Spermatogenesis

SPERMATOGENESIS

• Process of spermatozoa formation (sperm) from primitive germ cells (spermatogonia).
• Result of stimulation by testosterone
• Secreted from Leydig cells under LH influence.

STEPS:

• Initial essential steps are completed in testis.
• Remaining, formation of mature sperms is completed only in female genital tract

Initial phase: 

• Spermatogonia→ 1° spermatocytes→ 2° spermatocytes
• Maturation phase:
• Spermatids→spermatozoa (sperms)
• Release of sperms – Spermiation

Hormonal influence:

• Testosterone diffuses into tubules & increase its concentration.
• Binds to Androgen Binding Protein (ABP)
• Secreted from Sertoli cells under FSH influence.
• Except for “Maturation phase” all other phases are testosterone independent 

Step 1:

• Seminiferous tubules in testis – Starting point of process.
• Spermatogonia (primitive germ cells) divide & migrate centrally in seminiferous tubules

Step 2 – Maturation – Spermiogenesis:

• Maturation of spermatids to spermatozoa depends on testosterone
• Occurs in epididymis
• Exact location – Deep cytoplasmic folds of Sertoli cells
• Spermatids undergo morphological changes to form Sperm
• Motility develops at this stage.
• Sertoli cells provide nourishment to mature spermatozoa

Step 3 – Spermiation

• Process of release of mature spermatids from Sertoli cells after maturation

Hormonal influence:

• FSH – Produce cytoskeletal organization of protein necessary for sperm release. 
• Testosterone – Ectoplasmic specialization formation.
• Human sperm remains fertile for 24-72 hrs in female genital tract after ejaculation.

TEMPERATURE & SPERMATOGENESIS:

• Spermatogenesis requires temperature considerably lower than core body temperature.
• Location [Testes/Scrotum] important to produce viable sperm
• Specifically 1°-8 °C lower than normal body temperature of 37 °C (98.6 °F

Maintenance of temperature by:

• Circulating air in scrotum.
• Countercurrent heat exchange between spermatic arteries & veins.

Applied physiology:

• Cryptorchidism – Failure of testicular descent
• Exposes testis to higher temperature (intra-abodimal) 
• Resulting in defective spermatogenesis & infertility.

HORMONES INVOLVED IN SPERMATOGENESIS:

• FSH, LH, testosterone, estrogen, growth hormones, inhibin & activin.

Individual action:

1. FSH – 

• Responsible for initiation of spermatogenesis. 
• Binds with sertoli cells & spermatogonia
• Induces proliferation of spermatogonia. 
• Stimulates sertoli cells 
• Secrete androgen binding protein, inhibin, mullerian inhibing substance & estrogen.

2. LH –

• Stimulates Leydig cells to secrete testosterone.

3. Testosterone –

• Principle hormone – Directly stimulates and maintains spermatogenesis. 
• Responsible for sequencing remaining stages of spermatogenesis (after initiation by FSH). 

4. Estrogen –

• Formed from testosterone (by aromatase) in Sertoli cells. 
• Essential for spermiogenesis. 

5. Growth hormone – 

• Promotes early division of spermatogonia.

6. Inhibin –

• Important role in regulation of spermatogenesis
• By feedback inhibition of FSH secretion.

7. Activin –

• Stimulates FSH secretion & stimulates spermatogenesis.

Important metrics in spermatogenesis:

• Begins at an average age of 13 years.
• Healthy human adult form up to 120 million sperm each day from both of testis together.
• 1 primary spermatocyte gives rise to 4 spermatids.
• 1 Spermatogonium gives rise to 512 spermatids. 
• Formation of mature sperm from primitive germ cells require average of 74 days