Reproductive system in human
Humans reproduce sexually and are viviparous (produce young ones rather than eggs). It goes through various steps and can be studied in two-part- i.e. male reproductive system and the female reproductive system
The main processes in the reproductive system are the Formation of gametes, fertilization, pregnancy, and development of the embryo.
Adolescent stage or Puberty in humans:
Human sex organs begin to mature with puberty. The name ‘puberty’ is given to the changes that occur in boys and girls as they grow up from age 10 to 14 years. Here at this stage, another term is used as adolescent shows the attainment of puberty.
During adolescence, both males and females show different secondary sexual characteristics.
Male Reproductive System
In males the system is located in the pelvis region, it consists of pair of testes, accessory ducts (a pair of the epididymis, vasa differentia (singular: vas deferens), urethra), accessory glands, and external genitalia (penis).
It is a pouch-like structure arising from the lower abdominal wall and hanging between the legs. If the testes fail to descend into the scrotum, it causes sterility because sperm formation cannot occur at abdominal temperature.
Testes are the primary sex organs. They are soft, smooth, and pink in color, and oval in shape. The testes are 4.5 cm long, 2-3 cm wide and 2-4 cm thick.
- Testes are suspended into scrotum.
- Testes are covered by a dense fibrous coat called tunica albuginea. Inside this lining is tunica vasculosa which is richly supplied with blood vessels.
- The outside covering of each testis is called tunica vaginalis.
- The testes are divided into 200-300 lobules by the tunica albuginea. Each testicular lobule contains 1-4 highly coiled seminiferous tubules. The end of the seminiferous tubule converges to form short straight tubules called tubuli recti. These open into a network of wider, irregular tubules called rete testes.
- Each seminiferous tubule is lined with germinal epithelium. The majority of these cells are diploid spermatogenic cells which form diploid spermatogonia in the lumen of the seminiferous tubules by mitotic division.
- Spermatogonia grow into primary spermatocytes which undergo meiosis to produce haploid secondary spermatocytes that form spermatids which metamorphose into spermatozoa. This process is called spermatogenesis.
Spermatogenic cells of germinal epithelium (2n)
Primary Spermatocytes (2n)
Secondary Spermatocytes (n)
- Sertoli cells provide nourishment and shape to the developing spermatozoa.
- interstitial cells of Leyding are small groups of large, polygonal cells that lie in the connective tissue present between the seminiferous tubules. They secret androgens (male sex hormones), like testosterone, into the blood.
Epididymis– It is a long highly coiled tube attached to the testis and lies within the scrotal sac. It stores spermatozoa (sperms) and acts as a passage for their transport from the testis.
Vas deferens– The epididymis leads to vas deferens which ascend to the abdomen and loop over the urinary bladder. It receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct. Store and transport the sperm from the testis to the outside through the urethra.
Urethra– The urethra (about 15-20 cm) originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus. It passages both urine and semen.
It receives ducts of the Cowper and prostate glands. It is known as the urogenital duct as it provides an exit for urine from the bladder as well as semen from the Vas Deferentia.
There is some male accessory gland such as seminal vesicles, prostate, and paired bulbourethral glands. These secrete some lubricants rich in fructose, calcium, and certain enzymes.
External Genitalia: penis
The penis is the male external genital organ. Made of special tissue that helps in the erection of the penis to make it able for insemination. The enlarged end of the penis is called the glans penis and it is covered by a loose fold of skin called the foreskin or prepuce.
Semen is a mixture of seminal fluid. Seminal fluid comprises secretions from the seminiferous tubule, seminal vesicle, prostate glands, and bulbourethral glands. It is slightly alkaline in nature.
Female Reproductive System
The female reproductive system is located in the pelvic region. It consists of organs as follows- A pair of ovaries, accessory ducts (a pair of fallopian tubes, uterus, cervix, vagina), external genitalia, and accessory glands.
A pair of ovaries, which lie in the lower part of the abdominal cavity, are placed on each side of the lower abdomen. These are primary sex organs.
