Development of hamster tracheal epithelium: IV. Cell proliferation and cytodifferentiation in the neonate

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Abstract
The development of the tracheal epithelium was studied in neonatal hamsters beginning on the day of birth (day 1) and ending on day 8. Morphological changes were characterized and the proportions of cells (basal, secretory, ciliated) and mitotic indices were quantified along dorsal and ventral epithelial surfaces. Cellular proportions were stable throughout the week but the makeup of the dorsal and ventral epithelia was different. The ventral epithelium was composed of about 59% secretory cells, 39% basal cells, and 2% ciliated cells, whereas the dorsal epithelium was composed of about 52% secretory cells, 32% basal cells, and 16% ciliated cells. Mitotic indices were generally less than 1% and mitotic activity in secretory cells predominated proportionate to the ratios of secretory cells and basal cells in dorsal and ventral epithelia. During the first postnatal week changes occurred that were related to the maturation of basal cells and secretory cells. Glycogen was rapidly lost from both cell types during the first part of the week and the lateral cell membranes became increasingly complex. Apical microvilli had formed in the secretory cells by day 2 and hemidesmosomes were well developed in the basal cells. Rough and smooth endoplasmic reticulum membranes developed rapidly in the secretory cells, and mucous granules were abundant in some cells on days 4—8, especially in the ventral epithelium between the cartilage rings. The study shows that shifts in the proportions of basal, secretory, and ciliated cells do not occur in dorsal or ventral tracheal epithelium during the first postnatal week but the basal cells and secretory cells undergo rapid cytodifferentiation and functional maturation at this time.