The corpus luteum is a temporary endocrine structure formed from the ovarian follicle after ovulation. Its luteinized cells produce substantial progesterone, helping maintain the secretory state of the endometrium. Without sustained hormonal support it regresses; in early pregnancy, human chorionic gonadotropin helps preserve its steroid production.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 27.2, ovulation, corpus luteum and ovarian-cycle hormone changes
It illustrates that an endocrine gland need not be a permanently separate organ. A local tissue is reorganized into a hormone-secreting structure and later changes again. A measured progesterone concentration reflects both this production and removal from the blood, so it is not a direct count of luteal cells.
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