The Male Reproductive System

Humans reproduce sexually and are viviparous, meaning the young develop inside the female body and are born live. The reproductive process in humans involves several key events that occur after puberty (the stage of sexual maturation).

Key reproductive events in humans include:

• Gametogenesis: Formation of male gametes (sperms) and female gametes (ova).
• Insemination: Transfer of sperms into the female genital tract.
• Fertilisation: Fusion of sperm and ovum to form a zygote.
• Implantation: Formation and attachment of the blastocyst (early embryo) to the uterine wall.
• Gestation: Embryonic development within the uterus.
• Parturition: Delivery of the baby.

Note that sperm formation continues throughout most of a male's life, whereas ovum formation in females ceases around age 50 (menopause).

The male reproductive system is located in the pelvic region and consists of testes, accessory ducts, glands, and external genitalia.

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Testes

There is a pair of testes, which are the primary male sex organs. They are located outside the abdominal cavity within a sac called the scrotum.

• The scrotum helps maintain the temperature of the testes about $2-2.5^\circ\textsf{C}$ lower than the normal internal body temperature. This lower temperature is essential for the production of sperms (spermatogenesis).
• Each testis is oval-shaped, approximately $4-5$ cm in length and $2-3$ cm in width, and covered by a dense outer layer.
• Internally, each testis is divided into about 250 compartments called testicular lobules.

Each testicular lobule contains one to three highly coiled tubes called seminiferous tubules. Sperms are produced within these tubules.

The inner lining of each seminiferous tubule consists of two types of cells:

• Male germ cells (Spermatogonia): These cells undergo meiotic divisions to form sperms.
• Sertoli cells: These provide nourishment to the developing germ cells and sperms.

The regions outside the seminiferous tubules are called interstitial spaces. These spaces contain small blood vessels and interstitial cells or Leydig cells. Leydig cells are responsible for synthesising and secreting male hormones called androgens (e.g., testosterone). Immunologically competent cells are also present in these spaces.

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Accessory Ducts

The male accessory ducts transport and store sperms. They include:

1. Rete testis: A network of tubules within the testis where seminiferous tubules open into.
2. Vasa efferentia (Efferent ductules): Several ducts arising from the rete testis that exit the testis.
3. Epididymis: A highly coiled tube located along the posterior surface of each testis. Vasa efferentia open into the epididymis. Sperms mature and are stored here.
4. Vas deferens (Ductus deferens): Leads from the epididymis, ascends into the abdomen, and loops over the urinary bladder.

The vas deferens receives a duct from the seminal vesicle to form the ejaculatory duct. The ejaculatory duct opens into the urethra. The urethra originates from the urinary bladder and extends through the penis, opening to the outside via the urethral meatus.

These ducts collectively transport sperms from the testes to the outside of the body.

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External Genitalia

The male external genitalia is the penis. It is composed of specialized erectile tissue that facilitates erection, which is necessary for the transfer of sperms into the female reproductive tract (insemination).

The enlarged tip of the penis is called the glans penis, which is covered by a loose fold of skin known as the foreskin (or prepuce).

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Accessory Glands

The male accessory glands contribute secretions that form the seminal plasma. These glands include:

• A pair of seminal vesicles.
• A prostate gland (single).
• A pair of bulbourethral glands (Cowper's glands).

Seminal plasma is rich in fructose, calcium, and certain enzymes. Fructose provides energy for sperm motility. The secretions of the bulbourethral glands also help in the lubrication of the penis.

The seminal plasma combined with sperms constitutes semen.

The functions of the accessory ducts and glands are maintained by testicular hormones (androgens).

The Female Reproductive System

The female reproductive system is also located in the pelvic region and consists of a pair of ovaries, a pair of oviducts, uterus, cervix, vagina, and external genitalia. These structures, along with the mammary glands, are functionally integrated to support ovulation, fertilisation, pregnancy, childbirth, and child care.

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Ovaries

The ovaries are the primary female sex organs. There is a pair of ovaries located one on each side of the lower abdomen.

• Each ovary is about $2-4$ cm in length and is connected to the pelvic wall and uterus by ligaments.
• The ovaries produce the female gamete (ovum) and several steroid hormones (ovarian hormones like estrogen and progesterone).
• Each ovary is covered by a thin epithelium enclosing the ovarian stroma. The stroma is differentiated into a peripheral cortex and an inner medulla.

