NCERT Class 12 Biology Chapter 3 Notes Human Reproduction

Class 12 NCERT Notes for Human Reproduction: Human Reproduction in Class 12 is a crucial step in understanding the complex processes that govern the continuation of the human species. These comprehensive notes provide valuable insights into reproductive anatomy, physiology, and the underlying molecular mechanisms. From gametogenesis to fertilization and embryonic development, these notes aim to equip students with a profound understanding of human reproduction. Through the NCERT notes for CBSE Class 12 biology Human Reproduction, you will be able to solve all the doubts and questions of this chapter. These NCERT class 12 Biology Human Reproduction notes will help you score good marks in the exams.

CBSE Biology Notes for Class 12 Human Reproduction

We know that humans are sexually reproducing and viviparous. The reproductive events in humans include the formation of gametes called gametogenesis, i.e., when the sperm in males transfer into the female genital tract, and fusion with the female gametes, which is called fertilisation, leads to the formation of zygote. This is followed by the formation and development of the blastocyst and its attachment to the uterine wall, which is called implantation; the gestation period, in which embryonic development takes place; and finally, the delivery of the baby, which is parturition.

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Given below are the parts Male Reproductive System:

Testes:

• Situated outside the abdominal cavity in a pouch called the scrotum, which helps in maintaining the low temperature of testes necessary for the process called spermatogenesis.

• Each testis has around 250 testicular lobules and each of them contains highly coiled seminiferous tubules in which sperms are produced. Each seminiferous tubules is lined by two types of cells, spermatogonia (male germ cell) and Sertoli cells.

Leydig cells:

It is also called Interstitial cells. It is present around the seminiferous tubules that synthesize and secretes the hormone called androgen.

Ejaculatory duct:

Its work is to stores and transports the sperm from the testes to the outside through urethra that originates from the urinary bladder and extends through the penis to its external opening called as urethral meatus.

Penis:

It is male external genitalia. The enlarged end of the penis is called the glans penis is covered by a loose fold of skin called the foreskin.

Male accessory glands are:

• paired seminal vesicles

• prostate

• paired bulbourethral glands

Secretion of these glands forms the seminal plasma which contains fructose, calcium, and enzymes, and secretion of bulbourethral glands also help in the lubrication of the penis.

Given below are the parts Female Reproductive System:

Ovaries:

They are the primary female sex organ that produces female gamete; egg, and several other steroid hormones. Each ovary is covered by a thin epithelium that encloses the ovarian stroma, which is divided into two parts that are a peripheral cortex and an inner medulla.

Fallopian tube:

The fallopian tube extends from the periphery of the ovary to the uterus. The part closer to the ovary is a funnel-shaped structure called infundibulum and also have a finger-like projection called fimbriae.

Infundibulum leads to ampulla and joins with uterus with the isthmus. The uterus is pear-shaped in structure and is also called as womb.

Uterus:

It opens the vagina through a narrow cervix. The cavity of the cervix (cervical canal) along with the vagina forms the birth canal.

The wall of the uterus has three layers of tissue:

Myometrium:

It is the middle thick layer of smooth muscles that exhibit strong contraction during delivery of the baby.

Endometrium:

line the uterine wall and undergo cyclic changes during the menstrual cycle.

Female external genitalia includes:

1. Mons pubis- It is a cushion of fatty tissues covered by skin and pubic hair.

2. Labia majora- It is a fleshy fold that surrounds the vaginal opening.

3. 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 the hymen. The tiny finger-like projection present at the upper junction of two labia manora above the urethral opening is called the clitoris.

Mammary glands:

They are paired structures that contain glandular tissues and variable fats. Each glandular tissue contains 15-20 mammary lobes containing alveoli that secrete milk. Mammary ducts join to form mammary ampulla.

Gametogenesis:

It is the process of formation of male and female gametes in testes and ovary respectively is called as gametogenesis.

Gametogenesis is of two types:

1. Spermatogenesis- In males

2. Oogenesis- In females

Spermatogenesis:

It is the process through which sperm formation in males takes place. When testes are immature, male germ cells (spermatogonia) produce sperm that begins at the stage of puberty.

The spermatogonia present at the inner side of seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium contains total of 46 chromosomes and then spermatogonia forms a spermatocyte that undergoes meiotic division to reproduce the secondary spermatocytes having 23 chromosomes.

The spermatids are transformed into spermatozoa by the process called spermiogenesis. The sperm heads remain embedded in Sertoli cells and are released from seminiferous tubules by the process of spermiation.

Hormonal control of spermatogenesis-

1. Spermatogenesis is initiated due to an increase in the secretion of Gonadotropin-Releasing Hormone (GnRH) by the hypothalamus.

2. An increase in GnRH act on the anterior pituitary and stimulate the secretion of two gonadotropins, LH and FSH.

3. LH acts on Leydig cells and stimulates them to secrete androgens.

4. FSH acts on Sertoli cells, stimulates the secretion of some factors which help in spermiogenesis.

Structure of sperm:

Sperm is a microscopic structure composed of a head, neck, middle piece, and tail. The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure acrosome.

• Human male ejaculates about 200-300 million sperms during coitus. The seminal plasma along with the sperms constitutes the semen. The function of male sex secondary ducts and glands are maintained by androgen hormones.

Oogenesis:

It is the process of formation of mature female gametes is called oogenesis. It started during the embryonic development stage when millions of oogonia (gamete mother cells) are formed in each fetal ovary.

• The gametes mother cells start division and enter into prophase-I of meiotic division and get temporally arrested at that stage called primary oocytes.

