Human Reproduction-Notes

Human Reproduction-Notes

1Human Male Reproductive SystemOverview on Human Male Reproductive System The male reproductive system consists of a pair of testes, accessory ducts, glands and the external genitalia.

Testes or TesticlesTestes are located outside the abdominal cavity within a pouch known as the scrotum. The role of the scrotum is to maintain the low temperature of the testes (2oC−2.5oC lower than the normal body temperature) required for spermatogenesis.

1. Testis is oval-shaped with a length of 4-5 cm and a width of 2-3 cm. It is covered by a dense covering called tunica albuginea.
2. Internally, the testis is divided into about 250 compartments known as testicular lobules.
3. Each lobule contains 1-3 highly coiled structures called seminiferous tubules in which sperms are produced.
4. Seminiferous tubules are lined on their inside by two types of cells – male germ cells (spermatogonia) and Sertoli cells.
5. Male germ cells undergo meiotic divisions to form sperms
6. The function of Sertoli cells is to provide nutrition to the germ cells.
7. Interstitial spaces are present in outside regions of seminiferous tubules which have small blood vessels and interstitial cells or Leydig cells.
8. Leydig cells secrete and synthesise the testicular hormones called androgens.

Male accessory ducts Male accessory ducts include – rete testis, vasa efferentia, epididymis and vas deferens.

1. The intratesticular duct system starts with tubuli recti, these tubules connect the seminiferous tubules to the rete testis.
2. From rete testis, fine tubules 10-25 in number arise known as vasa efferentia that leave the testis and open into the epididymis.
3. Epididymis leads to vas deferens. Vas deferens ascends the abdomen and loops over the urinary bladder.
4. The Urinary bladder receives a duct from the seminal vesicle to form an ejaculatory duct. It runs through the prostate gland and opens into the urethra.
5. Urethra receives the ducts of the prostate gland and the bulbourethral glands (Cowper’s glands) a little ahead and runs through the penis to its external opening called the urethral meatus.

Accessory Glands The accessory glands of the male reproductive system include:

1. A pair of seminal vesicles, a prostate gland and a pair of bulbourethral glands (Cowper’s glands).
2. The secretion of all these male accessory glands is called seminal plasma.
3. Seminal plasma is composed of fructose, calcium and some enzymes. It mainly provides nutrition to the spermatozoa while travelling through the female reproductive tract.
4. Seminal plasma with sperms is called semen.
5. The bulbourethral glands secretion helps in the lubrication of the penis.

External Genitalia The external genitalia is the penis. It is made up of special erectile tissue that helps in the erection of the penis. The enlarged tip of the penis is called the glans penis. It is covered by a loose fold of skin called foreskin or prepuce.2Human Female Reproductive SystemOverview on Human Female Reproductive System The female reproductive system comprises a pair of ovaries, a pair of fallopian tubes, uterus, vagina, external genitalia and mammary glands.

Ovaries Ovaries are the main female sex organs which produce female gametes called eggs. They secrete the female sex hormones.

1. Ovaries are located on each side of the lower abdomen. They are two in number.
2. Each ovary is almond-shaped, 2-4 cm in length and 1.5 cm in width.
3. It is connected to the pelvic wall and uterus with the help of ligaments.
4. The ovary is covered by a thin epithelium. The epithelium encloses the ovarian stroma.
5. Stroma has two regions, i.e. peripheral cortex and inner medulla.

Fallopian Tubes The fallopian tube is about 10-12 cm long. It extends from the periphery of each ovary to the uterus.

1. The part closer to the ovary is a funnel-shaped infundibulum.
2. The edges of the infundibulum have finger-like projections called fimbriae.The fimbriae help in collection of the egg after ovulation.
3. Infundibulum leads to ampulla.
4. Ampula leads to isthmus which is the last part of the oviduct.

Uterus Uterus or womb is a pear-shaped muscular organ which is attached to the pelvic wall and is supported by ligaments.

1. Wall of the uterus consists of three layers of tissue.
2. The perimetrium is the outermost thin membranous layer of the uterus, myometrium is the middle thick layer of smooth muscles and endometrium is the innermost glandular layer.
3. Uterus opens into the vagina through the cervix. The cavity of the cervix is called the cervical canal, which along with vagina forms the birth canal.
4. Endometrium layer undergoes a cyclic change during the menstrual cycle.
5. The smooth muscles in myometrium contract during parturition to deliver the baby out.

