Fig. 1. Basic concepts in ectodermal organ morphogenesis.

A) Concept animal with different forms of ectodermal skin appendages. Endodermal organs are also shown. Modified from Chuong edit, 1998.¹ B) Chart showing the progression of ectodermal development into many different types of ectodermal organs. It also shows that the plasticity in ectoderm derived epithelial cells (i.e., multi-potentiality) gradually decreases, while the complexity of mesenchyme increases. C) Feather follicles undergo cyclic molting and regeneration. The follicles can change phenotypes between cycles. In subsequent generations of feathers, a symmetric downy feather and a contour feather emerge from the same feather follicle. Radial and bilateral feather symmetry can be determined by the topobiologcial arrangement of stem cells. Modified from Yue et al., 2006²², Chuong et al., 2000¹⁰² D) Fundamental issue in biology that can be addressed by skin appendage model and integrative biology approach.