The role of cell proliferation in the formation of scleral ectomesenchymal condensations underlying the conjunctival papillae was examined with in vivo tritiated thymidine labelling in chick embryos ranging in age from 8 days 0 h to 10 days 12 h. Percentages of labelled nuclei were determined in both ectomesenchyme and the deeper fibrous sclera for short-term and continuous tritiated thymidine incubations. During formation of the ectomesenchymal condensations the percentages of labelled nuclei were consistently higher within the condensations than in corresponding non-condensing ectomesenchyme between papillae. The consistent differences of labelling percentages observed within the condensing versus noncondensing ectomesenchyme were not found in the fibrous sclera at any stage. All areas of both the ectomesenchyme and fibrous sclera showed decreases in the percentages of labelled nuclei from 8 days Oh to 10 days 12h, although the decline in the ectomesenchymal condensations beneath papillae occurred more slowly than in areas between papillae. The data suggest that the conjunctival papillae directly influence the proliferation in the subjacent condensing ectomesenchyme but have no effect on the ectomesenchyme between papillae or any region of the deeper fibrous sclera. The observations of this investigation are discussed in relation to other studies of the development of the pre-ossicular mesenchyme.

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