Question

Describe spermatogenesis with the help of a diagram.

Answer 1

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

1. Multiplication phase
2. Growth phase
3. Maturation phase

1. 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.

2. 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.

3. Maturation phase:

• During this phase, one diploid primary spermatocyte divides meiotically to form four haploid spermatids.
• It is further divided into 2 phases -

1. Maturation-I:

• It involves meiosis-I which is also called a reductional division.
• The diploid primary spermatocyte divides to form two haploid secondary spermatocytes.

1. Maturation-II:

• It involves meiosis-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 -

(1) 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 -

1. Changes in the Centriole:

• The spermatid has two centrioles. During the process of spermiogenesis, one centriole moves anteriorly and occupies a position in the notch of the nucleus. It is called proximal 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 additional ring centriole is present in the posterior part of the middle piece. However, it's function is obscure.

2. 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.

3. 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.

(2) 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