Bihar Board - Class 12 Biology - Chapter 3: Human Reproduction Plants Long Answer Question

Q.1. Mention the changes taking place during the transition of a primary follicle to Graafian follicle in the oogonia.

A.1. Oogonia or the gamete mother cells are formed within each fetal ovary. No more oogonia are formed after birth. They enter into the prophase-I stage of meiotic division when they start cell division to approach the primary oocyte stage. These primary oocytes are girdled by a layer of granulosa cells to form the primary follicle which degenerates during the stages of birth to puberty. The primary follicles are encircled by more layers of granulosa cells and a new theca known as secondary follicles. The theca is subdivided into other theca externa and an inner theca interna which secretes estrogen. The secondary follicle is then transformed into a tertiary follicle characterized by antrum, which is a fluid-filled cavity. At this phase, the primary oocyte grows in size inside the tertiary follicle to complete the first meiotic division. The tertiary follicle finally transitions to form the Graafian follicle.

Q.2. Explain the role of pituitary gonadotropins during the follicular and ovulatory phases of the menstrual cycle. Describe the shifts in steroidal secretions.

A.2. The menstrual flow is due to the breakdown of the lining of the uterine endometrium and blood vessels which forms the liquid discharged from the vagina. The menstrual cycle is controlled through the pituitary gland by the hypothalamus. Changes in the ovary and uterus during the menstrual cycle is due to the fluctuation in the levels of ovarian and pituitary hormones. Towards the end of the menstrual phase, the pituitary FSH eventually increases which causes the development of the follicles inside the ovaries. Both the FSH and LH attain a peak level during the mid of the cycle. This speedy secretion of LH leads to LH surge which induces rupture of the Graafian follicle and hence the ovulation. During the maturation of follicles, more of estrogen is secreted causing a surge in FSH and LH from the anterior pituitary. The LH surge causes ovulation. The LH also induces luteinisation. The LH hormone causes the conversion of the empty follicle into the corpus luteum. The Corpus luteum produces steroid hormones – progesterone and estrogen. These hormones govern the growth and maintenance of the uterine endometrium for probable implantation.

Q.3. Explain in detail the various developmental stages of the zygote until implantation with suitable diagrams.

A.3. When the zygote moves through the isthmus of the oviduct, the mitotic division is initiated and is called the cleavage towards the uterus to form 2,4,8,16 daughter cells called blastomeres. It is an embryo containing 8 to 16 blastomeres from the morula. It continues to transform and divide into blastocysts as it further approaches the uterus. In the blastocyst, the blastomeres are organized into an outer layer referred to as the trophoblast and the inner cell mass, which is an inner collection of cells attached to the trophoblast. This layer gets attached to the endometrium and the inner cell mass transforms into the embryo. After attachment, the cells of the uterus rapidly divide and cover up the entire blastocyst. This causes the blastocyst to implant in the endometrium of the uterus which leads to conception.

Q.4. Explain the organization of the mammary gland .

A.4. One of the characteristics of the female mammals is that they possess functional mammary glands. They have paired structures, containing glandular tissues and fat that varies in individuals. The glandular tissue is organized into 15-20 mammary lobes in each breast, which possess alveoli which are a cluster of cells. These alveolar cells secrete milk that is stored in the lumens or cavities of the alveoli. The alveoli open into the mammary tubules. These tubules in each of the lobes combine to form the mammary duct. Many such mammary ducts join to form a mammary ampulla that is connected to the lactiferous ducts. Through these structures, milk is sucked.

Q.5. What is Reproduction? Explain how humans reproduce their young ones?

A.5. Reproduction is a biological process of producing young ones or offspring, which are identical to their parents. There are two different modes of reproduction and are classified mainly based on the involvement of the parents.

The two different modes of reproduction are:

Asexual Reproduction: This mode of reproduction involves only one parent and the new offspring produced is genetically similar to the parent.

Sexual Reproduction: This mode of reproduction involves the formation and transfer of gametes, followed by fertilization, the formation of the zygote and embryogenesis. It is very complex.

Humans reproduce their young ones through the sexual mode of reproduction.

Q.6.What is the menstrual cycle? Name the Hormones which control the menstrual cycle.

A.6.The menstrual cycle is defined as the natural process, which occurs in all females after reaching the age of their puberty.  During this period, an ovary releases a mature egg, which travels to the uterus, if the egg is not fertilized, the uterine lining sheds and a new cycle begins. Overall a menstrual cycle lasts for 28 days. This cycles may either last for 21 days or as long as 35 days in some individuals. 

The entire process of the menstrual cycle is controlled by the endocrine system and the hormones involved are FSH, LH, estrogen, and progesterone. Both FSH and LH hormones are produced by the gonadotropic cells and progesterone hormones are produced by the ovaries.

Q.7.Explain the hormonal regulation of the process of spermatogenesis in humans.
A.7:Hormonal control of spermatogenesis in human males is as follows

• Gonadotropin Releasing Hormone (GnRH) is released significantly from the hypothalamus during puberty.

• GnRH stimulates anterior pituitary to secrete gonadotropins, i.e. LH and FSH or Interstitial Cell Stimulating Hormone (ICSH).

• Luteinising Hormone (LH) acts on Leydig cells to stimulate the synthesis and secretion of androgens which then stimulate the process of spermatogenesis.

• Follicle Stimulating Hormone (FSH) acts on Sertoli cells and stimulates them to secrete inhibin which then stimulates the process of spermiogenesis.