Ch 02 - Human Reproduction12th Biology

The reproductive events in humans follow a strict sequence after puberty:

• Gametogenesis: Formation of gametes (sperms and ovum).
• Insemination: Transfer of sperms into the female genital tract.
• Fertilisation: Fusion of male and female gametes to form a zygote.
• Implantation: Development of blastocyst and its attachment to the uterine wall.
• Gestation: Embryonic development inside the womb.
• Parturition: Delivery of the fully developed baby.

• Testis: Oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm.
• Ovary: Approximately 2 to 4 cm in length, connected to the pelvic wall and uterus by ligaments.

The scrotum acts as a critical thermoregulator for the testes.

• It maintains the testicular temperature 2 to 2.5°C lower than the normal internal body temperature.
• Significance: This lower temperature is an absolute biological requirement for spermatogenesis (the synthesis of viable sperms). Normal body temperature would lead to mutations or cell death in developing sperms.

Based on anatomical structure:

• Each testis contains approximately 250 compartments called testicular lobules.
• Each of these lobules contains 1 to 3 highly coiled seminiferous tubules, which are the actual sites of sperm production.

The germinal epithelial lining of seminiferous tubules contains:

• Male Germ Cells (Spermatogonia): These are diploid cells that undergo meiotic divisions to finally produce haploid spermatozoa (sperms).
• Sertoli Cells: Also known as nurse cells, these provide essential nutrition and structural support to the dividing and developing germ cells.

The interstitial spaces contain small blood vessels, immunologically competent cells, and Leydig cells (Interstitial cells).

• Function: Leydig cells synthesize and secrete the testicular hormones known as androgens (primarily testosterone).
• Note: This secretion is triggered by Luteinizing Hormone (LH) from the pituitary gland.

Sperms travel through a specific sequence of accessory ducts:

1. Seminiferous Tubules (site of production)
2. Rete Testis (channels within the testis)
3. Vasa Efferentia (leaves the testis)
4. Epididymis (highly coiled tube for storage/maturation)
5. Vas Deferens (ascends into the abdomen)
6. Ejaculatory Duct (receives seminal vesicle duct)
7. Urethra (passes through the penis)
8. Urethral Meatus (external opening)

• The enlarged end of the penis is called the glans penis.
• It is covered by a loose, retractable fold of skin known as the foreskin (or prepuce).

Seminal plasma is the nutrient-rich fluid component of semen.

• Glands involved: Paired Seminal vesicles, a single Prostate gland, and paired Bulbourethral glands (Cowper's glands).
• Composition: It is highly rich in fructose, calcium, and certain enzymes.
• Special Note: The secretion of the bulbourethral glands specifically helps in the lubrication of the penis.

The ovary is covered by a thin epithelium that encloses the ovarian stroma (matrix).
This stroma is distinctly divided into two zones:

• Peripheral Cortex: The outer region, which houses the ovarian follicles at various stages of development.
• Inner Medulla: The central region, containing blood vessels, nerves, and connective tissue.

The 10-12 cm long fallopian tube is divided into three parts:

• Infundibulum: The proximal, funnel-shaped part closer to the ovary. Its edges possess finger-like projections called fimbriae, which sweep and collect the ovum after ovulation.
• Ampulla: The wider, middle section of the oviduct.
• Isthmus: The final part with a narrow lumen that directly joins the uterus.

The uterus (womb) consists of three layers:

• Perimetrium: The thin, external membranous covering.
• Myometrium: The thick, middle layer made of smooth muscle. It exhibits strong contractions during the delivery of the baby (parturition).
• Endometrium: The inner glandular layer lining the uterine cavity. It undergoes cyclical breakdown and regeneration during the menstrual cycle and is the site for blastocyst implantation.

Variables & Units:

• Spermatogonium = Diploid ($2n=46$) immature germ cell.
• Primary Spermatocyte = Diploid ($2n=46$) cell ready for meiosis.
• Secondary Spermatocyte = Haploid ($n=23$) cell post-reductional division.
• Spermatid = Haploid ($n=23$) cell post-equational division.

Conditions/Notes:

• The process begins at puberty due to the secretion of GnRH.
• Spermatids must still undergo differentiation (spermiogenesis) to become functional motile spermatozoa.

The birth canal is structurally formed by the combination of:

1. The Cervical canal (the cavity of the cervix)
2. The Vagina

Note: This combined passageway facilitates the expulsion of the fully developed foetus during parturition.

The female external genitalia comprises five main parts:

• Mons pubis: A cushion of fatty tissue covered by skin and pubic hair.
• Labia majora: Fleshy tissue folds extending from the mons pubis, enclosing the vaginal opening.
• Labia minora: Paired tissue folds situated directly under the labia majora.
• Hymen: A thin membrane partially covering the vaginal opening.
• Clitoris: A tiny, sensitive finger-like projection lying at the upper junction of the labia minora.

