Morphological changes at the basement membrane during some tissue interactions in the integument

Changes at the basement membrane have been reported at the time of tissue interactions during the development of a number of integumentary organs. During the development of vibrissa follicles of the mouse in vitro, gaps appeared in the lamina densa surrounding the dermal papilla at the time of the second, specific dermal message for follicle differentiation. Direct contacts between epithelial and mesenchymal cells were made through these gaps. This area has now been studied in vivo over the same period of follicle development. In addition, the epithelial part of the follicle has been separated from the dermal papilla by (i) enzyme treatment which destroys the lamina densa and (ii) chelating agents which leave the lamina densa attached to the dermal papilla. The resulting surfaces of epithelium and dermal papilla were examined by scanning and transmission electron microscopy. Small gaps first appeared in the lamina densa at the same substage of follicle development as in vitro and were present at the next substage. Processes from the dermal papilla mesenchyme cells traversed these gaps and made close contacts with the epithelial cells just prior to their differentiation into inner root sheath and hair. Instructive interaction through cell surfaces is suggested.