Describe what is happening during the female monthly cycle related to:
a. fsh, lh, estrogens, and progesterones
b. the follicles in the ovaries
c. the lining of the uterus

All women have to go through what is known as the menstrual cycle. This process allows a woman to be able to prepare for, and conceive, a baby, and carry him/her to full term. As such, there are a series of steps that the female body takes, both physically and physiologically, spurred by the rise and fall of hormones, to ensure the smoothness of the process and the possibility of conception.

The cycle is divided into 4 stages: the menstrual, the follicular, the ovulating, and the luteal stage. Each of them has a play of hormones that will play their part in helping the process be successful. These hormones are: FSH (Follicle Stimulating Hormone released by the Pituitary gland), LH (Luteinizing Hormone, also released by the Pituitary), estrogens, progesterone, and in the case of a pregnancy, hCG (Human Chorionic Gonadotropin, or the pregnancy hormone).

So, as to the question, the play of hormones, and how the cycle develops, would be like this:

1. Menstruation phase: After the late released egg is not fertilized during its timeframe of 24 hours, estrogen and progesterone levels drop, producing reactions in the body: the egg is sheded, the uterine wall lining is shedded, and blood, tissue and mucus comes out.

2. Follicular phase: Almost overlapping, the hypothalamus orders the Pituitary gland to start releasing FSH. This hormone will stimulate the ovaries to begin producing follicles, where the immature eggs are. Around 5 to 20 of them will begin getting matured for release. Only one, in rare ocassions 2, will end the process; the rest, will be reabsorbed by the ovaries. The follicle starts to release estrogens and progesterones that will signal the uterus to begin building up the walls with a lining rich in nutrients, in case of pregnancy.

3. Ovulating, or ovulation phase: As the maturation of the egg continues, the follicles get ready to release it into the fallopian tubes. As this happens, the Pituitary, stimulated by the rising levels of estrogens and progesterones, and by the work of the hypothalamus, releases LH, which will play a part not only in the final maturation of the egg, and the lining, but also in maintaining the levels of estrogen and progesterone so that a pregnancy is successful. At this point, the egg is released by the follicle and the process down the tube begins towards the uterus. It is here that pregnancy can take place.

4. Luteal phase: The follicle that releases the egg, becomes later on the corpus luteum, which will further produce estrogen and especially progesterone, to maintain the lining once the fertilized egg has implanted. If not, it will be reabsorbed and the declining levels of hormones will signal the body to initiate the menstrual phase.

the african cheetah.

this species has a relatively low level of genetic

variation because the population was reduced to a small size approxi-

mately 10,000 to 12,000 years ago.

chapter   outline

24.1

genes in populations and the hardy-

weinberg equation

24.2

factors that change allele and genotype

frequencies in populations

24.3     sources of new genetic variation

population   genetics

24

until   now,   we   have   primarily   focused   our   attention   on   genes  

within   individuals   and   their   related   family   members.   in   this  

chapter   and   chapters   25   and   26   ,   we   turn   to   the   study   of   genes   in  

a   population   or   species.   the   field   of

population   genetics

  is   con-

cerned   with   changes   in   genetic   variation   within   a   group   of   indi-

viduals   over   time.   population   geneticists   want   to   know   the   extent  

of   genetic   variation   within   populations,   why   it   exists,   and   how   it  

changes   over   the   course   of   many   generations.   the   field   of   popu-

lation   genetics   emerged   as   a   branch   of   genetics   in   the   1920s   and  

1930s.   its   mathematical   foundations   were   developed   by   theoreti-

cians   who   extended   the   principles   of   gregor   mendel   and   charles  

darwin   by   deriving   formulas   to   explain   the   occurrence   of   geno-

types   within   populations.   these   foundations   can   be   largely   attrib-

uted to three scientists: sir ronald fisher, sewall wright, and j. b. s.

haldane.   as   we   will   see,   support   for   their   mathematical   theories  

was provided by several researchers who analyzed the genetic com-

position   of   natural   and   experimental   populations.   more   recently,  

population   geneticists   have   used   techniques   to   probe   genetic  

variation   at   the   molecular   level.   in   addition,   staggering   advances  

in   computer   technology   have   aided   population   geneticists   in   the  

analysis   of   their   genetic   theories   and   data.   in   this   chapter,   we   will  

explore   the   genetic   variation   that   occurs   in   populations   and   con-

sider   the   reasons   why   the   genetic   composition   of   populations   may  

change over the course of several generations