ATS COACHING CLASSES
HUMAN REPRODUCTION
REVISION NOTES
Humans are sexually reproducing and viviparous. The reproductive cycle in humans is very complex and there are remarkable differences between the reproductive events in the males and the females.
The Male Reproductive System
The male reproductive system is differentiated into:
(a) Primary sex organs: it consists of a pair of testicles suspended in a scrotum.
(b) Secondary sex organs: these include a pair of ducts that are differentiated into rete testis, vasa efferentia, epididymis, and vas deferens, an ejaculatory duct and accessory glands.
(c) External Genetilia
• The testes are located outside the abdominal cavity in a pouch called the scrotum, which helps in keeping the testes at the low temperature required for spermatogenesis.
• Each testis has around 250 testicular lobules, each of which contains densely coiled seminiferous tubules in which sperms are created.
• Two types of cells line each seminiferous tubule: spermatogonia (male germ cell) and Sertoli cells.
• Leydig cells, which are found surrounding the seminiferous tubules, produce and secrete androgen hormone.
• The urethra, originates from the uranary bladder and extends through the penis to its external entrance, the urethral meatus, stores and transports sperm from the testes to the outside.
• The penis is the male genitalia. The glans penis is the expanded end of the penis that is covered by a loose flap of skin called the foreskin.
• Accessory glands in men include paired seminal vesicles, prostate glands, and paired bulbourethral glands. These glands secrete seminal plasma, which is rich in fructose, calcium, and enzymes. The production of bulbourethral glands also aids in penile lubrication.
The Female Reproductive System
The female reproductive system consists of:
(a) Primary sex organ that are the ovaries.
(b) Secondary sex organs that include Fallopian tube, uterus, cervix and vagina.
(c) External genetilia.
(d) Mammary glands.
• The ovaries are the major female sex organs, producing the female gamete as well as many steroid hormones. Each ovary has a thin epithelium that surrounds the ovarian stroma, which is separated into a peripheral cortex and an interior medulla.
• The Fallopian tube connects the ovary's periphery to the uterus. The closest component to the ovary is a funnel-shaped structure called the infundibulum, which has finger-like projections called fimbriae.
• The infundibulum leads to the ampulla, which connects to the uterus through the isthmus. The uterus, often known as the womb, is a pear-shaped structure.
• A small cervix allows the uterus to open the vagina. The birth canal is formed by the cavity of the cervix (cervical canal) and the vagina.
The uterus is internally lined by three layers that play an important role in pregnancy.
• Perimetrium: it is the external layer.
• Myometrium: it is the middle thick layer of smooth muscles that exhibit strong contraction during delivery of the baby.
• Endometrium: it lines the uterine wall and undergoes cyclic changes during menstrual cycle.
The external genitalia in females include:
• Mons pubis: it is a cushion of fatty tissues covered by skin and pubic hair.
• Labia majora: it is a fleshy fold that surrounds the vaginal opening.
• Labia manora: it is a paired fold of tissue under labia majora.
• The opening of the vagina is often partially covered by a membrane called hymen.
• The tiny finger-like projection present at the upper junction of two labia manora above the urethral opening is called clitoris.
The mammary glands
Mammary glands are paired structures that contain glandular tissues as well as different types of fat. Each glandular tissue has 15-20 mammary lobes with milk-secreting alveoli. The breast ducts connect to produce the mammary ampulla.
Gametogenesis
Gametogenesis refers to the process of formation of the male and female gametes in the testes and ovaries, respectively. In males and females, it is controlled by different hormones.
1. Male spermatogenesis
2. Female oogenesis
Spermatogenesis: Male germ cells (spermatogonia) in immature testes create sperm through spermatogenesis, which begins at puberty.
• Spermatogonia on the inner surface of seminiferous tubules proliferate and grow in quantity by mitotic division. Every spermatogonium has 46 chromosomes.
• Spermatogonia produces spermatocytes that divide meiotically to create secondary spermatocytes with 23 chromosomes.
• Spermiogenesis is the process through which spermatids are converted into spermatozoa. Sperm heads stay lodged in Sertoli cells and are liberated from seminiferous tubules during the spermiation process.
The process of spermatogenesis is under strict control of hormones that timely manages each of its step. The hormones are as follows:
• Spermatogenesis begins due to increasing Gonadotropin-releasing hormone (GnRH) released by the hypothalamus.
• GnRH acts on the anterior pituitary and induces the release of two gonadotropins, LH and FSH.
• LH increases the secretion of androgens by Leydig cells.
• FSH increases the release of certain substances that aid in spermiogenesis by acting on Sertoli cells.
Human Sperm structure:
Sperm is a microscopic structure made up of a head, neck, middle section, and tail. The sperm head has an extended haploid nucleus that is covered by a cap-like structure called an acrosome. During coitus, a human male ejaculates around 200-300 million sperms. The seminal plasma and sperms combine to form the semen. Androgen hormones regulate the activity of male sex secondary ducts and glands. Structure of sperm.
Oogenesis:
it is the process through which mature female gametes are formed.
• It begins during the embryonic development stage, when each foetal ovary produces millions of ogonia (gamete mother cells).
• The gametes' mother cells divide and reach prophase-I of meiotic division, where they are temporarily arrested as primary oocytes.
• When a primary oocyte is surrounded by a layer of granulosa cells, it is referred to be a primary follicle.
• At adolescence, each ovary has between 60,000 and 80,000 primary follicles.
The primary follicle is surrounded by several layers of granulosa cells, which change into secondary follicles, which contain a fluid-filled chamber called the antrum.
• The tertiary follicles mature into the Graafian follicle, which ruptures to release secondary oocytes (ovum) from the ovary during the ovulation process.
