Attenuated fibroblast sheath around the basement membrane zone in the trachea.

In the connective tissue underlying the epithelium of the airways, there are thin attenuated cells that appear to be fibroblasts. These cells are close to the basal lamina and delineate an area equivalent to the basement membrane zone (BMZ). Using the mean data from transversely and longitudinally sectioned attenuated fibroblasts, we estimate that the diameter of the attenuated fibroblast is approximately 28.0 microns and its thickness 0.55 microns. Serial sections indicate there are pores of various sizes in the attenuated fibroblast. Also, the presence of cell segments less than 5 microns in length suggests there are processes from the edge of the cell which would give it a stellate appearance. These large flat cells form a sheath covering 66 to 70% of the BMZ at an average distance of 1.9 microns from the basal lamina. In addition, they make contact with the basal lamina approximately 7,000 times per mm2 of basal lamina. The majority of the contacts with the basal lamina are beneath basal cells. Cells of the attenuated fibroblast sheath are probably associated with lung morphogenesis, wound healing, and development of the BMZ. Their existence as a sheath demonstrates an anatomical unit associated with fibroblast-epithelial cell interactions in airway epithelium.