- Ovaries produce ova, the process of formation of an egg in the ovary is known as oogenesis.
- Ovaries secrete female sex hormones (steroid hormones) – estrogen and progesterone.
- Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
- The ovaries are located near the kidneys and are attached to the abdominal cavity and uterus by ligaments called mesovarium.
- The ovaries are small almond-shaped bodies about 2-4 cm in diameter covered by thin single-layered cubical cells called the germinal epithelium.
- The epithelium encloses ovarian stroma which is divided into the peripheral dense cortex and inner loose medulla.
- The cortex comprises numerous oval or spherical masses of cells called ovarian or Graffian follicles.
Fallopian tubes (oviducts):
These are one pair. The proximal funnel-shaped end of each oviduct lies near the ovary and is called the infundibulum and bears finger-like projections called ‘fimbriae’ to help in the collection of the ovum after ovulation. The wider part of the infundibulum is called the ampulla. The last part of the oviduct is called the isthmus has a narrow lumen that joins the uterus.
The uterus is single and it is also called the womb. It is a pear-shaped, muscular, thick-walled organ. The uterus opens into the vagina through a narrow cervix and the cavity of the cervix is called the cervical canal it forms the birth canal.
- The wall of the uterus comprises three layers- the outer perimetrium, and middle myometrium- responsible for contraction during childbirth, and the inner glandular layer called endometrium –which goes through changes during the menstrual cycle.
- The endometrium layer is richly supplied with blood vessels. There is a sphincter muscle that closes the lower end of the uterus where it joins the vagina.
The female external genitalia includes the mons pubis, labia majora, labia minora, hymen, and clitoris. Through this organ, the penis is inserted during coitus for semen discharge. It serves as the birth canal during childbirth and also acts as a duct for the passage of uterine secretions and menstrual flow.
- Mons pubis is a cushion of fatty tissue covered by skin and pubic hair.
- The labia majora has fleshy folds of tissue, down from the mons pubis and surrounds the vaginal opening.
- The labia minora are paired folds of tissue found under the labia majora
- Hymen- The thin membrane which partially covers the opening of the vagina.
*** Hymen can be broken by a sudden fall or jolt, insertion of a vaginal tampon, or active participation in some sports like horseback riding, cycling, etc. In some women, the hymen remains even after coitus. In fact, the presence or absence of a hymen can not be a reliable indicator of virginity or sexual experience
- clitoris (external sensual organ)- a tiny finger-like structure that lies at the upper junction of the two labia minora above the urethral opening. It is also called Equivalent to the male penis.
The mammary gland is characteristic of all female mammals. The mammary glands are paired structures (breasts) that contain glandular tissue and fat.
- The glandular tissue of each breast is divided into 15-20 mammary lobes containing alveoli
- The cells of the alveoli secrete milk, which is stored in the cavities of the alveoli.
- The milk is sucked out through the lactiferous duct.
The primary sex organs the testis of males and the ovaries of females–produce gametes, i.e. sperms, and ovum, by the process called gametogenesis, which is termed spermatogenesis and Oogenesis respectively.
In Males, the process of formation of sperms is termed Spermatogenesis. The spermatozoa are male gametes produced by the testes. The sperms have three-part- head, neck, and tail.
- The tip of sperm is covered by Acrosome a cap-like structure, having enzymes that help the sperm to penetrate inside the egg during fertilization.
- The middle piece possesses numerous mitochondria, which produce energy for the movement of the tail which facilitates sperm motility essential for fertilization.
- The secretions of various accessory glands along with sperms from the semen.
- During each ejaculation, about 200-300 million sperms are discharged.
The process in which the formation of a mature female gamete takes place is known as oogenesis. Oogenesis starts in the embryonic stage of females. After birth oogonia are not formed or added, these get divided only.
- Following various steps of meiotic division, the primary oocytes formed.
- Each primary oocyte gets surrounded by a layer of granulosa cells and is called the primary follicle, these follicles degenerate after birth to puberty.
- So only 60,000-80,000 primary follicles are left in each ovary.
- A more layer of granulosa cells surrounds the primary follicle which is called the secondary follicle.
