Human Reproduction: NEET notes

1. The male reproductive system

NCERT §2.1

• The male system sits in the pelvis and has a pair of testes, a set of accessory ducts, accessory glands and the external genitalia.
• The testes lie outside the abdominal cavity inside a pouch, the scrotum, which keeps them about 2–2.5 °C cooler than the internal body temperature; this lower temperature is needed for sperm production.
• An adult testis is oval, roughly 4–5 cm long and 2–3 cm wide, and is divided internally into about 250 compartments called testicular lobules.
• Each lobule holds one to three tightly coiled seminiferous tubules, where sperms are made. Their lining carries two cell types: male germ cells (spermatogonia), which divide by meiosis to form sperms, and Sertoli cells, which nourish the developing germ cells.
• The spaces between the tubules (interstitial spaces) contain small blood vessels, Leydig (interstitial) cells that make androgens such as testosterone, and some immunologically competent cells.
• Sperms leave the testis through a duct sequence: rete testis → vasa efferentia → epididymis (on the back surface of the testis) → vas deferens. The vas deferens loops over the urinary bladder, receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct.
• The urethra starts at the urinary bladder, runs through the penis and opens outside at the urethral meatus; it therefore carries both urine and semen.
• The penis is made of special tissue that helps it become erect for insemination. Its enlarged end, the glans penis, has a loose fold of skin over it called the foreskin.
• There are three kinds of accessory glands: paired seminal vesicles, a single prostate and paired bulbourethral glands. Together their secretions make up the seminal plasma, rich in calcium, fructose and certain enzymes; the bulbourethral secretions also help lubricate the penis.

2. The female reproductive system

NCERT §2.2

• Located in the pelvic region, the female reproductive system consists of two ovaries and two oviducts, the uterus, cervix and vagina, and the external genitalia; a pair of mammary glands is functionally linked to it.
• Ligaments hold each ovary, about 2–4 cm long, to the pelvic wall and the uterus. A thin epithelium covers it, and inside lies the ovarian stroma, with an outer cortex and an inner medulla. The ovaries produce ova as well as several steroid hormones.
• Each oviduct (fallopian tube) measures about 10–12 cm. The funnel-shaped infundibulum lies nearest the ovary, and its finger-like fimbriae help collect the released ovum; next comes a wider ampulla, and last a narrow isthmus that joins the uterus.
• The uterus (womb) is a single organ shaped like an inverted pear and held to the pelvic wall by ligaments. It opens into the vagina through a narrow cervix; the cervical canal together with the vagina forms the birth canal.
• The uterine wall has three layers: the outer thin perimetrium, the thick middle myometrium of smooth muscle that contracts strongly during childbirth, and the inner glandular endometrium that lines the cavity and changes cyclically during the menstrual cycle.
• The external genitalia are the labia majora, labia minora, mons pubis, clitoris and hymen. Above the urethral opening lies the clitoris, a small finger-like structure.
• The hymen partly covers the vaginal opening. It can tear during a fall, a sudden jolt, insertion of a tampon, active sport such as horse riding or cycling, or first intercourse, and in some women it persists after intercourse, so it is not a reliable sign of virginity.
• The breasts are paired mammary glands made of glandular tissue with a variable amount of fat. In each breast the glandular tissue is split into 15–20 mammary lobes, and each lobe has clusters of milk-secreting cells called alveoli.
• Milk made in the alveoli flows along this route: alveoli → mammary tubules → mammary duct → wider mammary ampulla → lactiferous duct, which opens at the nipple.