The glandular tissue of each breast is divided into 15 to 20 mammary lobes containing clusters of milk-secreting cells.
Milk follows a specific ductal pathway to reach the nipple:

1. Alveoli: Clusters of cells within the mammary lobes that secrete milk.
2. Mammary Tubules: Cavities of alveoli open into these tubules.
3. Mammary Duct: Tubules from each lobe join to form this duct.
4. Mammary Ampulla: A wider sac formed by the joining of several mammary ducts.
5. Lactiferous Duct: Connects the ampulla to the nipple, through which milk is sucked out.

• Oogenesis is initiated during the embryonic development stage of the female fetus. Millions of gamete mother cells (oogonia) are formed before birth, and no more oogonia are added after birth.
• In contrast, spermatogenesis only begins later in life at the onset of puberty and continues continuously.

At puberty, increased GnRH from the hypothalamus stimulates the anterior pituitary to release LH and FSH:

• Luteinizing Hormone (LH): Acts on Leydig cells in the interstitial spaces, stimulating them to synthesize and secrete androgens (testosterone), which drive spermatogenesis.
• Follicle Stimulating Hormone (FSH): Acts on Sertoli cells to stimulate the secretion of specific factors that help in the process of spermiogenesis.

• Spermiogenesis: This is a cellular differentiation process where non-motile, spherical spermatids are transformed into mature, motile spermatozoa (sperms). No cell division occurs here.
• Spermiation: This is a mechanical release process. After spermiogenesis, the sperm heads are embedded in Sertoli cells. Spermiation is the final release of these sperms from the seminiferous tubules into the duct system.

A sperm is enveloped by a plasma membrane and consists of a Head, Neck, Middle Piece, and Tail.

• Acrosome (in the Head): A cap-like structure covering the anterior haploid nucleus. It is filled with enzymes (like hyaluronidase) essential for dissolving the ovum's membrane to facilitate fertilization.
• Middle Piece: Contains numerous mitochondria that generate ATP (energy) for the vigorous whipping movement of the tail, facilitating sperm motility.

During a single coitus, a human male ejaculates 200 to 300 million sperms.
For normal fertility, the ejaculate must meet two strict criteria:

• At least 60% of the sperms must possess normal shape and size.
• Of those 60%, at least 40% must show vigorous motility.

The gamete mother cells (oogonia) start dividing and enter into Prophase-I of the meiotic division.

• They get temporarily arrested at this specific stage.
• These arrested cells are subsequently called primary oocytes.

Due to follicular degeneration (atresia), only about 60,000 to 80,000 primary follicles are left in each ovary at the time of puberty.

When the first sperm physically comes into contact with the zona pellucida layer of the ovum, it induces immediate biochemical changes in the membrane.

• These changes permanently block the entry of any additional sperms.
• This barrier mechanism ensures monospermy (only one sperm fertilizes a single ovum). Simultaneously, the acrosomal secretions help the winning sperm penetrate the plasma membrane.

• Primary Follicle: A primary oocyte surrounded by a single layer of granulosa cells.
• Secondary Follicle: Surrounded by more layers of granulosa cells and a new theca layer.
• Tertiary Follicle: Characterized by a fluid-filled cavity called an antrum. The theca organizes into interna and externa. The primary oocyte completes meiosis I here.
• Graafian Follicle: The fully mature follicle. The secondary oocyte inside forms a new membrane, the zona pellucida.

The primary oocyte completes Meiosis I within the Tertiary Follicle.

• It is an unequal division, resulting in a large haploid secondary oocyte and a tiny first polar body.
• Advantage: The secondary oocyte retains the bulk of the nutrient-rich cytoplasm from the primary oocyte, which is vital for providing energy and resources for early embryonic development if fertilized.

Variables & Stages:

• Zygote = Single diploid cell ($2n=46$) formed by syngamy.
• Cleavage = Rapid mitotic divisions occurring as the zygote travels through the isthmus.
• Blastomeres = The resulting daughter cells from cleavage.
• Morula = A solid ball of 8-16 blastomeres.
• Blastocyst = A hollow structure with an outer trophoblast layer and an inner cell mass, ready for uterine implantation.

Conditions/Notes:

• The trophoblast attaches to the endometrium, while the inner cell mass differentiates into the actual embryo.

• Menarche: The first menstruation event that marks the biological beginning of the reproductive cycle at puberty.
• Menopause: The permanent cessation (stopping) of the menstrual cycles, which generally occurs around the age of 50 years in human females.

Variables & Chromosomes:

• Ovum = Always carries the X sex chromosome.
• Sperm = 50% carry the X chromosome, 50% carry the Y chromosome.
• Zygote (44+XX) = Develops into a female baby.
• Zygote (44+XY) = Develops into a male baby.

Conditions/Notes:

• The sex of the baby is determined strictly at the moment of fertilization by the genetic payload of the father's sperm, not the mother's ovum.

