What you all were waiting for has finally come! Yeah.
Sperm maturation and ejaculation
OK!! So the last time I was over here at Neurotopia we were talking about sperm in their infancy. We discussed spermatogenesis -- how we got from a diploid spermatogonial stem cell to a haploid spermatid -- and then spermiogenesis -- the process by which the spermatids acquire the features of spermatozoa: tails, acrosomes and the like.
We stopped there when the sperm had just graduated from diapers to their big kid undies, and we'll pick up where we left off.
As you may recall, the spermatozoa last we checked were still in the lumen of the seminiferous tubules. They've jettisoned excess cytosol and are ready move out of the testis.
Except...they can't swim!!! That's right: sperm in the testis are infertile. Sperm immotility is the one of the leading causes of male infertility, but worry not!! The sperm aren't supposed to be motile in the testis anyway. They acquire those superpowers in the epididymis and that's where they're headed next.
So how do they get there if they can't swim? The sperm are moved out of the seminiferous tubules by peristaltic contractions of a smooth muscle layer that lies just outside the Sertoli cells. From the seminiferous tubules, the sperm move through the rete testis and into the epididymis.
THIS IS WHERE THE MAGIC HAPPENS!
Figure 1: Histological sections of testis and epididymis. A) Seminiferous tubule (this should be looking familiar by now), B) Epididymal caput (the lumen looks pretty empty here but it's really just the the sperm are so diffuse), C) Epididymal corpus (sperm are the little purple spots with tails), D) Epididymal cauda (look at all those sperm!!).
The sperm first enter the caput or head of the epididymis. This region has a relatively narrow lumen and lots of cilia to keep the sperm movin' on through. The caput is subdivided into several segments, each of which has a different job to do as far as getting those sperm to grow up into swimmers. You can think of the caput as a sort of sperm assembly line. Though the sperm actually has all the necessary parts, each segment of the caput is in effect booting up a new functional program to get the sperm ready to go.
The sperm are relatively diffuse in the caput. In the next region, the body, or corpus, of the epididymis, the sperm are concentrated by removal of excess fluids.
Finally, they move into the cauda, or tail, which is a big sperm waiting room. By the time they get here they have traveled through about 6-7 meters of male reproductive tract in a journey (most of which is not under their own power) that takes roughly 20 days from the first division of the spermatogonial stem cell until they arrive in the cauda. Here in the cauda they hang about and twiddle their thumbs until they get the call.
Ready??? On to ejaculation!! It pretty much goes like this:
Well, OK, not exactly like that.
So how does it work really? Well, there are really two things that need to happen from the male's end of things in order for ejaculation to have a chance of getting those sperm in a position to find an egg. First, you need an erection. Second, you need to expel sperm into a receptive female reproductive.
So first, ERECTION. The reasons for this are sort of obvious I think, but the mechanisms vary widely between species. You've got this organ that needs to be long and stiff enough to deposit your sperm inside a female, but when you're not using it for that, you need to be able to tuck it away somewhere so it's not flapping about and banging into things and getting injured. Some animals solve this problem by having a penile bone (or cartilage) which keeps it hard all the time, but they can retract it into the body cavity or at least a more protected area of the body to keep it out of harm's way while they're not using it. Both dogs and bulls provide good examples of this strategy. (Speaking of dogs and bulls...any of you give your dogs "bully sticks" as a treat? Yeah, those are the cartilage from bull penises. They come in 6 and 12 inch lengths, though both are truncated from the total length of the penis "bone". Fancy that!)
Other species lack a penis bone (no, a "boner" actually has nothing to do with bones) and instead keep a small soft organ in a still relatively vulnerable area, but they can "inflate" it when needed. Both humans and ducks provide good examples of this strategy. (Seriously, check out those links - duck penises are six kinds of weird and wonderful.)
Aaaanyway, where were we? Oh yes, erections. In humans, erections are a result of "inflating" the penis with blood. The penis contains some specified tissue that acts as a reservoir for blood, which then becomes trapped in the reservoir while more keeps pouring in. The result: flaccid to turgid, just like blowing up a balloon. (Turgid has to be one of my all-time favorite words. Second only to "tumescent".)
