Follicular patterns during feather morphogenesis in relation to the formation of asymmetric feathers, filoplumes and bristles

The present microscopic study on follicles of regenerating feathers describes the pattern of morphogenesis responsible for the formation of symmetric and asymmetric feathers, filoplumes and bristles. Barb ridges are generated in the ventral locus of the follicle and merge into a rachis located in the dorsal side after helicoidal growth along the ramogenic collar. Asymmetric feathers result from the elongation of rami of larger size on one side with respect to the opposite side of the follicle before merging into the rachis. The increase in length of barb ridges along the collar may derive from the higher number of cells incorporated in the ramus, from the increase of cell size or elongation within barb ridges, or from the shifting of the ventral locus along the collar. The latter produces the formation of more elongated rami on one side of the follicle with respect to the rami with the same size formed on the opposite side. These processes determine an increase of the length of helical growth of barbs on one side of the follicle and the formation of the longer rami in asymmetric feathers. The change in pattern of barb ridge formation gives origin to bristles and filoplumes.