Table 1.

Comparison of mammary glands and feathers

Appendage Define class in evolution	Mammary gland Mammals	Feather Aves
Basic structures	Requires both epithelium and mesenchyme. Invaginates into dermis and branches	Requires both epithelium and mesenchyme. Protrudes out of skin surface and branches
Function	Chemical secretion (developed with endothermy) to moisten eggs and feed offspring. Display	Mechanical properties for endothermy, display, and flight
Developmental origins	Skin	Skin
Inductive events and inducers	Ectoderm–mesenchymal interactions, mammary gland stroma	Ectoderm–mesenchymal interactions, dermal papilla
Number of appendages	2–18 (hundreds in platypus)	20,000–100,000
Molecular regulator of induction  and morphogenesis	FGF-, BMP- and Wnt-signaling, etc., but not Hh signaling	FGF-, BMP-, Wnt, Hh-signaling, etc.
Mechanisms of branching  morphogenesis	Branches made by budding and elongation	Branches made by differential apoptosis from multi-layered epithelia
Regional specificity	Morphologically: yes. Functionally: none	Both morphological and functional specificity in flight, downy and contour feathers
Differentiation markers	B-casein, whey acidic protein	Feather beta keratin
Regenerative cycling	Partially. Do go through lactation and involution phases. Next phase starts morphologically equivalent to pubertal stage	Yes, go through growth, resting, molting and regenerative phases
Stem cells	Cap and other places	Follicle bulge
Hormonal regulation	Yes, for expanding milk production	Yes, for sexually dimorphic feathers