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Spermatogenesis is pretty straightforward. The testes contain many germ cells, called spermatogonia (singular: spermatogonium). The spermatogonia are constantly undergoing regular mitosis, so that the number of them never decrease. Does that make sense? Let me explain a little more... every time a spermatogonium goes through meiosis it turns into 4 sperm cells. So, if the spermatogonia only underwent meiosis, we would soon run out of spermatogonia. When spermatogonia go through mitosis first, however, each spermatogonium turns into two of them... then only one spermatogonium proceeds through meiosis and we are still left with the same number of spermatogonia that we started with.
The testes contain an arrangement of the spermatogonia and their progeny within them into tubules. These tubules, the seminiferous tubules, run throughout each testis. They have a lumen running through their middle, into which the newly-born spermatozoa are released. All of the spermatogonia lie in the outer edges of the seminiferous tubules. As they proceed through meiosis, their daughter cells get pushed closer and closer to the lumen of the seminiferous tubule. So the various cells in the progression from spermatogonium to spermatozoa lie in obvious layers through the tubules.
You can see that the spermatogonium, or germ cell, is at the
bottom. As it progresses through the two meiotic divisions, it gives rise to four
future-spermatozoa, each containing half of the normal human genetic content. These
four cells are the ones indicated by the line from the cells with only "N"
written within them. When these four cells are first born, they do not look like
spermatozoa yet. It takes a little bit of time for them to change from round cells
to the flagellated cells we think of as spermatozoa. The lumina of the seminiferous tubules are continuous with the rete testes, efferent ductules, and vas deferens lumina. You will see more about these structures in the semen web page. But, basically, once a spermatozoan makes it into the lumen of a seminiferous tubule, it is has a continuous path with the urethra available to it. It cannot traverse this pathway, however, until it matures and is specifically released. Therefore, it needs a storage and maturation site, and that site is within the epididymus. The maturation of the spermatozoa is explained in the semen web page. Males undergo spermatogenesis continually. They need to because millions of spermatozoa leave the body in every ejaculation. Your book notes that 120 million spermatozoa are in every single mL of semen, and between 2 to 6 mL of semen are released in each ejaculation. So, approximately 500 million spermatozoa (or, a half a billion) are released in each ejaculation. The only way to have so many spermatozoa available for semen is to continually make them and store them as they are made until the next ejaculation. |
© 2011 STCC Foundation Press |
That said, let's look at meiosis
in the testes and the actual production of spermatozoa.
Your book shows you those layers
in Figure 22.5 on page 840. I have copied a part of that figure here, and included
next to it the picture of cells going through meiosis. I rotated my drawing of cells
in meiosis to match the orientation of the developing spermatozoa in Figure 22.5.
Please note that the lumen of the seminiferous tubule is at the top of this drawing, while
the outer edge of the tubule is at the bottom.
Your book has a figure, Figure 22.6, that shows
the differentiation of the pre-sperm cell into the spermatozoan. Basically, the cell
has to organize its nucleus and enzymes into a head region, put tons of
mitochondria (for making ATP) into its midpiece, and develop a long flagellum
(your book calls it a "tail"). Once that is done, the spermatozoa (or
sperm cell) can wriggle its way free of the seminiferous tubule wall, and enter the lumen.