Human Reproduction

The Male Reproductive System

The human male reproductive system is responsible for the production and delivery of male gametes (sperm) and the production of male sex hormones.

The male reproductive system is located in the pelvic region and includes:

1. A pair of Testes
2. Associated Accessory Ducts
3. External Genitalia
4. Accessory Glands

*(Image shows a diagram of the human male reproductive system highlighting the major organs and ducts)*

Testes

• Pair of oval-shaped primary male sex organs.
• Location: Located outside the abdominal cavity within a pouch called the scrotum.
• Significance of scrotal location: The scrotum provides a temperature lower than the normal body temperature (about $2-2.5^\circ C$ lower), which is essential for spermatogenesis (sperm formation).
• Each testis is covered by a dense outer layer. Inside, each testis contains about 250 compartments called testicular lobules.
• Each testicular lobule contains 1-3 highly coiled tubules called seminiferous tubules. Seminiferous tubules are the site of spermatogenesis.
• Seminiferous tubules are lined by two types of cells:
  • Spermatogonia (male germ cells): Undergo meiosis to produce sperm.
  • Sertoli cells (sustentacular cells): Provide nourishment to the developing sperm.
• The region outside the seminiferous tubules is called the interstitial space. It contains small blood vessels and Interstitial cells (Leydig cells).
• Leydig cells secrete androgens, the male sex hormones (e.g., testosterone).

*(Image shows a cross-section of the testis indicating lobules and seminiferous tubules, and a magnified cross-section of a seminiferous tubule showing the arrangement of cells)*

Accessory Ducts

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

• Rete testis: A network of tubules located within the testis, receiving sperm from the seminiferous tubules.
• Vasa efferentia: A number of fine tubules connecting the rete testis to the epididymis.
• Epididymis: A highly coiled tube located along the posterior surface of each testis. Sperm mature and are stored here temporarily.
• Vas deferens: A tube that arises from the epididymis, ascends into the abdomen, loops over the urinary bladder, and receives a duct from the seminal vesicle.
• Ejaculatory duct: Formed by the joining of the vas deferens and the duct of the seminal vesicle. It passes through the prostate gland and opens into the urethra.
• Urethra: The common terminal duct for both urine and semen. It extends from the urinary bladder through the penis to the outside.

The accessory ducts from the rete testis up to the ejaculatory duct are located within the scrotum and pelvic region, while the urethra passes through the penis.

External Genitalia

• Includes the Penis.
• Penis: The external male copulatory organ. It is made up of special erectile tissue. The enlarged end is called the glans penis, which is covered by a loose fold of skin called the foreskin (prepuce).
• The erectile tissue fills with blood during sexual arousal, causing erection of the penis, which is necessary for insemination.

Accessory Glands

The male accessory glands secrete fluids that form the seminal plasma. Seminal plasma, along with sperm, constitutes semen.

• Paired Seminal Vesicles: Produce a fluid rich in fructose (energy source for sperm), prostaglandins (stimulate uterine contractions), and clotting factors.
• Prostate Gland: A single gland surrounding the urethra. Produces a milky fluid containing citrate, calcium, and enzymes.
• Paired Bulbourethral Glands (Cowper's glands): Located below the prostate. Produce a lubricating fluid that helps in the lubrication of the penis during coitus.

The secretions of these glands help in the transport of sperm and provide nourishment and protection to the sperm.

The Female Reproductive System

The human female reproductive system is responsible for the production of female gametes (ova), receiving sperm, providing a site for fertilisation, development of the embryo/foetus, childbirth, and lactation.

The female reproductive system is located in the pelvic region and includes:

1. A pair of Ovaries
2. Associated Accessory Ducts
3. External Genitalia

Associated with the female reproductive system is also a pair of Mammary Glands, which are involved in lactation (milk production).

*(Image shows a diagram of the human female reproductive system highlighting the major organs and ducts)*

Ovaries

• Pair of almond-shaped primary female sex organs.
• Location: Located one on each side of the lower abdomen, connected to the pelvic wall and uterus by ligaments.
• Functions:
  • Production of ova (female gametes) - Oogenesis.
  • Production of steroid ovarian hormones (Oestrogen and Progesterone).
• Each ovary is covered by a thin epithelium. Inside, it consists of the ovarian cortex and medulla. The cortex contains ovarian follicles in various stages of development.