Menstrual Cycle:
The cycle is divided into four phases:
1. Mensural phase: the cycle lasts from day 3-5days of a 28-day menstrual cycle.
• LH production from the pituitary gland's anterior lobe is diminished.
• Withdrawal of this hormone promotes corpus luteum degeneration, which reduces progesterone synthesis.
• Oestrogen production is also lowered during this time.
• The uterine endometrium degrades and menstruation begins.
• The menstrual flow is made up of endometrial fluids, blood, and an unfertilized ovum.
2. Follicular phase: In a 28-day cycle, this phase lasts from day 6-13 or 14 days.
• FSH, which is released by the anterior lobe of the pituitary gland, stimulates the ovarian follicle to secrete oestrogens.
• Oestrogen promotes the growth of endometrium.
• Rapid cell multiplication causes the endometrium to thicken, which is accompanied by a rise in uterine glands and blood vessels.
3. Ovulatory phase: Both LH and FSH reach their highest levels in the middle of the cycle (about 14th day).
• Oestrogen levels in the blood rise.
• There is rapid LH secretion that causes Graafian follicle to rupture and thus discharging the ovum.
• The hormone that is responsible for ovulation is LH.
4. Luteal phase: The phase lasts from day 15 to 28.
• In this phase, the corpus luteum secreates progesterone.
• The endometrium gets thickened and the uterine glands start their secretion.
Hormones controlling Mensural cycle:
• LH (leutinizing hormone) stimulates the corpus luteum to produce progestrone.
• The menstrual phase is triggered by an increase in oestrogen production.
• LH hormone stimulates ovulation.
• The proliferative phase is triggered by increased estrogen production.
• Progestrone synthesis induces the secretory phase.
Fertilisation and Implantation
Fusion of sperm with ovum is called fertilisation.
• FSH increases the production of oestrogens by ovarian follicles.
• Semen is discharged into the vagina during coitus (copulation).
• The motile sperms swim quickly to the junction of the isthmus and the ampulla of the fallopian tube. • The ovum also arrives, and gamete fusion occurs at the ampullary-isthmic junction.
• The sperms acrosome undergoes acrosomal activation and releases specific sperm lysins that breakdown the egg’s envelope locally and create a channel for sperm entry.
• The sperm lysins comprise hyaluronidase, a lysing enzyme that dissolves the hyaluronic acid polymers in the intercellular gaps that keep the corona radiata granulosa cells together; corona piercing enzyme (that dissolves the corona radiata); and acrosin (which dissolves the zona pellucida). The zone pellucida is then dissolved. As soon as a sperm enters into the egg, the latter exhibits a cortical response to prevent the admission of further sperms.
• During this process, the cortical granules under the plasma membrane of the egg release chemical substance between the ooplasm and the plasma membrane (vitelline membrane).
• These chemicals elevate the vitelline membrane above the surface of the egg. The raised vitelline membrane is referred to as the fertilisation membrane.
• The enlarged gap between the ooplasm and the fertilisation membrane, as well as the chemical contained in it, effectively prevent additional sperm from entering.
• But, if more than one sperm enters the secondary oocyte, the cell has too much genetic material to grow healthily.
• The haploid gametes fuse to produce a diploid zygote. As the zygote approaches the uterus, mitotic division begins and cleavage occurs, resulting in two, four, eight, or sixteen celled blastomeres.
• Blastomeres have 8 to 16 cells. More than 200 celled embryo is known as Morula. Morula divides to become blastocysts. The blastomeres in the blastocyst are organized into an exterior layer known as the trophoblast and an inner group of cells linked to the trophoblast known as the inner cell mass.
• The outer layer of a blastocyst is termed trophoblast, and it attaches to the endometrium of the uterus, resulting in implantation and pregnancy.
Pregnancy and embryonic development
• The finger-like extensions on the trophoblast following implantation are known as chronic villi.
• They along with the uterine wall, create a functional unit between the growing embryo and the mother body known as the placenta. Diagram showing human foetus in the womb.
• The placenta is connected to the foetus by an umbilical cord, which transports food and oxygen to the embryo.
• The placenta produces the hormones hCG (human chorionic gonadotropin), hPL (human placental lactogen), and relaxin in women exclusively during pregnancy.
• The inner cell mass (embryo) differentiates after implantation into an exterior layer called ectoderm and an inner layer called endoderm.
• Soon after, a mesoderm forms between the ectoderm and the endoderm.
• All tissues (organs) in adults are formed by these three layers.
• It is vital to highlight that the inner cell mass comprises stem cells, which have the ability to give rise to all tissues and organs.
• In humans, the embryo's heart develops after one month of pregnancy. Legs and fingers are established at the end of the second month.
• Major organs and external genital organs are fully matured by the end of the 12 weeks (first trimester). • In 5 months, the foetus makes its first movement, the hairs develop.
• By the conclusion of 24 weeks (second trimester) the body is covered in fine hair, eye lids, and eyeless.
• The foetus is completely formed at the end of 9 months.
Parturition and Lactation
• Delivery of the fully developed foetus is called as parturition.
• The fully grown foetus and placenta send out signals for parturition, generating moderate uterine contractions known as the Foetal ejection reflex.
• It causes the release of oxytocin from the maternal pituitary gland. Oxytocin acts on the uterine muscles and causes stronger contraction due to which child is expelled out through birth canal - parturition.
• The female mammary glands begin producing milk and continue to do so until the pregnancy is over, a process known as lactation.
• Colostrum is the milk produced during the first few days of breastfeeding and contains many antibodies. The process is called as lactation.
• Breast- feeding during the initial period of infant growth is recommended by doctor.