Human Reproduction PDF

CHAPTER 2 HUMAN REPRODUCTION 2.1 The Male Reproductive System 2.2 The Female Reproductive As you are aware, humans are sexually reproducing and System viviparous. The reproductive events in humans include formation of...

CHAPTER 2 HUMAN REPRODUCTION 2.1 The Male Reproductive System 2.2 The Female Reproductive As you are aware, humans are sexually reproducing and System viviparous. The reproductive events in humans include formation of gametes (gametogenesis), i.e., sperms in males 2.3 Gametogenesis and ovum in females, transfer of sperms into the female 2.4 Menstrual Cycle genital tract (insemination) and fusion of male and female gametes (fertilisation) leading to formation of zygote. This 2.5 Fertilisation and is followed by formation and development of blastocyst Implantation and its attachment to the uterine wall (implantation), 2.6 Pregnancy and Embryonic embryonic development (gestation) and delivery of the Development baby (parturition). You have learnt that these reproductive events occur after puberty. There are remarkable 2.7 Parturition and Lactation differences between the reproductive events in the male and in the female, for example, sperm formation continues even in old men, but formation of ovum ceases in women around the age of fifty years. Let us examine the male and female reproductive systems in human. 2.1 THE MALE REPRODUCTIVE SYSTEM The male reproductive system is located in the pelvis region (Figure 2.1a). It includes a pair of testes alongwith accessory ducts, glands and the external genitalia. 2024-25 HUMAN REPRODUCTION The testes are situated outside the abdominal cavity within a pouch called scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5o C lower than the normal internal body temperature) necessary for spermatogenesis. In adults, each testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm. The testis is covered by a dense covering. Each testis has about 250 compartments called testicular lobules Figure 2.1(a) Diagrammatic sectional view of male pelvis (Figure 2.1b). showing reproductive system Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells (Figure 2.2 ). The male germ cells undergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide nutrition to the germ cells. The regions outside the seminiferous tubules called interstitial spaces, contain small blood vessels and interstitial cells or Leydig cells (Figure 2.2). Leydig cells synthesise and secrete testicular hormones called Figure 2.1(b) Diagrammatic view of male reproductive system (part of testis is open to show inner details) androgens. Other immunologically competent cells are also present. The male sex accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens (Figure 2.1b). The seminiferous tubules of the testis open into the vasa efferentia through rete testis. The vasa efferentia leave the testis and open into epididymis located along the posterior surface 27 of each testis. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct (Figure 2.1a). These ducts store and transport the sperms from the testis to the outside through urethra. The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus. 2024-25  BIOLOGY Figure 2.2 Diagrammatic sectional view of seminiferous tubule The penis is the male external genitalia (Figure 2.1a, b). It is made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of penis called the glans penis is covered by a loose fold of skin called foreskin. The male accessory glands (Figure 2.1a, b) include paired seminal vesicles, a prostate and paired bulbourethral glands. Secretions of these glands constitute the seminal plasma which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands also helps in the lubrication of the penis. 2.2 THE FEMALE REPRODUCTIVE SYSTEM The female reproductive system consists of a pair of ovaries alongwith a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region (Figure 2.3a). These parts of the system alongwith a pair of the mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care. Ovaries are the primary female sex organs that produce the female 28 gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen (Figure 2.3b). Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The stroma is divided into two zones – a peripheral cortex and an inner medulla. 2024-25 HUMAN REPRODUCTION Figure 2.3 (a) Diagrammatic sectional view of female pelvis showing reproductive system The oviducts (fallopian tubes), uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus (Figure 2.3b), the part closer to the ovary is the funnel-shaped infundibulum. The edges of the infundibulum possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider 29 Figure 2.3 (b) Diagrammatic sectional view of the female reproductive system 2024-25  BIOLOGY part of the oviduct called ampulla. The last part of the oviduct, isthmus has a narrow lumen and it joins the uterus. The uterus is single and it is also called womb. The shape of the uterus is like an inverted pear. It is supported by ligaments attached to the pelvic wall. The uterus opens into vagina through a narrow cervix. The cavity of the cervix is called cervical canal (Figure 2.3b) which alongwith vagina forms the birth canal. The wall of the uterus has three layers of tissue. The external thin membranous perimetrium, middle thick layer of smooth muscle, myometrium and inner glandular layer called endometrium that lines the uterine cavity. The endometrium undergoes cyclical changes during menstrual cycle while the myometrium exhibits strong contraction during delivery of the baby. The female external genitalia include mons pubis, labia majora, labia minora, hymen and clitoris (Figure 2.3a). Mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labia majora are fleshy folds of tissue, which extend down from the mons pubis and surround the vaginal opening. The labia minora are paired folds of tissue under the labia majora. The opening of the vagina is often covered partially by a membrane called hymen. The clitoris is a tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening. The hymen is often torn during the first coitus (intercourse). However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling, etc. In some women the hymen persists even after coitus. In fact, the presence or absence of hymen is not a reliable indicator of virginity or sexual experience. 30 Figure 2.4 A diagrammatic sectional view of Mammary gland 2024-25 HUMAN REPRODUCTION A functional mammary gland is characteristic of all female mammals. The mammary glands are paired structures (breasts) that contain glandular tissue and variable amount of fat. The glandular tissue of each breast is divided into 15-20 mammary lobes containing clusters of cells called alveoli (Figure 2.4). The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli. The alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out. 2.3 GAMETOGENESIS The primary sex organs – the testis in the males and the ovaries in the females – produce gametes, i.e, sperms and ovum, respectively, by the process called gametogenesis. In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes. Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids (Figure 2.5). What would be the number of chromosome in the spermatids? The 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. Spermatogenesis starts at the age of Figure 2.5 Diagrammatic sectional view of a puberty due to significant increase in the seminiferous tubule (enlarged) secretion of gonadotropin releasing hormone (GnRH). This, if you recall, is a hypothalamic hormone. The increased 31 levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinising hormone (LH) and follicle stimulating hormone (FSH). LH acts at the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates 2024-25