Vagina or Birth Canal The vagina is a muscular tube-like structure that opens to the outside of the body. It receives spermatozoa during insemination and also serves as a birth canal. External Genitalia Female external genitalia have mons pubis, labia majora, labia minora, clitoris and hymen.

1. Mons pubis – It is a cushion of fatty tissue covered by skin and pubic hair.
2. Labia majora – They are fleshy folds of tissue that extend down from the mons pubis. It surrounds the vaginal opening.
3. Labia minora – These are paired folds of tissue under the labia majora.
4. Hymen – It is a membrane that covers the opening of vagina.
5. Clitoris – A tiny finger-like structure, which lies at the upper junction of the two labia minora above the urethral opening.

Mammary Glands Mammary glands are paired glands that contain glandular tissue and variable amount of fat.

1. The glandular tissue of each mammary gland is divided into 15-20 mammary lobes. These clusters of cells are called alveoli.
2. The cells of alveoli secrete milk. Milk is stored in the cavities of alveoli.
3. Alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct.
4. Mammary ducts join to form mammary ampulla.
5. The ampulla is connected to lactiferous duct through which the milk is sucked out.

3GametogenesisGametogenesis is the process of formation of male and female gametes in the testes and ovary respectively. It is of two types: 1. Spermatogenesis – males 2. Oogenesis – females Spermatogenesis In testes, spermatogonia produce sperms by the process of spermatogenesis. This begins at puberty. Steps in Spermatogenesis

• The spermatogonia present at the inner side of seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium has 46 chromosomes.
• Spermatogonia undergoes the first meiotic division to form spermatocytes. i.e secondary spermatocytes having 23 chromosomes.
• The secondary spermatocytes under the second meiotic division form haploid spermatids.
• The spermatids are transformed into spermatozoa through a process called spermiogenesis. In spermatogenesis, the sperm heads remain embedded in Sertoli cells.
• These are released from seminiferous tubules by the process of spermiation.

Hormonal Control of Spermatogenesis

• Spermatogenesis is initiated due to increased activity of gonadotropin-releasing hormone (GnRH) by the hypothalamus
• An increase in the amount of GnRH acts on the anterior pituitary and stimulate the secretion of two gonadotropins, LH and FSH.
• LH acts on Leydig cells and stimulates them to produce androgens.
• FSH acts on Sertoli cells and stimulates the secretion of some factors which help in the process of spermiogenesis.

Sperm A sperm is a microscopic structure that is composed of a head, neck, middle piece and tail. The sperm head has an elongated haploid nucleus. The anterior portion of sperm is covered by a cap-like structure acrosome. Human male ejaculate about 200-300 million sperms during single coitus. The seminal plasma along with the sperms forms the semen. The function of male sex secondary ducts and glands is maintained by androgens.

Oogenesis The process of the formation of mature female gametes is called oogenesis. It starts from the embryonic development stage. At this time millions of oogonia are formed in each fetal ovary. Steps in Oogenesis

• The gamete mother cells start dividing and enter into prophase-I of meiotic division and get temporarily arrested at this stage. These are called primary oocytes.
• The primary oocyte gets surrounded by a layer of granulosa cell and is called the primary follicle.
• By the time of puberty, about 60,000- 80,000 primary follicles are left in each ovary.
• The primary follicles get surrounded by more layers of granulosa cells forming the secondary follicles. These transform into tertiary follicles which has a fluid-filled cavity called the antrum.
• The tertiary follicles change into the mature follicle called the Graafian follicle. This ruptures to release secondary oocyte from the ovary by the process of ovulation.

Structure of Egg

4Menstrual Cycle The reproductive cycle in female primates is known as the menstrual cycle. It starts at puberty which is referred to as menarche. Phases of the Menstrual Cycle The menstrual cycle has 4 phases: (1) Menstrual Phase:

1. The menses take place on cycle days 3-5 in the menstrual cycle.
2. The production of LH is reduced.
3. This leads to degeneration of the corpus luteum and the progesterone production is reduced.
4. Production of oestrogen is also reduced during this phase.
5. The endometrium of the uterus breaks down and the menstruation begins.
6. The menstrual flows have cells of endometrium secretions, blood and unfertilised egg.

