pectoral fin fold

The functional webs of the paired fins. They arise as a consequence of apical fold (AER) expansion and of migration of the distal mesenchyme. The fin folds are delimited by a dorsal and a ventral striated epidermis. The cells of the epidermal basal stratum are positioned on a basement membrane. Directly beneath the basement membrane of each side there is an array of actinotrichs, fibers of the extracellular matrix which act as supportive elements of the larval fin folds.