A new type of extracellular fibril is described in the dermis of Bufo marinus, Rana pipiens, and Amblystoma punctatum. It is restricted in distribution to the dermal micropapillae and the region immediately below them in the stratum spongiosum. The fibrils (diameter = 200 to 750 A) are lateral aggregates of fine filaments and have a unique banding pattern characterized by absence of recognizable periodicity and by polarization in respect to the basement membrane. Their distal1 ends are anchored in the basement membrane, and their proximal ends converge in knots located in the middle region of the micropapillae. These anchoring fibrils seem to secure the minute outfoldings of the basement membrane along the dermal-epidermal junction.
Comparable, but less frequent, fibrils are also encountered along the proximal aspect of the basement membrane in the skin, lingual mucosa, and mucosa of the gastric fundus in the rat.