. Quick Revision Summary