510 



Special Vertebrate Organogenesis 



short duration. Soon tongue-like thickenings 

 of the basal cells of the epidermis grow 

 downward into the underlying corium, form- 

 ing the follicle with the dermal papilla 

 situated in its base. Other hairs of the face, 

 head, and trunk which arise later do not 



A 



e 







h.;-: 



Fig. 195. Sections illustrating early stages in hair 

 formation. Iron hematoxylin, 8^- X 210. A, Begin- 

 ning of a sensory hair (vibrissa), 22 mm. pig em- 

 bryo, lower jaw. Note aggregation of mesodermal 

 cells (primordium of dermal papilla) and elevation 

 of two-layered, overlying ectoderm. 5, Beginning 

 of a body hair, 17-day mouse embryo. Note increase 

 in ectodermal cells, to form the hair primordium, 

 and aggregation of underlying mesodermal cells to 

 form primordium of dermal papilla. C, Further 

 down growth of ectodermal cells in the formation of 

 the hair primordium. Same embryo as B. 



show such pronounced surface elevations of 

 the corium and overlying epidermis. (A 

 striking exception is the European hedgehog, 

 whose spines develop quite like the sensory 

 hairs of other mammals; Davies, '89.) Each 

 body hair begins as a minute epidermal nod- 

 ule which forms by local proliferation of cells 

 of the germinal or basal layer (Fig. 195/4). 

 Simultaneous with, or immediately follow- 



ing, the initial changes in the epidermis, a 

 condensation of cells in the underlying mes- 

 enchyme takes place to form the primordium 

 of the dermal papilla. By continued growth 

 downward of the epidermal tongue of cells 

 (Fig. 195C), the follicle is established with 

 the dermal papilla located in its base (Fig. 

 196/4 and B). The hair proper develops from 

 epidermal cells covering the dermal papilla. 



Although the ectodermal and mesodermal 

 components of the hair papilla have not been 

 submitted to experimental analysis com- 

 parable to that of the feather, the great 

 similarities in the formation of hairs and 

 feathers suggest strongly that the inductive 

 mechanisms involved are similar in these 

 two groups of skin derivatives. In fact, evi- 

 dence in support of this view is forthcoming 

 from Hardy's ('49) work on culturing mouse 

 skin in vitro. She noted that no epidermal 

 thickenings or "plugs" formed in the ab- 

 sence of mesoderm, and, furthermore, no 

 dermal papillae could be found in areas from 

 which the epidermis had been removed. 

 Normal hair differentiation was obtained 

 from cultures in which the epidermis and 

 dermis were not separated. Such results sug- 

 gest strongly that an interaction of ectoderm 

 and subjacent mesoderm is necessary for in- 

 itiating hair formation. It would seem, then, 

 highly probable that hairs, like feathers, are 

 products of embryonic induction. 



Epidermal Ridges. Since the initiation of 

 development of both feather and hair pri- 

 mordia appears to be dependent upon an 

 interaction between the ectoderm and the 

 subjacent mesoderm, one is prompted to 

 consider the possibility of the existence of 

 similar inductive relationships in the forma- 

 tion of other specializations of the epidermis, 

 such, for example, as the epidermal ridge 

 systems found on the under surface of the 

 hands and feet. In all of the primates the 

 skin of these regions is characteristically 

 marked with fine, parallel ridges presenting 

 a corrugated appearance. Hairs and seba- 

 ceous glands are absent, but sweat glands are 

 abundant and large. The minute details of 

 the ridges and the very definite patterns — 

 loops, whorls, arches — formed by them on 

 the tips of the digits and in consistent sites 

 on the palms and soles show regional as well 

 as individual variation. Even the skin of a 

 small area will show ridge details not found 

 elsewhere on the same or any other indi- 

 vidual. This, together with the fact that the 

 ridge pattern in all of its detail remains un- 

 changed throughout life, is the basis for 

 the use of epidermal ridge patterns (der- 



