NCERT Solutions For Class 12 Biology Chapter 3
Human Reproduction

Dogs are polyovulatory non-seasonal species. More than one egg is released from the ovary in each ovulation cycle. Thus, if a female dog gave birth to 6 puppies, it means that 6 eggs were released during ovulation and they all got fertilised.

In our society, women are often blamed for giving birth to daughters. However, this is a completely wrong statement as the sex of the child depends upon the sex chromosome of the sperm that has fused with the mother's egg giving rise to the zygote. All human beings have 23 pairs of chromosomes out of which, 22 pairs are autosomes and 23^rd pair is the sex chromosome. Females have 22 pairs of autosomes and XX chromosomes as their sex chromosome. In the case of males, 22 pairs are autosomes and there are two different kinds of sex chromosomes namely X and Y.  When gametes (sperms) are formed in males, some receive X sex chromosome, while others receive Y sex chromosome. Now, depending upon the sperm type (carrying either X or Y chromosome) that fuses with the ovum, the sex of the foetus is determined. Fertilisation with an X chromosome carrying sperm will result in a female child, while that with Y carrying sperm will result in a male child. Thus, it is incorrect to blame women for the gender of the child. 

Parturition is the process of delivery of the foetus (childbirth) due to vigorous contractions of the uterus at the end of nine months of pregnancy. This is induced by a complex neuroendocrine mechanism. The signals for parturition are generated from the fully developed foetus and placenta which generates mild uterine contractions. This triggers the release of hormone oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscles and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin. The other hormone that is involved in parturition is relaxin which is responsible for relaxation of ligaments of the pelvic region and thus; helps in childbirth.

Seminal plasma is a mixture of secretions from three different glands (seminal vesicles, prostate and paired bulbourethral glands) of male reproductive systems. Prostate gland secretes a milky white alkaline fluid which enhances the sperm motility. The secretion from bulbourethral gland is mucoid in nature and has lubricating effect. The combination of secretion from these three different glands constitutes seminal plasma. This plasma is rich in nutrients like fructose, calcium and certain enzymes and is a thick, viscous fluid.

1. Spermiogenesis: The final stage of spermatogenesis in which haploid spermatids, produced as a result of meiosis from spermatocytes, are converted into mature and motile spermatozoa or sperm is called spermiogenesis.
2. Spermiation: The process by which embedded sperms, produced as a result of spermiogenesis, are released from the Sertoli cells into the lumen of seminiferous tubules is called spermiation.

A single egg is released by human ovary every month by the process of ovulation.

Only a single egg is released in the ovary when the mother gives birth to identical twins. The blastomeres separate during early zygotic stage giving rise to identical twins.

When the twins born are fraternal, two eggs are released in the ovary during a single ovulation event and both are fertilised by two separate sperms leading to the formation of two zygotes.

The main accessory ducts of the male reproductive system are rete testis, vasa efferentia, epididymis and vas deferens. Their major function is storage and transportation of sperms from the testis to the outside through urethra.

The male accessory glands are seminal vesicles, prostate and paired bulbourethral glands. Their combined secretion is called seminal plasma which is rich in fructose, calcium and certain enzymes. They provide nutrients to the sperm and also serve as a lubricant of the penis.

The hormones involved in regulation of spermatogenesis are gonadotropin releasing hormone (GnRH), luteinizing hormone (LH), follicle stimulating hormone (FSH) and androgens (testosterone). 

Functions of Hormones:

1. Spermatogenesis starts at the onset of puberty due to a significant increase in the secretion of gonadotropin releasing hormone (GnRH), which is released from the hypothalamus.
2. The increased level of GnRH results in the secretion of gonadotropin hormone namely luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on Leydig cells and stimulates synthesis and secretion of androgens (testosterone).
3. Androgens stimulate the process of spermatogenesis.
4. FSH acts on the Sertoli cells and stimulates the secretion of factors that are also involved in the process of spermiogenesis.

Two major functions of testis:

(a) Production of sperms in the highly coiled seminiferous tubules

(b) Secretion of a male hormone called testosterone by interstitial cells

Two major functions of ovary:

(a) Production of female gamete (ovum) every month by the process of oogenesis

(b) Production and secretion of female sex hormones, estrogen and progesterone, during the ovarian cycle

A series of cyclic physiological changes that occur inside the female reproductive tract of the primates from the beginning of one menstrual period to the beginning of the next is called the menstrual cycle. In humans, this cycle consists of about 28/29 days. One ovum is released during the middle of each cycle and the endometrium of the uterus also undergoes a cyclic series of events. The major events of the menstrual cycle are as follows:

(a) Menstrual phase (Days 1-5): During days 1-5, there is a low level of female sex hormones in the body, causing the uterine lining to disintegrate and its blood vessels to rupture. A flow of blood, known as menses, passes out of the vagina during a period of menstruation.

(b) Follicular Phase (Day 6-13): There is increased production of estrogen by growing ovarian follicle which causes endometrium of uterus to thicken, and become vascular and granular. The primary follicle in the ovary grows to become a fully mature Graafian follicle. This is called the proliferative phase of the menstrual cycle. These changes in the ovary and uterus are induced by changes in the levels of pituitary and ovarian hormones. Gonadotrophins like luteinising hormone (LH) and follicle-stimulating hormone (FSH) increase gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles. LH and FSH attain their peak level in the middle of the cycle (14^th day). The maximum level of LH during the mid-cycle induces rupture of Graafian follicle and results in ovulation at the end of follicular or proliferative phase.