(2) Follicular Phase:

1. The follicular phase includes cycle days 6-13 or 14 in a 28 days cycle.
2. The FSH stimulates the ovarian follicle to secrete oestrogens.
3. Oestrogen stimulates the proliferation of the endometrium lining of the uterine wall.
4. The endometrium becomes thick by the rapid multiplication of cell and this is accompanied by an increase in uterine glands & blood vessels.

(3) Ovulatory Phase:

1. Both LH & FSH attain a peak level in the middle of the menstrual cycle (about the 14th day).
2. Oestrogen concentration in the blood increases.
3. Rapid secretion of LH ruptures the Graafian follicle and the egg is released.
4. LH leads to ovulation.

(4) Luteal Phase:

1. Luteal phase includes cycle days 15 to 28.
2. Progesterone is secreted by corpus luteum
3. Endometrium wall thickens.
4. Uterine glands become secretory.

Hormonal Control of Menstrual Cycle

1. FSH stimulates the ovarian follicles to produce estrogens.
2. LH stimulates corpus luteum to secrete progesterone.
3. Menstrual phase occurs by the increased production of oestrogens.
4. LH leads to ovulation
5. Proliferative phase occurs due to an increase in the production of oestrogens.
6. Secretory phase occurs due to increased production of progesterone.

5Fertilization and ImplantationFertilization The process of fusion of sperm with an egg is known as fertilisation. Process of Fertilization

• During copulation, semen is released into the vagina. The motile sperm swims rapidly to reach the junction of the isthmus and ampulla of the fallopian tube. The egg also reaches here and fusion of gametes takes place.
• The acrosome of the sperm undergoes an acrosomal reaction and releases certain sperm lysins. These lysins dissolve the egg envelopes and help in the penetration of sperm.
• These sperm lysins contain a lysing enzyme called hyaluronidase.

Cortical reaction:

1. After the entry of sperm into the egg, the egg shows a cortical reaction to check the entry of more sperms.
2. In this reaction, the cortical granules which are present beneath the egg’s plasma membrane release some chemical substances.
3. These substances elevate the vitelline membrane above the egg surface. The elevated vitelline membrane is called the fertilization membrane.
4. The increased space and the chemical effectively checks the entry of other sperms.
5. During polyspermy is when more than one sperm enters the secondary oocyte, the resulting cell has too much genetic material to develop normally.

Implantation

• The haploid gametes (sperm and egg), fuse together to form a diploid zygote.
• As the zygote moves towards the uterus, the mitotic division starts and because of cleavage, it changes into 2, 4, 8, 16 celled blastomeres.
• The blastomeres having 8 to 16 cells are called the morula. Morula divides further to change into blastocyst.
• The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
• The trophoblast attaches to the endometrium of the uterus, leading to implantation leading to pregnancy.

6Pregnancy and Embryonic Development

• Chorionic villi are the finger-like projections on the trophoblast after implantation. This along with the uterine wall forms a functional unit called the placenta between the developing embryo and maternal body. The placenta is attached to the foetus with the help of the umbilical cord that transports food and oxygen to the embryo.

• Hormones hCG (human chorionic gonadotropin), hPL (human placental lactogen) and relaxin are produced in a woman only during the time of pregnancy by the placenta.
• After implantation, the inner cell mass differentiates into an outer layer, ectoderm and an inner layer endoderm.
• A mesoderm appears between the ectoderm and the endoderm. These three layers give rise to all tissues and organs in adults.
• The inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.

• In human beings, after one month of pregnancy, the heart of the embryo is formed. By the completion of the 2nd-month limbs and digits are formed. In 12 weeks, major organs and external genital organs are developed. The first movement of the foetus is observed during the 5 months. By the end of 24 weeks, the body is covered with fine hair, eyelids and eyelashes are developed. At the end of 9 months, the foetus is fully developed.

7Parturition and LactationParturition

• The process of delivery of a fully developed foetus is known as parturition.
• The signals for parturition originate from the fully developed foetus and placenta. This induces mild uterine contractions called foetal ejection reflex.
• The foetal ejection reflex triggers the release of oxytocin from the maternal pituitary.

Lactation

• After delivery, the mammary glands of females start producing milk. This happens at end of pregnancy by the process of lactation.
• The milk produced during the initial few days is called colostrum, which contains several antibodies.