Spermatogenesis : The formation of the sperms from the germ cells in the testes is called spermatogenesis.

· The structural & functional unit of testes is seminiferous tubule.

· It is provided with germinal epithelium which is made up of germ cells.

· Spermatogenesis is a continuous process but for the convenience of study, it is divided into two steps -

1. Spermatocytogenesis

2. Spermiogenesis

(1) Spermatocytogenesis :- It results in the formation of four haploid spermatids from one diploid germ cell.

· It is further divided into three phases

(i) Multiplication phase

(ii) Growth phase

(iii) Maturation phase

(i) Multiplication phase : During this phase, the germ cells divide repeatedly by mitosis.

· It results in the formation of more cells which are called spermatogonia. The spermatogonia are diploid.

(ii) Growth phase : This phase results in the formation of primary spermatocyte.

· It is a phase of short duration which does not involve any division.

· The spermatogonia exhibit growth due to an accumulation of proteins & chromatin substances.

· The primary spermatocytes are also diploid.

(iii) Maturation phase : During this phase, one diploid primary spermatocyte divides meiotically to form four haploid spermatids.

· It is further divided into 2 phases -

Maturation-I : It involves meiosis-I which is also called a reductional division

· The diploid primary spermatocyte divides to form two haploid secondary spermatocytes

Maturation-II : It involves meiodis-II which is also called an equational division.

· The two secondary spermatocytes divide to form four spermatids.

(2) Spermiogenesis :- During this phase, a non-motile & tail-less spermatid transforms into a motile & tailed sperm.

· There is no division during the spermiogenesis and one spermatid forms only one sperm. It is also called as spermatogenesis.

· The changes during the spermiogenesis result in the formation of the head, middle piece & tail in the sperm.

· The various changes during spermiogenesis are of 2 types -

(i) Changes in Cytoplasm : During spermiogenesis, most of the cytoplasm of the spermatid is visiting in the form of cytoplasmic residue.

· The other changes in the cytoplasm are as follows -

(a) Changes in the centriole : The spermatid has two centrioles. During the process of speriogenesis, one centriole moves anteriouly and occupies a position in the notch of the nucleus. It is called procimal centriole.

· The second centriole assumes the slightly posterior position and forms the neck of the sperm. It is called distal centriole.

· Both the centrioles in the sperm are situated at a right angle to each other, The distal centriole gives rise to an axial filament which is found in the middle piece & tail of the sperm.

· The axial filament exhibits 9 + 2 fibrillar arrangement out of the 9 fibres are peripheral & double while the 2 fibres are central & single.

· In the sperms of some species, an addition ring centriole is present in the posterior part of the middle piece. However, it's function is obscure.

(ii) Changes in Mitochondria : During the process of spermiogenesis, all the mitochondria of the spermatid fuse to form a nebenkern or Jensen's sheath.

· The nebenkern along with little cytoplasm forms a spiral sheath around the anterior end of the axial filament which is called as manchette.

· The nebenkern provides required energy to the sperm.

(iii) Formation of Acrosome : The anterior most part of the sperm is called an acrosome. It is formed by the Golgi body during the spermiogenesis.

· Firstly, one of the vacuoles of the Golgi body develops an acrosomal granule.

· Gradually, the acrosomal granule accumulates lytic enzymes and forms a cap-like structure on the anterior end of the nucleus.

· Soon this cap gets bounded by a single unit plasma membrane and forms an acrosome.

· The best parts of the Golgi body are lost in the form of Golgi rest.

(3) Changes in the Nucleus : There is a loss of water and unwanted substances from the nucleus during spermiogenesis.

· Nucleolus & RNA are lost gradually.

· The nucleus slightly elongates and become narrow.

· The remaining DNA with nucleoproteins get condensed in the nucleus.

· Golgi body Proacrosomal granule Acrosomal granule