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Accessory Ducts

The female accessory ducts include the oviducts, uterus, and vagina.

1. Oviducts (Fallopian tubes): A pair of tubes, each about $10-12$ cm long, extending from the periphery of each ovary to the uterus.
   • The part closer to the ovary is funnel-shaped and called the infundibulum.
   • The edges of the infundibulum have finger-like projections called fimbriae, which help collect the ovum after ovulation.
   • The infundibulum leads to a wider part called the ampulla.
   • The last part, the isthmus, is narrow and connects to the uterus. Fertilisation usually occurs in the ampullary region.
2. Uterus (Womb): A single, pear-shaped muscular organ supported by ligaments. It is where the embryo develops.
3. Cervix: The narrow lower part of the uterus that opens into the vagina. The cavity of the cervix is the cervical canal.
4. Vagina: A muscular tube extending from the cervix to the outside of the body. The cervical canal along with the vagina forms the birth canal during parturition.

The wall of the uterus has three layers:

• Perimetrium: External, thin membranous layer.
• Myometrium: Middle, thick layer of smooth muscle, responsible for strong contractions during childbirth.
• Endometrium: Inner, glandular layer that lines the uterine cavity. It undergoes cyclical changes during the menstrual cycle and is the site of implantation of the fertilised ovum.

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External Genitalia

The female external genitalia, collectively called the vulva, are located around the vaginal opening. They include:

• Mons pubis: A cushion of fatty tissue covered by skin and pubic hair.
• Labia majora: Fleshy folds extending from the mons pubis, surrounding the vaginal opening.
• Labia minora: Paired folds of tissue located under the labia majora.
• Hymen: A membrane that partially covers the opening of the vagina. Its presence or absence is not a reliable indicator of virginity.
• Clitoris: A tiny, finger-like structure located at the upper junction of the two labia minora, above the urethral opening. It is analogous to the male penis and is highly sensitive.

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Mammary Glands

Mammary glands (breasts) are characteristic of all female mammals and are considered secondary sexual characteristics. They are paired structures containing glandular tissue and variable amounts of fat.

The glandular tissue in each breast is divided into $15-20$ mammary lobes, which contain clusters of milk-secreting cells called alveoli. The milk produced by alveolar cells is stored in their lumens.

Alveoli open into mammary tubules. Tubules from different alveoli within a lobe join to form a mammary duct.

Several mammary ducts join to form a wider mammary ampulla, which is connected to the lactiferous duct. Milk is released through the lactiferous duct.

Gametogenesis

Gametogenesis is the process by which the primary sex organs (testes and ovaries) produce haploid gametes (sperms and ovum, respectively).

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Spermatogenesis

This is the process of sperm formation in males. It begins at puberty in the testes, specifically within the seminiferous tubules.

Steps involved:

1. Multiplication Phase: Immature male germ cells called spermatogonia (diploid, $2n=46$ chromosomes) on the inner wall of seminiferous tubules multiply by mitotic division.
2. Growth Phase: Some spermatogonia grow in size and are called primary spermatocytes (diploid, $2n=46$ chromosomes).
3. Maturation Phase (Meiosis):
   • Each primary spermatocyte undergoes the first meiotic division (Meiosis I), which is a reductional division. This produces two equal, haploid (n=23 chromosomes) secondary spermatocytes.
   • Each secondary spermatocyte then undergoes the second meiotic division (Meiosis II), which is a mitotic-like division. This produces four equal, haploid (n=23 chromosomes) spermatids.
4. Spermiogenesis: The spermatids are non-motile. They are transformed into motile spermatozoa (sperms) through a process of differentiation called spermiogenesis.

After spermiogenesis, the heads of the sperms become embedded in Sertoli cells for nourishment and support. Finally, sperms are released from the seminiferous tubules by the process called spermiation.

Spermatogenesis is initiated at puberty due to increased secretion of Gonadotropin Releasing Hormone (GnRH) from the hypothalamus. GnRH stimulates the anterior pituitary to release two gonadotropins:

• Luteinising Hormone (LH): Acts on Leydig cells to stimulate the synthesis and secretion of androgens (e.g., testosterone). Androgens stimulate spermatogenesis.
• Follicle Stimulating Hormone (FSH): Acts on Sertoli cells to stimulate the secretion of factors that help in spermiogenesis.

