I think you are missing some basic knowledge or misunderstanding something.

Its pretty simple in the end:

You have a bunch of Ad cells (spermatogonia) that undergo mitosis ... thats all they do. And they either form new Ad cells that stay in this "pool" and keep dividing or they form a type Ap cell.. Now this Ap cell leaves this "pool" to undergo a series of mitotic divisions untill they form type B cells and these cells undergo one more mitotic division to form primary spermatocytes. And thise cells ondergo two meiotic divisions to form spermatides in the end.

(4 spermatids for each primary spermatocyte).

So all together: Ad cells, they simply divide to form new cells (they keep the population of cells in order) and at some point a few of these cells form Ap cells in stead of Ad cells.. Those Ap cells leave the pool of Ad cells to start their journey to become spermatides.

So spermatogonia either stay as Ad cells to keep the population of cells in order or they become Ap type spermatogona to make spermatides...

I hope you get the idea now? In fact its just one big collection of cells that divide the entire time and either they form a Ap cell or Ad cell....

This diagram actually makes more sense, since the spermatogonium divides into two cells: a spermatogonium (that doesn't undergo

However, isn't the "additional spermatogonia" and the "primary spermatocyte" in this diagram **100% genetically identical**

==> that primary spermatocyte that starts meiotic division comes from type Ap , that aditional cell is indeed also a type Ap cell and will also undergo some mitotic divisiosn to form such a primary cell.. SO yes, they are the same.

In fact: the shedule shows only halve of what happens: you need to double it... You have indeed two primary cells ...

(this is just an example on how 1 Ad cells becomes 2 Ap cells by mitotic divion... so those two cells "leave the pool" of Ad cells.)

Off course: the figure could also just mean (or wants to state) that beside forming a type Ap cell, you can also form Ad cells that keep dividing.. This is just a figure to show what happens , how spermatides are formed. The part in where you have Ad cells just dividing by mitotic division is often not showed because its not that intersting to show, because the only thing that happens is that they divide...

I hope this made it clear?

**Edited by pito, 23 April 2011 - 02:23 PM.**