The first cell-lineage specification and restriction in mammalian development occurs in preimplantation embryos, when blastocysts form from compacted morulae. They consist of an outer cell population, the trophectoderm, from which the trophoblast compartment of the extra-embryonic tissues develops and an inner cell population, the inner cell mass or embryoblast, from which all tissues of the em...

Although mosaicism was shown to be a normal feature in cleaving embryos, its consequences for the late preimplantation stages are unknown. We performed blastocyst immunosurgery, followed by fluorescent in-situ hybridization (FISH), to determine the number of cells and degree of mosaicism in the inner cell mass (ICM) of human blastocysts. Of 47 ICM samples analysed, 20 had aneuploid cells, and t...

Rabbit inner cell masses (ICMs) have been prepared by immunosurgery from 96-h-old blastocysts. Their properties are described. It is shown that ICMs prepared this way retain their developmental potentialities. In particular if they are injected into host blastocysts, they appear to be able to participate in the development of the composite blastocysts thus obtained: upon implantation into a fos...

The development of the bank vole Clethrionomys glareolus is described from implantation to the formation of the foetal membranes. The embryonic development of this species combines features of primitive rodent species, for example Geomys bursarius and highly specialized ones, for examples Mus musculus. The egg-cylinder is formed by invagination into the blastocoelic cavity of the inner cell mas...