Epigenesis in developing avian scales. II. Cell proliferation in relation to morphogenesis and differentiation in the epidermis

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Abstract
Throughout the early development of chicken scale epidermis, consisting of discrete placode and interplacode cell populations, morphogenesis and differentiation were examined from the standpoints of cell proliferation. From day 9.25 to day 11, active cell proliferation was observed only in the interplacode region, whereas the number of constituent cells increased considerably within the placode. After 26 hr of continuous labeling with [3H]thymidine (3H‐TdR) in ovo, three regions could be recognized: (1) unlabeled placode on the distal edge, (2) labeled placode in both lateral and proximal sites to the unlabeled placode, and (3) extensively labeled interplacode on the most proximal site of the scale. Examination of the distribution pattern of labeled cells has demonstrated that a fraction of the cells in the interplacode transits and annexes only to the proximal side of the placode. Cells transit after mitosis, accompanied with changes in morphology and proliferation activity. During the period of cell transition, orientation of mitosis paralleled completely along the proximodistal axis of the scale. Cell marking with carbon particles was carried in organ cultures from day 10, and the changes in the position of marked cells were observed. The results confirm those obtained after continuous labeling with 3H‐TdR. Various combinations of pulse‐chase experiments with 3H‐TdR clearly demonstrated the establishment of a cell lineage in which cells were aligned along the axis of the scale according to order of their birth. Along with a sudden resumption of active proliferation after day 11, a new class of cells (suprabasal cells) differentiated upward from the basal cells on the top region of the hump scale. At the first stage of development of suprabasal cells from day 11 to day 11.75, the direction of mitoses became preferentially polarized in the direction of axis vertical to the basal lamina. The important coordination of cell proliferation in relation to morphogenesis and cell differentiation is discussed.