histamine: normal tissues 



sheet of tissue up to 2 feet square which is extremely rich in both mast cells 

 (Fig. 6) and in histamine. As Table III indicates, the histamine value rises 

 with each successive step in isolating the tissue which contains the mast cells. 



Table III 



Partition of histamine between parenchyma and capsule of ox liver. 

 The mast cells are also predominantly in the capsule (cf Figs. 5 and 6) 



Estimation of histamine content per mast cell in ox liver capsule 



An estimate of the histamine content of the individual mast cell in ox liver 

 capsule was arrived at as follows. Rectangles of cleaned liver capsule were 

 cut out with a sharp scalpel around a 6x8 cm. rectangle of k Perspex\ These 

 samples were then weighed, extracted and their histamine contents estimated. 

 Adjacent strips from the same capsule were mounted as tissue spreads on 

 microscope slides, sufficient tissue being draped over the edges of the slide to 

 prevent shrinkage of the spread during fixation and staining. 



A horizontal projection microscope was next set up in a darkened room to 

 throw an image of a portion of a prepared spread on to white paper on which, 

 for convenience, a rectangle had been drawn proportionate to the size of the 

 rectangles of tissue used for the pharmacological assays. The actual size of 

 the rectangle in the microscope field was measured by replacing the histological 

 preparation with a stage micrometer, a slide on which a scale in absolute units 

 is engraved for projection at any selected magnification. 



However, mast cells are so numerous m ox liver capsule (Fig. 6) that 

 difficulty was encountered not only in transmitting sufficient light through the 

 specimen to show up the individual cells on the paper but also in counting 

 them in the darkened room with any degree of accuracy. This was overcome 

 by throwing a second beam of ultra-violet light on to the paper and covering 

 the image of each mast cell as it was counted with a dot of fluorescent paint. 

 As a check, these dots were again counted by serially quenching their 

 fluorescence with drawing ink, ten fields, chosen at random, being counted 

 twice over for each tissue spread. 



In this way the number of mast cells in a sheet of tissue of known histamine 

 content was obtained, calculation then showing each mast cell in ox liver 

 capsule to contain approximately 25 \i\xg. histamine base. In a later experiment 



73 



