Ontogeny of small-granule APUD cells in hamster lung: A morphological study

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Abstract
Development of small-granule APUD cells and cell clusters was studied in 13-day to 15-day fetal hamster lungs by periodic acid-Schiff (PAS)-lead hematoxylin staining, monoamine fluorescence, and transmission electron microscopy. We examined 11-day and 12-day fetal, early postnatal, and adult animals only by PAS-lead hematoxylin. Precursors of small-granule cells first appear as PAS-negative clear cells in proximal airways of 13-day lung, occurring singly or in clusters of 2–25 cells and standing out among their undifferentiated, glycogen-laden, PAS-positive neighbors. By 14 days, developing small-granule cell clusters are prominent in main and lobar bronchi, extending 2–3 airway generations into the periphery. Clear-cell clusters, similar to those seen in 13-day lung, appear in peripheral airways and reach within one generation of developing terminal sacs. By 15 days, a few small, small-granule cell clusters are located at bronchioloalveolar junctions. Comparatively mature clusters occur in proximal airways; they are characterized by specific formaldehyde-induced monoamine fluorescence demonstrable after exposure in vitro to 5-hydroxytryptophan. In early postnatal stages, PAS-positive granules are resolvable toward the base of some endocrine cells. Ultrastructurally, pulmonary APUD cells contain numerous membrane-limited granules (180-nm diameter) of varying electron density. In 13-day lung, granules sparsely populate the cytoplasm of clear cells, but as the cells mature, the granule population increases and becomes concentrated in the basal cytoplasm. Fetal development of smallgranule cells is therefore compressed into the last 4 days before birth. Most clusters appearing in neonatal lungs are not yet fully mature, and not all subtypes of this population are present until some time later.

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