3. Gametogenesis: spermatogenesis

NCERT §2.3

• Gametogenesis is the making of sperms in the testis and ova in the ovary. In males it begins at puberty.
• Spermatogonia on the inner wall of the seminiferous tubules multiply by mitosis. Each is diploid with 46 chromosomes. Some of them become primary spermatocytes, which enter meiosis.
• Meiosis I of one primary spermatocyte gives two equal, haploid secondary spermatocytes with 23 chromosomes each; meiosis II of these gives four equal, haploid spermatids.
• Spermiogenesis is the change of a spermatid into a sperm (spermatozoon); no division occurs. Spermiation is the release of sperms from the seminiferous tubules after spermiogenesis.
• Hormonal control: at puberty the hypothalamus markedly increases its release of GnRH, which acts on the anterior pituitary to release LH and FSH.
• LH acts on Leydig cells and makes them produce androgens, which in turn stimulate spermatogenesis. FSH acts on Sertoli cells and makes them release factors that help spermiogenesis.
• A sperm has a head, neck, middle piece and tail, all wrapped in a plasma membrane. The head has an elongated haploid nucleus capped in front by the acrosome, which carries enzymes needed for fertilisation.
• The middle piece is packed with mitochondria that supply energy for the tail's movement, which gives the sperm the motility it needs to reach the egg.
• About 200–300 million sperms are released in one ejaculation. Normal fertility requires that at least 60 per cent of them be of normal shape and size, and that at least 40 per cent move vigorously.
• Sperms from the seminiferous tubules travel through the accessory ducts, and the secretions of the epididymis, vas deferens, seminal vesicles and prostate are needed for their maturation and motility. Sperms plus seminal plasma make up semen. The ducts and glands depend on androgens to work.

4. Gametogenesis: oogenesis

NCERT §2.3

• Oogenesis starts before birth: during embryonic development, a couple of million oogonia (gamete mother cells) form in each foetal ovary, and no new oogonia are added after birth.
• Oogonia begin meiosis and halt in prophase I; these arrested cells are primary oocytes. A primary oocyte surrounded by a layer of granulosa cells is a primary follicle.
• Many follicles degenerate between birth and puberty, so each ovary has only about 60,000–80,000 primary follicles left at puberty.
• A secondary follicle forms when a primary follicle is surrounded by additional layers of granulosa cells and acquires a new theca. Once a fluid-filled cavity called the antrum appears, the follicle is tertiary, and its theca has become organised into theca interna and theca externa.
• Inside the tertiary follicle the primary oocyte grows and finishes meiosis I. The division is unequal: it gives a large haploid secondary oocyte that keeps most of the cytoplasm and a tiny first polar body.
• The tertiary follicle matures into the Graafian follicle, in which the secondary oocyte lies enclosed by a covering called the zona pellucida.
• At ovulation the Graafian follicle ruptures and releases the secondary oocyte (the ovum). Meiosis II of this cell is completed only if a sperm enters it.
• Net result: one primary spermatocyte gives four sperms, but one primary oocyte gives only one functional ovum, with the rest of the chromosomes discarded in polar bodies.

5. The menstrual cycle

NCERT §2.4

• Females of primates (monkeys, apes and humans) show a reproductive cycle called the menstrual cycle. The first menstruation at puberty is menarche.
• In humans the cycle repeats roughly every 28–29 days, and one ovum is normally released around its middle.
• Menstrual phase: bleeding lasts about 3–5 days as the endometrial lining and its blood vessels break down and leave through the vagina. It happens only when the released ovum was not fertilised; missing a period may point to pregnancy but can also come from stress or poor health.
• Follicular (proliferative) phase: primary follicles grow into a fully mature Graafian follicle, and the endometrium rebuilds itself by proliferation. These changes are driven by pituitary and ovarian hormones.
• During the follicular phase LH and FSH rise gradually, promoting follicle growth and estrogen secretion by the growing follicles.
• LH and FSH both peak mid-cycle, around day 14. The sharp LH surge causes the Graafian follicle to rupture, which is ovulation (ovulatory phase).
• Luteal (secretory) phase: the remains of the ruptured follicle form the corpus luteum, which secretes large amounts of progesterone. Progesterone keeps the endometrium in shape, and a maintained endometrium is needed for implantation.
• If fertilisation happens, the cycle's events stop and there is no menstruation through pregnancy. If it does not, the corpus luteum degenerates, the endometrium breaks down and a new cycle begins with menstruation.
• Cycles stop around 50 years of age; this is menopause. Regular cycles mark the normal reproductive span between menarche and menopause.