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Structure Of Sperm

A sperm is a microscopic structure composed of four main parts:

1. Head: Contains an elongated haploid nucleus. The anterior portion of the head is covered by a cap-like structure called the acrosome. The acrosome contains enzymes (like hyaluronidase and acrosin) that are essential for fertilisation, helping the sperm penetrate the egg's layers.
2. Neck: A short region connecting the head to the middle piece.
3. Middle Piece: Contains numerous mitochondria arranged spirally. Mitochondria provide the energy (ATP) for the movement of the sperm tail.
4. Tail: A long, slender structure that provides motility to the sperm, allowing it to swim in the female reproductive tract towards the ovum.

The entire sperm body is enveloped by a plasma membrane.

During ejaculation, a human male typically releases $200-300$ million sperms. For normal fertility, at least $60\%$ of these sperms must have normal shape and size, and at least $40\%$ must show vigorous motility.

The accessory ducts and glands contribute to the maturation and motility of sperms. The secretions of the epididymis, vas deferens, seminal vesicle, and prostate are crucial for this.

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Oogenesis

This is the process of formation of a mature female gamete (ovum). It is significantly different from spermatogenesis.

• Oogenesis begins during embryonic development. Millions of gamete mother cells (oogonia, diploid) are formed in the fetal ovary. No new oogonia are added after birth.
• Oogonia start division and enter into prophase I of meiosis, getting temporarily arrested at this stage. These cells are called primary oocytes (diploid, $2n=46$).
• Each primary oocyte gets surrounded by a layer of granulosa cells, forming a primary follicle.
• A large number of primary follicles degenerate from birth to puberty. At puberty, only $60,000 - 80,000$ primary follicles remain in each ovary.
• Primary follicles become surrounded by more layers of granulosa cells and a new layer called the theca, becoming secondary follicles.
• The secondary follicle transforms into a tertiary follicle, characterised by a fluid-filled cavity called the antrum. The theca layer is organised into inner theca interna and outer theca externa.
• The primary oocyte within the tertiary follicle grows and completes its first meiotic division just before ovulation. This is an unequal division, resulting in a large haploid secondary oocyte ($n=23$) and a tiny first polar body. The secondary oocyte retains the bulk of the cytoplasm. The fate of the first polar body (whether it divides or degenerates) is variable.
• The tertiary follicle matures into the Graafian follicle.
• The secondary oocyte in the Graafian follicle develops a new protective membrane called the zona pellucida around it.
• Ovulation occurs when the mature Graafian follicle ruptures to release the secondary oocyte from the ovary.
• The second meiotic division (Meiosis II) of the secondary oocyte begins but is arrested at metaphase II. It is completed only upon the entry of a sperm during fertilisation. Completion of Meiosis II is also unequal, producing a mature haploid ovum (ootid) and a second polar body.

Key differences from spermatogenesis:

• Oogenesis begins in the fetus, spermatogenesis starts at puberty.
• Oogonia formation stops after birth, spermatogonia continue to divide.
• One primary oocyte produces one ovum and two or three polar bodies; one primary spermatocyte produces four sperms.
• Oogenesis involves unequal cytoplasmic division resulting in one large ovum.
• Oogenesis includes periods of arrest in development (Prophase I and Metaphase II).

Menstrual Cycle

The reproductive cycle in female primates (monkeys, apes, and humans) is called the menstrual cycle. It begins at puberty (menarche) and repeats approximately every $28/29$ days.

One ovum is typically released during the middle of each cycle (ovulation).

The menstrual cycle is regulated by complex interactions between pituitary hormones (FSH and LH) and ovarian hormones (estrogen and progesterone). It can be divided into several phases:

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Menstrual Phase

• This phase marks the beginning of the cycle and lasts for about $3-5$ days.
• It is characterised by menstrual flow, which consists of blood, tissue fluid, mucus, and the disintegrated endometrial lining of the uterus. This material is expelled through the vagina.
• Menstruation occurs when the released ovum is not fertilised.
• Lack of menstruation may indicate pregnancy, but it can also be caused by other factors like stress or poor health.

