Gametogenesis 

Gametogenesis is the biological process through which organisms produce specialized sex cells, or gametes, for reproduction. In humans and other animals, it occurs in the reproductive organs: sperm are produced in males through spermatogenesis in the testes, and eggs are produced in females through oogenesis in the ovaries. Gametogenesis involves cell division and differentiation, ultimately resulting in haploid cells that carry half the genetic material of the parent. This process is crucial for sexual reproduction, as it allows for genetic diversity when male and female gametes unite during fertilization.

1.0Introduction

• The primary sex organs – the testis in the males and the ovaries in the females–produce gametes, i.e., sperms and ovum, respectively, by the process called gametogenesis 
• GnRH, FSH, LH regulates gametogenesis. Besides this hormone vitamin E is also essential for gametogenesis. 
• Deficiency of vitamin E leads to sterility. Vitamin A is also required for the formation of healthy gametes. 
• Gametogenesis is divided in three stages: 
• (i) Multiplication phase 
• (ii) Growth phase 
• (iii) Maturation phase 
• Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells or PGCs, which are extra-Gonadial in origin. 
• Spermatogenesis occurs in the seminiferous tubules of the testes and oogenesis occurs in the follicles of the ovary. 
• Formation of gametes starts at puberty.

2.0Spermatogenesis

• The process of formation of spermatozoa in the seminiferous tubules of testis. The wall of seminiferous tubules is lined by cuboidal spermatogonia (male germ cells derived from primordial germ cells) and columnar Sertoli cells. 
• At puberty spermatogonia divide to form spermatozoa and Sertoli cells provide nourishment to dividing & developing germ cells. 
• Sertoli cells form a 'blood testes barrier' and protect the sperm from the immune system of the body. Sertoli cells function as an endocrine gland which secretes. 
• (i) Inhibin hormone: It gives negative feedback to the pituitary gland (mainly) and hypothalamus. 
• (ii) Androgen binding protein: It concentrates testosterone in seminiferous tubules to aid spermatogenesis. 

Spermatozoa are formed in the wall of the seminiferous tubules of the testes. 

A. The spermatogonia (type A) or germ cells (44 + X Y) divide mitotically, to give rise to more spermatogonia of type A and also spermatogonia of type B. 

B. The spermatogonia (type B) (44 + XY) enlarge, to form primary spermatocytes. 

C. The primary spermatocytes (44 + XY) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division: it reduces the number of chromosomes to half. D. Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time there is no reduction in chromosome number. 

E. Each spermatid (22 + X or 22+Y) gradually changes its shape to become a spermatozoon. This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis. 

F.  In human beings, the maturation phase is the longest phase of spermatogenesis.

Sperms are liberated from Sertoli cells of seminiferous tubules by the process called spermiation.

Male Reproductive Hormones

• Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH). 
• GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinising hormone (LH) and follicle stimulating hormone (FSH). 
• LH acts at the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis.
• FSH acts on the Sertoli cells and stimulates spermatogenesis.

• FSH: Binds with FSH receptors attached to the Sertoli cells in seminiferous tubules. This causes these cells to grow and secrete various spermatogenic substances and androgen binding proteins (ABP). 
• LH/ICSH: It stimulates the Leydig cells to secrete testosterone. 
• Inhibin: It is secreted by Sertoli cells in response to excess spermatogenesis. The inhibin gives a negative feedback to the anterior pituitary (Mainly) and hypothalamus (Negligible), this results in suppression of synthesis and release of FSH (Spermatogenesis decreases). 
• Testosterone: Secreted by Leydig cells. 
• It is essential for : 

(1) Sperm production 

(2) Development of secondary sexual characters 

(3) It also gives –ve feedback to hypothalamus and anterior pituitary in its excess concentration to suppress GnRH, LH release. 

(4) It is secreted in the fetal stage in as low as 30 ng/ml plasma concentration to cause descent of testis in the last trimester of intrauterine life. Note: 65-74 days are required to complete the cycle of spermatogenesis in human beings.

3.0Structure of Sperm

It is a microscopic structure composed of a head, neck, a middle piece and a tail. Plasma membrane envelops the whole body of sperm. 

• Head: The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome. Acrosome is filled with lytic enzymes called sperm lysins that help in fertilization of ovum. 
• Neck: It contains a proximal & distal centriole (or Basal body). An axial filament begins just behind the distal centriole & passes through the middle piece and extends into the tail.
• Middle Piece: It is also called the energy chamber. It contains numerous spirally arranged mitochondria (Nebenkern sheath), which produce energy for the movement of the tail that facilitate sperm motility essential for fertilization. Ring centriole (function unknown) is present at the distal end of the middle piece. It is not a true centriole. 
• Tail: The longest part of sperm. Sperm moves by tail. Distal centriole, present in the neck, acts as the basal body and produces axonemes.

Note : The human male ejaculates about 200 to 300 million sperms during a coitus of which, for normal fertility, at least 60 per cent sperms must have normal shape and size and at least 40 per cent of them must show vigorous motility. 

4.0Oogenesis

• The process of formation of a mature female gamete is called Oogenesis. 
• Like spermatogenesis, oogenesis process also can be divided into three stages: (i) Multiplication phase (ii) Growth phase (iii) Maturation phase (i) Multiplication Phase : 
• During the embryonic development stage, primordial germ cells or gamete mother cells repeatedly divide by mitosis to form a large number of diploid oogonia in each foetal ovary; no more oogonia are formed and added after birth. (ii) Growth Phase : 
• Like spermatogenesis, in this process oogonia grows in size and forms primary oocytes. 
• The growth phase is the longest phase in oogenesis in oviparous animals. 
• During the growth phase, the size of the egg increases many times. Yolk and cortical granules are formed during this phase. (iii) Maturation Phase : 
• It is the longest phase in humans. 
• In contrast to males the initial steps in egg production occur prior to birth. By the time the fetus is 25 weeks old, all the oogonia that she will ever produce are already formed by mitosis. 
• These diploid cells develop into primary oocytes, begin the first steps of the first meiotic division, proceed up to diplotene (prophase-I) and then stop any further development. The oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives very little amount of cytoplasm but one full set of chromosomes. 
• The first polar body may undergo degeneration due to lack of cytoplasm or may divide. The polar body is, therefore, formed merely to get rid of unwanted chromosomes. 
• Whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further, it awaits the arrival of the spermatozoa for completion of second meiotic division.
• Entry of the sperm restarts the cell cycle breaking down MPF (Metaphase promoting factor and turning on the APC (Anaphase promoting complex) which results in completion of meiosis II. The secondary oocyte retains the bulk of the nutrient rich cytoplasm of the primary oocyte and becomes Ovum/Ootid. A small second polar body is also formed. 
• The centriole of oocyte degenerates at the time of second maturation division hence centriole is absent in ovum. 

5.0Some Important Differences Between Oogenesis and Spermatogenesis

• One primary spermatocyte gives rise to four spermatozoa whereas, one primary oocyte forms only one ovum. 
• When the primary spermatocyte divides, its cytoplasm is equally distributed between the two secondary spermatocytes formed. However, when the primary oocyte divides, almost all its cytoplasm goes to the daughter cell which forms the secondary oocyte. The other daughter cell (first polar body), receives half the chromosomes of the primary oocyte, but almost no cytoplasm.