This tissue differentiates into excretory tubules, first the anterior part, then the middle, and finally the most posterior. Embryos need to filter wastes out of their blood, beginning very early. They don't need to walk, or think, but they do need to excrete. So heart and kidneys develop much earlier than brain or limbs.

Notice the contrast between heart and kidney in respect to how they solve this problem of being needed very early. Embryos form a succession of different kidneys, and destroy the old ones. They make the first kidney out of the most anterior parts of the intermediate mesoderm; and they make the final, adult kidney out of the most posterior part of the intermediate mesoderm. They use different organs in the embryo than in the adult.

The heart, in contrast uses the same organ, and keeps improving it, making it more and more complicated as well as bigger. The poor heart keeps pumping blood all this time. For an analogy, imagine starting out with a car whose motor was like of a lawn-mower (one cylinder, 2-cycle, etc.), and gradually rebuilding this motor to convert it into a regular 6-cylinder, 4-cycle car engine. and doing all this rebuilding while the car was driving down the road propelled by this same engine! That's what happens with the heart!

In contrast, we first make a very simple pair of kidneys (like a lawn-mower engine), called the pronephros and use them to filter the blood of the early embryo. Then we build a better pair of kidneys, mesonephros, and bring them on line during most of embryonic development. In mammals, birds and reptiles, we then build a third, even better pair of kidneys metanephros, which we use after birth, and for the rest of our lives. The first 2 pairs of kidneys are then destroyed.

Special ducts (=pipes) carry urine from these different kidneys. The pronephric ducts "grow" rearward from the two pronephros kidneys, mostly by cell locomotion, analogous to the formation of capillaries, and sort of like a multicellular equivalent of axon formation, except that the structure being formed is multicellular and becomes hollow. Experiments support the idea it is guided by an adhesion gradient. It connects to the cloaca (rearmost part of the digestive tract). In mammals, the cloaca later becomes split into the rectum (dorsal) and the bladder (ventral), and of course the kidney ducts connect to the latter. For historical reasons, "Wolffian duct" is another name for the pronephric duct.

The second embryonic kidneys (mesonephros) connect to these two pronephric ducts. In males, the seminferous tubules of the testis become connected to some of these mesonephric tubules, and sperm will eventually exit the body via these tubules and via the pronephric duct, which will then be called the vas deferens. In other words, the sperm ducts were embryonic kidney ducts. A different pair of ducts, called the ureters, develop to connect the mesonephros to the bladder. In Amphibians, the sperm still go down the urine duct, since their adult kidneys are equivalent to our mesonephros, and they don't form ureters or metanephric kidneys (but some of them jump better than we can).

In female mammals, etc., the pronephric duct degenerates. The female sex ducts (oviducts, Fallopian tubules and ureter) develop from entirely separate tissue of the lateral plate. They are called "Mullerian ducts". In males, these start development, but then degenerate. Notice how the sex ducts of both sexes begin development in the other sex, but then degenerate. The male sex ducts are NOT homologous to the female sex ducts.

The gonads themselves are formed from the same tissue in both sexes: Testis (testes plural) in males and ovaries in females. They are both formed from thickenings in the wall of the coelomic cavity, which makes then lateral plate mesoderm. These thickenings are called genital ridges.

The actual future sperm and egg cells, however, do NOT develop from cells of the genital ridge. Sperm and egg cells develop from special cells called "primordial germ cells", that migrate to the genital ridges from some other part of the body (like Aztecs migrating to Tenochtitlan, or something!),
and as if that weren't strange enough, the places from which the primordial germ cells start out differs widely between different kinds of animals. In mammals, they originate in the yolk sac, in birds from a crescent shaped region of the zona pellucida in front of where the body forms, in flies and nematodes from special cells at the extreme posterior end. There may or may not be any kinds of animals in which the future egg and sperm cells start out located in the same place as the gonads; and please tell me if you hear of any. If you kill or remove the primordial germ cells from an embryo, or keep them from getting to the genital ridges, then that animal will be sterile. Grafts of chicken primordial germ cells from one egg to another will result in animals that produce eggs or sperm that have different genes than the parent.

One of the world leaders of research on primordial germ cells is out at the NIEHS in the Res. Triangle Park, and one year a student in this course did research with him, and we had PGCs crawling around in dishes. We were trying to prove whether or not they were guided by chemotaxis, but didn't succeed.