Unit 2 : Human Reproduction

1. The Male Reproductive System

The male reproductive system is located in the pelvis region and consists of primary sex organs, secondary sex organs (accessory ducts and glands), and external genitalia.

Primary Sex Organ and Accessory Structures

• Testes: The primary male sex organs situated outside the abdominal cavity within a pouch called the scrotum.
• Scrotum: Maintains a temperature 2 to 2.5 degrees Celsius lower than the normal internal body temperature, which is essential for spermatogenesis.

Internal Anatomy of Testes

Each testis has about 250 compartments called testicular lobules. Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced.

• Male Germ Cells (Spermatogonia): Undergo mitotic and meiotic divisions to form sperms.
• Sertoli Cells: Provide nutrition to the germ cells and developing sperms.
• Interstitial Cells (Leydig Cells): Present in the intertubular spaces and synthesize and secrete male hormones called androgens (testosterone).

Accessory Ducts and Glands

The male accessory ducts include rete testis, vasa efferentia, epididymis, and vas deferens. They store and transport sperms from the testes to the outside.

• 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.
• Urethra: Originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus.
• Accessory Glands: Include paired seminal vesicles, a prostate gland, and paired bulbourethral glands. Their secretions constitute the seminal plasma, which is rich in fructose, calcium, and certain enzymes. The secretions of bulbourethral glands also help in the lubrication of the penis.

External Genitalia

• Penis: The male external genitalia made up of special erectile tissue that helps in coitus. The enlarged end of the penis is called the glans penis, covered by a loose fold of skin called the foreskin.

2. The Female Reproductive System

The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina, and the external genitalia located in the pelvic region.

Primary Sex Organs

• Ovary: The primary female sex organ that produces the female gamete (ovum) and several steroid hormones (ovarian hormones). Each ovary is connected to the pelvic wall and uterus by ligaments.

Accessory Ducts

• Oviducts (Fallopian Tubes): Each is about 10-12 cm long and extends from the periphery of each ovary to the uterus. It is divided into three parts: infundibulum (funnel-shaped with finger-like projections called fimbriae that help collect the ovum), ampulla (wider part), and isthmus (narrow lumen joining the uterus).
• Uterus (Womb): Single, inverted pear-shaped organ supported by ligaments attached to the pelvic wall. The uterine wall has three layers of tissues: perimetrium (external thin membrane), myometrium (middle thick layer of smooth muscle), and endometrium (inner glandular layer that lines the uterine cavity and undergoes cyclical changes during the menstrual cycle).
• Cervix and Vagina: The uterus opens into the vagina through a narrow cervix. The cavity of the cervix is the cervical canal which along with the vagina forms the birth canal.

External Genitalia

The female external genitalia include the mons pubis, labia majora, labia minora, hymen, and clitoris.

• 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.
• Labia minora: Paired folds of tissue under the labia majora.
• Clitoris: A tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening.
• Hymen: A membrane that often covers the opening of the vagina partially.

Mammary Glands

Paired structures containing glandular tissue and variable amount of fat. The glandular tissue is divided into 15-20 mammary lobes containing clusters of cells called alveoli. The alveoli secrete milk, which is stored in the cavities of alveoli, opens into mammary tubules, joins to form a mammary duct, several of which join to form a wider mammary ampulla connected to the lactiferous duct through which milk is sucked out.

3. Gametogenesis

The process of formation of gametes (sperms and ova) in the primary sex organs (testes and ovaries) is called gametogenesis.

Spermatogenesis

In testes, immature male germ cells (spermatogonia) produce sperms by spermatogenesis, which begins at puberty.

• Spermatogonia multiply by mitotic division and increase in number. Each spermatogonium is diploid (2n) and contains 46 chromosomes.
• Some spermatogonia called primary spermatocytes periodically undergo meiosis.
• The primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid (n) secondary spermatocytes.
• The secondary spermatocytes undergo the second meiotic division to form four equal, haploid spermatids.
• Spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis.
• After spermiogenesis, sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation.

Structure of a Sperm

A mature sperm is composed of a head, neck, middle piece, and tail.

• Head: Contains a haploid nucleus and an apical cap-like structure called the acrosome, which is filled with enzymes that help fertilize the ovum.
• Middle Piece: Possesses numerous mitochondria, which produce energy for the movement of tail that facilitate sperm motility essential for fertilization.