*(Image shows a cross-section of an ovary illustrating primary follicles, growing follicles, Graafian follicle, ovulation, corpus luteum)*

Accessory Ducts

The female accessory ducts transport the ovum and provide sites for fertilisation and embryonic development. They include:

• Oviducts (Fallopian tubes): A pair of tubes extending from the periphery of each ovary to the uterus. Each oviduct is about 10-12 cm long.
  • Infundibulum: The funnel-shaped part close to the ovary, with finger-like projections called fimbriae, which help collect the ovum after ovulation.
  • Ampulla: The wider, curved central part. This is the usual site of fertilisation.
  • Isthmus: The narrow part that joins the uterus.
• Uterus (Womb): A single, inverted pear-shaped organ located in the pelvic cavity between the urinary bladder and the rectum. It is where the embryo develops during pregnancy. The uterus wall has three layers:
  • Perimetrium: Thin external membranous layer.
  • Myometrium: Middle thick layer of smooth muscle. Strong contractions of the myometrium cause labour during childbirth.
  • Endometrium: Inner glandular layer lining the uterine cavity. It undergoes cyclical changes during the menstrual cycle and is shed during menstruation. It is also the site of implantation of the embryo.
• Cervix: The narrow lower part of the uterus that opens into the vagina. The cavity of the cervix is the cervical canal.
• Vagina: A muscular tube extending from the cervix to the outside. It serves as the passage for menstrual flow, the receptacle for sperm during coitus, and the birth canal during childbirth.

External Genitalia

The female external genitalia, collectively called the vulva or pudendum, include:

• Mons pubis: A cushion of fatty tissue covered by skin and pubic hair.
• Labia majora: Fleshy folds of tissue extending down from the mons pubis and surrounding the vaginal opening.
• Labia minora: Paired folds of tissue located beneath the labia majora.
• Hymen: A thin membrane that partially covers the opening of the vagina. It can be torn during first coitus or other activities. It 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 homologous to the penis and is erectile.

The vaginal opening and urethral opening are located posterior to the clitoris.

Mammary Glands

• Pair of functional mammary glands are characteristic of all female mammals.
• Contain glandular tissue and variable amount of fat.
• The glandular tissue is divided into 15-20 mammary lobes containing clusters of cells called alveoli. Alveolar cells secrete milk, which is stored in the lumens of alveoli.
• Alveoli open into mammary tubules, which join to form mammary ducts. Several mammary ducts join to form a wider mammary ampulla, which is connected to the lactiferous duct through which milk is suckled out.
• Development of mammary glands is stimulated by hormones like oestrogen and progesterone. Milk production is stimulated by prolactin, and milk ejection by oxytocin.
• Function: Lactation (milk production) to nourish the newborn.

*(Image shows a diagram of a mammary gland highlighting its internal structure)*

Gametogenesis

Gametogenesis is the process of formation of haploid gametes from diploid germ cells in the gonads (testes and ovaries). It involves meiosis.

Spermatogenesis

Spermatogenesis is the process of formation of sperm (male gametes) from spermatogonia (male germ cells) in the seminiferous tubules of the testes. It begins at puberty.

Stages of Spermatogenesis:

1. Spermatogonia (2n): Diploid male germ cells located on the inner wall of seminiferous tubules. They divide by mitosis to increase in number. Some spermatogonia grow in size and are called primary spermatocytes (2n).
2. Meiosis I: Primary spermatocytes undergo meiosis I (reductional division) to form two equal, haploid cells called secondary spermatocytes (n).
3. Meiosis II: Secondary spermatocytes undergo meiosis II (equational division) to form four equal, haploid cells called spermatids (n).
4. Spermiogenesis: The process of transformation of non-motile spermatids into mature, motile spermatozoa (sperm) (n). During spermiogenesis, spermatids elongate, form a head, neck, middle piece, and tail. The sperm heads become embedded in the Sertoli cells for nourishment.

After spermiogenesis, sperm are released from the seminiferous tubules by a process called spermiation.

From one spermatogonium, one primary spermatocyte is formed. From one primary spermatocyte, four spermatids (and thus four sperm) are formed.

Spermatogenesis is stimulated by hormones: GnRH (from hypothalamus), FSH and LH (from pituitary), and androgens (from Leydig cells).

