THE MAST CELLS 



of the dog. However, for present purposes it is probably more instructive to 

 examine the origin and distribution of tissue mast cells in a limited number of 

 higher vertebrates than to compile a catalogue of the minor variations which 

 have been recorded. We shall therefore examine briefly conditions in rat and 

 man. For references to mast cells in other species the reader is referred to the 

 comprehensive review of the literature by Michels (1938). 



Mast cells of the rat. The careful studies of Maximow (1910) and Alfejew 

 (1924) indicate that tissue mast cells are first recognizable in the early embryo 

 of the rat as small lymphoid-like cells with few granules and arise either from 

 fixed mesenchymal precursors or from basophilic non-granular wandering 

 cells which settle down and undergo differentiation in the loose subcutaneous 

 and intramuscular connective tissues of the head and neck. Mast cells appear 

 somewhat later than the other granulocytes and at first multiply heteroplastic- 

 ally, later by homoplastic mitotic division. By the end of intrauterine life the 

 topographical distribution of the tissue mast cells has spread to include all the 

 loose connective tissues (Laguesse, 1919; Urtubey, 1948; Arvy, 19566). Webb 

 (1935) states that the number of mast cells in the dermis of the white rat 

 increases tenfold from 5 days before to 19 days after birth, the count then 

 dropping slightly and remaining constant up to the fifty-first day. A sharp 

 increase in tissue mast cells up to the tenth day was also noted by Fish (1949). 

 Bates (1935), however, records a slight but progressive fall in the number of 

 tissue mast cells from birth onwards to 78 weeks with concomitant increase 

 in the number and size of the granules in the cells that remain. The cells are 

 common in the serous membranes and can be recovered in small yield from 

 the peritoneal fluid (Webb, 1931; Padawer and Gordon, 1955). 



Mast cells are rare in the parenchymatous organs of the rat in contrast 

 to their great abundance in connective tissue and peritoneum. Lungs, spleen, 

 liver and kidney are virtually devoid of mast cells. However, they are by no 

 means uncommon in the oesophagus and in the forestomach, where they 

 appear to arise chiefly from precursors around the deeper vessels and, on 

 occasion, to migrate up to and even through the mucosa (Sansonow, 1909; 

 Cambel et al, 1952). They are comparatively rare in the rest of the alimentary 

 tract (Arvy, 1956/?; Mota et al, 1956). 



There are no mast cells within the brain substance of the rat, though they 

 are common in the meninges (Waldeyer, 1875) and in the sheaths of peripheral 

 nerves (Riley, 1953o ; Riley and West, 1955a; Padawer, 1957). 



Mast cells in man. Mast cells appear late in the human embryo as in other 

 species (Ehrlich, 1877; Holmgren, 1946; Kitanishi, 1956). During the course 

 of intrauterine life small cells containing a few metachromatic granules are 

 seen at sites of extramedullar haemopoiesis and in organs which are well 

 developed before birth, e.g. liver and gall bladder, eye, heart muscle and vessels, 



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