Primordial Germ Cells – Genital System Development

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the development of the genital system and some common abnormalities. The pattern we?re going to follow is we?ll look, first of all, at the migration and significance of the primordial germ cells. We will then look at the development and shifts in position, the migration, as it were, of the ovaries and testis. We?ll look at the development of the external genitalia in males and females.

    00:24 And then we?ll have a look at some of the most common abnormalities that we can get in this developing system in humans. So let?s start off, first of all, with the origin of the primordial germ cells, the PGCs. Now, this actually starts out on the yolk sac.

    00:39 As we can see in the embryo on the left, amniotic cavity is at the top, and the yolk sac is at the bottom and it?s out in the tissue of this part of the yolk sac that the primordial germ cells originate. They then have to migrate quite a long distance into the body over the gut and into the developing gonad, which is part of the urogenital ridge. This is also covered in the lecture on the development of the kidneys. They invade the gonadal ridge by about six weeks after fertilization. If it was to happen that for some reason they did not arrive, then the gonadal ridges themselves would not develop normally. So they?re essential to an interaction between the gonads and the primordial germ cells. If a primordial germ cell becomes lost or strayed during the course of this development and it ends up in the wrong position, its development may well be abnormal. In particular, it may form a kind of tumor called a teratoma. This contains a variety of differentiated cell types, including for instance, blood, muscle, bone, and even teeth on some occasions. How did it get to the developing site of the gonad? Well, the answer is that there are developmental cues that they can follow. It may well be that the structure of the gut and its surrounding mesoderm is arranged to provide a track along which they can follow. There may also be chemical signals which are arising from the gonad, and attracting the primordial germ cells towards them.

    02:12 Of course, we must wonder why they begin in this distant site compared to most cells in the body. And the answer may be to protect them from the developmental signals that we?ve looked at in a number of other lectures, which could influence what cells turn into the primordial germ cells by contrast with other cells, they have to remain completely undifferentiated so they can give rise to the next generation. The mesothelium, now, that?s the thickening outer mesodermal wall of the developing gonad. So it?s not a true epithelium. The mesothelium will begin to invade into the underlying mesoderm. And this is known as the primary sex cords or primitive sex cords. Alongside the mesonephric ducts, which we explored during the development of the kidney, we also have a new set of ducts called the paramesonephric ducts, and these run parallel. In the early stages, these are present in both male and female embryos.

    About the Lecture

    The lecture Primordial Germ Cells – Genital System Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Primordial Germ Cells – Genital System Development

     John McLachlan, PhD

    John McLachlan, PhD

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