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Human Reproduction Male and Female Reproductive Systems ● Human beings reproduce sexually and are viviparous. ● In humans, the reproductive phase starts after puberty. ● It involves: ○ Gametogenesis ○ Insemination ○ Fertilisation ○ Implantation ○ Gestation ○ Parturition The Male Reproductive System ● It is located in the pelvic region. ● It consists of: ○ A pair of testes ○ Accessory glands and ducts ○ External genitalia Testes ● Situated within the scrotum, which protects the testes and also helps in maintaining the temperature. ● Each testis is 4 to 5 cm in length, and 2 to 3 cm in width, and has about 250 compartments called testicular lobules. ● Testicular lobules have seminiferous tubules which are the sites of sperm formation. ● Seminiferous tubules are lined by two types of cells: ○ Male germ cells − They undergo meiosis to form sperms. ○ Sertoli cells − They provide nourishment to the germ cells. ● Region outside the seminiferous tubules is called the interstitial space, which contains Leydig cells (interstitial cells). The Leydig cells produce androgens. Accessory Ducts and Glands ● Accessory ducts include: ○ Rete testis ○ Vasa efferentia ○ Epididymis ○ Vas deferens ● The seminiferous tubules open into the vasa efferentia through the rete testis. ● The vasa efferentia open into the epididymis, which leads to the vas deferens. The vas deferens opens into the urethra along with a duct from the seminal vesicle called the ejaculatory duct. ● It is also called womb, and is pear shaped. ● It is connected to the pelvic walls by ligaments. ● The uterine wall consists of: ○ External perimetrium ○ Middle myometrium ○ Internal endometrium, which lines the uterine cavity ● The endometrium undergoes changes during the menstrual cycle. Cervix and Vagina ● The cervix connects the uterus to the vagina. ● The cervix and the vagina constitute the birth canal. External Genitalia ● Consists of: ○ Mons pubis − Fatty tissue covered by skin and pubic hair ○ Labia majora − Extends from mons pubis and surrounds the vaginal opening ○ Labia minora − Fold of skin beneath the labia majora ○ Hymen − Partially covers the vaginal opening ○ Clitoris − Lies at the junction of labia minora Mammary Glands ● Present in all female mammals ● It is paired and is glandular. ● Each breast contains 15 to 20 mammary lobes with alveoli which secrete milk. ● The alveoli open into the mammary tubules, which unite to form a mammary duct. ● Many mammary ducts constitute the mammary ampulla, which is connected to the lactiferous duct. Gametogenesis The testis and ovary produce the male and female gametes respectively by gametogenesis (spermatogenesis in males and oogenesis in females). Spermatogenesis ● In males, sperms are produced by the spermatogonia (immature germ cells), which are present in the inner walls of the seminiferous tubules. ● Spermatogonia increase in number by mitosis. These are diploid. ● Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. ● After the first meiotic division, two haploid and equal secondary spermatocytes are formed. ● These further undergo meiosis to give rise to four haploid spermatids. ● These spermatids are converted into sperms by spermiogenesis. ● The sperm head gets embedded in the Sertoli cells after spermiogenesis and is released from the seminiferous tubules by spermiation. ● Spermatogenesis starts at puberty by the action of the gonadotropin releasing hormone (GnRH), which in turn causes the release of two gonadotropins called Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH). ● LH acts on Leydig cells and causes them to release androgens, which stimulate the process of spermatogenesis while the FSH acts on the Sertoli cells, which help in spermiogenesis. Structure of a Sperm ● Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche). ● In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. ● During the middle of the menstrual cycle, one ovum is released (ovulation). ● The cycle starts with the menstrual flow (3 to 5 days), caused due to the breakdown of the endometrium of the uterus. Blood vessels in liquid state are discharged, but this occurs only when the ovum is not fertilised. ● It is followed by the follicular phase.In this phase, the primary follicles mature into the Graffian follicles. This causes the regeneration of the endometrium. These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins (LH and FSH) increases. This causes follicular growth and the growing follicles produce oestrogen. ● The LH and FSH are at their peak in the middle of the cycle (14th day), and cause the rupture of the Graffian follicles to release ovum. This phase is called the ovulatory phase. ● The remains of the Graffian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. ● In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle. ● In humans, the menstrual cycle ceases to operate at the age of 50 years. This phase is known as the menopause. Fertilisation and Implantation ● During coitus, the semen is released into the vagina, passes through the cervix of the uterus and reaches the ampullaryisthmic junction of the fallopian tube. ● The ovum is also released into the junction for fertilisation to occur. ● The process of fusion of the sperm and the ovum is known as fertilisation. ● During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum. ● The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum. ● This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body. ● Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote. ● Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8, 16 daughter cells called blastomeres. ● The 8−16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus. ● The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass. ● The trophoblast gets attached to the uterine endometrium, and the process is called implantation. This leads to pregnancy. ● The inner cell mass gets differentiated to form the embryo. Pregnancy, Parturition and Lactation Pregnancy ● After implantation, the trophoblast forms fingerlike projections called chorionic villi, surrounded by the uterine tissue and maternal blood. ● The chorionic villi and the uterine tissue get integrated to form the placenta, which helps in supplying the developing embryo with oxygen and nutrients, and is also involved in