MAST CELL IN EVOLUTION 



ally with the connective tissues (Lazarenko, 1925). In certain species the 

 basophilic cytoplasmic bodies stain strongly with the periodic acid-Schiff 

 method for a neutral mucopolysaccharide (Ohuye, 1952; Ohuye and Horikawa, 

 1956) 'which forms or contributes to the connective tissues and basement 

 membranes' (Wigglesworth, 1956). 



With the evolution of an 'open' type of circulation, as in arthropods, more 

 typical mast cells become demonstrable around the walls of the smaller arteries 

 whose contents empty direct into the tissue spaces. Over a century ago Hackel 

 (1857) described an adventitial cuff, a 'Zellgewebe', surrounding the smaller 

 blood vessels of the crayfish. By injecting a suitable vital stain into the 

 pericardial sac or primitive heart of the crayfish, myriads of cells filled with 

 metachromatic granules can be demonstrated in Hackel's perivascular sheath. 

 Only rarely does such a tissue basophil escape into the circulation (Hardy, 1892). 



VERTEBRATES 



However, it is at the next evolutionary level that tissue mast cells appear 

 in an abundance which makes it difficult to escape the conclusion that their 

 number and situation foreshadow something of their function in higher 

 organisms. The tissues of many Ichthyopsida (fishes and amphibians) are 

 teeming with mast cells in contrast to the virtual absence of mast cells in the 

 blood. So intimately is the distribution of the mast cells bound up with the 

 evolution of the blood vascular system that it is worth while pausing for a 

 moment to review briefly the development of the blood-forming tissue itself 

 and the progressive specialization of its component parts (Jordan, 1926; 

 Jordan and Speidel, 1930). 



Evolution of the blood-vascular system 



In the lancelet amphioxus (branchiostoma) the few free mesenchymal cells 

 in the blood are derived from undifferentiated precursors scattered throughout 

 the body. With the advent of the primitive craniate fishes, the jaw-less cyclo- 

 stomes, there begins a process of segregation and specialization of blood- 

 forming tissue which may be traced progressively forwards to the mammals. 

 From being distributed diffusely throughout the wall of the intestine, as in the 

 hagfish, the haemopoietic tissue becomes aggregated in the so-called spiral 

 valve of the larval lamprey and forms a compact nodule in the wall of the 

 stomach of the African lung fish. In the cartilaginous fishes the active tissue 

 is now attached to the stomach by a mesentery and may thus more properly be 

 termed a spleen. At the amphibian level, especially in the tailless anurans, 

 the spleen is concerned almost exclusively with the production of erythrocytes 

 and thrombocytes, granulopoiesis still being carried on at former sites of blood 



