Hi Vicenta,
Nishida (1987; Dev. Biol. 121: 526) found that in Halocynthia embryos the
B7.5 cell pair (which exists at the 64-110 cell stage) gives rise to larval
muscle and a few cells in the ventral trunk which they called
endoderm. The "endodermal" cells were later shown to express biochemical
markers of muscle, not gut, and were suggested to be precursor cells of the
adult heart (Whittaker 1990; Biol Bull 178). It was later shown that
during post-metamorphic development, these so called "trunk ventral cells"
(TVCs) migrate and differentiate to form both heart muscle and pericardium,
as well as the whole latitudinal body wall muscle (Hirano & Nishida, 1997;
Dev. Biol. 192: 199).
So to answer the question, it is the B lineage at the 8-cell stage which
gives rise to heart. But also to gut, larval tail muscle, larval
mesenchyme (juvenile tunic cells), and a bit of notochord.
As an alternative, you might consider looking at the development of heart
excitability during regeneration or reconstitution instead of
embryogenesis. Check out Sister F. M. Scott 1959 (Acta Embryologiae et
Morph. Exp. 2: 209-26) --she scrambled the tissues of ascidian zooids and
watched them reorganize themselves. She observed that a beating heart
could develop from chunks of mushed-up pericardium, which (I think) can be
experimentally isolated from other organs by pinching off a segment of body
wall around it. There may be other kinds of tunicates that do this trick
as well or better than the species Scott used----I do not know. Anyway,
seems like it might be a useful system.
/2001 +0000, you wrote:
>Hi all,
>I am Vicenta Salvador and I am interested in the development of
>excitability in the ascidian heart muscle. To start a project I first need
>to find the precursors to larval/adult heart muscle. The literature
>concerning the fates of the cells at the 8-cell ascidian embryo does not
>provide information as to which cells give rise to the heart. Would anyone
>know where/when to find them?
>
>Any suggestions will be appreciated
>Greetings from the Bamfield Marine Station!
>
>Vicenta
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