Skip to main content

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.

Comments

Post a Comment

Popular posts from this blog

BONY PELVIS

By Oba Mike Image credit: sites.google. com Introduction The bony pelvis or simply pelvis is a basin-like structure composed of the hip bones, pubic symphysis, sacrum and coccyx. Functionally, the pelvis provides support to the vertebral column, pelvic and lower abdominal organs. It also connects the bones of the lower limbs to the axial skeleton. Functions of the Pelvis 1. Supports the vertebral column. 2. Protects pelvic and lower abdominal organs. 3. Connects the bones of the lower limbs to the axial skeleton. 4. Transmits the weight of the body to the lower limbs while standing and to the seat while sitting. 5. Helps in the attachment of several muscles. 6. Supports childbearing in females. Components of the Bony Pelvis The bony pelvis is formed by four different structures: 1. 2 hip bones (pelvic girdle) 2. Pubic symphysis  3. Sacrum 4. Coccyx Hip bones The hip bones also called pelvic or coxal bones are the two bones that form the pelvic girdle. They form the anterior and latera
Post a Comment
Read more

CONNECTIVE TISSUES

By Oba Mike Introduction Connective tissues are the most abundant tissues in the body. As the name suggests, the major function of connective tissue is to bind together, support and strengthen cells and other body tissues. Functions of Connective tissues 1. They give structural and functional support to cells and other tissues. 2. Protection and defense against pathogens. 3. Stores energy. 4. Transport of substances within the body. 5. Insulation. Structure of Connective Tissues Connective tissues consist of two basic elements: 1. Cells 2. Extracellular matrix Cells Connective tissue cells originate from embryonic cells called mesenchyme . The cells vary according to the type of tissue and include the following: A. Fibroblasts B. Adipocytes (fat cells) C. Macrophages D. Mast cells E. Blood cells ( erythrocytes, leukocytes, thrombocytes). Fibroblasts They are large flat cells with branching processes. They are the most numerous connective tissue cells and are present in all the general
Post a Comment
Read more