Neurotopia

Sci recently did a fantastic series on female reproduction, and she's asked me to guest-post a complementary series on male reproduction. I cannot tell you how excited I am to do this. I feel very famous posting on Neurotopia, so thanks to Sci and Evil and nLTP for inviting me.

This first installment is basic anatomy so let's start with a picture. I really really wanted to use these interactive digital renditions, but could not afford to shell out $250.00 for the licensing. You should really check them out though.

Because we're doing cock-and-balls at bargain basement prices, here's what you get on-blog:

Figure 1. Exactly what you thought it would be.

Since the reproductive system exists for the purpose of reproducing let's take the tour in order that a sperm will travel to make his or her (yes, her) way toward the ultimate goal of fertilization.

The sperm starts in the male gonad, otherwise known as the testis, and various other things more suited for the locker room than this esteemed blog. The testes are external in most mammals, or at least spend part of their time outside the body cavity. In humans the testes are located in the scrotum which hangs outside the body. (Weirdly, the wikipedia entry for "scrotum" notes that this particular anatomical feature is also known as the "cod". While I can think of several other euphemisms for the scrotum, including "St. Gregory's sack", "cod" didn't immediately spring to mind, which makes one wonder if the author of the article was a Blackadder fan.)

While it may seem unwise on evolution's part to hang such an important and delicate organ out there flapping in the breeze there's actually a very good reason for it: sperm are very sensitive to temperature and body temp is high enough to kill a lot of them off. Cryptorchidism, a congenital condition in which the testes are retained within the body cavity, is also associated with a higher incidence of testicular cancer. The testes can be held closer to the body when ambient temperatures get a bit too chilly by contraction of the cremaster muscle. This is also the case when experiencing fear, or during ejaculation. Some other animals can fully retract their testes through the inguinal canal into the body cavity for safe-keeping short periods when they're feeling threatened.

The testis is a pretty awesome organ, comprised mainly of a bunch of coiled loops called seminiferous tubules.
Figure 2: Anatomy of the testis.

The seminiferous tubules contain germ cells, which give rise to sperm, and Sertoli cells, which act as the nurse cells to provide nutrients to the developing sperm as they shed their extraneous cytosolic material in preparation for the big race. Outside of the seminferous tubules is the interstitial space, comprised of hormone-producing Leydig cells, some vasculature, and the connective tissue. When you section through the testis, you can see a remarkable organized structure within each tubule.

Figure 3: Cross section through testis.

The circular structure in the center is the seminiferous tubule. Within you will notice several concentric rings of different cell types. We'll talk more about this in the spermatogenesis post coming up.

Basically, the structure looks like this:

Figure 4: Pseudo-colored scanning electron micrograph of seminiferous tubule.

Purple cells are various stages of spermatocytes (cells that will give rise to sperm). They are surrounded by Sertoli cells in green, and the yellow cells at the center of the tube are spermatozoa that are ready to be on their way to the next organ.

Spermatozoa travel through the lumen of the seminiferous tubules to the rete testis which is basically a big collecting duct that all the seminiferous tubules empty into. The rete testis in turn empties into a series of efferent ducts that transport the sperm out of the testis and into the epididymis.

The epididymides (that's plural for epididymis) are segmented coiled tubes (there are a LOT of coiled tubes in the male reproductive tract) that form a C shape around the testes. The part that connects with the efferent ducts is the caput or head of the epididymis, the next region is the corpus or body, and the last bit is the cauda or tail. Different epididymal segments are responsible for different phases of sperm maturation, so you can think of the sperm acquiring new skills or powers as they pass through the epididymis. We'll get into specifics in a later post. Then they are stored until they are needed in the cauda.

Figure 5: The epididymis.

Figure 6: Section through epididymis, showing caput (head), corpus (body), and cauda (tail). The testis is not shown in this image.

From the epididymis, sperm travel through ANOTHER TUBE -- the vas deferens. Nothing much exciting happens here like it does in the seminiferous tubules or the epididymis as far as we know - this one's just for transport. But finally, after going through all those other tubes, the sperm leave the scrotum -- huzzah!

Why is this a big deal? Well, remember that the sperm maturing as they pass through the epididymis, and then when they reach the cauda, they sort of hang about waiting for something exciting to happen. And what sort of happening might be exciting to a sperm? You guessed it - ejaculation!! Woo hoo! More about this in a subsequent post. The vasa deferens are incidentally the tubes that are snipped and tied off during a vasectomy.

Via the vas deferens, the sperm travel out of the scrotum, over the pubic bone, take a somewhat circuitous route hooking over the nephric ducts which transport urine from the kidneys, and meet up with seminal vesicle behind the bladder.

Figure 7: Vas deferens and seminal vesicle.

The seminal vesicle is where the body makes - you guessed it - semen! Or at least the majority of it. Somewhere between 60-75% of the total volume of semen in a given ejaculate comes from the seminal vesicle; the rest comes from the prostate and bulbourethral gland, with sperm making up a tiny percentage of the final volume. Semen serves several purposes. It provides a major energy source to the sperm in the form of fructose (most of this is made in the seminal vesicles). Remember, sperm have to travel light in order to be the first to meet the egg, so they carry very little energy stores with them.

Just the below the seminal vesicle the vas deferens meets up with the urethra. It is surrounded by the prostate gland, which secretes still more goodies into the semen. The prostate contributes more sugars, as well as proteolytic enzymes, and it is alkaline, which will help balance the acidic vaginal environment upon copulation.

Just below the prostate is the teeny-tiny bulbourethral gland, whose secretions comprise the majority of pre-ejaculate fluids and function largely in lubricating the urethra.

The urethra is the final passageway out of the body. It is the same tube for expelling urine, and it runs from the bladder, joins with the vas deferens just below all those glands then out through the penis.

That's all for now. Don't worry, we'll talk a lot more about the penis in the up-coming ejaculation post.