*(Image shows a flowchart or diagram of spermatogenesis showing spermatogonia $\rightarrow$ primary spermatocyte $\rightarrow$ secondary spermatocytes $\rightarrow$ spermatids $\rightarrow$ sperm, indicating mitosis, meiosis I, meiosis II, and spermiogenesis)*

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Structure Of Sperm

A mature sperm is a microscopic, motile structure consisting of:

• Head: Contains the haploid nucleus. The anterior portion of the head is covered by a cap-like structure called the acrosome. The acrosome contains enzymes (e.g., hyaluronidase, acrosin) that help in the fertilisation of the ovum. The acrosome is derived from the Golgi apparatus of the spermatid.
• Neck: A short region connecting the head to the middle piece. Contains the centrioles.
• Middle Piece: Contains numerous mitochondria arranged spirally around the axial filament. Mitochondria provide energy for the movement of the tail.
• Tail: A long, slender structure that provides motility to the sperm, enabling it to swim in the female reproductive tract.

*(Image shows a diagram of a sperm highlighting its parts)*

Oogenesis

Oogenesis is the process of formation of a mature ovum (female gamete) from oogonia (female germ cells) in the ovaries. Oogenesis begins during foetal development.

Stages of Oogenesis:

1. Oogonia (2n): Diploid female germ cells in the foetal ovary. They divide by mitosis to form millions of oogonia. No more oogonia are formed after birth.
2. Primary oocytes (2n): Oogonia enter the prophase I of meiosis and get arrested at this stage. These are called primary oocytes. Each primary oocyte is surrounded by a layer of granulosa cells, forming a primary follicle. A large number of primary follicles degenerate during childhood (follicular atresia). At puberty, only 60,000-80,000 primary follicles remain in each ovary.
3. Meiosis I: Primary oocytes complete meiosis I just before ovulation (starting from puberty). Meiosis I is unequal, producing a large, haploid cell called the secondary oocyte (n) and a small, haploid cell called the first polar body (n). The secondary oocyte retains most of the cytoplasm.
4. Meiosis II: The secondary oocyte enters meiosis II but gets arrested at metaphase II. Meiosis II is completed only if the sperm penetrates the secondary oocyte during fertilisation. This division is also unequal, producing a large, haploid ovum (n) and a small, haploid second polar body (n). The first polar body may also divide to form two polar bodies.

From one oogonium, one primary oocyte is formed. From one primary oocyte, one ovum and two or three polar bodies are formed.

Polar bodies are very small and degenerate. They contain minimal cytoplasm. Oogenesis results in the formation of only one functional ovum from each primary oocyte.

Development of ovarian follicles and oogenesis are regulated by hormones: GnRH, FSH, LH, Oestrogen, Progesterone.

*(Image shows a flowchart or diagram of oogenesis showing oogonia $\rightarrow$ primary oocyte $\rightarrow$ secondary oocyte + first polar body $\rightarrow$ ovum + second polar body (after fertilisation), indicating mitosis, meiosis I, meiosis II, and stages of follicular development)*

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Feature	Spermatogenesis	Oogenesis
Site	Seminiferous tubules (Testes)	Ovaries (within follicles)
Timing	Starts at puberty, continues throughout life	Starts during foetal development, arrested at puberty, resumes after puberty, completed only upon fertilisation, ends at menopause
Gametes per primary cell	4 sperm per primary spermatocyte	1 ovum per primary oocyte
Meiosis I Division	Equal	Unequal (produces secondary oocyte and first polar body)
Meiosis II Division	Equal (produces spermatids)	Unequal (produces ovum and second polar body)
Growth Phase	Short	Prolonged
Cytokinesis	Equal	Unequal
Polar Bodies	Absent	Present (1st and 2nd polar bodies)

Menstrual Cycle

The menstrual cycle is a cyclical series of changes that occur in the female reproductive tract, primarily in the ovary and uterus, preparing the uterus for possible pregnancy. It occurs in primate females (monkeys, apes, and humans).

The menstrual cycle is typically about 28/29 days long. It starts with menstruation and is regulated by hormones from the hypothalamus, pituitary, and ovaries.

Phases of the Menstrual Cycle:

The menstrual cycle is divided into four main phases:

