Heterotypic cell contacts and basal lamina morphology during hair follicle development in the mouse: a light, scanning, and electron microscopic study at the site of tissue interaction

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Abstract
A study was made of the interface between the epithelial hair matrix and the mesenchymal dermal papilla in developing vibrissa follicles in the mouse. The objective was to examine the hypothesis that the instruction from the dermal papilla mesenchyme, which is known to lead to differentiation of the hair matrix cells into inner root sheath and hair, is mediated by heterotypic cell contacts. The upper lip skin of 14.0- to 15.5-day embryos was separated into epithelium and mesenchyme after ethylene diamine tetraacetic acid or trypsin treatment, and sometimes tape-stripped or cryofractured before scanning electron microscopy. All methods were compared and evaluated. The chelating agent produced clean preparations of intact basal lamina on mesenchyme, proving its usefulness for complex interfaces. Transmission electron microscopy of untreated and of separated tissue confirmed that gaps appeared only in the previously intact lamina densa surrounding the dermal papilla and persisted for three stages of follicle development. Mesenchymal cell processes from the papilla were found in close contact with epithelial hair matrix cells through these gaps just prior to and during differentiation of the hair matrix cells, but not at earlier stages. These contacts could therefore be involved in transmission of the instruction.