(c) Ovulatory Phase (Day 15-28): This Graafian follicle after rupture turns into corpus luteum. There is increased production of progesterone hormone from the developing corpus luteum. This results in a doubling of endometrium thickness and the uterine glands mature. A thick mucoid secretion is produced and is called secretory phase of the cycle. The endometrium is ready to implantation of a fertilised ovum however in case pregnancy does not occur, the corpus luteum degenerates and the uterine lining breaks down due to low levels of sex hormones. This results in menstruation. While menstruation is going on, the anterior pituitary begins to increase its production of FSH and a new follicle begins to mature.

Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche (onset of menstruation) and menopause (menstruation stops).

 Gonadotrophins like Luteinizing hormone (LH), follicle stimulating hormone (FSH) and ovarian hormones like progesterone and estrogen regulate the menstrual cycle. 

1. False, androgens are not produced by Sertoli cells. Sertoli cells are found inside the seminiferous tubules. Androgen is produced by interstitial cells or Leydig cells found in the regions outside the seminiferous tubules.
2. True
3. False. Leydig cells are not found in ovary. Leydig cells are found in the regions outside the seminiferous tubules, called interstitial spaces.
4. True
5. False, oogenesis does not take place in corpus luteum. Oogenesis takes place in Graafian follicle found in the ovary.
6. True
7. True

(a) Corpus luteum: Corpus luteum is the transformed and ruptured Graafian follicle after ovulation phase. During the luteal phase of the menstrual cycle, corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium and making it ready for implantation of the fertilised ovum and other events of pregnancy. 

(b) Endometrium: The endometrium functions as the inner lining of the uterus, preventing adhesions between the opposed walls of the myometrium. During the menstrual cycle, the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This provides optimal environment for the implantation of a blastocyst upon its arrival in the uterus. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the developing embryo.

(c) Acrosome: Acrosome is a cap-like structure found at the head of a sperm that comes in contact with the ovum at the time of fertilisation and releases a hydrolytic enzyme, called hyaluronidase, that hydrolyses the outer membrane of the ovum and thus, helps in the process of fertilisation.

(d) Sperm tail: The sperm tail is made of protein fibres that contracts on the alternative sides, performing a wave-like movement. This imparts motility to the sperm and facilitates the movement of the sperm inside the female reproductive tract so that it can reach the ovum to fertilise it .

(e) Fimbriae: Fimbriae are finger-like projections found at the edges of infundibulum, which is the ovarian end of the fallopian tube). This structure helps in the collection of ovum after ovulation with the help of ciliary movement

The process of formation of a mature female gamete is called oogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells or oogonia are formed in each fetal ovary. These cells start dividing and enter prophase-I of the meiotic division and get temporarily arrested at that stage. They are called primary oocytes at this stage. The primary oocytes then get surrounded by a layer of granulosa cells and this structure is called primary follicle. Many of the primary follicles degenerate from birth to puberty phase. About 60,000-80,000 such primary follicles remain at the onset of puberty. The primary follicle gets surrounded by more layers of granulosa cells and a new theca. This structure is then called secondary follicle. Out of thousands of such follicles, only about 400 ever mature because a female produces only one egg per month during her reproductive years.

The secondary follicle soon transforms into a tertiary follicle with a fluid-filled cavity called antrum with the further organisation of theca layer. It is during this stage that the primary oocyte grows in size and completes its first meiotic division. It is an unequal division that results in the formation of a large haploid secondary oocyte and a tiny polar body. The secondary oocyte retains most of the nutrient-rich cytoplasm. This tertiary follicle changes into a mature Graafian follicle. The secondary oocyte forms a new membrane, called zona pellucida, surrounding it. The Graafian follicle ruptures and the secondary oocyte or ovum is released from the ovary. This process is called ovulation.

The ovarian cycle is under the control of gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinising hormone (LH). It is the interplay between these two hormones that control the ovulation cycle.

The process of production of male gametes (sperms) in the primary male sex organ testes is called spermatogenesis. This process starts at the onset of puberty, where immature male germ cells or spermatogonia are converted into sperms. The diploid spermatogonia which are present on the inside wall of seminiferous tubules undergo mitotic division and increase their number. Out of these, some are called primary spermatocytes and they undergo meiotic division periodically. A primary spermatocyte completes the first meiotic division leading to the formation of two equal, haploid cells called secondary spermatocytes (having only 23 chromosomes). The secondary spermatocytes undergo the second meiotic division and thus produce four equal, haploid spermatids. Spermatids also have 23 chromosomes and are thus; haploid. The process of spermiogenesis further transforms these spermatids into spermatozoa (sperms) after which the sperm head gets embedded in the Sertoli cells. As the last step, they are released from the seminiferous tubules by the process called spermiation.

Seminiferous tubules are highly coiled structures found in each lobule of testes and are 1-3 in numbers. The combined length of all the tubules found in the testes is approximately 250 m. Each seminiferous tubule is lined on its inside by two types of cells namely

• Male germ cells or spermatogonia
• Sertoli cells

It is packed with cells undergoing spermatogenesis. The Sertoli cells not just provide the support and nourishment (nutrients) to the sperms, but also regulate the spermatogenic cells. The regions outside the seminiferous tubules are called interstitial spaces. They contain small blood vessels and interstitial or Leydig cells. Leydig cells are polyhedral in shape. They synthesise and secrete testicular hormones called androgens.

Diagrammatic sectional view of seminiferous tubule  

(a) sexually 

(b) viviparous 

(c) internal

(d) haploid 

(e) diploid 

(f) ovulation 

(g) luteininsing hormone 

(h) fertilisation 

(i) fallopian tube 

(j) blastocyst 

(k) placenta (umbilical cord)