Follicular phase
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The follicular phase (or proliferative phase) is the beginning phase of the estrous cycle in animals. It begins with the regression of the corpus luteum (called luteolysis) and ends with ovulation. The main hormone controlling this stage is estradiol.
Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland and is essential for follicular growth until the antrum develops. FSH secretion is highest and most important during the first week of the follicular stage of the menstrual cycle. FSH induces estrogen and progesterone secretion by activating aromatase and p450 enzymes and causes negative feedback on Gonadotropin-releasing hormone (GnRH) secretion. FSH also induces the proliferation of granulosa cells and the expression of LH receptors on these granulosa cells. Luteinizing hormone (LH) is secreted by the anterior pituitary gland and is required for growth of preovulatory follicle as well as the luteinization and ovulation of the dominant follicle. During the follicular phase of the menstrual cycle, LH induces androgen synthesis by thecal cells, stimulates proliferation, differentiation, and secretion of follicular thecal cells and increases LH receptor expression on granulosa cells. The preovulatory LH surge starts the first meiotic division in oocytes and initiates luteinization of thecal and granulosa cells. The resulting corpus luteum produces high levels of progesterone and some estrogen. This estrogen produced at the level of the ovary is crucial for the development of the antrum and maturation of the Graafian follicle. Estrogen levels are highest at the end of the follicular phase, directly preceding ovulation.
[edit] Female Human Follicular Phase
During the follicular phase, the follicles within the ovary develop and begin to secrete the hormone estrogen. As follicles continue to develop, estrogen levels in the blood increase. In a 28-day mentrual cycle, the follicular phase normally takes place from days 6 to 13.
[edit] References
- Bagnell, C. 2005. "Animal Reproduction". Rutgers University Department of Animal Sciences.
- H.Cohen. Molsen Medical Informatics Project. McGill Medicine 2000, accessed via http://sprojects.mmi.mcgill.ca/.
- D. Silverthorn. Chapter 26: Reproduction and Development. Human Physiology, Third edition 2004 p 810-818 (Female Reproduction).
- Loucks A, Verdun M, Heath E (1998). "Low energy availability, not stress of exercise, alters LH pulsatility in exercising women.". J Appl Physiol 84 (1): 37-46. PMID 9451615.
- D. Silverthorn. Chapter 23 Endocrine control of growth and metabolism. Human Physiology, Third Edition 2004.
- Nelson- Science 9. Chapter 7, Hormones and the Reproductive Cycle, Pg. 220