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Follicular Phase

• Follows the menstrual phase.
• In the ovary, primary follicles grow and develop into a mature Graafian follicle. This is stimulated by increasing levels of FSH and LH from the pituitary gland.
• The developing follicles secrete increasing amounts of estrogen.
• Simultaneously, in the uterus, the endometrium begins to regenerate and proliferate under the influence of estrogen.
• This phase lasts for about $10-14$ days.

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Ovulation

• Occurs around the middle of the cycle (typically day 14 in a 28-day cycle).
• Both LH and FSH reach their peak levels at this time.
• A rapid surge in LH level, called the LH surge, is critical. It induces the rupture of the mature Graafian follicle, releasing the secondary oocyte (ovum) from the ovary.

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Luteal Phase

• Follows ovulation.
• The remaining parts of the Graafian follicle in the ovary transform into a temporary endocrine structure called the corpus luteum, under the influence of LH.
• The corpus luteum secretes large amounts of progesterone, along with some estrogen.
• Progesterone is essential for maintaining the thickened and glandular endometrium, preparing it for possible implantation of a fertilised ovum.
• If fertilisation occurs, the corpus luteum persists (due to hCG secreted by the developing embryo) and continues to secrete hormones vital for maintaining pregnancy. Menstrual cycles stop.
• If fertilisation does not occur, the corpus luteum degenerates after about $14$ days.
• Degeneration of the corpus luteum leads to a sharp decline in progesterone and estrogen levels. This causes the disintegration of the endometrial lining, leading to menstruation and the start of a new cycle.

The cessation of menstrual cycles around the age of 50 is called menopause. The period between menarche and menopause is the normal reproductive phase in a female's life, characterised by cyclical menstruation.

Maintaining proper hygiene during menstruation is important, including regular cleaning and timely changing of sanitary products.

Fertilisation And Implantation

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Fertilisation

Insemination is the process where semen, containing sperms, is released by the male penis into the female vagina during copulation (coitus).

Motile sperms travel rapidly through the cervix, enter the uterus, and reach the ampullary region of the fallopian tube. The ovum released during ovulation also travels to the ampullary region.

Fertilisation, the fusion of a sperm with an ovum, typically takes place in the ampullary region of the fallopian tube. It can only occur if sperms and the ovum are present in the ampulla simultaneously. This explains why not every act of copulation results in pregnancy.

When a sperm contacts the zona pellucida layer of the ovum, it induces changes in the ovum's membrane that prevent the entry of additional sperms, ensuring monospermy (fertilisation by only one sperm).

Enzymes from the sperm's acrosome help it penetrate the zona pellucida and the ovum's plasma membrane to enter the cytoplasm.

The entry of the sperm into the ovum's cytoplasm triggers the completion of the second meiotic division of the secondary oocyte. This unequal division produces a haploid ovum (ootid) and a second polar body.

The haploid nucleus of the sperm fuses with the haploid nucleus of the ovum to form a diploid zygote ($2n=46$).

Sex Determination

The sex of the future baby is determined at the moment of fertilisation by the type of sperm that fertilises the ovum.

• Human females have XX sex chromosomes; all ova carry an X chromosome.
• Human males have XY sex chromosomes; sperms carry either an X chromosome ($50\%$) or a Y chromosome ($50\%$).
• If an X-carrying sperm fertilises the ovum (X), the zygote will be XX, developing into a female.
• If a Y-carrying sperm fertilises the ovum (X), the zygote will be XY, developing into a male.

Therefore, the sex of the baby is determined by the father (specifically, by which type of sperm fertilises the egg), not the mother.

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Implantation

The zygote, while moving through the isthmus of the oviduct towards the uterus, undergoes rapid mitotic divisions called cleavage. This produces a solid ball of cells called the morula, which contains $8-16$ blastomeres (daughter cells).

The morula continues to divide and develops into a hollow ball of cells called the blastocyst as it reaches the uterus.

The blastocyst has two distinct layers of cells:

• Trophoblast: The outer layer, which attaches to the uterine wall.
• Inner cell mass: A group of cells attached to the trophoblast, which will differentiate to form the embryo.

