Ultrastructure of mouse ectoplacental cone cells.

Ectoplacental cone of the Muridae is an excrescence formed by proliferation of the polar trophectoderm of the bastocyst. Four types of trophoblast cells are seen in mouse ectoplacental cone on day 8 of the pregnancy: (1) trophoblast-1 at the base of the cone are polyhedral, compactly arranged and contain large nucleoli, (2) trophoblast-2 in the middle of the cone enclose several heterolysosomes, erythro-and leucophagosomes, (3) trophoblast-3, also in the middle, have several membrane-bound osmiophilic granules, (4) trophoblast-4 at the periphery of the cone are oblong and enclose many pleomorphic bodies. These morphological differences appear to be correlated with functional adaptation.