82 KERATIN AND KERATINIZATION 



penetrate within the papillae of the follicles where the epidermal and 

 mesenchymal cells are separated by the single basement membrane. 

 The meshwork of collagen constitutes the " glassy membrane " visible in 

 the light microscope (Horstmann, 1957 and Montagna, 1956). 



The one-sided attachment of the cells to the basal membrane also 

 establishes an intracellular polarity which is revealed by the often 

 asymmetrical arrangement of the cell contents. The small cluster of 

 vesicles (Fig. 36, p. 81) referred to as the Golgi complex, tends to lie 

 distal to the nucleus and mitochondria m may be more common nearer 

 the basal membrane. 



The germinal cells, by their persistent cell division, maintain the 

 population of keratinizing cells. In this sense they conserve an embryonic 

 character which is emphasized by the generalized nature of their cell 

 contents. The cytoplasm of embryonic cells contains mitochondria, many 

 clusters of smooth-surfaced y-cytomembranes (p. 46), vacuoles containing 

 phospholipid and vast numbers of the small dense RNP particles. The 

 more specialized structures, such as the a-cytomembranes of secretory- 

 cells and the specialized inclusions, which later distinguish differentiated 

 cells, are rare. The similarity of this cytology to that of the germinal 

 cells of the skin is obvious. The surface of attachment of these latter cells 

 is, however, a specialized feature distinguishing them from the earliest 

 embryonic cells. 



There are various opinions about the detailed course of the process 

 whereby the basal layer both maintains itself and supplies cells to form 

 the differentiated layers. A common view is that there is some asymmetry 

 in the division of a basal cell in the sense that two unlike cells result; 

 one, referred to as a " stem cell," remaining attached and preserving a 

 generalized character, the other free to move up and enter the stream of 

 differentiating cells. This cell may also be capable of further divisions. 

 Mitoses are to be seen among the matrix cells some distance from the 

 basal layer in the hair follicle and it has been maintained that dividing 

 cells are also to be seen well above the basal layer in the epidermis 

 (Thuringer, 1924). Most observers agree now that nearly all dividing cells 

 in epidermis are found in the basal layers and critical opinion (Hanson, 

 1947 and Leblond, 1951) holds that the earlier observations were unreliable 

 on the grounds that it is easy to be mistaken when examining oblique 

 sections. It seems more likely that the widely accepted view, that cell 

 division and cell differentiation are mutually exclusive, applies to the 

 epidermis. Cell division appears to cease in cells in which cytoplasmic 

 fibrils have commenced to accumulate, which, on the face of it, means that 

 the cells' synthetic activities have swung over from producing materials 

 needed for division to producing keratin precursors. The factors control- 

 ling mitosis are discussed in the next chapter. 