6. Fertilisation and implantation

NCERT §2.5

• Semen is deposited in the vagina during copulation; this is called insemination. The motile sperms swim through the cervix and the uterus until they reach the fallopian tube's ampullary region.
• The ovum too is carried to the ampullary region, and fertilisation happens there. It can occur only when the ovum and the sperms arrive in this region together, which is why not every act of intercourse results in pregnancy.
• When a sperm touches the zona pellucida, it triggers changes in the membrane that block entry of any more sperms, so only one sperm fertilises an ovum.
• Acrosomal secretions help the sperm pass through the zona pellucida and plasma membrane into the ovum's cytoplasm.
• Entry of the sperm triggers the secondary oocyte to complete meiosis II. This division, also unequal, produces a haploid ovum (ootid) and a second polar body. The haploid sperm and ovum nuclei then fuse, forming the diploid zygote.
• Sex is fixed at fertilisation. The female gives only X-bearing ova, while sperms carry either X or Y (50 per cent each). An X sperm gives an XX (female) zygote and a Y sperm gives an XY (male) zygote, so the father's sperm decides the baby's sex.
• The zygote divides by mitosis (cleavage) while moving through the isthmus towards the uterus, forming 2, 4, 8, 16 cells called blastomeres. The 8–16 cell stage is the morula.
• The morula keeps dividing and becomes the blastocyst as it moves further into the uterus. Its cells are arranged into an outer layer, the trophoblast, and an inner cell mass attached to it.
• The inner cell mass will give rise to the embryo, while the trophoblast becomes attached to the endometrium. The uterine cells then divide rapidly and cover the blastocyst, so it becomes embedded in the endometrium; this is implantation, and it leads to pregnancy.

7. Pregnancy and embryonic development

NCERT §2.6

• After implantation, finger-like chorionic villi grow out of the trophoblast and are surrounded by uterine tissue and maternal blood. The villi and uterine tissue interlock to form the placenta, the structural and functional link between embryo and mother.
• The placenta supplies oxygen and nutrients to the embryo and removes carbon dioxide and excretory wastes. It is connected to the embryo by the umbilical cord, which carries substances to and from the embryo.
• The placenta is also an endocrine tissue, producing hCG (human chorionic gonadotropin), hPL (human placental lactogen), estrogens and progestogens, among others. Later in pregnancy the ovary secretes relaxin.
• hCG, hPL and relaxin are found in a woman only while she is pregnant. Pregnancy also raises the maternal blood levels of estrogens, progestogens, cortisol, prolactin and thyroxine several times over; this hormonal rise sustains the growing foetus, brings about metabolic changes in the mother and keeps the pregnancy going.
• Right after implantation, the inner cell mass differentiates into three layers: outer ectoderm, middle mesoderm and inner endoderm, which together give rise to all tissues and organs of the adult.
• The inner cell mass contains stem cells, which can give rise to every tissue and organ.
• Human pregnancy lasts about 9 months. The heart forms by the end of the first month, and the heartbeat can be heard with a stethoscope as the first sign of a growing foetus. Limbs and digits appear by the end of the second month.
• Most major organ systems, including well-developed limbs and external genital organs, have formed by week 12, the end of the first trimester.
• The first foetal movements are felt in the fifth month, when hair also appears on the head. By the end of about 24 weeks (second trimester), fine hair covers the body, the eyelids separate and eyelashes form. At the end of nine months the foetus is fully developed and ready for delivery.

8. Parturition and lactation

NCERT §2.7

• Parturition, the delivery of the foetus, is brought about by strong uterine contractions when pregnancy ends. A complex neuroendocrine mechanism controls it.
• The signals that start it come from the placenta and the fully developed foetus, and they produce mild uterine contractions called the foetal ejection reflex.
• This reflex triggers the release of oxytocin from the mother's pituitary. Oxytocin acts on the uterine muscle to cause stronger contractions, which in turn prompt still more oxytocin release, a positive feedback loop.
• The loop continues, with contractions growing stronger, until the baby is pushed out through the birth canal. The placenta is expelled from the uterus soon after birth.
• The mammary glands differentiate during pregnancy and begin producing milk towards its end; making milk is called lactation and it helps the mother feed her newborn.
• The milk of the first few days is colostrum. It contains several antibodies that are essential for building resistance in the newborn, which is why breastfeeding in this early period is recommended by doctors.