1. Menstrual Phase:
   • Occurs if fertilisation does not take place.
   • Lasts for about 3-5 days.
   • Involves the shedding of the thickened lining of the uterus (endometrium) and its blood vessels, causing bleeding that flows out through the vagina. This is called menstruation.
   • This phase is caused by the decrease in levels of progesterone and oestrogen.
2. Follicular Phase (Proliferative Phase):
   • Starts after menstruation and lasts for about 10-14 days (up to ovulation).
   • Primary follicles in the ovary grow and develop into a fully mature Graafian follicle. This is stimulated by FSH (from pituitary).
   • The growing follicles secrete oestrogen. Oestrogen stimulates the proliferation of the uterine endometrium, making it thicker and more vascularised again.
   • FSH and oestrogen levels increase during this phase.
3. Ovulation:
   • Occurs around the middle of the cycle (typically around day 14 in a 28-day cycle).
   • A surge of LH (Luteinizing Hormone) from the pituitary occurs (LH surge).
   • The LH surge induces the rupture of the mature Graafian follicle and the release of the secondary oocyte (ovum) from the ovary (ovulation).
4. Luteal Phase (Secretory Phase):
   • Starts after ovulation and lasts for about 14 days (until the next menstruation).
   • The ruptured Graafian follicle is transformed into a yellowish mass called the corpus luteum under the influence of LH.
   • The corpus luteum secretes large amounts of progesterone (and some oestrogen). Progesterone is essential for maintaining the uterine endometrium, making it suitable for implantation of the embryo.
   • If fertilisation and implantation occur, the corpus luteum persists and continues to secrete progesterone (supported by hCG hormone from the developing placenta).
   • If fertilisation does not occur, the corpus luteum degenerates after about 14 days. The decrease in progesterone and oestrogen levels leads to the breakdown of the endometrium, causing menstruation and the start of a new cycle.

*(Image shows a combined diagram illustrating the changes in ovary (follicle growth, ovulation, corpus luteum), uterus (endometrium thickness), and the levels of pituitary hormones (FSH, LH) and ovarian hormones (Oestrogen, Progesterone) over a 28-day cycle)*

Hormonal Regulation of the Menstrual Cycle:

• Hypothalamus: Secretes GnRH (Gonadotropin Releasing Hormone), which stimulates the pituitary gland.
• Pituitary Gland: Secretes FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone). FSH stimulates follicular growth; LH triggers ovulation and corpus luteum formation.
• Ovaries: Secretes Oestrogen and Progesterone. Oestrogen stimulates endometrial proliferation; Progesterone maintains the endometrium and supports pregnancy.

These hormones interact in complex positive and negative feedback loops to regulate the cyclical changes.

Fertilisation And Implantation

Fertilisation is the fusion of male and female gametes, leading to the formation of a zygote. Implantation is the process by which the early embryo attaches to the uterine wall, marking the beginning of pregnancy.

Fertilisation

• It is the process where the sperm fuses with the ovum.
• Location: Fertilisation usually occurs in the ampulla of the fallopian tube.
• Insemination: During coitus, semen is released into the vagina (insemination). Sperm travel through the cervix and uterus into the fallopian tubes.
• Sperm encounter the secondary oocyte (ovum) in the ampulla.
• The sperm undergoes capacitation (physiological changes that make it capable of fertilising the egg).
• The acrosome of the sperm releases enzymes (acrosomal enzymes) that help the sperm penetrate the outer layers of the ovum: the corona radiata and the zona pellucida.
• Penetration of the zona pellucida by a sperm induces changes in the membrane, preventing polyspermy (fertilisation by more than one sperm).
• The sperm nucleus fuses with the nucleus of the secondary oocyte. This triggers the completion of meiosis II by the secondary oocyte, forming a mature ovum and a second polar body.
• The nuclei of the sperm and ovum fuse to form a diploid zygote (2n).

*(Image shows a diagram illustrating sperm approaching and penetrating the ovum layers (corona radiata, zona pellucida) and the fusion of nuclei to form a zygote)*

Implantation

• After fertilisation, the zygote undergoes rapid mitotic divisions called cleavage as it moves through the isthmus of the oviduct towards the uterus.
• The zygote forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8-16 blastomeres is called a morula.
• The morula continues to divide and develops into a blastocyst as it reaches the uterus.
• The blastocyst is a hollow ball of cells with an outer layer called the trophoblast and an inner group of cells attached to the trophoblast, called the inner cell mass. The inner cell mass will develop into the embryo. The blastocyst cavity is filled with fluid.
• Implantation: The process by which the blastocyst attaches to the wall of the uterus (endometrium).
  • The trophoblast layer of the blastocyst attaches to the endometrium.
  • The uterine cells divide rapidly and cover the blastocyst.
  • The blastocyst becomes embedded in the endometrium.
• Implantation usually occurs about 7 days after fertilisation.
• After implantation, pregnancy begins. The trophoblast layer gives rise to the placenta, and the inner cell mass differentiates to form the three germ layers of the embryo.

*(Image shows a diagram illustrating the stages of development from zygote, cleavage (2, 4, 8 cell), morula, blastocyst, and the blastocyst embedding into the uterine wall)*

Fertilisation and implantation are critical events that initiate pregnancy and lead to the development of a new individual.

