Embryonic cells

Muscle tissue is unique in its ability to shorten or contract. The human body has three basic types of muscle tissue histologically classified into smooth, striated, and cardiac muscle tissues. Only the striated muscle tissue is found in all skeletal muscles. The type of cells which compose the muscle tissue are known as contractile cells. They originate from mesenchymal cells which differentiate into myoblasts. Myoblasts are embryonic cells which later differentiate into contractile fiber cells. 

Homeoboxes code for homeodomains, sequences of 60 amino acids that function as the DNA-binding regions of transcription factors. Each homeo-box gene in Drosophila is expressed only in its own characteristic subset of embryonic cells, and almost every embryonic cell contains a unique combination of homeodomain proteins. 

All cancers are diseases of abnormal cell proliferation, development and death. During the earliest stages of human life all of the embryonic cells divide constantly and differentiate to form the specialised tissues and organs. 

Embryonic cell cycles regulation at G2/M by patterning cues... 

In summary, transcription of the Cdc25-type phosphatase encoded by string limits the progression of many embryonic cell cycles, which are rapid (1—3 h) and require little cell growth. The massive ofi-regulatory region of string acts as a sophisticated pattern sensor that is influenced by a wide variety of cell type-specific transcription factors (Fig. 2A). 

Regulation of the embryonic cell proliferation by Drosophila cyclin D and cyclin E complexes... 

The Xenopus system has proven instrumental in determining the mechanism controlling exit from mitosis at the metaphase/anaphase transition. Studies in this area have relied heavily on extracts prepared from fully mature oocytes/ unfertilized eggs that are arrested at metaphase of the second meiotic division. Upon Ca2+ addition, anaphase is initiated and the extract enters the first embryonic cell cycle to replicate DNA. The activity responsible for metaphase arrest was discovered by Masui at the same time as MPF (Masui Markert 1971), and given the name cytostatic factor (CSF). CSF has never been purified... 

Special control of the early embryonic cell cycles in the early mouse embryo... 

The results show that CSF activity fluctuates after oocyte activation. Inactivation of CSF proceeds in two steps first, CSF is transiently down-regulated by a mechanism independent from Mos degradation and MAP kinase inactivation to allow exit from the M II arrest. Second, the disappearance of CSF activity after the transition to the first embryonic cell cycle requires inactivation of the MAP kinase pathway. 

The very beginning of the first mitotic cell cycle of the mouse embryo seems to be controlled by the mechanisms characteristic for both meiotic and mitotic cell cycles. Active MAP kinase, its substrate p90rsk and the CSF activity itself could influence the cellular processes within the one-cell embryo. Indeed, we have observed that despite the entry into the interphase (as judged by the low activity of MPF) some proteins are actively phosphorylated as during the meiotic M phase (e.g. 35 kDa complex Howlett et al 1986, Szollosi et al 1993), the nuclei and the microtubule interphase network start to form only 1.5 hours after activation (Szollosi et al 1993). This delay in the phenomena characteristic for the interphase could be linked to the mixed meiotic/mitotic character of this early period. This delay probably allows the correct transformation of the sperm nucleus into the male pronucleus. In species like Xenopus or Drosophila the transitional period between the meiotic and the mitotic cell cycle control is probably much shorter since it is proportional to duration of the short first cell cycle of these rapidly cleaving embryos. Mammalian embryos are perhaps the most suitable to study this transition because of the exceptionally long first embryonic cell cycle. 

The entry into the first mitotic M phase at the end of the first embryonic cell cycle requires activation of MPF. In the mouse one-cell embryo this activation is fully autonomous from the nucleus (Ciemerych 1995, Ciemerych et al 1998). It proceeds within the cytoplasts obtained either by enucleation or by bisection of the embryo. Other autonomous phenomena are the cortical activity, or the deformation of the one-cell embryo, directly preceding the entry into first mitosis (Waksmundzka et al 1984) and the cyclic activity of K+ ion channels (Day et al 1998). The role of the cortical activity remains unknown however, the fact that it directly precedes the entry into the first mitotic M phase suggests that it could be linked to the activation... 

Do later embryonic cell cycles differ from the somatic ones ... 

This suggests that cyclin A2 is not essential for the early embryonic cell cycles. Also D-type cyclins seem to be dispensable for the early mouse embryo cell cycle progression since embryonic stem (ES) cells do not express them at all before differentiation (Savatier et al 1996). We do not know, however, whether the D-type cyclins are also absent in the early embryo. These observations suggest that not only could the first cell cycles of the mouse embryo have specific modifications, but also further embryonic cell cycles are specifically modified as well. Mammalian embryonic cell cycles are probably modified often during development. Such studies could allow us to determine a profile of a minimal cell cycle in mammals which must, however, be much more complex than a simple S M phase embryonic cell cycle of amphibians or insects. 

Hird SN, White JG 1993 Cortical and cytoplasmic flow polarity in early embryonic cells of Caenorhabditis elegans. J Cell Biol 121 1343-1355... 

Sulston JE, Horvitz HR 1977 Post-embryonic cell lineages of the nematode Caenorhabditis... 

Sulston JE, Schierenberg E, White JG, Thomson JN 1983 The embryonic cell lineage of the... 

These experiments clearly demonstrated that despite the occurrence of extensive DNA damage, particularly DSBs, the embryonic cells kept progressing throughout the cell cycle unaffected, demonstrating that the checkpoint pathway that normally prevents cell progression following DNA damage was not operational in these cells. 

The major rapeseed storage proteins are a 12S type globulin, called cruciferin, which makes up 60% of the total protein, and a 2S type albumin, called napin, which makes up 20% of the total protein [9, 10]. Both of these proteins are formed in embryonic cells and stored in specialized vacuoles known as protein bodies. 

Kintner, C. Regulation of embryonic cell adhesion by the cadherin cytoplasmic domain. Cell 69 225-236,1997. 

There are four transgenic mouse models that have been broadly evaluated the TSPpSB, the Tg.AC, the Hras2 and the XPA. Each of these has its own characteristics. Each of these merits some consideration. They are each made by either zygote injection or specific gene targeting in embryonic cells (McAnulty, 2000 French et al., 1999). 

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