Reproduction in human
Humans reproduce sexually and are viviparous (produces young ones rather than eggs). It goes through various steps and can be studied in two-part-
- Reproductive system i.e. male and female
- Formation of gametes, fertilization, pregnancy, and development of the embryo.
Human sex organs begin to mature with puberty. The name ‘puberty’ given to the changes that occur in boys and girls as they grow up during age 10 to 14 years. Here at this stage another term is used as adolescent shows the attainment of puberty.
During puberty, the body grows rapidly and both primary and secondary reproductive organs grow and become mature and by this secondary sexual characters appear. During adolescence, both males and females show different secondary sexual characters. The reproductive phases go through- Gametogenesis, Menstrual Cycle, Fertilisation and Implantation, Pregnancy and Parturition, Embryonic Development, and Lactation. So there is a difference in the male and female reproductive systems.
THE MALE REPRODUCTIVE SYSTEM
In males the system is located in the pelvis region, it consists of pair of testes, a pair of the epididymis, of vasa differentia (singular: vas deferens), urethra, penis, and accessory glands.
Testes– These are extra-abdominal, i.e. present outside the abdomen within a pouch made up of skin and connective tissue called scrotal sac or scrotum.
- The scrotum acts as a thermoregulatory, the temperature of testes at about 2-3°C lower than the body.
- The sperm formation takes place in the seminiferous tubule.
- The inner lining of seminiferous tubules by two types of cells called male germ cells (spermatogonia) and Sertoli cells.
- The male germ cell involves in meiotic division-leading sperm formation.
- Sertoli cells provide nutrition to these germ cells.
- Leydig cells– synthesize and secrete testicular hormones called androgens
- body temperature.
- The testis is covered by a dense covering. Each testis has about 250 compartments called testicular lobules, each lobule contains one to three highly coiled seminiferous tubules.
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 ascends to the abdomen and loops 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 sperms 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.
Penis– The penis is the male external genital organ. Made by special tissue that helps in the erection of the penis to make it able for insemination. The enlarged end of the penis called the glans penis and it is covered by a loose fold of skin called the foreskin.
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.
Female Reproductive System
The female reproductive system consists of organs as follows- A pair of ovaries, a pair of fallopian tubes, uterus, vagina, and external genitalia. It is located in the pelvic region.
A Pair of ovaries, which lie in the lower part of the abdominal cavity, placed one on each side of the body.
- Ovaries produce ova, the process of formation of egg in the ovary is known as oogenesis.
- Ovaries secrete female sex hormones (steroid hormones) – oestrogen and progesterone.
- Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
Fallopian tubes (oviducts)– These are one pair. The proximal funnel-shaped end of each oviduct lies near the ovary and is called infundibulum bears finger-like projections called ‘fimbriae’ help in the collection of the ovum after ovulation. The wider part of the infundibulum called the ampulla. The last part of the oviduct is called the isthmus having a narrow lumen that joins to the uterus.
Uterus– It 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, middle myometrium- responsible for contraction during childbirth, and the inner glandular layer called endometrium –which goes through changes from 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.
Vagina– The female external genitalia include 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 having 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 hymen can not be a reliable indicator of virginity or sexual experience
- clitoris (external sensual organ)- tiny finger-like structure 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 alveoli secrete milk, which is stored in the cavities of 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, respectively, by the process called gametogenesis.
- Spermatogenesis- In Male, the process of formation of sperms is termed Spermatogenesis.
- The spermatozoa are male gametes produced by the testes. The sperm having 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.
- middle piece possesses numerous mitochondria, which produces 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.
- Oogenesis- The process in which the formation of a mature female gamete takes place is known as oogenesis. This is started in the embryonic stage of females. After birth oogonia are not formed either 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.
- More layer of granulosa cells surrounds primary follicle then it is called secondary follicles.
- 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 as ovulation.
Menstrual Cycle in Human 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.
- The onset of menstruation in a female is called menarche around the age of 11-13 years. About the age of 45-50 years’ 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 mature 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 with the uterine lining, which leads to the next menstrual cycle.
- Lack of menstruation may indicate the 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 enters 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.
- 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 the male.
- The zygote having 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 called Morula and it divides continuously transforms into a blastocyst and blastocysts get embedded in the endometrium of the uterus and it is termed as implantation, leads to pregnancy.
- The placenta is an association between maternal and foetal 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 foetus and uterine wall.
- The embryo enclosed in a sac called an amnion which is filled with amniotic fluid, that protects 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 foetal 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.
- Later days the foetus gets development day by day, the first sign of growing foetus can be noticed by listening to the heart sound through the stethoscope.
- The foetus 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 foetus can be felt and the appearance of hair on the head can be observed during the fifth month.
- End of the second trimester eye-lids separate, and eyelashes are formed.
- Foetus gets fully developed after nine months.
Parturition – The process of delivery of foetus is called parturition.
- The uterus undergoes occasional contractions (labor), the amnion bursts and the amniotic fluid starts discharged.
- 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 mother’s milk which is produced in the initial days of childbirth.
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