Hello and welcome to Guest Post TWO! And Sci is so glad that Ambivalent Academic is covering meiosis, because I sure as heck didn't want to do it. 🙂
Male Reproduction Part II - Spermatogenesis
Last we left off we took a basic tour through the male reproductive tract in the way in which a sperm will encounter it. So now let's talk sperm!
How are they made? How do they make their way through all these convoluted tubes? How do they finally achieve all their spermy aspirations?
Sit down and buckle up - I'm about to tell you.
The beginning of a sperm is just the same as the beginning of an egg: as a diploid primordial germ cell (PGC) that is set apart shortly after a new zygote is formed. Our sperm and eggs (or rather the cells that will give rise to them) are specified while we're still in the womb, and long before the embryo even has a body axis. Pretty cool. I'm working up a cool post on embryonic sex determination in which we will talk more about this.
Eventually they end up in the gonad and they proliferate some and fill up the seminiferous tubules and mostly don't do much in the way of differentiation until puberty.
Remember the seminiferous tubule? Remember the concentric rings of different cell types found within? Those rings of cell types represent stages of spermatogenesis, or making of the sperm. The least differentiated cells appear in the outermost ring, while the most differentiated cells, or rather the tails of the spermatozoa are in the very center of the seminiferous tubule, with the intermediately differentiated cells arranged in a nice organized progression in between. So neat and tidy!
Figure 1: Seminiferous tubules with concentric rings of differentiated germ cells.
Interlaced throughout the seminferous tubule (excepting the empty lumen in the middle) is the cytoplasm of the Sertoli cells. Sertoli cells are like big sperm nurseries. Germ cells are actually kind of needy weeny little cells that need a whole lot of pampering as they grow up into sperm. The Sertoli cells form tight junctions with one another, making the seminiferous tubule an immune-privileged environment and protecting the germ cells from the marauding immune system. The Sertoli cells also produce some stuff that helps concentrate androgens like testosterone near the developing germ cells so they can differentiate properly. The Sertoli cells also feed the germ cells and when they finally differentiate into swimmers, the Sertoli cells clean up the mess of all the extra cellular debris that the sperm discard. Basically, the germ cells spend most of their time sucking up resources, making big messes, and requiring molly-coddling, while the Sertoli cells feed them and change their diapers. Leydig cells, living it up in the interstitium, send the occasional child support check in currency of hormones but otherwise have little to do with raising the brats. It's one big dysfunctional family down here in the testis.
Figure 2: Sertoli cell with developing germ cells. The outside of the seminiferous tubule is oriented at the bottom of the image and the lumen is at the top.
So what do those germ cells get up to?
In the outermost ring of the seminiferous tubule you will find spermatogonia. You can think of these guys a sperm-producing stem cells. They have one job and one job only: to make the legions of cells that will eventually become sperm.
Actually it's not quite that simple, because they also have to make more copies of themselves so that the stem cell population is never used up. Because, dudes, you guys go through A LOT of sperm, the vast majority of which never actually fertilize anything, and you keep making MORE! Good things sperm is cheap! So it's really important to have stem cells, which make more of themselves before making sperm, so that you never run out.
How do they do it? Like most stem cells, by asymmetric divisions. When they divide, one daughter cell is is also a spermatogonial stem cell, just like the original (type A spermatogonium), and the other daughter cell will proceed towards the eventuality of making sperm (it is now a type B spermatogonium).
It should be pointed out that this asymmetric division of the spermatogonium is a mitotic division, just like the division of any other somatic cell in the body. The cell starts with 46 chromosomes (23 pairs in humans, one of each pair from mom and one from dad), replicates all these chromosomes, does not reshuffle any of them, and redistributes them at division into two daughter cells with identical chromosomal makeup. Just like any other cell division in the body.
I'm stressing this point because in just a minute we're going about a different kind of cell division that is unique to germ cells (sperm and egg). Meiosis is the type of cell division that reduces the number of chromosomes to haploid (23 total chromatids instead of 23 pairs of chromosomes) so that the germ cells (like a sperm) can recombine with other germ cells (like an egg) at fertilization resulting in a diploid zygote 46 total chromosomes or 23 pairs.
But we're not quite there yet. We're still on the type B spermatogonia, who will then spend some time growing up and differentiating into a primary spermatocyte (this is the concentric ring of cells that lies just inside the spermatogonia). Spermatocytes are the ones that undergo meiosis.
Figure 3: Meiosis.
Just before the cell divides, it lets those chromosome do something pretty unique. It's called "crossing over". This is when all those chromosomes are tangled up with each other, and some of them do indeed cross over one another. Not only that but they actually swap DNA!! Whoa! What effect does this have? Well, you've effectively shuffled the cards in the genetic deck. Now your chromosome that came from mom originally carries some of the information that was originally on dad's chromosome, and vice versa. This is a really really important factor in sexual reproduction. It contributes to the expression of new phenotypes for natural selection to act on.
OK, so the primary spermatocyte has shuffled the deck and its daughter cells, the secondary spermatocytes, now contain one chromosome from each original pair, but those chromosomes have been reshuffled so that each chromosome contains new combinations of genetic information from both mom and dad. The first meiotic division is complete...but we're not done yet.
The secondary spermatocytes undergo the second meiotic division which will separate each chromosome into the two individual sister chromatids, held together up to this point by a centromere. The cells that result from this division contain only a single copy of any given bit of genetic information, and they are called spermatids.
One other interesting feature of meiotic divisions is that the nuclei divide completely, but the cells actually remain connected by cytoplasmic bridges. They remain in this syncitial arrangement, sharing their juice boxes and bunk beds with all their siblings, until they become spermatozoa.
Figure 4: Spermatogenesis
But we're still. not. done. What we've got here are a bunch of round haploid nuclei, sharing its cytoplasm with a bunch of other nuclei. They're not the sleek little swimmers that primed to find an egg.
The differentiation of these secondary haploid secondary spermatocytes into spermatozoa (these guys have tails and pointy heads and they look like they're ready to swim!) is a process called spermiogenesis.
No more DNA replication or cell divisions or complicated reshuffling of genetic material. The process of spermiogenesis involves jettisoning any superfluous baggage which gets gobbled up by the Sertoli cells. The cells also develop a tail and repackage their DNA, replacing big bulky histone spools with fast-and-light protamines. The tiny volume of the remaining cytoplasm is comprised mostly of mitochondria for generating swimming energy, and the acrosome which will release proteolytic enzymes if that sperm is lucky enough to find an egg. When all these changes are complete we have a spermatozoan. The connection with its syncitial siblings and the Sertoli cell is dissolved, and the spermatozoan is released to lumen of the seminiferous tubule.
Figure 5: Spermiogenesis
Phew! This is getting really long and I think I've bombarded you with a lot of information already...and we haven't even gotten these sperm out of the testis yet.
Stay tuned for Sperm Maturation, Ejaculation, Fertilization, and if you're very very lucky, Sex Determination of the Embryo.