1. Menstrual Phase (Days 1-5): Breakdown and shedding of the endometrial lining and blood vessels (menses) due to lack of fertilization.
2. Follicular / Proliferative Phase: The endometrium regenerates through rapid proliferation, induced by rising estrogen.
3. Ovulatory Phase (Day ~14): Endometrium remains intact while the ovum is released due to the LH surge.
4. Luteal / Secretory Phase: Endometrium is further thickened and maintained by large amounts of progesterone secreted by the corpus luteum to prepare for implantation.

During the mid-cycle (around the 14th day) of the menstrual cycle, the secretion of Luteinizing Hormone (LH) from the anterior pituitary increases rapidly, reaching its maximum level.

• This peak is called the LH Surge.
• Function: It directly induces the rupture of the mature Graafian follicle, resulting in the release of the secondary oocyte from the ovary, a process known as ovulation.

The corpus luteum is formed from the remnants of the ruptured Graafian follicle immediately after ovulation (during the luteal phase).

• Function: It acts as a temporary endocrine gland, secreting large amounts of progesterone.
• Significance: Progesterone is absolutely essential to maintain the thickened endometrium of the uterus, which is necessary for the successful implantation of the fertilized ovum and to halt the menstrual cycle during pregnancy.

• Site: Fertilisation takes place in the ampullary region of the fallopian tube.
• Condition: It can only occur if the ovum and the motile sperms are transported simultaneously to this ampullary region.
• Implication: This strict timing is the physiological reason why not all copulations lead to fertilisation and pregnancy.

Immediately after implantation, the inner cell mass differentiates into the three primary germ layers that give rise to all adult tissues and organs:

1. Ectoderm: The outer layer.
2. Endoderm: The inner layer.
3. Mesoderm: A middle layer that appears soon after, between the ectoderm and endoderm.

• Special Note: The inner cell mass contains specialised stem cells which possess the pluripotency (potential) to generate all the tissues and organs in the body.

• Chorionic Villi Formation: After implantation, finger-like projections called chorionic villi appear on the surface of the trophoblast.
• Integration: These villi are immediately surrounded by the uterine tissue and maternal blood.
• Interdigitation: The chorionic villi and uterine tissue become physically interdigitated (interlocked) with each other.
• Result: This joint structural and functional unit between the developing foetus and the maternal body is called the placenta.

The placenta acts as a temporary endocrine tissue, secreting several hormones (hCG, hPL, estrogens, progestogens) to support foetal growth.

• Exclusive to Pregnancy: Human chorionic gonadotropin (hCG), human placental lactogen (hPL), and Relaxin are produced only during pregnancy.
• Origin of Relaxin: Later in pregnancy, Relaxin is specifically secreted by the ovary, NOT the placenta.
• Note: Maternal blood levels of other hormones (cortisol, prolactin, thyroxine) also increase several-fold to support maternal metabolic changes.

Human gestation lasts an average of 9 months. The developmental milestones are:

• 1st Month: The heart is formed (first sign of a growing foetus, audible via stethoscope).
• 2nd Month: The foetus develops limbs and digits.
• 12 Weeks (End of 1st Trimester): Most major organ systems are formed (limbs and external genitalia are well-developed).
• 5th Month: First movements of the foetus and appearance of hair on the head.
• 24 Weeks (End of 2nd Trimester): The body is covered with fine hair, eye-lids separate, and eyelashes are formed.

• The glandular tissue of each breast is divided into 15 to 20 mammary lobes.
• These lobes contain clusters of milk-secreting cells called alveoli, which open into mammary tubules.

Parturition is the vigorous contraction of the uterus causing the delivery of the foetus.

• Trigger: Signals originate from the fully developed foetus and the placenta, inducing mild uterine contractions known as the foetal ejection reflex.
• Hormonal Release: This reflex triggers the maternal pituitary gland to release Oxytocin.
• Positive Feedback Loop: Oxytocin acts on the uterine muscle (myometrium) causing stronger contractions $\rightarrow$ this stimulates further oxytocin secretion $\rightarrow$ leading to increasingly stronger contractions.
• Result: This continuous stimulatory reflex leads to the expulsion of the baby and placenta through the birth canal.

• Definition: Towards the end of pregnancy, the mammary glands differentiate and begin lactation. The specific milk produced during the initial few days of lactation is called colostrum.
• Biological Significance: Colostrum is highly rich in nutrients and contains abundant antibodies (specifically IgA).
• Function: These maternal antibodies are absolutely essential to develop passive resistance (immunity) in the newborn baby against common environmental infections.

Different mammals have vastly different gestation (pregnancy) periods:

• Dogs: Approximately 2 months (~60-65 days).
• Cats: Approximately 2 months (~60-65 days).
• Elephants: Approximately 22 months (one of the longest in the animal kingdom).