Intercalary muscle cell renewal in planarian pharynx

Planarian cell renewal is achieved as a result of proliferation and differe ntiation of totipotent undifferentiated cells called neoblasts. The absence of mitosis within the planarian pharynx raises the question as to how cell renewal and growth occur within this organ. Two explanations have been adv anced: one proposes that new cells remain close to the base of the pharynx, which then grows by distal displacement of older cells, and the other sugg ests that the new cells are intercalated between older cells throughout the pharynx. The second alternative, however, does not explain how new cells e nter the pharynx or how they reach their final destination. In this study o f myosin heavy-chain gene expression within planarian pharynx, a row of dif ferentiating myocytes was detected all along the pharynx parenchyma. Accord ing to the hybridization pattern, all these myocytes appeared to be at earl y stages of differentiation. These data favour an intercalary model for mus cle cell renewal within the pharynx. According to this model, neoblasts at the base of the pharynx would enter the pharynx, where they would start dif ferentiation to myocytes, move to the subepithelial musculature and interca late between the old muscle cells. The possible application of this interca lary model to other pharynx cell types is also discussed.