Endocrine-like cells in the terminal bronchioles and saccules of human fetal lung: an ultrastructural study.

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Abstract
An ultrastructural study of Kulschitsky cells was made on lung tissue from five human fetuses of gestational ages between 18 and 25 weeks. The cells were found to occur with relative frequency throughout the bronchi and bronchioles, as well as in the developing saccules. This report describes in detail the cells of the terminal bronchioles and saccules, where their long cytoplasmic processes weave through the epithelium and come into contact with intercellular spaces and the lumen. The cytoplasmic organelles have an orderly arrangement comparable to secreting cells. Dense core vesicles characterise the cytoplasm but clear vesicles and some with less dense, diffuse cores are also present, and may be seen outside the plasma membrane and in the interstitial spaces. The K-cells are usually colsely associated with capillaries or smooth muscle cells across the basement membrane. In addition, the rare occurrence of a small unmyelinated nerve fibre in the vicinity of a K-cell in a terminal bronchiole raises the possibility that the cells fulfill a type of 'mini receptor' role in addition to their probable vasoactive one. Evidence is given that K-cells divide in human fetal lung.