The Male Reproductive System

The male reproductive system, located in the pelvic region, is responsible for the production and delivery of male gametes (sperms). It comprises a pair of testes, accessory ducts, glands, and external genitalia.

Testes: These are the primary male reproductive organs, oval-shaped and situated outside the abdominal cavity within a pouch called the scrotum. The scrotum maintains a temperature 2-2.5°C lower than the normal body temperature, which is crucial for spermatogenesis. Each testis, about 4-5 cm long and 2-3 cm wide, is covered by a dense covering and divided into approximately 250 testicular lobules. Each lobule contains one to three highly coiled seminiferous tubules where sperm production occurs. The inner lining of these tubules consists of spermatogonia (male germ cells) that undergo meiotic division to form sperms, and Sertoli cells, which provide nutrition to the germ cells. The interstitial spaces between tubules contain interstitial cells or Leydig cells, which synthesize and secrete testicular hormones called androgens.

Male Sex Accessory Ducts: These include the rete testis, vasa efferentia, epididymis, and vas deferens. Seminiferous tubules connect to vasa efferentia via the rete testis. Vasa efferentia lead to the epididymis, a coiled tube located on the posterior surface of the testis, which then leads to the vas deferens. The vas deferens ascends to the abdomen, loops over the urinary bladder, receives a duct from the seminal vesicle, and opens into the urethra as the ejaculatory duct. These ducts store and transport sperms to the outside through the urethra.

Urethra: Originating from the urinary bladder, the urethra extends through the penis and opens externally as the urethral meatus. It serves as the common passage for both urine and semen.

External Genitalia: The penis is the external male genitalia, adapted for erection to facilitate insemination. Its enlarged tip, the glans penis, is covered by a loose fold of skin called the foreskin.

Male Accessory Glands: These include paired seminal vesicles, a prostate gland, and paired bulbourethral glands. Their secretions, along with sperms, form the seminal plasma, which is rich in fructose, calcium, and certain enzymes. The secretions of the bulbourethral glands also aid in lubricating the penis.

The Female Reproductive System

The female reproductive system, situated in the pelvic region, includes a pair of ovaries, oviducts (fallopian tubes), uterus, cervix, vagina, and external genitalia. It is designed to support ovulation, fertilization, pregnancy, childbirth, and infant care. Mammary glands are also considered part of the female reproductive system for their role in nurturing the newborn.

Ovaries: These are the primary female sex organs, producing the female gamete (ovum) and ovarian hormones. Located one on each side of the lower abdomen, they are connected to the pelvic wall and uterus by ligaments. Each ovary is covered by epithelium and has a stroma divided into a peripheral cortex and an inner medulla.

Female Accessory Ducts: This system includes the oviducts (fallopian tubes), uterus, and vagina.

• Oviducts (Fallopian Tubes): Approximately 10-12 cm long, they extend from the ovarian periphery to the uterus. The part near the ovary is a funnel-shaped infundibulum with finger-like projections called fimbriae that help capture the ovum after ovulation. The infundibulum leads to the wider ampulla, and the last section, the isthmus, connects to the uterus.
• Uterus: A single, inverted pear-shaped organ, supported by ligaments. It opens into the vagina through a narrow cervix. The cervical canal, along with the vagina, forms the birth canal. The uterine wall consists of three layers: the outer perimetrium, the middle muscular myometrium (responsible for strong contractions during childbirth), and the inner glandular endometrium (undergoes cyclical changes during the menstrual cycle).
• Vagina: A muscular canal that extends from the cervix to the outside.

External Genitalia: Collectively known as the vulva, these include the mons pubis (a fatty cushion covered by skin and pubic hair), labia majora (fleshy folds surrounding the vaginal opening), labia minora (paired folds under the labia majora), hymen (a membrane partially covering the vaginal opening), and the clitoris (a small, sensitive, finger-like structure at the upper junction of the labia minora).

Mammary Glands: These are paired structures (breasts) containing glandular tissue and fat. The glandular tissue is divided into 15-20 mammary lobes, each containing clusters of cells called alveoli. Alveoli secrete milk, which is collected in mammary tubules, ducts, and ampullae, and finally ejected through lactiferous ducts. These glands develop and produce milk after childbirth to nourish the newborn.

Gametogenesis

Gametogenesis is the process of gamete formation: spermatogenesis in males and oogenesis in females. These processes are initiated at puberty.

Spermatogenesis

Spermatogenesis is the process of sperm formation, occurring in the seminiferous tubules of the testes. It begins at puberty, triggered by an increase in GnRH secretion from the hypothalamus. GnRH stimulates the anterior pituitary to release Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).

• LH: Acts on Leydig cells, stimulating the synthesis and secretion of androgens, which in turn promote spermatogenesis.
• FSH: Acts on Sertoli cells, stimulating the secretion of factors that aid in spermiogenesis.

Process:

1. Spermatogonia: The diploid (46 chromosomes) spermatogonia, located along the inner wall of seminiferous tubules, multiply by mitotic divisions.
2. Primary Spermatocyte: Some spermatogonia differentiate into diploid primary spermatocytes, which undergo the first meiotic division (reductional division).
3. Secondary Spermatocytes: The first meiotic division results in two equal, haploid (23 chromosomes) secondary spermatocytes.
4. Spermatids: Each secondary spermatocyte undergoes the second meiotic division, producing four equal, haploid spermatids.
5. Spermiogenesis: Spermatids are transformed into spermatozoa (sperms) through this process.
6. Spermiation: Mature sperms are released from the seminiferous tubules by being embedded in Sertoli cells and then released.

