Gametogenesis is the process by which male and female sex cells or gametes, i.e., sperms and ova are formed respectively in the male and female gonads (testes and ovaries). The gametes differ from all other cells (= somatic cells) of the body in that their nuclei contain only half the number of chromosomes found in the nuclei of somatic cells.
Meiosis forms the most significant part of process of gametogenesis. Gametogenesis for the formation of sperms is termed spermatogenesis, while that of ova is called oogenesis.
Mammalian spermatogenesis is a regular, long and extremely complex process of cellular differentiation by which a spermatogonial “stem-cell” is gradually transformed into a highly differentiated haploid cell ‘Spermatozoon.”
This differentiation involves three distinct classes of germinal cells—the spermatogonia, the spermatocytes, and the spermatids, which usually are arranged in concentric layers in the seminiferous tubules.
In the adult mammals spermatogenesis is a continuous process, which can be divided into two distinct phases and each characterized by specific morphological and biochemical changes of nuclear and cytoplasmic components.
Spermatogenesis occurs in the seminiferous tubules of the testes. The seminiferous tubules are lined by germinal epithelium. The germinal epithelium consists largely of cuboidal primary or primordial germ cells (PGCs) and contains certain tall somatic cells called Sertoli cells (=nurse cells). Spermatogenesis includes formation of spermatids and formation of spermatozoa.
The two phases include:
(i) formation of spermatids (mitosis and meiosis)
Formation of spermatids:
This phase of spermatogenesis is further subdivided into three phases like:-
(a) Multiplication phase.
(b) Growth phase.
(c) Maturation phase
(a) Multiplication phase.
This phase is also known as proliferation and renewal of spermatogonia. During this phase the diploid spermatogonia which are situated at the periphery of the seminiferous tubule, multiply mitotically to form spermatocytes and also to give rise to new spermatogonial stem cells and enter the phase of growth. The stem cell Spermatogonia (2N) are of two types:
- Type A spermatogonia and type B spermatogonia. Type A spermatogonia serve as stem cells which divide to form type A spermatogonia and type B spermatogonia.
- Type B spermatogonia are the precursors of sperms.
(b) Growth phase:-
Each type B spermatogonium actively grows to a larger primary spermatocyte by obtaining a nourishment from the nursing cells. During this phase, a limited growth of spermatogonia takes place; their volume becomes double and they are now called primary spermatocytes which are still diploid in number. Now these primary spermatocytes enter into the next phase namely, maturation phase.
(c) Maturation phase:-
Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form, four haploid spermatids, ultimately form four from each primary spermatocyte.
The primary spermatocyte enter into the prophase of meiotic or maturation division. Meiotic prophase starts pairing is a very complex process characterised by an ordered series of chromosomal rearrangements which are accompanied by molecular changes. During meiosis, first nuclear DNA duplicates, each homologous chromosome (synapsis) and longitudinally spilts up into two chromatids, both of which remain joined by a common centromere. By chiasma formation mutual exchange of some chromosome material between two non-sister chromatids of each homologous pair (tetrad) occurs (crossing over) to provide an almost indefinite variety of combinations of paternal and maternal genes in any gamete.
Lastly, two chromosomes of each homologous pair (tetrad) migrate towards opposite poles of the primary spermatocyte. Now each pole of primary spermatocyte has haploid set of chromosomes. Each set of chromosome is surrounded by the nuclear membrane developed from the endoplasmic reticulum. The first meiotic division, as a rule, is followed by the division of cytoplasm (cytokinesis) which divides each primary spermatocyte into two haploid, secondary spermatocyte.
Each secondary spermatocyte undergoes second meiotic or maturation division which is a simple mitosis and produces four haploid spermatids. These are non-functional male gametes. To become functional spermatozoa, they have to undergo a complex process of cytological and chemical transformations; a process usually referred to as spermiogenesis.
Spermiogenesis: The transformation of spermatids into spermatozoa is called spermiogenesis or spermateliosis. The spermatozoa are later on known as sperms. Thus four sperms are formed from one spermatogonium.
Steps of spermiogenesis: (Rest part of study materials after registration)
Details heading of rest part study materials:-
- Steps of spermiogenesis:-
- Formation of head.
(a) Changes in the nucleus.
(b) Golgi phase.
- Schematic diagram
(c) Cap phase:
(d) Acrosome phase.
- Formation of the tail of the spermatozoon
- Characteristic features od spermatogenesis
- Important Note by AKB Sir.
- Hormonal Control .
- Clinical significance.
- Spermatids are attacked by Gossypol
- Significance of Spermatogenesis.
- Biochemical Changes in Spermatogensis.
- Control of Spermatogenesis.
- Differences between spermatogenesis and spermiogenesis
- Spermatozoon (Sperm) structure with diagram
- Multiplication phase
- Growth Phase.
- Oocyte Content.
- Maturation phase.
- Important Note by AKB Sir.
- Hormonal control of Oogenesis with diagram
- Significance of Oogenesis.
- Structure of Mature Egg(Ovum) with diagram
- Similaritites in Spermatogenesis and Oogenesis
- Differences between Spermatogenesis and Oogenesis
- Differentiation of Gametes (Sperms and Ova)