Human Reproduction Notes

1.1. Male Reproductive System

Located in the pelvis region, it includes primary sex organs, accessory ducts, glands, and external genitalia.

• Primary Sex Organ: Testes (pair) - produce sperms and testosterone.
• Accessory Ducts: Rete testis, Vasa efferentia, Epididymis, Vas deferens.
• Accessory Glands: Seminal vesicles, Prostate gland, Bulbourethral glands (Cowper's glands).
• External Genitalia: Penis, Scrotum.

Table 1.1: Components and Functions of the Male Reproductive System.

1.2. Female Reproductive System

Located in the pelvic region, it includes primary sex organs, accessory ducts, and external genitalia.

• Primary Sex Organ: Ovaries (pair) - produce ova (eggs) and female hormones (estrogen, progesterone).
• Accessory Ducts: Oviducts (fallopian tubes), Uterus, Vagina.
• External Genitalia: Vulva (Mons pubis, Labia majora, Labia minora, Hymen, Clitoris).

Table 1.2: Components and Functions of the Female Reproductive System.

2.1. Spermatogenesis

The process of formation of sperms from spermatogonia in the seminiferous tubules of the testis.

• Starts at puberty (~12-16 years).
• Spermatogonia: Present on inner wall of seminiferous tubules, multiply by mitosis. Some differentiate into Primary Spermatocytes (2n).
• Meiosis I: Primary Spermatocyte (2n) completes meiosis I, forming two equal Secondary Spermatocytes (n).
• Meiosis II: Secondary Spermatocytes (n) undergo meiosis II, forming two Spermatids (n) each, totalling four spermatids from one primary spermatocyte.
• Spermiogenesis: Spermatids are non-motile and round. They transform into motile Spermatozoa (sperms).
• Spermiation: After spermiogenesis, sperm heads become embedded in Sertoli cells and are finally released from the seminiferous tubules.

Structure of a Sperm:

• Microscopic, motile structure. Consists of Head, Neck, Middle Piece, and Tail.
• Head: Contains an elongated haploid nucleus. Anterior portion is covered by a cap-like structure, the Acrosome.
• Acrosome: Contains enzymes (hyaluronidase, acrosin) that help penetrate the egg layers during fertilization.
• Middle Piece: Contains numerous mitochondria that provide energy for sperm motility essential for fertilization.
• Tail: Long, slender, provides motility.

2.2. Oogenesis

The process of formation of mature female gametes (ova) in the ovaries.

• Starts during fetal development (unlike spermatogenesis which starts at puberty).
• Fetal Stage: Oogonia (mother egg cells) multiply by mitosis and enter meiosis I, getting arrested at Prophase I. These are called Primary Oocytes.
• Each primary oocyte gets surrounded by a layer of granulosa cells, forming a Primary Follicle.
• At Puberty: Many primary follicles degenerate (follicular atresia). Remaining primary follicles develop into Secondary Follicles (more layers of granulosa cells, new theca layer).
• Tertiary Follicle: Characterized by an antrum (fluid-filled cavity). The primary oocyte within it grows in size, completes meiosis I, forming a large, haploid Secondary Oocyte and a tiny First Polar Body.
• Secondary oocyte retains most of the cytoplasm of the primary oocyte.
• The tertiary follicle matures into a Graafian Follicle. The secondary oocyte forms a new membrane, zona pellucida, around itself.
• Ovulation: Graafian follicle ruptures to release the secondary oocyte from the ovary.
• The secondary oocyte is arrested at Metaphase II. It completes meiosis II only upon sperm entry (fertilization).
• If fertilized, it forms a large Ovum (egg) and a Second Polar Body.

2.3. Hormonal Control of Gametogenesis

Both spermatogenesis and oogenesis are regulated by hormones from the hypothalamus, pituitary, and gonads.

• Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH).
• Pituitary (Anterior): GnRH stimulates anterior pituitary to secrete Gonadotropins: Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH).

In Males (Spermatogenesis):

• LH: Acts on Leydig cells, stimulating them to secrete androgens (testosterone). Androgens stimulate spermatogenesis.
• FSH: Acts on Sertoli cells, stimulating secretion of factors (e.g., ABP - Androgen Binding Protein) which help in spermiogenesis.

In Females (Oogenesis & Menstrual Cycle):

• LH & FSH: Stimulate follicular development and secretion of estrogens.
• LH Surge: A rapid increase in LH levels during the mid-cycle (around day 14) induces ovulation (rupture of Graafian follicle).
• LH also stimulates the formation of corpus luteum from ruptured follicle and its secretion of progesterone.

