Molecular differentiation in the preimplantation mouse embryo

Abstract

THE primary observable event of differentiation in the mammalian embryo is the formation of the trophectoderm and the inner cell mass (ICM). Differentiation first becomes evident in the mouse embryo during early blastulation and is clear and apparently irreversible by the 60-cell blastocyst stage. The 45 or so trophectoderm cells surround a blasto-coelic cavity containing the ICM, they pump fluid, are phagocytic, are interconnected through junctional complexes and can elicit an implantation response in the uterus¹. The trophectoderm gives rise to the ectoplacental trophoblast, primary and secondary trophoblastic giant cells and probably also to extra-embryonic ectoderm^1,2. The cells of the ICM are less evidently differentiated, being rounded, sticky cells somewhat like cleavage stage blasto-meres, but having lost the capacity to form trophoblast^3,4. The ICM gives rise to embryonic ectoderm, and to both extra-embryonic and embryonic endoderm^1,2,5. Although the fully differentiated cells of the blastocyst are distinguishable by structural and functional criteria, adequate markers of differentiation are not available for the morula stage, when the determinative events leading to differentiation are presumed to occur. Thus, attempts to detect blastomeres with a commitment to one or the other differentiated cell type have relied for an index of commitment on the subsequent behaviour of genetically or physically marked blastomeres combined in various spatial relations with other distinguishable blastomeres. This experimental approach is open to criticism. Frequently, many cell divisions must elapse before suitable genetic markers can be detected with sufficient sensitivity, and the procedures involve artificial manipulation and therefore possibly modification of blastomeres to test their potency^2,6. The detection of a cell marker which is produced in situ in the earliest stages of differentiation, and one which is detectable by highly sensitive techniques would circumvent both of these problems. We describe here the detection of tissue specific markers which may prove to fulfil these criteria.