When a primordial follicle begins the changes leading to the formation of a mature follicle, the layer of squamous follicular cells becomes cuboidal, as in this figure. In addition, the follicular cells proliferate and become multilay-
ered. A follicle undergoing these early changes is called a primary follicle. Thus, an early primary follicle may still be unilaminar, but it is surrounded by cuboidal cells, and this distinguishes it from the more numerous unilaminar primordial follicles that are surrounded by squamous cells.
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