Gametogenesis in human beings

Introduction

Gametogenesis is the process of formation of a diploid zygote by the fusion of a haploid male gamete with the haploid female gamete. Haploid is the term used for a single set of unpaired chromosomes and the diploid is called so when the chromosomes occur in pairs( 23 pairs). In the males, spermatogenesis is alternatively used instead of gametogenesis, similarly, in females, it is termed as oogenesis. Human beings have  46 chromosomes in 23 pairs hence they are called diploid. A normal eukaryotic gamete is made up of diploid cells, with one set of chromosomes derived from each parent. However, after meiosis, the number of chromosomes will be halved.

Table Of Content

1. Introduction

2. Gametogenesis

3. Spermatogenesis

4. Phases of spermatogenesis

    4.1 Phase1: Formation of primary spermatogonia and secondary                                      spermatogonia

    4.2 Phase2. release of spermatids into tubules( spermiation)

5. Phrases of Oogenesis

Gametogenesis

Gametogenesis is the formation of matured gamets from the precursor cells. Gametogenesis in a male is the spermatogenesis whereasx in a femal it is called oogenesis. 

 

Spermatogenesis

The development  of  sperms from the imature spermatocytes is known as spermatogenesis. It starts between the ages 14-19 years in a healthy and normal male. The process requires approximately 70 days , during which a series of multiplications and hormonal changes occur in the male reproductive system. On an average, a healthy male produces around ~200 million sperms a day. Spermatogenesis occur inside  the testes - the seminiferous tubules. The processc is led under the influence of gonadotropin releasing hormone ( GNRH) produced from the hypothalamus (pituitary gland).Some of th eother harmones mediating the gametgenesis are  In addition, luteinizing hormone ( LH) acts on Leydig cells of the testicles that stimulates the formation of testosterone . The testosterone plays a vital role in nourishing the testes. 

 

Phases of spermatogenesis:

Phase1: Formation of primary spermatogonia and secondary spermatogonia

Testosterone hormone is released by gonads (by the Leydig cells in testes in men and by the ovaries in women), although small quantities are also produced by the adrenal glands in both sexes. TSH combines with FSH to stimulate the Sertoli cells of the testes. As a result, Sertoli cells produce a hormone known as inhibin. Inhibin assists in the upregulation of testosterone binding globulin receptors. The upregulation of these receptors will again stimulate the cells resulting in producing testosterone. Spermatogonium ( immature sperms) activates the Sertoli cells, simultaneously the mitotic division within the seminiferous tubules results in the formation of the primary spermatocytes. The primary spermatogonia enter into the first meiotic division to form the secondary spermatocytes. Once again, the secondary spermatocytes are subjected to the second meiotic division, to produce 4 haploid spermatids. During this phase,  spherical spermatids gets into a matured tadpole-like spermatocytes and this is known as spermiogenesis. The sperm`s cell body is elongated to form a properly shaped sperms. 

Phase2. release of spermatids into tubules( spermiation)

In the second phase, the sperm heads become embedded in the Sertoli cells to gain energy and nourishment. With this energy, they are finally released from the seminiferous tubules through spermiation. Spermiation happens as a result of the breaking down of cytoplasmic bridges around the cells. The spermatids are released into the lumen of the seminiferous tubule. Simultaneously, the spermatids undergo continuous remodelling. Spermiogenesis help in converting the immature spermatogonia into the mature spermatozoa. The spermatozoa travel down and ultimately reach the epididymis of seminiferous tubules. From the seminiferous tubule, the mature spermatozoa move into the rete testis, where the excess of fluid is removed to make the spermatic fluid concentrated and sticky. After they get thicker, they move down to the epididymis where they get stored finally.

 Phases of Embryogenesis

 

 

 

 

 

Phrases of Oogenesis

 The process by which a matured female gamete is formed from its immature primary oocyte is called oogenesis. Oogenesis is a continuous process right from birth until the menopause (last menstrual cycle). However, physiologically significant phases of oogenesis start from the time of the first menstrual cycle; menarche. Oogenesis occurs inside the ovaries -female gonads through a series of cell divisions, growth, maturation, and hormonal activities. Following are the series of phases in oogenesis.

Phase 1:It is initiated during the embryonic phase of a female when a couple of a million oogonia (gamete mother cells) are formed in an individual fetal ovary. At this stage, cells start dividing by the process called prophase-I resulting in the formation of primary oocytes.

Phase 2: In this stage, each primary oocyte is encircled by granulosa cells to form primary follicles. There are 2million primary follicles at the time of birth.  But, as the girl attains her puberty, a large number of these follicles are degenerated to form 60,000-80,000 primary follicles from each ovary.

Phase 3: The secondary follicles are formed by primary follicles. At the same time, the secondary follicles undergo further maturation to form tertiary follicles. The tertiary follicles float in a fluid-filled space called the antrum. The antrum space has a theca layer organized into an internal theca from inside and an external theca from the outside.

Phase 4: When the follicles are completely matured, they result in the formation of large haploid secondary oocytes along with a tiny first polar body. The tertiary follicles grow further to form Graffian follicles. The graffian follicle ruptures and releases the matured secondary oocyte or ovum from the ovary and this is known as ovulation.



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