Introduction to Human General Embryology Developmental Genetics

Facts associated with spermatogenesis include:

1. 
   Spermatogonia divide repeatedly, via mitotic division to produce cells called Primary spermatocytes, still with 46 chromosomes.
   

2. 
   The Primary Spermatocytes undergo 1^st meiotic division to produce Secondary Spermatocytes.
   

3. 
   The Secondary Spermatocytes undergo 2^nd meiotic division that produces Spermatids, which then undergo mature into the Spermatozoa, or sperm cells. These sperm cells have 23 chromosomes each, half the number needed to initiate human development.
   

4. 
   Phases of gametogenesis include:
   

1. 
   Multiplication Phase (Spermatocytogenesis)
   
2. 
   Growth phase
   
3. 
   Maturation phase
   
4. 
   Spermiogenesis.
   

The events associated with spermatocytogenesis include:

1. 
   Sperm mother cells present in germinal epithelium of seminiferous tubules divide repeatedly by mitosis to form large number of diploid rounded sperm mother cells which are called as spermatogonia.
   

2. 
   Some of these sex cells move towards the lumen of seminiferous tubules and enter the growth phase. These cells are called primary spermatocytes.
   

3. 
   The primary spermatocytes are diploid and contain (44 + XY) chromosomes.
   

4. 
   Some of the sex cells produced by the division of spermatogonia remain in their original condition and continue to divide giving rise to primary spermatocytes. Such cells are known as stem cells.
   

The facts associated with the growth phase include:

1. 
   The primary spermatocytes are diploid and contain (44 + XY) chromosomes.
   

2. 
   During this phase the spermatocyte as well as its nucleus enlarges in size.
   

3. 
   It gets ready to undergo maturation division.
   

1. 
   Each diploid primary spermatocyte undergoes meiosis I, which is a reduction division.
   
2. 
   Two daughter cells are formed each with 'n' number of chromosomes.
   

3. 
   The daughter cells are called secondary spermaotcytes.
   

4. 
   The secondary spermatocytes are haploid and much smaller comparatively, containing (22+X) or (22+Y) chromosomes. 
   

5. 
   The secondary spermaotcytes undergo the second meiotic division. This results in the formation of four daughter cells known as spermatids.
   

6. 
   The final stage of spermatogenesis, which sees the maturation of spermatids into mature, motile spermatozoa, is called Spermiogensis or Spermiolysis.
   

The steps involved are:

1. 
   Formation of an acrosome which covers half of the nuclear surface. It contains enzymes to assist in penetration of the egg and its surrounding layers during penetration.
   

2. 
   Condensation of the nucleus.
   

3. 
   Formation of the neck, middle piece and tail.
   

4. 
   Shedding of most of the cytoplasm referred to as the Residue.
   

5. 
   The time required for a spermatogonium to develop into a mature spermatozoon is approximately 74 days.
   

6. 
   Fully developed spermatozoa enter the lumen of seminiferous tubules.
   

7. 
   Spermatozoa are pushed towards the epididymis by contractile elements in the wall of the seminiferous tubules.
   

8. 
   The spermatozoa are initially only slightly motile, but obtain full motility in the epididymis.
   

9. 
   Clinical correlates include:
   

1. 
   Abnormal Spermatozoa are seen frequently.
   

2. 
   Up to 10% of all spermatozoa have observable defects.
   

3. 
   Structurally, the head and tail may be abnormal; may be giants or dwarfs; or may be joined.
   

4. 
   Following non-disjunction of chromosomes, some sperm cells may have 22 or 24 chromosomes instead of 23 and such has been attributed to some genetic disorders. E.g. Monosomy and Trisomy.