The trophoblast layer attaches to the endometrium (uterine lining). After attachment, the uterine cells divide rapidly and surround the blastocyst, causing it to become embedded in the endometrium.

This process of embedding the blastocyst in the uterine wall is called implantation, and it leads to the initiation of pregnancy.

Pregnancy And Embryonic Development

After implantation, the embryo starts developing within the uterus. Pregnancy is the period of embryonic development, with an average duration of about 9 months in humans, known as the gestation period.

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Placenta

Following implantation, finger-like projections called chorionic villi appear on the trophoblast. These villi become surrounded by the uterine tissue and maternal blood.

The chorionic villi and uterine tissue interdigitate to form the placenta. The placenta is a vital structural and functional unit between the developing embryo (foetus) and the mother's body.

Functions of the placenta:

• Facilitates the supply of oxygen and nutrients from the mother to the foetus.
• Facilitates the removal of carbon dioxide and excretory/waste materials from the foetus.
• Connected to the embryo via the umbilical cord, which transports substances to and from the foetus.
• Acts as an endocrine tissue, producing several hormones necessary to maintain pregnancy.

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Hormonal Changes During Pregnancy

The placenta produces important hormones during pregnancy, including:

• Human chorionic gonadotropin (hCG): Maintains the corpus luteum and its hormone production.
• Human placental lactogen (hPL): Involved in metabolic changes in the mother to support fetal growth.
• Estrogens
• Progestogens (Progesterone)

In the later stages of pregnancy, the ovary also secretes relaxin, which helps relax pelvic ligaments and the cervix during childbirth.

Levels of hCG, hPL, and relaxin are exclusively high in pregnant women.

Additionally, the levels of other hormones like estrogens, progestogens, cortisol, prolactin, and thyroxine increase significantly in the maternal blood during pregnancy. These elevated hormone levels are crucial for supporting fetal growth, inducing metabolic adaptations in the mother, and maintaining the pregnancy.

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Embryonic Development

Immediately after implantation, the inner cell mass of the blastocyst differentiates into three primary germ layers:

1. Ectoderm: The outer layer.
2. Endoderm: The inner layer.
3. Mesoderm: Develops between the ectoderm and endoderm shortly after.

These three germ layers give rise to all the tissues and organs of the body in the adult.

The inner cell mass contains stem cells, which are pluripotent, meaning they have the potential to differentiate into various tissues and organs.

Major features of embryonic development during human pregnancy (approximate timings):

• End of 1st month: Heart is formed. Listening to the heart sound is the first sign of a growing fetus.
• End of 2nd month: Limbs and digits develop.
• End of 12 weeks (First Trimester): Most major organ systems are formed. External genital organs are well-developed.
• During 5th month: First movements of the fetus are felt, and hair appears on the head.
• End of 24 weeks (Second Trimester): Body is covered with fine hair, eyelids separate, and eyelashes are formed.
• End of 9 months: The fetus is fully developed and ready for birth.

Parturition And Lactation

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Parturition

Parturition is the process of childbirth, the expulsion or delivery of the fully developed foetus from the uterus at the end of pregnancy.

It is initiated by complex neuroendocrine mechanisms involving signals from the fully developed foetus and the placenta.

These signals induce mild uterine contractions called the foetal ejection reflex.

The foetal ejection reflex triggers the release of oxytocin hormone from the mother's pituitary gland.

• Oxytocin acts on the uterine muscles, causing stronger contractions.
• These stronger contractions, in turn, stimulate further release of oxytocin, creating a positive feedback loop (stimulatory reflex).

This positive feedback leads to progressively stronger and stronger uterine contractions, ultimately forcing the baby out of the uterus through the birth canal (cervix and vagina).

After the baby is delivered, the placenta is also expelled from the uterus (called the afterbirth).

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Lactation

Lactation is the process of milk production by the mammary glands. It begins towards the end of pregnancy, and the mammary glands undergo differentiation during pregnancy in preparation.

Lactation enables the mother to feed the newborn baby.

The milk produced during the initial few days of lactation is called colostrum. Colostrum is yellowish fluid rich in antibodies (like IgA) that provide passive immunity to the newborn, helping them develop resistance to infections.

Breast-feeding the newborn, especially with colostrum, is highly recommended by health professionals for the healthy growth and development of the baby.

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