Mitotic Activity during the Early Differentiation of the Scleral Bones in the Chick

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ABSTRACT The dispositions of mitoses in the mesenchyme beneath the conjunctiva of parts of a 7-, 8-, 9-, and 10-day embryo have been ascertained. For the purpose of analysis, the number of mitoses, cell population density, and mitotic index have been found in small segments of annuli which include parts of the rings of bone primordia, and the mesenchyme between them. Twenty-three primordia were studied. It is found that the cell population density increases from about 4−14 × 105 per cubic mm within the primordia, but only from about 4−9 × 105 in the mesenchyme between them. Mitotic activity increases sharply within young primordia from about 12-27 mitoses per 1,000 cells, and thereafter decreases. It is found that this mitotic activity probably occurs in three waves of decreasing intensity. Between the primordia mitotic activity is at a markedly lower level and probably follows a simpler pattern. The cells involved in this mitotic activity in the primordia are particularly those just beneath the conjunctival papilla, and the number of mitoses decreases the greater the distance from the conjunctiva. In the 8-and 9-day embryos, the number of mitoses is inversely proportional to the distance. Calculation indicates that in each of the three waves of cell division, the cells that divide do so only once; and that this mitotic activity is adequate to account for the increase in cell population density which is found. The cause of the increase in the number of cell divisions in the primordia is not clear, but the result is probably the production of blastemas, the cells of which become the osteoblasts of the scleral bones.