Must-know facts

1. Scrotal temperature is about 2–2.5 °C below internal body temperature; this is needed for spermatogenesis.
2. Each testis has about 250 testicular lobules, each with 1–3 seminiferous tubules.
3. Sertoli cells nourish germ cells and respond to FSH; Leydig cells make androgens and respond to LH.
4. Seminal plasma is rich in calcium, fructose and certain enzymes.
5. Oviduct length is about 10–12 cm; its parts from ovary to uterus are infundibulum (with fimbriae), ampulla, isthmus.
6. Uterine wall layers from outside: perimetrium, myometrium (contracts at delivery), endometrium (changes with the cycle).
7. One primary spermatocyte → 2 secondary spermatocytes → 4 spermatids → 4 sperms; spermatogonia and primary spermatocytes are diploid (46), later stages haploid (23).
8. Spermiogenesis = spermatid to sperm; spermiation = release of sperms from the seminiferous tubules.
9. A normal ejaculate has 200–300 million sperms; for fertility, at least 40% must show vigorous motility and at least 60% must be of normal shape and size.
10. Oogonia form only in foetal life; at puberty each ovary retains about 60,000–80,000 primary follicles.
11. Primary oocytes stay arrested in prophase I; meiosis I is completed inside the tertiary follicle, giving the secondary oocyte and the first polar body.
12. The fluid-filled antrum is the hallmark of the tertiary follicle; the secondary follicle is marked by extra granulosa layers and a new theca.
13. The ovulated cell is a secondary oocyte; meiosis II is completed only after sperm entry, releasing the second polar body.
14. The LH surge around day 14 triggers ovulation; the corpus luteum's progesterone maintains the endometrium.
15. Fertilisation occurs in the ampullary region of the fallopian tube.
16. Morula = 8–16 blastomeres; the blastocyst has an outer trophoblast and an inner cell mass; the trophoblast attaches to the endometrium.
17. Only during pregnancy do women produce hCG, hPL and relaxin; late in pregnancy, relaxin is secreted by the ovary.
18. Heart forms by end of month 1; most organ systems by end of week 12; first movements and head hair in month 5; eyelids separate by about week 24.
19. The foetal ejection reflex triggers oxytocin release, and oxytocin and uterine contractions boost each other by positive feedback.
20. Colostrum carries several antibodies that give the newborn resistance.

Common traps

Key terms

Seminiferous tubule

Coiled tubule inside a testicular lobule where sperms are produced.

Sertoli cell

Supporting cell lining the seminiferous tubule that feeds developing germ cells.

Leydig cell

Interstitial cell of the testis that synthesises and secretes androgens.

Spermiogenesis

Remodelling of a round spermatid into a motile sperm without cell division.

Spermiation

Release of finished sperms from the seminiferous tubule.

Acrosome

Enzyme-filled cap on the sperm head that helps it penetrate the ovum.

Primary follicle

A primary oocyte surrounded by a single layer of granulosa cells.

Graafian follicle

The mature follicle that ruptures at ovulation to release the secondary oocyte.

Zona pellucida

Covering around the secondary oocyte; sperm contact with it blocks entry of further sperms.

Corpus luteum

Structure formed from the ruptured follicle that secretes progesterone in the luteal phase.

Menarche

The first menstruation, at puberty.

Menopause

Permanent end of menstrual cycles, around 50 years of age.

Ootid

The haploid ovum formed when the secondary oocyte completes meiosis II after sperm entry.

Morula

Early embryo of 8–16 blastomeres.

Blastocyst

Embryo stage with an outer trophoblast and an inner cell mass, which implants in the endometrium.

Implantation

Embedding of the blastocyst in the endometrium, which starts pregnancy.

Chorionic villi

Finger-like outgrowths of the trophoblast that interlock with uterine tissue to form the placenta.

Foetal ejection reflex

Mild uterine contractions set off by signals from the mature foetus and placenta that start oxytocin release.

Colostrum

Milk of the first few days after birth, rich in antibodies.