The Endocrine System, Female Physiology Before Pregnancy, and the Female hormones

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Female Hormonal system:

  1. Gonadotropin-releasing hormone (GnRH) from the hypothalamus that stimulates FSH and LH secretion from the anterior pituitary. GnRH is inhibited by circulating estrogen and progesterone
  2. LH is secreted from the basophillic cells of the anterior pituitary gland and stimulates the development of the corpus luteum in the ovaries.
  3. FSH is secreted from the basophillic cells of the anterior pituitary gland in response to GnRH and stimulates the follicles in the ovaries.
  4. Estrogen and Progesterone are the steroid hormones secreted by the follicle and corpus luthium of the ovary. 


Corpus luteum
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Section of the ovary. 1. Outer covering. 1’. Attached border. 2. Central stroma. 3. Peripheral stroma. 4. Bloodvessels. 5. Vesicular follicles in their earliest stage. 6, 7, 8. More advanced follicles. 9. An almost mature follicle. 9’. Follicle from which the ovum has escaped. 10. Corpus luteum.


Monthly Menstrual or Ovarian Cycle
File:MenstrualCycle3.png
Follicular Development
At the beginning of the monthly cycle, no mature follicles or lutea are present.
Estrogen and progesterone are at the lowest levels. No inibition for GnRH. The GnRH stimulates FSH and LH secretion from the anterior pituitary.


FSH stimulates 12 to 14 primary ovarian follicles. The follicles are surrounded by granulosa cells, that secrete fluid in the centre - the fluid filled antrum that expands the follicle. The ANTRIAL FOLLICLE -  fluid is rich in estrogen which diffuses in the blood and results in a progressive rise in estrogen levels. The follicle continues to increase in size stimulated by LH, FSH and estrogen. Proliferation of the granulosa continues with growth of the surrounding layer of thecal cells derived from the stroma of the ovary. - Vesicular Follicle


At one week - one of the follicles dominates and outgrows the others. The remaining follicles undergo atresia and degenerate. Stimulated by FSH and Estrogen. Estrogen stimulates the increase of receptors on the granulosa and thecal cells, providing positive feedback.
The GnRH is now inhibited, but the dominant follicle continues to develop as it uses its own estrogen as stimulus


OVULATION
12 days after initiation of follicular development - the pituitary produces a surge of LH secretion. The thecal cell begin to produce progesterone for the first time. The thecal cells secrete fluid into the vesicle. Weakness in the wall of the ovary develops and the wall raptures, and the follicle evaginates and the oocyte and the surrounding layers of granulosa tissue - referred to as the corona radiata, leave the vessicle and enter the fallopian tubes.


CORPUS LUTEUM
The structure of the follicle remaining on the surface of the ovary after ovulation contains layers of granulosa and thecal cells. The high concentration of LH before ovulation turns these cells into LUTEIN CELLS, become yellow and are called the CORPUS LUTEUM
Secrete progesterone - after 12 days the LH levels are at  a minimum and the Menstruation begins

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