Structure of Sperm

Introduction

Sperm is the male gamete essential in human reproduction. Sperms are characterised by a unique structure and chemical composition help in penetrating against the wall of the ovum.  Spermatogenesis is the term used for the formation of sperms. Let us understand the process of spermatogenesis through each step.  

 

Spermatogenesis

Table of Contents

1. Introduction

2. Spermatogenesis

3. Structural and functional characteristics of sperms

4. Anatomical description of sperm 

    4.1 Head of the sperm  -structure, and functions

    4.2 Mid-piece - the body of sperm

    4.3 Tail

Spermatogenesis is the process of formation of matured sperms from the immature spermatogonia. Testosterone plays a vital role in the production, growth, and maintenance of sperm cells. Hypothalamus produces a hormone, gonadotropin-releasing hormone (GnRH) responsible for the release of sperms out of the testes. Spermatogonia are the immature germ cells found inside the testes.  Spermatogonium are located in the inner wall of a seminiferous tubules.  Each individual spermatogonia have a  diploid set of chromosomes, i,e  46  chromosomes in 23 pairs. In the 1st phase of spermatogenesis, the primary spermatocytes constantly undergoes meiosis resulting in the formation of 2 equal, haploid cells known as secondary spermatocytes. As a result of meiosis, the developing spermatocytes become half of the diploid chromosomes (23 chromosomes ). In the next stage, each secondary spermatocytes further undergo second meiotic division to produce 4 equal, haploid spermatids. At this stage, they are called spermatids. The spermatids are transformed into spermatozoa or sperms and are ready to release from seminiferous tubules, the process is called the spermiation where the sperm heads become embedded in the Sertoli cells to gain energy and nourishment.

Phases of Spermatogenesis

 

Structural and functional characteristics of sperms

A sperm has an oval-shaped head with a smooth surface resembling the shape of an egg. A mature sperm measure about 2.5 to 3.5 um (um=micrometers) in diameter and 4.0 to 5.5 um in length hence the length to width ratio of a sperm is around 1.50 to 1.70. Acrosome is a cap-like sheath covering the head of the sperm. Acrosome forms about 40% to 70%of the oval-shaped head. The motile capacity of the sperms is regulated by various chemical pathways. A slender and straight middle section of a mature sperm is called the body. Body portion is rich in mitochondria that help in supplying energy to the sperm during the penetration into the ovum.  Tail is lengthy and irregular. It caries between the sperms. Tail help in driving the sperm forward.  Let us discuss the detailed structure of each segment of the sperm with the help of an image.

 

Anatomical description of sperm 

Head of the sperm  -structure, and functions

The head is the most important part of the sperm cell as the necessary genetic material of the sperm is located here. The head of the sperm releases  23 haploid set of chromosomes comes in contact with the 23 chromosomes found in the ovum. Together, the acrosome with an acrosomal cap form the acrosomal body. Acrosomial body contains a Golgi complex along with an enzyme acrosin. Acrosome also contains polysaccharides such as mannose, hexosamine, and galactose to supply energy.  The acrosome plays a major role in identifying the most suitable egg for fertilization as it has a capacity to sense it. The secretions released by acrosome help to stick to the surface of  ovum. The process of sperm coming in contact with the egg as well as penetration into the egg is mediated by a process called chemotaxis. Acrosome also helps in avoiding the double fertilisation, however, in rare cases( twins and triplets). The process of sperm being attracted and penetrated into the most suitable ovum is known as acrosomal reaction.

 

Mid-piece - the body of sperm

It measures hardly about 10% of the total length. It is a junction between the head and the tail. Midpiece has a tightly packed mitochondria assist in pushing the sperm though the reproductive canal.

 

Tail

It is the lengthiest, slender and very active. A tail makes up about 80% of the total length of the sperm. Longer the tail, better the ability to fertilize the ovum. Tail is again divided into several segments such as connecting piece midpiece and the axial filament.  The connecting piece help in connecting the flagellum to the sperm head. The mid-piece of the tail contains mitochondria and the Principal piece or the axial filament along with the end piece assist in producing wavy movement of the sperm.



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