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Medical Definition of Secondary follicle
1. A follicle in which the oocyte attains its full size and is surrounded by an extracellular glycoprotein layer (zona pellucida) that separates it from a peripheral layer of follicular cells permeated by one or more fluid-filled antra; the theca of the follicle develops into internal and external layers. Synonym: folliculus ovaricus vesiculosus, graafian follicle, secondary follicle. (05 Mar 2000)