Re: Which part of the testis does the FSH & LH stimulate?

Hello,

you have asked a very complex and interesting question. As for being
confused, I can assure you that this does not only happen on the
undergraduate level, but also among physicians and biologists. Diseases of
the hormonal glands (not only reproductive glands, of course) are 
complex enough to require their own specialist among physicians, the
endocrinologist. So, if hormones and their interactions sometimes make you
all confused, you are in very good company. 

Therefore, your question requires a small assay on sexual hormones rather
than a straightforward answer. I will try to be as concise as possible. 

First of all, terminology. "Androgen" is not a substance. Rather, the
androgens are a class of substances that exert "androgenic" effects. These
effects are, among others the growth of the testes during puberty, the
production of sperm cells (see below), the function and maturation of the
male genitalia, and the male libido. Other effects are seen as "secondary"
male gender traits: The deeper voice; the male pattern of hair growth
(pubic, axillary, facial); the male muscle distribution and muscle mass
(most anabolic steroids are also androgens). Finally a number of less
clear-cut (and sometimes, not entirely undisputed) "typically male" traits
are ascribed to testosterone: We men get heart disease earlier; our hair
gets thinner, sometimes before we turn 30 or even 20. And not a few people
see "typically male" behaviour as a consequence of testosterone - e.g.,  a
higher degree of aggressiveness and competitiveness. So the name "androgen"
is no coincidence: it is derived from the Ancient Greek "aner", man, and
the root "-gen", to become, but also to make. If this weren't MadSci but a
tabloid headline, it would run: "An androgen is what makes a man a man."
 
The prototype substance of the androgens is, of course, testosterone. Any
substance, that mimics the effects mentioned above is called an androgen -
be it natural or artificial, chemically similar to testosterone or not. You
name one of them, DHT (Dihydrotestosterone), in your question. DHT is a
close cousin of testosterone, namely testosterone plus two hydrogen atoms.
DHT is produced once testosterone has entered a target cell and is the
chemical form in which testosterone becomes biologically active (in the
cell nucleus). A cell that does not possess the chemical machinery to
transform testosterone into DHT is unable to respond to testosterone. 

The two other hormones you refer to in your question, LH (luteinising
hormone) and FSH (follicle stimulating hormone) are not androgens. They are
secreted by the hypophyseal gland (which is attached to the underside of
the brain) and are regulators of both female and male sexual physiology.
They are very different from testosterone in their chemical structure: They
are proteins, while testosterone belongs to the large group of steroid
hormones. These include not only the male but of course also the female sex
hormones; then, the glucocorticoids (among these, cortisol is the most
prominent); and finally, the mineralocortcoids, which regulate our
salt-and-water metabolism. All steroid hormones are chemically derived from
the (in)famous substance cholesterol.

I leave aside here the function of LH and FSH in the regulation of the
female reproductive system (veeery complex), since you only asked me for
the effects in males. LH and FSH have two distinct effects in the testis: 

LH directly stimulates the production of testosterone in the testis,
specifically, in the so-called Leydig cells (named after the German
anatomist Franz von Leydig, 1821-1908). These cells are situated between
the "tubuli contorti", long, winding tubules inside the testis, where the
production and maturation of sperm cells takes place. Leydig cells produce
about 6 - 8 mg of testosterone a day in an adult healthy male. 

The primary target for testosterone in the testis is another population of
cells, the Sertoli cells (named after Enrico Sertoli, Italian Physiologist,
1842-1910). These cells sit inside the tubuli contorti and are like nurses
to the maturing sperm cells, feeding and nurturing them. To be able to do
their job, they need testosterone. But to be able to receive the
testosterone signal, they need to have Androgen binding protein (that's
where the ABP in your question comes into play) in their cytoplasm - and
stimulating the production of  t h i s  ABP is what FSH does to the Sertoli
cells. 

In other words, LH makes Leydig cells produce testosterone to stimulate the
Sertoli cells. FSH, in turn, gives the Sertoli cells the capability to
respond to this stimulus. 

I think this should answer your question - a lot of things I left out of
the discussion entirely: the fact that the adrenal glands can produce
testosterone, too; what exactly testosterone does inside the cell; how the
secretion of LH, FSH, and testosterone is finely regulated in the body;
what effects androgens can have in girls and women (yes, they produce it,
too); what there is to say about androgens as drugs (think of doping in
sports, for example, or all the "anti-aging" nonsense - this stuff can be
really dangerous!), what happens if the production of testosterone fails or
goes out of tune - and, and, and. 

But I hope you know enough now to delve into all these questions on your own.

Have fun exploring!

Sincerely yours, 
Jens Peter Bork

P. S. This answer does not contain any literature references, but you can
read about all of this in any good biology, physiology, biochemistry or
undergraduate Medical textbook. I used my old student textbooks (which are
in German, of course) myself! As for images, any Google image search for
"Leydig cells" and "Sertoli cells" will give you ample material.