Figure 2.

Oral siphon amputation activates cell division in the branchial sac, migration of progenitor cells to the blastema, and progenitor cell differentiation into circular muscle fibers. A. A diagram of the oral siphon showing the position of the amputation plane (dashed line). B. C. EdU incorporation into the transverse vessels (TV) of the branchial sac of a control (B) and an oral siphon amputated animal (C) after a 24 hr EdU pulse. Scale bar: 20 μm in B, C. D, E. EdU labeled cells in longitudinal columns (outlined by red dashed lines in D) and the regeneration blastema (RB) of an oral siphon following a 2-day chase. D and E show bright field and fluorescence images of the same regenerating siphon. Scale bar: 400 μm in D, E. F. EdU labeling is scattered through cells in the regeneration blastema (RB) and in stigmata (S) of the branchial sac following a 24 hr pulse and 4 days of chase. G. Rhodamine-phalloidin labeling of regenerating circular muscle bands after 8 days of chase. H. EdU labeling is associated with horizontal lines of circular muscle fibers (CM) (inset 2X magnification of boxed area) after a 24 hr pulse followed by 8 days of chase. Scale bar: 40 μm in F-H. I. Diagram of a sectioned oral siphon body wall showing epidermis (ep), endodermis (ed), a blood cell (bc), and bundles of circular muscle fibers (cm). After Millar (1953). J. Section of a regenerating oral siphon through a horizontal plane showing bundles (dashed circle) of EdU labeled circular muscle cells (CM). Scale bar: 25 μm. K, L. Low levels of EdU incorporation in the oral siphon of a control following a 24 hr pulse and 8 days of chase. K and L show bright field (K) and fluorescence (L) images of the same siphon. Scale bar: 500 μm.