2.4. Differences between Spermatogenesis and Oogenesis

Table 2.1: Key Differences between Spermatogenesis and Oogenesis.

4.1. Fertilization

The process of fusion of male and female gametes (sperm and ovum) to form a diploid zygote. It usually occurs in the ampullary-isthmic junction of the fallopian tube.

• Copulation: Release of semen into the vagina (insemination).
• Sperms travel through cervix, uterus, and reach the ampullary-isthmic junction.
• Ovum released by ovary also reaches this junction.
• Capacitation: Physiological changes in sperm within the female reproductive tract that make it capable of fertilizing an egg. Increases motility and enables acrosome reaction.
• Acrosome Reaction: On contact with the zona pellucida layer of the ovum, the acrosome of the sperm releases enzymes (e.g., hyaluronidase, acrosin). These enzymes help the sperm digest its way through the corona radiata and zona pellucida.
• Sperm binds to specific receptors on the zona pellucida, inducing cortical reactions in the ovum (blocks polyspermy).
• Plasma membrane of sperm fuses with plasma membrane of secondary oocyte.
• Entry of sperm into cytoplasm induces completion of meiosis II by secondary oocyte, forming a haploid ovum and second polar body.
• Nucleus of sperm and ovum fuse to form a diploid zygote.

4.2. Implantation

The process by which the blastocyst attaches to the endometrium of the uterus.

• The zygote undergoes rapid mitotic divisions called cleavage as it moves through the isthmus towards the uterus.
• This forms 2, 4, 8, 16-celled stages called blastomeres. The embryo with 8-16 blastomeres is called a morula.
• The morula continues to divide and transforms into a blastocyst, which moves further into the uterus.
• The blastocyst has an outer layer of cells called trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
• The trophoblast layer attaches to the endometrium (uterine wall), and the inner cell mass differentiates to form the embryo.
• After attachment, the uterine cells rapidly divide and cover the blastocyst.
• As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is implantation. It typically occurs about 7 days after fertilization.

5.1. Placenta

The placenta is a temporary organ that develops in the uterus during pregnancy, providing oxygen and nutrients to the growing baby and removing waste products from the baby's blood.

• Formed by interdigitation of chorionic villi (from trophoblast) and uterine wall.
• Provides large surface area for exchange between mother and embryo.
• Acts as an endocrine organ, producing hormones like hCG, hPL, estrogens, progesterone, relaxin.

Table 5.1: Functions of the Placenta.

5.2. Pregnancy Hormones

• During pregnancy, levels of estrogens, progestogens, cortisol, prolactin, thyroxine, etc., increase several folds in maternal blood.
• These hormones support fetal growth, metabolic changes in the mother, and maintenance of pregnancy.
• Progesterone: Essential for maintaining pregnancy. High levels inhibit uterine contractions.
• hCG: Maintains corpus luteum (which continues to produce progesterone). Basis for pregnancy tests.
• hPL: Human Placental Lactogen, helps in mammary gland development and metabolic changes.

5.3. Embryonic Development

After implantation, the inner cell mass differentiates into three primary germ layers: ectoderm, endoderm, and mesoderm. These layers give rise to all tissues and organs.

Table 5.2: Key Milestones of Human Embryonic Development.

6.1. Parturition (Childbirth)

The process of delivery of the fetus from the uterus. It's induced by a complex neuroendocrine mechanism.

• Fetal Ejection Reflex: Fully developed fetus and the placenta send signals (mild uterine contractions).
• This triggers release of oxytocin from the maternal pituitary.
• Oxytocin: Acts on uterine muscles, causing stronger contractions. This in turn stimulates more oxytocin release (positive feedback loop).
• Strong uterine contractions lead to expulsion of the baby.
• After delivery, the placenta is also expelled (afterbirth).

6.2. Lactation

The production of milk by the mammary glands.

• Mammary glands differentiate during pregnancy.
• Prolactin (from anterior pituitary) stimulates milk production.
• Milk is produced towards the end of pregnancy.

• Breastfeeding is highly recommended as it helps the baby develop resistance against diseases.
• Milk Ejection Reflex: Suckling of the baby stimulates sensory nerves, leading to oxytocin release, which causes contraction of myoepithelial cells around alveoli, forcing milk into ducts.