Segration of Cell lines in the Embryo

In all multi cellular organism, the cleavage of the egg gives rise to cells which differ from one another and which, through successive cell divisions, will eventually give rise to homogeneous Cell populations (cell lines) each endowed with its own specific developmental program. This not only implies a process of sorting out of molecules (either pre-existing in the egg before fertilization or being synthesized in the course of development) into the various blastomers; but also of cells recognizing one another and coordinating their movements, their rate of cleavage, their metabolic activities, and the like.

The dichotomy between the two cell lines involves:

a) That in the somatic cell line, the genes which in the unicellular organism code for the surface structures responsible for the recognition of and interaction between cells of the two gametic types, are silenced. The evidence for this is indirect. The formation of mouse chimaeras shows that genetically male and female embryonic cells do not discriminate one another as different. Also, hybrid hystotypic aggregates can be formed in culture from such species as far as apart as chick and mouse. However, the possibility should be taken into consideration that in vitro conditions may alter the organization of the cell surface in such a way that some of its properties such as the species-specificity are lost while the tissue-specificity is retained. These observations are compatible with the view that the structures discriminating between male and female are not expressed at the surface of these cells.

b) The retention of a largely depressed genome by the cells of the germ line. This is inferred from the fact that in the oocyte, the complexity of the transcripts is several-fold greater than in the somatic cells. But there is no such direct evidence in the case of the male germ cells, it has been shown that at least in Drosophila, spermatocytes exhibit lampbrush chromosomes comparable to those of the oocyte.

The emergence of multicellular organism has required the establishment of cell junctions; not only as a means of holding the cells together, but as a vehicle of functional coordination between cells.

A classical example of a very precocious segregation of the somatic from the germ line is that of Ascaris. In this nematode while the lineage cells of the germ line retain their full chromosomes complement, in the cells of the somatic line pieces of chromosomes are lost; the loss amounts to about 27% of the total DNA of the cell. Interestingly, about one-half of the eliminated DNA consists of repetitive sequences and the other half of unique sequences.

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