Pregnancy And Embryonic Development

Pregnancy is the period from implantation of the blastocyst in the uterus until childbirth. It is a period of significant growth and development of the embryo and foetus.

The average duration of human pregnancy is about 9 months (gestation period).

Placenta

• The placenta is a vital structure that forms a connection between the developing embryo/foetus and the maternal body.
• It is formed by finger-like projections called chorionic villi (from the trophoblast) which are surrounded by uterine tissue and maternal blood vessels.
• Functions of Placenta:
  • Provides oxygen and nutrients to the embryo/foetus.
  • Removes carbon dioxide and excretory waste products from the foetus.
  • Functions as an endocrine gland, producing several hormones (e.g., human Chorionic Gonadotropin - hCG, human Placental Lactogen - hPL, oestrogens, progestogens, relaxin) essential for maintaining pregnancy.
• The placenta is connected to the embryo/foetus by the umbilical cord, which transports substances to and from the embryo.

*(Image shows a diagram illustrating the placenta embedded in the uterine wall, chorionic villi, maternal blood vessels, and the umbilical cord connecting to the foetus)*

Hormonal Changes During Pregnancy

• Pregnancy is associated with increased production of several hormones.
• hCG (human Chorionic Gonadotropin): Produced by the placenta. Maintains the corpus luteum in the early stages of pregnancy, which continues to secrete progesterone. Presence of hCG in urine is the basis of pregnancy tests.
• hPL (human Placental Lactogen): Produced by the placenta. Stimulates mammary gland development and helps in nutrient transfer to the foetus.
• Oestrogens and Progestogens: Levels increase significantly during pregnancy, secreted by the ovary and later by the placenta. Essential for maintaining the uterine endometrium and supporting foetal growth.
• Relaxin: Secreted by the ovary and placenta. Relaxes the pelvic ligaments at the end of pregnancy, facilitating childbirth.

Increased levels of other hormones like cortisol, prolactin, and thyroxine are also observed during pregnancy, supporting maternal metabolism and foetal growth.

Embryonic Development

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

Major Events in Embryonic Development:

• Within a week of implantation, the inner cell mass differentiates into the three germ layers.
• Development of limbs and digits.
• Formation of major organs (organogenesis).
• By the end of the first month, the embryo's heart is formed (can be detected by listening to heart sound).
• By the end of the second month, limbs and digits develop.
• By the end of 12 weeks (first trimester), most major organ systems are formed. The foetus resembles a miniature human.
• During the second trimester (up to 24 weeks), the first movements of the foetus are felt, and hair appears on the head.
• By the end of about 9 months, the foetus is fully developed and ready for delivery.

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Pregnancy is a complex process requiring precise hormonal control and intricate embryonic development to ensure the healthy growth of the foetus.

Parturition And Lactation

Parturition is the process of childbirth, where the fully developed foetus is expelled from the uterus. Lactation is the process of milk production by the mammary glands to nourish the newborn.

Parturition

• The average gestation period in humans is about 9 months.
• Parturition is induced by complex neuroendocrine mechanisms.
• The signals for parturition originate from the fully developed foetus and the placenta, which induce mild uterine contractions. This is called foetal ejection reflex.
• Foetal ejection reflex triggers the release of Oxytocin from the maternal pituitary gland.
• Oxytocin acts on the uterine muscles and causes stronger uterine contractions, which in turn stimulate further secretion of oxytocin (positive feedback loop).
• The increasing contractions lead to the dilation of the cervix and the expulsion of the baby from the uterus through the birth canal (vagina).
• After the baby is delivered, the placenta and remaining umbilical cord (afterbirth) are also expelled from the uterus.

*(Image shows diagrams illustrating the progression of labour, birth of the baby, and expulsion of the placenta)*

Lactation

• Lactation is the production of milk by the mammary glands, which differentiate during pregnancy.
• Milk production (lactogenesis) is stimulated by the hormone Prolactin, secreted by the anterior pituitary.
• Milk ejection (let-down reflex) is stimulated by the hormone Oxytocin, secreted by the posterior pituitary in response to suckling by the baby. Suckling is a neuroendocrine reflex.
• The milk produced during the initial few days of lactation is called colostrum.

Colostrum:

• A yellowish fluid secreted by the mammary glands during the initial days of lactation.
• It contains antibodies (especially IgA) that provide passive immunity to the newborn, protecting it from infections.
• It also contains other nutrients essential for the baby's health.

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Breastfeeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby because it provides essential nutrients and antibodies. Breastfeeding also helps in the contraction of the uterus after childbirth (due to oxytocin release).