A light- and electron-microscopic study of primordial germ cells in the early mouse embryo

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Abstract
The primordial germ cells (PGC’s) of the early mouse embryo have been identified in semi-thin and ultra-thin plastic sections on the combined bases of location and a distinctive set of morphological features. These cells originate extra-gonadally and their path of migration during the stages investigated agrees with results of previous histological and experimental studies. At 8–9 days of development, PGC’s are observed among the endoderm cells of the mid- and hindgut and the yolk stalk; at 10–11 days, PGC’s are seen in the dorsal mesentery, the dorsal coelomic lining, and in the rudimentary genital ridge; by 13 days, the gonad is abundantly populated with germ cells. During the migratory period the PGC’s appear as small (10–12 pm diameter), oval, basophilic cells which have a large nucleo-cytoplasmic ratio. These cells possess numerous free ribosomes and polysomes, a filamentous ground cytoplasm, and few profiles of endoplasmic reticulum. Mitochondria display small oval and thread-like profiles. A Golgi complex is not prominent in the PGC’s. The densely granular and fibrillar nucleus is often oval in outline but sometimes shows irregular contours. A large nucleolus is present. Although located among groups of cells that exhibit extensive cell junctions, the PGC’s have not been found to share such junctions with neighboring cells. Furthermore, the PGC’s possess small cytoplasmic processes that contain numerous microfilaments. These observations are interpreted as morphological correlates to the migratory activity of these cells.In the early PGC’s located in the gut, regions of the endoplasmic reticulum where membranes are closely apposed, perhaps fused, often present the appearance of annulate cisternae. In addition, compact aggregates of granulofibrillar material are found in the cytoplasm of these same cells. Neither of these structures is detected in the mesenteric PGC’s, the gonadal germ cells, or any other cell of the embryo at the stages studied. It is suggested that these two cytoplasmic elements may be related to further differentiation of the germ cells.The germ cells of the developing gonad are large, oval cells which possess a nucleus of very round contour. Both the nucleus and the cytoplasm retain a densely granular appearance. The expanded cytoplasm contains a large Golgi complex but still few profiles of endoplasmic reticulum. Free ribosomes and polysomes are abundant. The germ cells share extensive gap junctions with one another and with adjacent stromal cells.