Oogenesis

The process of formation of a mature female gamete is called oogenesis, which is markedly different from spermatogenesis.

• Oogenesis initiates during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth.
• These cells start division and enter prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes.
• Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle.
• A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty, only 60,000 to 80,000 primary follicles are left in each ovary.
• The primary follicle gets surrounded by more layers of granulosa cells and a new teca to form a secondary follicle.
• The secondary follicle soon transforms into a tertiary follicle characterized by a fluid-filled cavity called the antrum.
• The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division resulting in an unequal division forming a large haploid secondary oocyte and a tiny first polar body.
• The tertiary follicle further changes into the mature follicle or Graafian follicle.
• The secondary oocyte forms a new membrane called zona pellucida surrounding it.
• The Graafian follicle ruptures to release the secondary oocyte from the ovary by the process called ovulation.

4. Menstrual Cycle

The reproductive cycle in female primates (e.g., monkeys, apes, and human beings) is called the menstrual cycle. The first menstruation begins at puberty and is called menarche. In human females, menstruation is repeated at an average interval of about 28/29 days.

Phases of the Menstrual Cycle

1. Menstrual Phase: The cycle starts with menstrual flow, which lasts for 3-5 days. The flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation occurs only if the released ovum is not fertilized. If pregnancy does not occur, the cycle repeats.
2. Follicular Phase: During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually in the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles.
3. Ovulatory Phase: Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle is called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation).
4. Luteal Phase: The remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilized ovum and other events of pregnancy. In the absence of fertilization, the corpus luteum degenerates, which causes disintegration of the endometrium leading to menstruation, marking a new cycle.

In human beings, menstrual cycles cease around 50 years of age; that is termed menopause.

5. Fertilization and Implantation

During copulation (coitus), semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube where fertilization takes place.

Process of Fertilization

Fertilization is the fusion of a sperm with an ovum. Fertilization can only occur if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction.

• A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms (ensuring polyspermy is prevented).
• The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
• This induces the completion of the second meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid).
• Soon the haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.

Sex Determination

The sex of the baby is determined during fertilization. The human male possesses one X and one Y chromosome, while females have two X chromosomes. Therefore, the sperm can carry either an X or a Y chromosome, whereas all eggs carry an X chromosome. If a sperm carrying an X chromosome fertilizes the egg, the zygote develops into a female (XX), and if a sperm carrying a Y chromosome fertilizes the egg, the zygote develops into a male (XY).

Cleavage and Implantation

• As the zygote moves through the isthmus of the oviduct towards the uterus, it undergoes mitotic division called cleavage for formation of daughter cells called blastomeres.
• The embryo with 8 to 16 blastomeres is called a morula.
• The morula continues to divide and transforms into blastocyst as it moves further into the uterus.
• The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass.
• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.
• The uterine cell divides rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation and it leads to pregnancy.

6. Pregnancy and Embryonic Development

Following implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated jointly to form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.

Functions of Placenta

• Facilitates supply of oxygen and nutrients to the embryo.
• Removes carbon dioxide and excretory/waste materials produced by the embryo.
• Acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
• Note that during pregnancy, the levels of hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased manifold in the maternal blood. Increased production of these hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.

Embryonic Development

• The inner cell mass contains certain cells called stem cells which have the potency to give rise to all tissues and organs.
• First Month: The heart's formation is the first sign of a growing foetus.
• Second Month: The foetus develops limbs and digits.
• Third Month (First Trimester): Most of the major organ systems are formed (e.g., limbs and external genital organs are well-developed).
• Fifth Month: First movements of the foetus and appearance of hair on the head.
• Sixth Month (Second Trimester): The body is covered with fine hair, eye-lids separate, and eyelashes are formed.
• End of Nine Months: The foetus is fully developed and ready for delivery.

7. Parturition and Lactation

The average duration of human pregnancy is about 9 months which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus is called parturition.

Mechanism of Parturition

• Parturition is induced by a complex neuroendocrine mechanism.
• The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called the fetal ejection reflex.
• This triggers release of oxytocin from the maternal pituitary.
• Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.
• The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions leading to expulsion of the baby out of the uterus through the birth canal.

Lactation

• The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy by the process called lactation.
• The milk produced during the initial few days of lactation is called colostrum, which contains several antibodies (especially IgA) absolutely essential to develop resistance for the new-born babies.
• Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.