Ultrastructural characteristics of 5BrdU labeling retention cells including stem cells of regenerating feathers in chicken.

Feathers regenerate from stem cells localized in a region of the follicle indicated as the bulge of the collar. Stem cells are slow cycling cells and some of these cells can be identified after labeling experiments using 5-bromo-deoxyuridine to detect label retaining cells (5BrdU LRCs). The present electron microscopic analysis of 5BrdU LRCs has described the ultrastructural characteristics of small cells present in the bulge region of the follicle in regenerating feathers of chickens that include stem cells. Labeled feather stem cells are smaller than 10 lm in average diameter, possess large nuclei with high nuclear/cytoplasmic ratio, and contain evenly distributed ribosomes, sparse bundles of intermediate filaments, scarce or no endoplasmic reticulum, and few mitochondria. The nuclei are mainly euchromatic with a variable amount of heterochromatin clumps and the nucleoli show developed granular and fibrillar components. These features indicate that feather stem cells are transcriptionally active cells for ribosomal and proteins synthesis. The cell surface of feather stem cells often shows small and irregular folds resembling microvilli in contact with the surrounding cells. The latter characteristics may be related to the exchange of molecules and/or with the migration of stem cells among other epithelial cells of the collar epithelium.