![]() ![]() Our observations in part support tailbud regionalization, but they also show that small cell groups in one central region can give rise to progeny populating several tissues in a single embryo: the neural tube, notochord, somitic muscle, epidermis, and other identified and unidentified cells. Embryos were examined several days later to record what tissues the group’s progeny had populated. In one set of experiments, we labeled groups averaging nine adjacent cells in three different regions. We have used photoactivation of fluorescence ( Vincent and O’Farrell, 1992 Kozlowski et al., 1997) to label small neighborhoods of tailbud cells in the developing frog embryo. Expression of some of these markers correlates generally with cell fate, so that whether or not the tailbud has mosaic cell fates, the tailbud is certainly not a homogeneous mass of cells. Within the last decade, molecular markers have been used to show that different regions in the tailbud have non-overlapping patterns of gene expression ( Gont et al., 1993 Beck and Slack, 1998 Knezevic et al., 1998). In addition, this work has been used to argue that different tissues in the tail can be mapped back to the immediately postgastrula embryo ( Pasteels, 1942 Gont et al., 1993). ![]() Fate mapping, tissue extirpation and tissue transplantation techniques were used to show that different regions of the tailbud are not only fated, but also committed, to different tissue types well before overt tail outgrowth that is, the tailbud shows mosaic, or regionalized, development. In this view, the tailbud does not contain a ‘blastema’ as originally defined, and the tailbud mesenchyme is already partitioned by germ layers. This view sparked experimental work supporting the opposite notion, that tail tissue formation is a simple continuation of processes begun at gastrulation ( Pasteels, 1939, 1943 Spofford, 1945, 1948 Smithberg, 1954 Bijtel, 1958 Gont et al., 1993). ![]()
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