Median DM Cells

MESODERMALLY-EXPRESSED GENES DETERMINING BROAD AREAS AND SPECIFIC TISSUES IN THE MESODERM 

tinman, a Homeobox Gene Required for the Formation of Visceral Musculature, Dorsal Somatic Muscles, and the Heart 

Initial tinman expression occurs at cellular blastoderm in the prospective mesoderm and is confined to the trunk mesoderm where it is initally modulated in a peri-t)dic pair-rule pattern (Bodmer et ah, 1990 Azpiazu and Frasch. 1993). During invagination and dorsal migration of the mesoderm, the entire trunk mesoderm expresses tinman mRNA in a uniform pattern. However, shortly before the mesoderm reaches the dorsal border of the ectoderm, tinman expression decreases in the ventral portions and its levels increase in the dorsaimost cells of the mesoderm. Be- 

Yin and Frasch, 1998). Together with the zinc finger homeobox gene zfli-I, tinman is also required for normal formation of the gonadal mesoderm (Boyle et al., 1997 Broihier et al., 1998). It is likely that the functions of tinman in these developmental processes are required during its early phase of expression in the entire trunk mesoderm. 

Ectopic expression experiments have also been performed to study the functional potential of the tinman gene. Perhaps surprisingly, the consequences of prolonged ubiquitous tinman expression in the mesoderm or in the whole embryo were relatively mild. The main effect was a transient increase in the number of heart precursors during stage 11, but the formation of the heart, visceral muscles, and body wall muscles appeared not to be disturbed (Yin and Frasch, 1998). These results indicate that coregulators, whose expression or activity is spatially restricted, are obligatory for all known aspects of tinman function. 

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