Gametogenesis in human beings

Introduction

Gametogenesis is the process of formation of a diploid zygote by the fusion of 2 haploid gametes derived  one from male and the female each. Diploid cells contain two complete sets (2n) of chromosomes whereas haploid cells contains  half the number of (n) chromosomes . In the males, spermatogenesis is alternatively used for gametogenesis, similarly, in females, it is termed as oogenesis. Gametogenesis is closely linked with exchnage of chromosomes between the 2 parents. The human cells contain 46 chromosomes which are found in 23 pairs among which, 22 pairs are called somatic chromosomes and the last pair is the set of sex chromosomes with the configuration of XY for male and  XX for female respectively. A normal eukaryotic gamete ( after the fusion ) is made up of diploid cells, with one set of chromosomes derived from each parent. However, after meiosis, the number of chromosomes become halved. Let us explore more on the process of gametogenesis in males and females.

 

Gametogenesis

Gametogenesis is the formation of matured gamets from the precursor cells. Gametogenesis in a male is the spermatogenesis whereas 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 cell multiplications and diifferentiations occur under the influence of male androgen secreted by the male reproductive system. On an average, a healthy male produces around ~200 million sperms a day.

 

Oogenesis

Oogenesis is the  the differentiation of the ovum or egg cell into a matured cell competent to further develop when fertilized.It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.

 

Spermatogenesis

Phases of spermatogenesis:

Spermatogenesis occurs inside the testes more precisely, in the seminiferous tubules. It take place under the influence of gonadotropin-releasing hormone ( GNRH) produced from the hypothalamus (pituitary gland). Additionally,  luteinizing hormone ( LH) acts on Leydig cells of the testicles to produce testosterone which plays a vital role in nourishing the testes. The phases of spermatogenesis is divided into 2 phases as explained below.

 

Phase1: Spermiogenesis

It is also known as phase of formation of spermatogonium. Spermatogonia are the poorly differentiated and immature sperm cells under the influence of androgens. Androgens (sex hormones)  are primarily released by gonads (by the Leydig cells in testes of men and by the ovaries in women), nevertheless, a small quantity  of androgens is also produced by the adrenal glands found in both male and females. 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 in the testes resulting in the production of testosterone. Spermatogonium (immature sperms) activates the Sertoli cells, while the mitotic division of immature sperm cells within the seminiferous tubules results in the formation of the primary spermatocytes. Gradually, primary spermatogonia enter into the first meiotic division to form the secondary spermatocytes and these secondary spermatocytes further undergoes second meiotic division, to produce 4 haploid spermatids. During this phase,  spherical spermatids grows into a matured tadpole-like spermatocytes and the process is termed as spermiogenesis. The sperm’s cell body is elongated to form a complete sperm with other parts along with it. 

 

Phase2. Release of spermatids into tubules(spermiation)

In the second phase, the sperm heads are 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 spermatids which ultimately release the spermatids  into the lumen of the seminiferous tubule. Simultaneously, the spermatids undergoes constant remodeling to form spermatogonia. Spermiogenesis help in converting the immature spermatogonia into the mature spermatozoa. The spermatozoa travel down to 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.

 

 

 

Oogenesis

Phrases of Oogenesis

The process by which a matured female gamete is formed from its immature primary oocyte is called oogenesis. It is a continuous process in females right from birth until the menopause (last menstrual cycle). However, physiologically the significant phases of oogenesis start from the time of the first menstrual cycle called menarche. Oogenesis occurs inside the ovaries -female gonads through a series of cell divisions under the influence of estrogen and progesterone. The 4 phases of oogenesis are explained below.

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: During this stage, each primary oocyte is encircled by granulosa cells to form primary follicles. Roughly 2 million primary follicles are present at the time of birth. But, as a girl attains her puberty, numerous follicles are degenerated to finally produce 60,000-80,000 primary follicles from each ovary.

Phase 3: The secondary follicles are formed by primary follicles while they undergo further maturation to form tertiary follicles. The tertiary follicles float in a fluid-filled space called the antrum. The antrum space is made up of 2 theca layers where antrum furnishes one layer as an internal theca from inside and another layer called external theca from the outside.

Phase 4: When the follicles are completely matured, they are modified into 2 types of oocytes; the large haploid secondary oocytes along with a tiny first polar body. The tertiary follicles grow further to form Graaffian follicles. The graaffian follicle ruptures and releases the matured secondary oocyte or ovum from the ovary and this is known as ovulation. Matured cells are released during the ovulation and they are called ovum or egg cells as shown in the picture below.

 

 Phases of Embryogenesis

Read more

1 .     

Human reproduction      

2.

Gametogenesis                     

3.

Infertility

4.

 Menstrual cycle                                          

 

Questions

  1. What is gametogenesis?

  2. How many somatic chromosomes are found in a human cell?

  3. What is the difference between the chromosomes of males and females?

  4. Explain the phases of spermatogenesis.

  5. Describe oogenesis in detail.

 



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