= Luteinization
<Luteinization> is the differentiation and reorganization of follicular cells into the steroid-secreting cells of the <corpus luteum>. <Granulosa cells> and <theca cells> change their endocrine activity, with increased capacity for <progesterone> production. In the normal ovarian cycle, the <luteinizing hormone> surge coordinates this process with <ovulation>. Release of the <oocyte> and transformation of the remaining follicular tissue are nevertheless distinct events.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section27.2: ovulation,corpus-luteum formation and steroid secretion]{ref}
A functional experiment examines what the differentiated cells actually make. In a 1991 study, human luteinized granulosa cells were cultured for 48 hours with or without <human chorionic gonadotropin> or <follicle-stimulating hormone>. The investigators measured several steroids and assayed <aromatase> activity separately. <Progesterone> production was prominent and responsive to stimulation, demonstrating endocrine activity in the luteinized preparation.<Regulation of steroid and steroid sulfate production and aromatase activity in cultured human granulosa-luteal cells by J. Tapanainen and others>[Abstract:48h culture,steroid production and separate aromatase assay]{ref}
This motivates a useful distinction between cell state and one of its outputs. Increased <progesterone> is evidence of altered steroid production, but one concentration measurement does not by itself prove that a whole follicle has ruptured, that an oocyte was released, or that every structural step of luteinization occurred. Comparing <histology> with biochemical measurements makes the interpretation more precise.
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