Page 93 - 86 human physiology part-2

Chapter 16

Structure

The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres
large) lined with milk-secreting epithelial cells and surrounded by myoepithelial cells. These alveoli
join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into
openings in the nipple. The myoepithelial cells can contract, similar to muscle cells, and thereby push
the milk from the alveoli through the lactiferous ducts towards the nipple, where it collects in
widenings (sinuses) of the ducts. A suckling baby essentially squeezes the milk out of these sinuses.

The development of mammary glands is controlled by hormones. The mammary glands exist in
both sexes, but they are rudimentary until puberty when - in response to ovarian hormones - they begin
to develop in the female. Estrogen promotes formation, while testosterone inhibits it.

At the time of birth, the baby has lactiferous ducts but no alveoli. Little branching occurs before
puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses
of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels
of estrogen and progesterone cause further branching and differentiation of the duct cells, together with
an increase in adipose tissue and a richer blood flow.

Colostrum is secreted in late pregnancy and for the first few days after giving birth. True milk
secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the
presence of the hormone prolactin. The suckling of the baby causes the release of the hormone
oxytocin which stimulates contraction of the myoepithelial cells.

The cells of mammary glands can easily be induced to grow and multiply by hormones. If this
growth runs out of control, cancer results. Almost all instances of breast cancer originate in the lobules
or ducts of the mammary glands.

STRUCTURE LOCATION & DESCRIPTION FUNCTION
Upper chest one on each side
containing alveolar cells (milk
Breasts production), myoepithelial cells Lactation milk/nutrition for newborn.
(contract to expel milk), and duct
walls (help with extraction of milk).
During childbirth, contractions of the uterus
will dilate the cervix up to 10 cm in diameter
The lower narrower portion of the
Cervix to allow the child to pass through. During
uterus.
orgasm, the cervix convulses and the external
os dilates
Small erectile organ directly in front
Clitoris Sexual excitation, engorged with blood.
of the vestibule.
Extending upper part of the uterus on Egg transportation from ovary to uterus
Fallopian tubes
either side. (fertilization usually takes place here).
Thin membrane that partially covers
Hymen
the vagina in young females.
Outer skin folds that surround the
Labia majora Lubrication during mating.
entrance to the vagina.
Labia minora Inner skin folds that surround the Lubrication during mating.

308 | Human Physiology