SPERMATOGENESIS

By Oba Mike

Introduction

Spermatogenesis is the formation of spermatozoa (sing. Spermatozoon) in the seminiferous tubules of the testis. It takes place during the reproductive period, which begins at the age of puberty (12-16years) and continues even to old age.

Location

It occurs in the seminiferous tubules of the testis.

Duration

64 - 74 days

Stages

Spermatogenesis can be divided into three stages:

1. Spermatocytosis

2. Meiosis

3. Spermiogenesis

Spermatocytosis

Spermatocytosis is the conversion of a spermatogonium (stem cell) into primary spermatocyte. It marks the beginning of spermatogenesis.

Spermatogonia are stem cells located in the walls of the seminiferous tubules near the basement membrane. These cells are similar to oogonia in the ovary and arise from primordial germ cells that migrated from the yolk sac. 

Each spermatogonium is diploid and contains 46 chromosomes (44+X+Y).

Prior to the division into primary spermatocyte, the spermatogonium first divides into type A and type B. The type A remains as stem cell while type B undergoes division and differentiation into primary spermatocyte.

The primary spermatocyte formed is also diploid (44+X+Y).

Spermatocytosis takes about 16 days.

Meiosis

The primary spermatocyte undergoes first and second meiotic divisions into secondary spermatocytes and spermatids respectively.

In the first meiotic division, the primary spermatocyte divides into two secondary spermatocytes. This division is a reduction reaction and reduces the chromosome number to half. Hence, each of the two newly formed secondary spermatocyte is haploid having 23 chromosomes (22+X or 22+Y).

In the second meiotic division, the two secondary spermatocytes divide to form four spermatids. This division is an equation division and retains the chromosome number. Hence, the newly formed spermatids are haploid containing 23 chromosomes (22+X or 22+Y).

Meiosis takes about 24 days.

Spermiogenesis

Spermiogenesis is the differentiation or metamorphosis of spermatids into spermatozoa. 

Note that spermiogenesis is neither a mitotic or meiotic division. It is simply the growth of the spermatids into spermatozoa.

During spermiogenesis, the spermatid which is just a circular cell, gradually transforms into spermatozoon (sperm cell) with the famous head, neck and tail shape.

Major events in spermiogenesis include:

Condensation of nucleus

Formation of acrosome

Formation of flagellum (tail)

Rearrangement of organelles

Shedding of excess cytoplasm.

The organelles form the various parts of the sperm cell as shown in the picture below.

Draw this diagram

Summary

Spermatogenesis involves the conversion of a single diploid spermatogonium (stem cell) into four haploid spermatozoa. This is unlike oogonia where a single oogonium forms only one ovum.

A healthy adult male produces about 1500 sperm cells per second. This brings daily production to over 100 million sperm cells.

Maturation and Capacitation of Spermatozoa

The sperm cells formed in the seminiferous tubules are immature, immotile (cannot move) and incapable of fertilizing an ovum.

From the testis, they are carried through the efferent ductules into the epididymis where they are stored till maturation.

Spermatozoa however, acquire the ability to fertilize an ovum only after they are already in the female genital tract. This final step in their maturation is called capacitation.

Capacitation occurs when a spermatozoon comes in contact with the zona pellucida of the oocyte. During this process, the membrane surrounding the acrosome breaks down releasing lysosomal enzymes. This is called acrosomal reaction. The enzymes help in digesting the zona pellucida to enable penetration of the spermatozoon into the oocyte.

ASSIGNMENT

Write on the structure of a mature spermatozoon.