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ISOTOPIC LABELLING OF THE MOUSE BLASTOCYST

In the implanting blastocyst of the mouse, the first trophoblast cells to penetrate the uterine epithelium contain large basophil inclusions. It has been suggested that these inclusions originate at an earlier stage within cells of the inner mass which move into the trophoblast just after disintegration of the zona pellucida (Wilson, 1963; Potts & Wilson, 1967). An attempt has been made to prove that these cells and their inclusions, 'primary invasive cells' (Wilson, 1963), are of embryonic origin and that they do penetrate the maternal tissues some time before general epithelial disintegration occurs.

The experimental procedure was as follows. Morulae and early blastocysts were collected at 72 to 76 hr post coitum from hybrid stock females. Eggs were cultured at 37·5° C in Krebs-bicarbonate-Ringer with added glucose and bovine serum albumin (both at 0·1%) and antibiotics. The same solution was used for all manipulations. Either 0·1 µc/ml of tritiated thymidine