Genetic Analysis of Early Development

The individual blastomeres of the mammalian embryo appear to be totipotent up to the 8-cell stage (Tarkowski, 1959 Tarkowski and Wroblewka, 1967 Moore et ah, 1968). By the blastocyst stage, two cell types have formed the outer trophoblast layer and the inner cell mass. These two populations have been shown to have restricted developmental capacities (Gardner, 1971). In mammalian embryos there is a characteristic grouping of the inner embryonic cell mass to one side 

Nesbitt and Gartler (1971) have shown that female mice heterozygous for Cattanach s translocation (Cattanach, 1962) have an inactive normal X chromosome which can be distinguished from cells containing an inactive translocated X chromosome. They interpreted their results to mean that X chromosome activation occurs around the time of implantation but before the determination of the primordial cell pools for the various tissues, within the inner cell embryonic mass. Comparisons of allophenic and X inactivation mosaic mice indicate a tendency for a coherent clonal growth of the inner ceU mass in the development of the mammalian blastocyst (Nesbitt and Gattler, 1971). 

It cannot be ruled out that RNA transcription is unimportant for normal development prior to blastocyst formation. The fertilized egg contains many enzymes, and the activity of these has been shown to change considerably through preimplantation development. For example, the activity of hypoxanthine-guanine phosphoribosyltransferase, adenine phosphoribosyltransferase, and hexokinase increases markedly from the fertilized egg to the blastocyst stage (Brinster, 1968a Epstein, 1970)  

RNA Containing Poly(A) Sequences in Preimplantatian Rabbit Embryos 

Since the discovery by Hadjivassiliou and Brawerman (1966) that rat liver microsomes contain polyadenylic acid [poly(A)] sequences, there has been much recent evidence which indicates that a high proportion of the messenger RNA s (mRNA s) of mammalian cells contain poly(A) sequences which are covalently linked to the mRNA molecule. Most of the rapidly labeled nonribosomal RNA components of mouse sarcoma 180, HeLa, and L cell polysomes have been shown to contain adenylate-rich sequences (Lee et at, 1971 Edmonds et al., 1971 Darnell et at, 1971a Sheldon et at, 1972 Perry et al., 1972). 

---