Follicular phase


Follicular phase

The follicular phase (or proliferative phase) is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. It ends with ovulation. The main hormone controlling this stage is estradiol. [cite paper | author = Bagnell, C | title = Animal Reproduction | publisher = Rutgers University Department of Animal Sciences | year = 2005 ]

Hormonal events

Follicle recruitment

Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland. FSH secretion begins to rise in the last few days of the previous menstrual cycle, cite book | first=Toni | last=Weschler | year=2002 | title=Taking Charge of Your Fertility | pages=pp.359-361 | edition=Revised Edition | publisher=HarperCollins | location=New York | id=ISBN 0-06-093764-5 ] and is highest and most important during the first week of the follicular phase. cite book | author = Dee Unglaub Silverthorn | title = Human physiology: an integrated approach | publisher = Pearson/Benjamin Cummings | location = San Francisco | year = 2004 | id = ISBN 0-13-102015-3 | edition = Third edition | pages = Chapter 26: Reproduction and Development, and Chapter 23 Endocrine control of growth and metabolism ] The rise in FSH levels recruits five to seven tertiary-stage ovarian follicles (this stage follicle is also known as a "Graffian follicle" or "antral follicle") for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance.

FSH induces the proliferation of granulosa cells in the developing follicles, and the expression of luteinizing hormone (LH) receptors on these granulosa cells. Under the influence of FSH, aromatase and p450 enzymes are activated, causing the granulosa cells to begin to secrete estrogen. This increased level of estrogen stimulates production of gonadotropin-releasing hormone (GnRH), which increases production of LH. LH induces androgen synthesis by thecal cells, stimulates proliferation, differentiation, and secretion of follicular thecal cells and increases LH receptor expression on granulosa cells.

Estrogen surge

Two or three days after LH levels begin to increase, usually by day seven of the cycle, cite book | author = Stenchever Droegemuller Herbst Mishell| title = Comprehensive Gynecology | publisher = Mosby | location = St. Louise, Missouri | year = 2001 | id = ISBN 0-323-01402-X | edition = Fourth edition | page = page 87 ] one (or occasionally two) of the recruited follicles has emerged as dominant. Many endocrinologists believe that the estrogen secretion of the dominant follicle has increased to a level that GnRH production is suppressed, which lowers the levels of LH and FSH. This slowdown in LH and FSH production leads to the "atresia" (death) of most of the recruited follicles, though the dominant follicle continues to mature. Estrogen levels will continue to increase for several days (on average, six days, but this varies widely).

These high estrogen levels initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. Crypts in the cervix are also stimulated to produce fertile cervical mucus. This mucus reduces the acidity of the vagina, creating a more hospitable environment for sperm. [Weschler (2002), p.57] It also has a characteristic texture that helps guide sperm through the cervix [Weschler (2002), illustrations p.59 and p.3 of color insert] and to the fallopian tubes, where they wait for ovulation. [cite web | last = Ellington | first = Joanna | title = Sperm Transport to the Fallopian Tubes | work = Frequently Asked Questions with Dr. E | publisher = INGfertility Inc | year = 2004 | url = http://www.ingfertility.com/FAQs.html#Sperm_Leaking_Out_After_Intercourse-_Lessons_in_Sperm_Transport_Through_the_Cervix_ | accessdate = 2006-08-13 ] In addition, basal body temperature may lower slightly under the influence of high estrogen levels. [Weschler (2002), pp.54,306,310]

LH surge and ovulation

Estrogen levels are highest right before the LH surge begins. The short-term drop in steroid hormones between the beginning of the LH surge and the event of ovulation may cause mid-cycle spotting or bleeding. [Weschler (2002), p.65] Under the influence of the preovulatory LH surge, the first meiotic division of the oocytes is completed. The surge also initiates luteinization of thecal and granulosa cells. Ovulation normally occurs 30 (± 2) hours after the beginning of the LH surge (when LH is first detectable in urine). [cite journal | author= Susan B. Bullivant, Sarah A. Sellergren, Kathleen Stern, et al | title= Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone | journal=Journal of Sex Research| month=February | year=2004 | volume=41 | issue=1 | pages=82–93 | url=http://www.findarticles.com/p/articles/mi_m2372/is_1_41/ai_n6032944 | pmid=15216427]

Follicular waves

The time from recruitment of tertiary stage follicles to ovulation is normally about two weeks, comprising days 1-14 of an idealized 28-day menstrual cycle. However, it is common for the follicular phase to last much longer, as sometimes no dominant follicle is selected. In this case, normally more tertiary stage follicles are recruited and the process begins again. One study of women with regular menstrual cycles found that none of them ovulated after the first wave of follicle development: 68% of cycles showed ovulation after the second wave, while 32% of cycles showed ovulation after the third wave. [cite journal | author = Baerwald A, Adams G, Pierson R | title = A new model for ovarian follicular development during the human menstrual cycle. | journal = Fertil Steril | volume = 80 | issue = 1 | pages = 116–22 | year = 2003 | pmid = 12849812 | doi = 10.1016/S0015-0282(03)00544-2] Fertility awareness sources may refer to multiple waves of follicular development as a "split peak" because of the characteristic pattern of cervical mucus changes that is sometimes observed. [Weschler (2002), pp.131-2]

References


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Look at other dictionaries:

  • follicular phase — the first part of the ovarian cycle, lasting from the end of the menstrual phase until ovulation and corresponding to the proliferative phase of the uterine cycle; it is characterized by the development of a dominant ovarian follicle, which… …   Medical dictionary

  • follicular phase — a stage of the menstrual cycle, from onset of menstruation to ovulation. Cf. luteal phase. * * * …   Universalium

  • follicular phase — a stage of the menstrual cycle, from onset of menstruation to ovulation. Cf. luteal phase …   Useful english dictionary

  • Proliferative phase — can refer to: * a phase of wound healing * another name for the follicular phase of the menstrual cycle …   Wikipedia

  • menstrual phase — the phase of the human menstrual cycle, following the luteal phase and occurring only if fertilization has not taken place. The corpus luteum regresses and is shed through menstruation and growth begins for the ovarian follicle, leading to the… …   Medical dictionary

  • luteal phase — noun the second half of the menstrual cycle after ovulation; the corpus luteum secretes progesterone which prepares the endometrium for the implantation of an embryo; if fertilization does not occur then menstrual flow begins • Syn: ↑secretory… …   Useful english dictionary

  • proliferative phase — the phase of the uterine cycle, corresponding to the follicular phase of the ovarian cycle, during which the functional layer of the endometrium is repaired and proliferates under the influence of estrogen following menstruation …   Medical dictionary

  • luteal phase — Physiol. a stage of the menstrual cycle, lasting about two weeks, from ovulation to the beginning of the next menstrual flow. Cf. follicular phase. * * * …   Universalium

  • Menstrual cycle — See also: Menstruation and Menstruation (mammal) The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women. This article focuses on the human menstrual cycle. The menstrual cycle, under the… …   Wikipedia

  • Ovulation — is the process in a female s menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg). Ovulation also occurs in the estrous cycle of other female mammals,… …   Wikipedia


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