. The thyroid gland in health and disease. network. Mitotic figures and recentdivision forms of nuclei are commonly to be seen in the glands ofthe new-born and young. Lying between the vesicles and filling up the spaces left bythe approximation of the more or less rounded vesicles is theintervesicular parenchyma. The cells in this situation areessentially the same in structure and in function as those lining 10 HISTOLOGY AND PHYSIOLOGY the vesicles. They are the reserve parenchyma cells from whichnew acini are formed as occasion may demand. These cells also, if not actually identical with thos
. The thyroid gland in health and disease. network. Mitotic figures and recentdivision forms of nuclei are commonly to be seen in the glands ofthe new-born and young. Lying between the vesicles and filling up the spaces left bythe approximation of the more or less rounded vesicles is theintervesicular parenchyma. The cells in this situation areessentially the same in structure and in function as those lining 10 HISTOLOGY AND PHYSIOLOGY the vesicles. They are the reserve parenchyma cells from whichnew acini are formed as occasion may demand. These cells also, if not actually identical with those of theparathyroid, any rate very similar to them in structureand appearance. The intervesicular parenchyma varies greatlyin amount at different age periods. The foetal thyroid is largelycomposed of it ; it is plentiful in the new-born and in youngsubjects (fig. 3), but with advancing age the gland becomes lesscellular and more vesicular. In the same organ some parts maybe more cellular than others ; some parts more vesicular than. l%-€>/S 0 A*# Fig. 2.—Section of normal thyroid of wild rat in state of active secretion. Thecolloid is undergoing absorption and newly formed secretion is being poured into thelymph spaces, x 250. others. Occasionally the cavities of the vesicles are separatedonly by the two rows of cells forming their walls (fig. 1), and bythe vascular network surrounding them ; but much more com-monly they arc separated by a layer of cells of variable thickness(fig. 3). When the vesicles are small the intervesicular parenchymais more plentiful ; when they are larger it is correspondinglydiminished. Two sections from the same gland may thus varyconsiderably in appearance. Some authors divide the parenchyma cells of the thyroid into STRUCTURE 11 principal cells and colloid cells. Such a division is unneces-sary and misleading since the different appearances representdifferent stages of the cells activity. All parenchyma cells,whether lining the vesicl
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