Biology 12

198 MHR • Unit 2 Homeostasis

Female Reproductive Hormones

Typically, puberty begins at age nine or 10 in North

American girls but significantly later in many

countries. A 1997 study showed some girls begin

puberty as early as three years old. Although this is

unusual, there is a trend toward earlier onset of

puberty than has historically been the case. To a

certain extent, this lowered age can be attributed to

better diet, but many scientists are concerned. They

are looking for other social or environmental

factors that may be influencing this trend.

Puberty is triggered by the hypothalamus, which

secretes releasing factors to begin the production of

the reproductive hormones. Hormone levels

typically rise gradually from ages eight to 12, and

then rise sharply in the early teens. These

hormones stimulate the development of the female

secondary sexual characteristics, which include the

development of breasts, the growth of hair around

the genitals and under the arms, the widening of

the hips, and an increase in body fat.

While the female reproductive system is not

affected by season or location, the reproductive

hormones follow a cyclical pattern. The release of

an ovum is timed to coincide with the changes in

the uterus that make it receptive to the fertilized

ovum. This menstrual cycleis usually about

28 days long, although it commonly varies from

20 to 45 days and can differ from month to month.

The menstrual cycle is controlled by the

hypothalamus, which excretes releasing factors for

the FSH and LH that are released from the anterior

pituitary gland. The ovary is composed of many

groups of cells called follicles, each of which

contains a single ovum. Although a woman is born

with over two million follicles, only approximately

The first menstruation does not occur until a girl has at

least 17 percent body fat. This accounts for the fact that

some young athletes and dancers have delayed

menstruation. In older women, menstruation stops if body

fat is less than 22 percent. This prevents pregnancy if a

woman’s body is unable to carry a baby.

BIO FACT

oviduct

ovary

uterus

urinary

bladder

pubic symphysis

urethra

clitoris

labium minora

labium majora

vaginal orifice

ureter

fimbriae

cervix

rectum

vagina

anus

Figure 6.26The ovaries release one egg a month; fertilization occurs in the

oviduct, and development occurs in the uterus. The vagina is the birth canal

and the organ of sexual intercourse.