Human beings reproduce sexually. The reproductive events in humans include gametogenesis, insemination, fertilization, implantation, gestation and parturition. Human beings are viviparous, i.e. they give birth to babies.
Male Reproductive System
The male reproductive system is located in the pelvis region. It consists of a pair of testes; along with accessory ducts, glands and the external genitalia.
Testis: A pair of testes is situated outside the abdominal cavity; suspended in a skin-pouch called scrotum. The scrotum helps maintaining the low temperature of the testes (2 – 2.5°C lower than body temperature). The lower temperature is necessary for spermatogenesis. In an adult, each testis is oval in shape. It is about 4 to 5 cm long and about 2 to 3 cm wide. Each testis has about 250 compartments; called testicular lobules.
Seminiferous Tubules: Each lobule contains one to three highly coiled seminiferous tubules. Sperms are produced in the seminiferous tubules. A seminiferous tubule is lined on the inside by two types of cells; called the male germ cells (spermatogonia) and Setoli cells. The male germ cells undergo meiosis; resulting in sperm formation. Sertoli cells provide nutrition to the germ cells.
Interstitial Spaces: The regions outside the seminiferous tubules are called interstitial spaces. The interstitial spaces contain small blood vessels and interstitial cells or Leydig cells. The Leydig cells synthesize and secrete the testicular hormones; called androgens. Other immunologically competent cells are also present in the interstitial spaces.
Accessory Ducts: These ducts include rete testis, vasa efferentia, epididymis and vas deferens. The seminiferous tubules open into the vasa efferentia through rete testis. The vasa efferentia leave the testis and open into epididymis. Epididymis is located along the posterior surface of each testis. The epididymis leads to vas deferens. The vas deferens 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. These accessory 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.
Penis: It is the male external genitalia. The penis is made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of penis is called the glans penis. The glans penis is covered by a loose fold of skin; called foreskin.
Accessory Glands: These glands include paired seminal vesicle, a prostate gland and paired bulbourethral glands. Secretions from these glands constitute the seminal plasma. The seminal plasma is rich in fructose, calcium and certain enzymes. The secretion from bulbourethral glands also helps in lubrication of the penis.
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 pelvic region. These parts of the system along with a pair of the mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care.
Ovaries: These are the primary female sex organs that produce the female gamete and several steroid hormones. Ovaries are located one on each side of the lower abdomen. An ovary is about 2 to 4 cm long. It 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, i.e. a peripheral cortex and an inner medulla.
Accessory Ducts: The oviducts (fallopian tubes), uterus and vagina constitute the female accessory ducts.
Fallopian Tubes: Each fallopian tube is about 10 to 12 cm long. It extends from the periphery of each ovary to the uterus. The part of oviduct; closer to the ovary is funnel shaped and is called infundibulum. There are finger-like projections; called flmbriae; at the edges of the infundibulum. Fimbriae help in collection of ovum after ovulation. The infundibulum leads to a wider part of the oviduct; called ampulla. The last part of the oviduct is called isthmus. The isthmus has a narrow lumen and its joins the uterus.
Uterus: There is a single uterus; which is shaped like an inverted pear. The uterus is supported by ligaments attached to the pelvic wall. The uterus opens into the vagina through a narrow cervix. The lumen of the cervix is called cervical canal. The cervical canal; along with the vagina; forms the birth canal. There are three layers of tissues in the wall of the uterus. The external layer is thin and membranous and is called perimetrium. The middle layer is thick and made up of smooth muscles, and is called myometrium. The inner layer is glandular layer and is called endometrium. The uterine cavity is lined by the endometrium. The endometrium undergoes cyclical changes during menstrual cycle. The myometrium exhibits strong contraction during child birth.
External Genitalia: The external genitalia in females include mons pubis, labia majora, labia minora, hymen and clitoris. The cushion of fatty tissue; covered by skin and pubic hair; is called the mons pubis. The fleshy folds of tissue; which extend down from the mons pubis and surround the vaginal tissue; are called labia majora. The paired folds of tissue under the labia majora is called the labia minora. The opening of the vagina is usually covered partially by a membrane called hymen. There is a tiny finger-like structure at the upper junction of the two labia minora; above the urethral opening. This structure is called the clitoris.
The hymen is usually torn during the first coitus. It can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in sports, etc. The hymen may persist in some women even after coitus. Hence, presence or absence of hymen is not a reliable indicator of virginity or sexual experience.
Mammary Gland: Mammary glands are paired structures that contain glandular tissue and variable amount of fat. The glandular tissue in each breast is divided into 15 to 20 mammary lobes. Each lobe contains clusters of cells; called alveoli. The cells of alveoli secrete milk. Milk is stored in the lumen of the alveoli. The alveoli open into mammary glands. The tubules of each lobe join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla. A mammary ampulla is connected to lactiferous duct. Milk is sucked out through the lactiferous duct.