cells first become different kinds of cell—muscle cells nerve cells reproductive cells etc. approximately four days after fertilization.
cells first become different kinds of cell—muscle cells nerve cells reproductive cells etc. approximately four days after fertilization.
when do cells first become different kinds of cell—muscle cells nerve cells reproductive cells etc?
approximately four days after fertilization
Na
Na
schmid and seipel argue that the origin of muscle cells is a monophyletic trait that occurred concurrently with the development of the digestive and nervous systems of all animals and that this origin can be traced to a single metazoan ancestor in which muscle cells are present. they argue that molecular and morphological similarities between the muscles cells in cnidaria and ctenophora are similar enough to those of bilaterians that there would be one ancestor in metazoans from which muscle cells derive. in this case, schmid and seipel argue that the last common ancestor of bilateria, ctenophora, and cnidaria was a triploblast or an organism with three germ layers and that diploblasty, meaning an organism with two germ layers, evolved secondarily due to their observation of the lack of mesoderm or muscle found in most cnidarians and ctenophores. by comparing the morphology of cnidarians and ctenophores to bilaterians, schmid and seipel were able to conclude that there were myoblast-like structures in the tentacles and gut of some species of cnidarians and in the tentacles of ctenophores. since this is a structure unique to muscle cells, these scientists determined based on the data collected by their peers that this is a marker for striated muscles similar to that observed in bilaterians. the authors also remark that the muscle cells found in cnidarians and ctenophores are often contests due to the origin of these muscle cells being the ectoderm rather than the mesoderm or mesendoderm. the origin of true muscles cells is argued by others to be the endoderm portion of the mesoderm and the endoderm. however, schmid and seipel counter this skepticism about whether or not the muscle cells found in ctenophores and cnidarians are true muscle cells by considering that cnidarians develop through a medusa stage and polyp stage. they observe that in the hydrozoan medusa stage there is a layer of cells that separate from the distal side of the ectoderm to form the striated muscle cells in a way that seems similar to that of the mesoderm and call this third separated layer of cells the ectocodon. they also argue that not all muscle cells are derived from the mesendoderm in bilaterians with key examples being that in both the eye muscles of vertebrates and the muscles of spiralians these cells derive from the ectodermal mesoderm rather than the endodermal mesoderm. furthermore, schmid and seipel argue that since myogenesis does occur in cnidarians with the help of molecular regulatory elements found in the specification of muscles cells in bilaterians that there is evidence for a single origin for striated muscle.
muscle