Metamorphosis of summer flounder, Paralichthys dentatus: Cell proliferation and differentiation of the gastric mucosa and developmental effects of altered thyroidal status

Summer flounder, like most marine fishes studied to date, are stomachless a t first feeding, and subsequently acquire gastric function during the proce ss of metamorphosis. Stomach formation is controlled largely by thyroxine ( T-4) In the present work we sought to understand gastric organogenesis in t erms of cell proliferation and its relationship to histological differentia tion. The objectives of the study were (1) to obtain a developmental patter n of cell proliferation in the gastric mucosa and to relate that pattern to the progress of gastric differentiation; and (2) to understand the regulat ory role of T-4 On cell proliferation and histological differentiation by a ltering the thyroidal status of the developing larvae. We observed that (1) in normally developing larvae, cell proliferation increased by early metam orphic climax (MC), remained high until mid-MC, and decreased to basal leve ls by late MC; concomitantly, the gastric glands appeared and differentiate d in the fundic mucosa, and were complete by late MC; (2) T-4 accelerated t he differentiation of gastric glands and mucus neck cells, while inhibiting the concomitant increase in cell proliferation observed in controls; and ( 3) the goitrogen thiourea inhibited both cell proliferation Emd gastric dif ferentiation compared to controls. These results indicate that T-4 is neces sary for the three-fold increase in cell proliferation that occurs in early metamorphic climax, but that high T-4 levels promote differentiation at th e expense of proliferation. The observed effects would be consistent with t he normal, metamorphosis-related increase in whole body T-4. (C) 2001 Wiley -Liss,Inc.