Page 311 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Oogenesis and the Menstrual Cycle In addition to general changes in the body
and mood, the following changes occur in the
Oogenesis. The development of the female gametes ovaries, uterus and cervix during the men-
(ova) extends from the oogonium stage to the pri- strual cycle (! A):
mary oocyte stage (in the primordial follicle), Day 1: Start of menstruation (lasting about
starting long before birth. Oogenesis therefore oc- 2–6 days).
curs much sooner than the corresponding stages Days 1–14 (variable, see above): The follicu-
of spermatogenesis (! p. 306). The fetal phase of
oogenesis is completed by the first week of gesta- lar phase starts on the first day of menstrua-
tion; these oocytes remain latent until puberty. In tion. The endometrium thickens to become
the sexually mature female, a fertilizable ovum prepared for the implantation of the fertilized
Hormones and Reproduction Menstrual cycle. After the start of sexual matu- inant follicle, which produces increasing quan-
ovum during the luteal phase (! A5), and
develops in the graafian follicles approximately every
28 days.
about 20 ovarian follicles mature under the in-
fluence of FSH. One of these becomes the dom-
ration, a woman starts to secrete the following
tities of estrogens (! A4 and p. 300). The small
hormones in a cyclic (approx.) 28-day rhythm
cervical os is blocked by a viscous mucous
(! A1, A2). Gonadoliberin (= gonadotropin-re-
plug.
leasing hormone, Gn-RH) and dopamine (PIH)
Day 14 (variable, see above): Ovulation. The
are secreted by the hypothalamus. Follicle-
amount of estrogens produced by the follicle
stimulating hormone (FSH), luteinizing hor-
(! A2). The increased secretion of LH in re-
the anterior pituitary. Progesterone, estrogens
sponse to higher levels of estrogen leads to
(chiefly estradiol, E 2) and inhibin are secreted
11 mone (LH) and prolactin (PRL) are released by increases rapidly between day 12 and 13
by the ovaries. Gn-RH controls the pulsatile ovulation (! A1, A4; see also p. 300). The basal
secretion of FSH and LH (! p. 300), which in body temperature (measured on an empty
turn regulate the secretion of estradiol and stomach before rising in the morning) rises
progesterone. The female sex functions are about 0.5!C about 1–2 days later and remains
controlled by the periodic release of hor- elevated until the end of the cycle (! A3). This
mones, the purpose of which is to produce a temperature rise generally indicates that ovu-
fertilizable egg in the ovaries each month lation has occurred. During ovulation, the cer-
(! A4) and produce an environment suitable vical mucus is less viscous (it can be stretched
for sperm reception (fertilization) and implan- into long threads—spinnbarkeit) and the cervi-
tation of the fertilized ovum (nidation) (! A5). cal os opens slightly to allow the sperm to en-
This cyclic activity is reflected by the monthly ter.
menses (menstruation) which, by definition, Days 14–28: The luteal phase is character-
marks the start of the menstrual cycle. ized by the development of a corpus luteum
(! A4), which secretes progesterone, (! A2);
Girls in Central Europe usually have their first men- an increase in mucoid secretion from the
strual period (menarche) around the age of 13. By uterine glands also occurs (! A5). The endo-
about age 40, the cycle becomes increasingly irregu-
lar over a period of up to 10 years (climacteric) as the metrium is most responsive to progesterone
end of the reproductive period nears. The last around the 22nd day of the cycle, which is
menses (menopause) generally occurs around the when nidation should occur if the ovum has
age of 48–52. been fertilized. Otherwise, progesterone and
estrogens now inhibit Gn-RH secretion
A menstrual cycle can last 21–35 days. The sec- (! p. 300), resulting in degeneration of the
ond half of the cycle (luteal phase = secretory corpus luteum. The subsequent rapid decrease
phase) usually lasts 14 days, while the first half
(follicular phase = proliferative phase) lasts in the plasma concentrations of estrogens and
progesterone (! A2) results in constriction of
7–21 days. Ovulation separates the two phases endometrial blood vessels and ischemia. This
(! A). If the cycle length varies by more than
2–3 days, ovulation generally does not occur. ultimately leads to the breakdown and dis-
charge of the uterine lining and to bleeding,
298 Such anovulatory cycles account for 20% of all i.e., menstruation (! A5).
cycles in healthy females.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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