Embryogenesis stages

Evidence for the involvement of a Cdc2/Cyc B complex in the mitotic stages of germ cell development comes from studies in Drosophila. Cyc B transcripts are abundant and uniformly distributed in the early Drosophila embryo (Whitfield et al., 1989, 1990 Lehner and O Farrell, 1990b). Maternally derived Cyc B transcripts are also concentrated at the posterior pole of the oocyte. Later, during embryogenesis, Cyc B tran-... 

Simon No, there is a defect already in embryogenesis. At the early stage you don t see these cells at all which would normally express CEV3. We can t identify any stem cell at an early stage of development. 

Arsenic can traverse placental barriers as little as 1.7 mg As+5/kg body weight at critical stages of hamster embryogenesis, for example, can produce fetal death and malformation. 

It is well established that actinomycin D inhibits DNA-directed RNA synthesis by binding to guanosyl residues in the DNA molecule. This disrupts the transcription of genetic information and thereby interferes with the production of essential proteins. DNA synthesis may also be inhibited, being reduced by 30% to 40% in utero. It is clear that in the initial stages of embryogenesis, synthesis of RNA for protein production is vital, and it is not surprising that inhibition of this process may be lethal. 

Hyaluronan is prominent in the very earliest stage of embryogenesis, in maintenance of the undifferentiated state, with its removal required prior to the onset of differentiation.85 The presence of HA inhibits the process of differentiation, permitting expansion of primordial cell masses. In organotypic cultures of rat keratinocytes, HA suppresses epidermal differentiation,86 must be removed before the program of differentiation can be initiated. 

Figure 7.12 The stages of mammalian embryogenesis indicating the periods of greatest susceptibility to teratogens.

There are marked differences in the rate and duration of proteolysis of cyclins in different organisms. At the early stages of amphibian embryogenesis, cyclin destruction is only of brief duration and cyclin concentrations are kept high, as one would expect, because they are needed. To the contrary, in cultured fibroblasts, proteolysis continues for several hours and is only terminated at the onset of the S phase, when all B-type cyclins have been completely removed. 

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