- After that, the tertiary follicle further changes into the mature follicle or Graafian follicle and it gets ruptures to release the secondary oocyte i.e. ovum is termed ovulation.
Menstrual Cycle in Females
In a human female, the fertility period ranges from the age of puberty, i.e. about 11-13 years up to menopause, i.e. 45-50 years.
Important facts about the Menstrual cycle:
- The onset of menstruation in a female is called menarche around the age of 11-13 years. At the age of 45-50 years,’ the permanent stoppage of menstruation in a female is called menopause.
- During the menstrual cycle, an ovum gets matured and releases once after every 28 days.
- The menstrual cycle starts with the menstrual flow, the menstrual flow results due to the breakdown of the endometrial lining of the uterus and its blood vessels which forms a liquid that comes out through the vagina the process continues for 3-4 days.
- Again the Graafian follicle matures from day 5th to 13th. The Graafian follicle also produces a hormone, estrogen, which stimulates the uterus to prepare itself for receiving the ovum. Ovulation takes place 13-14 days after menstruation.
- Ruptured Graafian follicle for release of ovum from the corpus luteum which secretes the hormone, progesterone. After that, the ovum goes to the uterus by fallopian tube for around 13 or 14 days.
- At the end of the 28th day, the ovum gets rejected by the uterine lining, which leads to the next menstrual cycle.
- Lack of menstruation may indicate pregnancy.
Fertilization and implantation:
The process of fusion of sperm with egg is called fertilization. After insemination (semen released in the vagina) the motile sperms swim very fast and enter the uterus and reach the fallopian tube (ampullary region).
- Fertilization takes place in the ampullary region.
- During fertilization, sperm connects with the zona pellucida layer of the egg, and the entry of another sperm is now blocked.
- The sperm consists of the acrosome, which helps it to enter the ovum’s cytoplasm. These unite to form a zygote which has an X chromosome from a female and X or Y from a male.
- The zygote having a XX chromosome will be female while the XY chromosome will be male.
- After it, the zygote divides mitotically, moves to the uterus, it forms 2,4,8,16, daughter cells called blastomeres.
- Embryo 8 to 16 days is called Morula and it divides continuously and transforms into a blastocyst blastocysts get embedded in the endometrium of the uterus and it is termed implantation, which leads to pregnancy.
The placenta is an association between maternal and fetal tissue meant for some extremely important physiological exchange. Placenta connects the developing embryo to the uterus, and the umbilical cord is a structure that serves as the blood-vascular connection between the fetus and the uterine wall.
- The embryo is enclosed in a sac called an amnion which is filled with amniotic fluid, that protects the embryo from external shock and prevents any damage. Through the Placenta oxygen and food are supplied from the maternal blood to the fetus.
- It also transports carbon dioxide and excretory waste from the fetal blood to the maternal blood. Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc. In the later
- In the phase of pregnancy, a hormone called relaxin is also secreted by the ovary.
- The hormones like hCG, hPL, and relaxin are produced in women only during pregnancy.
- In later days the foetus gets development day by day, the first sign of a growing fetus can be noticed by listening to the heart sound through the stethoscope.
- The fetus develops limbs and digits, by the end of the second month of pregnancy.
- Most of the major organ systems are formed, after the first trimester (3 months).
- The first movements of the fetus can be felt and the appearance of hair on the head can be observed during the fifth month.
- End of the second-trimester eyelids separate, and eyelashes are formed.
- Foetus gets fully developed after nine months.
- The uterus undergoes occasional contractions (labor), the amnion bursts and the amniotic fluid starts to discharge.
- The uterus contracts vigorously, expelling the baby and the baby’s lungs start functioning and the baby takes its first breath.
- After birth, the placenta gets discharged from the uterus.
Lactation– The mammary glands of the female undergo differentiation during the time of pregnancy and start producing milk by the end of pregnancy and the process of producing milk is called lactation.
Colostrum– Contains several antibodies essential for developing resistance for newborn babies, it is found in the mother’s milk which is produced in the initial days of childbirth.
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