• Each primary oocytes get surrounded by a layer of granulosa cell than it is called the primary follicle. At puberty, about 60,000- 80,000 primary follicles are left in each ovary.

• The primary follicle gets surrounded by more layers of granulosa cells called secondary follicles that transform into tertiary follicles that contains fluid-filled cavities called the antrum.

• The tertiary follicles further change into the mature follicle called Graafian follicle, which rapture to release secondary oocytes (ovum) from the ovary by the process of ovulation.

Menstrual cycle:

The reproductive cycle in female primates is called the menstrual cycle. It starts at puberty and is called menarche.

There are Four Phases of Menstrual Cycle (MC):

1. Menstrual Phase:

• In 28 days menstrual cycle, the menses take place on a cycle of 3-5 days.

• The production of LH from the anterior lobe of the pituitary gland is reduced.

• The withdrawal of this hormone causes degeneration of the corpus luteum and, therefore progesterone production is reduced.

• Production of oestrogen is also reduced in this phase.

• The endometrium of the uterus breaks down & menstruation begins.

• The cells of endometrium secretions, blood & unfertilised ovum constitute the menstrual flow.

2. Follicular Phase:

• This phase usually includes cycle days 6-13 or 14 in a 28 days cycle.

• The follicle-stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete oestrogens.

• Oestrogen stimulates the proliferation of the endometrium of the uterine wall.

• The endometrium becomes thicker by rapid cell multiplication and this is accompanied by an increase in uterine glands & blood vessels.

3. Ovulatory Phase:

• Both LH & FSH attain a peak level in the middle of the cycle (about the 14th day).

• Oestrogen concentration in blood increases.

• Rapid secretion of LH induces rupturing of graffian follicle and thereby the release of the ovum.

• In fact, LH causes ovulation.

4. Luteal Phase:

• Includes cycle days 15 to 28.

• Corpus luteum secretes progestrone.

• Endometrium thickens.

• Uterine glands become secretory.

Hormonal Control of Menstrual Cycle:

(i) FSH stimulates the ovarian follicles to produce oestrogens.

(ii) LH stimulates the corpus luteum to secrete progesterone.

(iii) Menstrual phase is caused by the increased production of oestrogens.

(iv) LH causes ovulation

(v) Proliferative phase is caused by the increased production of oestrogens.

(vi) Secretory phase is caused by increased production of progesterone.

Fertilisation and Implantation

The process of fusion of sperm with an ovum is called fertilisation.

During coitus (copulation) semen is released into the vagina. The motile sperms swim rapidly to reach the junction of the isthmus and ampulla of the fallopian tube. The ovum also reaches there and fusion of gametes takes place at the ampullary-isthmic junction.

In this acrosome of sperm undergoes an acrosomal reaction and releases certain sperm lysins which dissolve the egg envelopes locally and make the path for the penetration of sperm.

These sperm lysins contain a lysing enzyme hyaluronidase which dissolves the hyaluronic acid polymers in the intercellular spaces which hold the granulosa cells of corona radiata together; corona penetrating enzyme that dissolves the corona radiata and acrosin which dissolves the zona pellucida. Then it dissolves the zona pellucida.

Cortical reaction:

(a) Immediately after the entry of sperm into the egg, the latter shows a cortical reaction to check the entry of more sperm.

(b) In this reaction, the cortical granules present beneath the egg’s plasma membrane release chemical substances between the ooplasm and the plasma membrane vitelline membrane.

(c) These substances raise the vitelline membrane above the egg surface. The elevated vitelline membrane is called the fertilization membrane.

(d) The increased space between the ooplasm and the fertilization membrane and the chemical present in it effectively check the entry of other sperm.

(e) If polyspermy occurs, that is more than one sperm enters the secondary oocyte, the resulting cell has too much genetic material to develop normally

The haploid gametes fuse together to form a diploid zygote. As the zygote moves towards the uterus, the mitotic division starts and forms cleavage to change into 2, 4,8,16 celled blastomeres.

• The blastomeres with 8 to 16 cells are called the morula. Morula divides to change into blastocysts.

• 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 outer layer of the blastocyst is called the trophoblast that attach with the endometrium of the uterus, called implantation that leads to pregnancy.

Pregnancy and embryonic development:

The finger-like projections on trophoblasts after implantation called is called chronic villi that along with the uterine wall form a functional unit between the developing embryo and maternal body called the placenta.

The placenta is attached with fetus with an umbilical cord that transports food and oxygen to the embryo.

• Hormones hCG (human chorionic gonadotropin)

• hPL (human placental lactogen)

• Relaxin

They are produced in women only during pregnancy by the placenta.

After implantation, the inner cell mass; embryo differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm.

These three layers give rise to all tissues; organs in adults. It is important to note that the inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.

In humans, after one month of pregnancy, the embryo’s heart is formed. By the end of 2nd-month limbs and digits are formed. By the end of 12 months, major organs and external genital organs are well developed.

The first movement of foetus is observed in 5 months. By the end of 24 weeks, the body is covered with fine hair, eyelids, and eyelashes are formed. At the end of 9 months fetus is fully developed.

Parturition And Lactation

• Parturition is the process of delivery of a fully developed foetus is called parturition.
• Signals for parturition originate from the fully developed fetus and placenta inducing mild uterine contractions called Foetal ejection reflex.
• It triggers the release of oxytocin from the maternal pituitary.
• The mammary glands of females, start producing milk, to the end of pregnancy by the process of lactation.
• The milk produced during the initial few days of lactation is called colostrum, which contains several antibodies.

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