"Cells that give rise to the gametes in both male and female mammals (including humans) originate within the primary ectoderm of the embryo during the second week of development.. They then detach from the ectoderm and migrate by ameboid movement into an extraembryonic structure called the yolk sac. At first, they may be distinguished within a mass of extraembryonic mesoderm at the caudal end of the embryo, and then within the endoderm of the yolk sac wall. These cells are called the primordial germ cells, and their lineage constitutes the germ line. Primordial germ cells can easily be recognized during their migration because of their distinctive pale cytoplasm and ovoid shape and because they specifically stain intensely with reagents that localize the enzyme alkaline phosphatase."
Thus begins Chapter One of Human Embryology by William J Larsen. Several associations immediately spring forward in my mind. Number one is, wow, ectoderm. Ectoderm makes the nervous system and a lot of the skin. And it makes the germ cells, at least something known as "primary" ectoderm does.. maybe there is more than just one ectoderm... I imagine if there is more than one sort of ectoderm I'll find out as I go along..
Second thought is, hmmnn, they are amoeba-like. How curious. I recall reading about macrophages and neutrophils, fibroblasts too, and how they use the same locomotive means, streaming into their little pseudopods and hauling themselves through the body, looking for bad guys and garbage to eat.. There is something about that way of a single cell moving deliberately that definitely has always grabbed my imagination. I recall an account I read somewhere while learning about integrins, how live skin cells separated in a blender and strewn over the bottom of a petri dish, would begin to crawl around, feeling for others.. when they found each other, would reunite and become tissue again.
Third thought: the germ cells form, then they have the sense to get the heck out of the construction zone until the coast is clear. Very wise.
The text continues:
"Between 4 and 6 weeks, the primordial germ cells migrate by ameboid movement from the yolk sac to the wall of the gut tube and from the gut tube via the mesentery to the dorsal body wall. In the dorsal body wall, these cells come to rest on either side of the midline in the loose mesenchymal tissue just deep to the membranous lining of the coelimic cavity. Most of the primordial germ cells populate the region of the body wall adjacent to the 10th thoracic vertebral level that will form the gonads. The primordial germ cells continue to multiply by mitosis during their migration. A few cells may become stranded along the route of migration or at inappropriate sites in the dorsal body wall. Occasionally, stray germ cells of this type may give rise to a type of tumor called a tetroma."So they wander back to the body, using established supply routes. Good enough route I suppose, in that just about everything inside the body (i.e., organ system) hangs off the inside of the thoracic wall eventually..
They amoeba their way back in, and know where to go. They increase their numbers during their long journey. Some get lost along the way. A few of those may turn into troublemakers a long way into the future.
The next bit of text is titled:
"The Germ Cells Induce the Formation of the Gonads in the Dorsal Body Wall".So, they arrive on the dorsal body wall in front of T10, somehow see it as the end of the trail, and start to set up a permanent settlement. They find tissue there that is willing to be morphed into germ cell condos, or 'gonads'. Later, the gonads themselves slide down through the body and end up where they're supposed to, male or female or in-betweener.
Take-away point from the little story: Ectoderm makes nervous system, skin, and germ cells. This is an important point.