The determination of trophoblast and embryoblast cells during cleavage in the mammal: New trends in the interpretation of the mechanisms

Cleavage and blastocyst formation in the mammal are characterized by precocious differentiation of the trophoblast, a population of cells which are specialized for functions related to the formation of an intimate physiological interrelationship between embryo and mother, as characteristic for these secondarily alecithal eggs: 1. transport of fluids, nutrients and metabolites; 2. hormone production ; 3. formation of a cellular contact at implantation. Early trophoblast cells are in fact the first cells which unequivocally exhibit signs of differentiation (morphological, cell physiological and biochemical), while this is not so clear for embryoblast cells (in spite of the fact that embryoblast-, or "ixmer cell mass"-, specific peptides have been described, see VAl'<BLERKOlVI: et al. 1976). This precocious process of differentiation of trophoblast cells attracts the interest for various reasons as discussed earlier (DENKER1976). Of particular interest are peculiarities found in the molecular genetics of trophoblast: e.g. maternal genes seem to be expressed preferentially (paternal X chromosome inactivated preferentially, see TAKAGIand SASAKI1975) in normal trophoblast, while hydatidiform moles appear to be exclusively of androgenetic origin (YA"NIASHITA et al. 1979). Thus it may be expected that the process of determination of trophoblast may perhaps also show peculiar features (DENKER1976). For the mechanism behind this early process oi determination, two theories have been proposed which are contradictory (Fig. I):