The Seminiferous Tubules, Leydig & Sertoli Cells

Human males produce widely varying numbers of sperm... sperm production typically varies by as much as 50%, even in age-matched groups.  Sperm production rates of between 30-400 million spermatozoa/day have been reported... and human males are generally considered to be relatively infertile in comparison to their counterparts among most domestic animal species.  A fertile ejaculate contains a minimum of around 70-100 million sperm... or at least 20 million sperm per milliliter.  In fact, you can now purchase a home male fertility kit that tests sperm count, for about $40.

The testes contain 2 major cell types which are involved in the production of androgens and spermatogonia; the Leydig (interstitial) cells and the Sertoli (nurse or sustenacular) cells, respectively. The Leydig cells produce androgens in response to LH.  The Sertoli cells produce androgen binding proteins (ABP), in response to FSH.  ABP helps maintain an androgen rich microenvironment in the seminiferous tubules.  High levels of androgens in the seminiferous tubules are essential for spermatogenesis.  The major physiological structures involved in spermatogenesis are the seminiferous tubules.

Spermatogenesis is divided into the 2 processes outlined below.

Spermatocytogenesis (Part 1):

Primitive spermatogonia lay dormant in the testes until the time of puberty.  As androgens rise during puberty, the spermatogonia begin to divide.  As each spermatogonium divides, it forms 2 cells... a dormant spermatogonium (Spermatogonium A) and an active spermatogonium (Spermatogonium B).  The dormant spermatogonium (A) sits and rests along the periphery of the seminiferous tubule, as an undifferentiated stem cell which will divide some time in the future.  The active spermatogonium (B) begins a series of 4 mitotic divisions that will lead to the production of 16 "primary spermatocytes."  In our diagram, we just see on primary spermatocyte undergoing further divisions; remember, however, that 16 primary spermatocytes are produced from each active spermatogonium!  Each of the 16 "primary spermatocytes" now undergoes meiosis.  Remember that meiosis consists of a series of 2 reduction divisions.  At the end of meiosis I, we have 32 "secondary spermatocytes."  At the conclusion of meiosis II, we have 64 "spermatids."  All of the cells mentioned so far still have that spherical appearance of a typical cell (ie. they do not look like sperm yet!)  As the cells undergo divisions, they move towards the lumen of the seminiferous tubule, but each cell remains attached to a Sertoli cell (nurse, sustenacular cell) by a fine "microtubule" which provides the spermatocytes with nutrients.  Each Sertoli cell may have "microtubule" attachments to nearly 200 spermatocytes.  The formation of these bridges between Sertoli and round spermatocytes depends on androgen and FSH.

Spermiogenesis (Part 2):

Spermiogenesis is the morphological transformation of spermatids to spermatozoa.  This process is androgen-dependent.