So how do you get it to inflate when you need to?
Well, the biochemical mechanisms that kick off this process are poorly understood at this point, but it's safe to say that there is significant sensory input required, and that a whole slew of molecules are known to be downstream effectors of arousal, including epinephrine, norepinephrine, acetylcholine, prostaglandins and nitric oxide, and probably a bunch more. These are really not my area of expertise so I'm going to leave them at that.
The physiological result of this biochemical concoction (heh. con-cock-tion.) is that blood flow into the corpora cavernosa, and corpus spongiosum, two tissues (actually three since the corpora cavernosa are paired) that contain quite a lot of empty space in between a network of connective tissue.
Figure 2: Cross-section through the penis showing corpora cavernosa and corpus spongiosum.
During arousal dilation of the arteries that feed into these areas leads to a net increase in total blood volume, while a concurrent restriction of the veins reduces outflow. As the corpora become engorged, outflow is further restricted by pressure from the expanding corpora on the veins.
Meanwhile, the testicles are drawn closer to the body via contraction of the cremaster muscle (heh. cream-master. I like to imagine that some anatomy department submitted names for all these parts and voted on the ones that made it into text books.).
Now, it is possible to deposit sperm in an environment permissible to fertilization. *Ahem.* I think you all know how the next bit goes, so I'll not narrate the next few minutes. Use your imagination. It's more fun that way. I'll wait.
The urinary sphincter contracts to prevent mixing of urine with the semen (because this would kill sperm) and to prevent retrodgrade ejaculation, in which there is backflow of semen into the bladder (this probably wouldn't be good for you either). Sperm arrive via the vas deferens and are mixed with secretions from the seminal vesicles and prostate gland. The bulbourethral gland secretes pre-ejaculate fluid to lubricate the urethra and neutralize pH.
At this point, proceeding towards ejaculation is still, at least nominally, a voluntary response, meaning that it is under conscious control. Having no first-hand experience with this process, but doing my damnedest to bring you information on this most interesting of processes, I have conducted a very unscientific survey regarding the relative control men are able to exert over delaying the process at this stage and results indicate that...your mileage may vary.
So...on to ejaculation. Ejaculation is a parasympathetic response (ergo not under conscious control, no matter how hard you guys try to focus on naming the baseball hall of famers or working through your thriteen times tables), in which serotonin appears to play a major role. I don't do neurotransmitters and this more up Sci's alley so maybe she can fill you in on what serotonin's doing to initiate this parasympathetic response. I'll tell you about the outcomes. (heh. out-comes. jeez, I am killing myself here.)
Semen is ejected via rhythmic contractions of the smooth muscle surrounding the urethra. Once the smooth muscle contractions begin ejaculation appears to proceed entirely involuntarily.
These smooth muscle contractions are strongly correlated to orgasm in most men. However, ejaculation can occur without orgasm (think wet dreams, which are entirely normal), and orgasm can occur without ejaculation (anejaculation), though this is usually indicative of some physiological perturbation.
Upon expelling the semen and completion of orgasm, the smooth muscles surrounding both the urethra and the blood vessels in the corpora relax. Dilation of vessels allow for outflow of the blood that has accumulated in the penis, and it once again becomes flaccid. This is a good thing. Remember you don't want to go around bumping into things and injuring it.
After ejaculation, most men experience a refractory period before they can begin the process of arousal, erection and ejaculation again. This is presumably a refractory period on erection rather than mustering the troops to produce more semen. For further info on secondary ejaculates and sperm quality in other mammals see Sci's most recent Friday Weird Science post.
Speaking of refractory periods, I'm going to need one before the next post on sperm capacitation. You thought I was done didn't you? (heh) Oh, not by a long shot. While the guys may be feeling pretty wiped out after all this, those sperm still have a lot of work to do. Their journey is far from over. The final stages of getting those sperm on their way occurs inside the female reproductive tract and we'll talk about that next time.
Acknowledgments: The author would like to thank B.H. for important input on this post.