. The thyroid gland in health and disease. 300 /x. In shape theymay be rounded, long, oval, polyhedral, or occasionally they have side branches, or several vesicles mayintercommunicate. In health each vesicle is lined by a single layer of epitheliumwhich rests directly on the vascular connective tissue is no basement membrane. The vesicular epitheliumvaries in height from low cuboidal (9 /a) (fig. 1) to low cohunnar STRUCTURE 9 (13 iJi) (fig. 2), according to the state of secretory activity of thegland. The vesicles contain the colloid substance which is sochar
. The thyroid gland in health and disease. 300 /x. In shape theymay be rounded, long, oval, polyhedral, or occasionally they have side branches, or several vesicles mayintercommunicate. In health each vesicle is lined by a single layer of epitheliumwhich rests directly on the vascular connective tissue is no basement membrane. The vesicular epitheliumvaries in height from low cuboidal (9 /a) (fig. 1) to low cohunnar STRUCTURE 9 (13 iJi) (fig. 2), according to the state of secretory activity of thegland. The vesicles contain the colloid substance which is socharacteristic of the thyroid, and forms more than half of itstotal bnlk. When the organ is in the resting stage of secretionthe vesicles are distended with colloid and the epithelium liningthem is flattened out in proportion to the degree of distension(fig. 1). When, on the other hand, the gland is in a state ofactive secretion the vesicular cells are cuboidal or even columnarin form and there is but little colloid in the vesicles (fig. 2).. Fig. 1.—Section of normal thyroid of wild rat in colloid or resting state. flat vesicular epithelinm and vesicles distended with colloid. The protoplasm of the cells is retiform in structure and oftencontains highly refractive granules of varying size lying near thevesicular edge of the cell. The nuclei are spherical andshow a fine chromatin 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 si
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