Structure of a Sperm: A sperm is microscopic, consisting of a head (containing an elongated haploid nucleus and the acrosome, which has enzymes for fertilization), a neck, a middle piece (rich in mitochondria for energy), and a tail (for motility). For normal fertility, at least 60% of ejaculated sperms (about 200-300 million per ejaculation) must have normal shape and size, and at least 40% must show vigorous motility.

The accessory ducts and glands contribute secretions essential for sperm maturation and motility, forming the semen.

Oogenesis

Oogenesis is the process of formation of mature female gametes (ova). It is initiated during embryonic development.

• Oogonia: Within the fetal ovaries, millions of gamete mother cells (oogonia) are formed. They start dividing and enter prophase-I of meiosis, becoming arrested as primary oocytes.
• Primary Follicle: Each primary oocyte is surrounded by a layer of granulosa cells, forming a primary follicle. Many of these degenerate before puberty.
• Puberty: By puberty, only 60,000-80,000 primary follicles remain in each ovary. These develop into secondary follicles, characterized by new layers of granulosa cells and a theca.
• Tertiary Follicle: The secondary follicle transforms into a tertiary follicle, which has a fluid-filled cavity called the antrum.
• Meiosis Completion: The primary oocyte within the tertiary follicle completes its first meiotic division, resulting in a large, haploid secondary oocyte and a small, haploid first polar body. The secondary oocyte retains most of the cytoplasm.
• Ovulation: The tertiary follicle matures into a Graafian follicle. It ruptures to release the secondary oocyte (ovum) from the ovary in a process called ovulation.

Oogenesis differs from spermatogenesis in that the formation of gametes begins before birth, only one ovum is typically released per menstrual cycle, and the process involves unequal cytokinesis.

Menstrual Cycle

The menstrual cycle is the reproductive cycle in female primates, occurring at approximately 28/29-day intervals. It begins at puberty with the first menstruation, called menarche, and typically ends around the age of 50 with menopause. The cycle involves cyclical changes in the ovaries and uterus, regulated by pituitary and ovarian hormones.

Phases of the Menstrual Cycle:

1. Menstrual Phase (Days 1-5): Characterized by the breakdown of the uterine endometrium and blood vessels, leading to menstrual flow, which exits through the vagina. Menstruation occurs if the ovum is not fertilized.
2. Follicular Phase (Approximately Days 6-13): Primary follicles in the ovary grow into a mature Graafian follicle, simultaneously stimulating the regeneration of the endometrium through proliferation. FSH and estrogen levels increase, with estrogen promoting endometrial growth.
3. Ovulatory Phase (Around Day 14): A surge in LH (LH surge), along with FSH, triggers the rupture of the Graafian follicle and the release of the ovum (ovulation).
4. Luteal Phase (Approximately Days 15-28): The remnants of the Graafian follicle transform into the corpus luteum, which secretes large amounts of progesterone. Progesterone is essential for maintaining the endometrium, preparing it for implantation. If fertilization does not occur, the corpus luteum degenerates, leading to a drop in progesterone and estrogen levels, causing the breakdown of the endometrium and initiating menstruation for the next cycle.

During pregnancy, all events of the menstrual cycle are suppressed, and menstruation does not occur.

Fertilisation And Implantation

Fertilisation: This process occurs in the ampullary region of the fallopian tube. During coitus (insemination), sperms are deposited into the vagina, travel through the cervix and uterus, and reach the fallopian tubes. For fertilization to occur, sperms and the ovum must be present in the ampulla simultaneously. Upon contact with the ovum's zona pellucida, sperms induce changes that block other sperm entries. Acrosomal enzymes help the sperm penetrate the zona pellucida and plasma membrane, triggering the completion of the secondary oocyte's meiotic division, forming a haploid ovum (ootid) and a second polar body. The nuclei of the sperm and ovum fuse to form a diploid zygote. The sex of the baby is determined at this stage by the sperm's sex chromosome (X for female, Y for male).

Cleavage and Blastocyst Formation: The zygote undergoes rapid mitotic divisions, called cleavage, as it travels towards the uterus. This forms 2, 4, 8, 16 daughter cells, resulting in a morula. The morula continues to divide and develops into a blastocyst, characterized by an outer layer called the trophoblast and an inner group of cells known as the inner cell mass.

Implantation: The trophoblast layer of the blastocyst attaches to the uterine endometrium. The inner cell mass differentiates into the embryo. Following attachment, uterine cells cover the blastocyst, embedding it within the endometrium. This process, called implantation, initiates pregnancy.

Pregnancy And Embryonic Development

Pregnancy is the period of embryonic development, lasting approximately nine months in humans. After implantation, finger-like projections called chorionic villi develop on the trophoblast, interdigitating with the uterine tissue and maternal blood to form the placenta. The placenta serves as a vital structural and functional unit, facilitating the exchange of oxygen, nutrients, carbon dioxide, and waste materials between the developing embryo (fetus) and the mother. It is connected to the embryo via the umbilical cord.

The placenta also acts as an endocrine tissue, producing hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, and progestogens. Relaxin, secreted by the ovary, is also present during later stages of pregnancy. These hormones are crucial for supporting fetal growth, maintaining the uterine lining, and facilitating metabolic changes in the mother.

Embryonic Differentiation: The inner cell mass differentiates into three germ layers: outer ectoderm, inner endoderm, and the mesoderm formed in between. These layers give rise to all tissues and organs in the adult. The inner cell mass contains stem cells with the potential to differentiate into all cell types.

Key Milestones of Embryonic Development: