Figure 8-3. Development of the human epidermis and hair follicle. A, epidermis of 2. 1 -mm embryo; 

 B, epidermis of 16-mm embryo; C, epidermis of 32-mm embryo; D, epidermis of 85-mm embryo; E 

 to H, progressive stages in the development of the follicle and hair. (After Patten, 1946) 



of the snout. It is not strengthened by a bony core as in the 

 cow but is seated on a bony knob of the skull. The pointed 

 tip is produced and maintained by wear. 



Bmbryological development The ectoderm is at first a 

 simple cuboidal epithelium which gradually becomes strati- 

 fied (Figure 8-3). Hair begins its development as thicken- 

 ings of the epidermis. Each such thickening grows down into 

 the dermis as a strand which carries the stratum germina- 

 tivum before it. A dermal papilla now forms which projects 

 upward into the strand. The cells overlying the papilla form 

 the matrix from which the hair grows upward through the 

 strand. The strand is thus converted into a follicle. Seba- 

 ceous glands form from the wall of the follicle. The sweat 

 glands form as ingrowths from the epidermis. 



The soft keratin of the stratum corneum is formed by ac- 

 cumulation of granules within the cells which pass through 

 a number of steps. Being cellular, the stratum corneum des- 

 quamates at its surface. 



The mammary glands are generally considered to be modi- 



fied sweat glands, although they might just as easily be viewed 

 as sebaceous glands. Pertinent to this question is the obser- 

 vation that in man the apocrine sweat gland of the arm pit 

 and the nipple undergo cyclic changes which can be cor- 

 related with the menstrual cycle. Unlike the sweat glands, 

 the mammary glands are compound and alveolar, and they 

 are related to mammary hairs. In man about eight hair fol- 



scale (hord keratin) 



strotum germinativum of epidermis 



Figure 8-4. Section through two scales of a pangolin, Manis tricuspis. 

 (After Weber) 



SKIN 



219 



