Breast: anatomy, embryology, histology, prolactin and lactation

Overview

The breast (mammary gland) is a modified sweat gland of the skin, present in both sexes but developed in women at puberty and fully mature only in pregnancy and lactation. PGET questions test:

• position, lobes, ligaments, blood supply and lymph drainage;
• embryology (milk line, extra nipples, Poland syndrome);
• histology (ducts, lobules, myoepithelial cells);
• the hormones of milk production and ejection and the content of milk.

Anatomy

Position and shape

• Lies in the superficial fascia over the pectoralis major, with parts on serratus anterior and external oblique.
• Base extends from the 2nd to the 6th rib, and sideways from the sternal edge as far as the midaxillary line.
• The axillary tail (of Spence) passes through the deep fascia at the lower edge of pectoralis major into the axilla.
• A layer of loose tissue, the retromammary space, separates the gland from the deep fascia, so a normal breast moves over the muscle.

Structure

• 15-20 lobes arranged like spokes around the nipple; the gland has no capsule.
• Each lobe drains by its own lactiferous duct, which widens into a lactiferous sinus just behind the nipple and then opens separately on the nipple.
• Suspensory ligaments (Cooper's ligaments): fibrous bands from the skin to the deep fascia that separate the lobes.
• Areola: pigmented skin around the nipple with sebaceous glands. In pregnancy it darkens and the glands enlarge as Montgomery's tubercles.

Blood supply

Veins follow the arteries to the axillary and internal thoracic veins; intercostal veins connect with the vertebral venous plexus (a route for spread to the spine).

Lymphatic drainage

Axillary nodes are described in three levels by their relation to pectoralis minor: below, behind and above it.

Nerve supply

Anterior and lateral cutaneous branches of the 4th to 6th intercostal nerves. The nipple usually lies at the T4 dermatome. These sensory nerves carry the suckling signal to the hypothalamus.

Embryology

• In the embryo a band of thickened ectoderm, the milk line (mammary ridge), runs from the axilla to the groin on each side.
• Normally only a small part in the pectoral region persists and grows down into the mesenchyme to make the ducts; the rest disappears.
• At puberty, ovarian oestrogen makes the female breast grow, mainly by fat deposition and duct lengthening.

Histology

• A compound tubuloalveolar gland. The terminal duct lobular unit (TDLU) (terminal duct plus its lobule of small ductules) is the functional unit and where most fibroadenomas, cysts and carcinomas arise.
• Ducts and alveoli have two cell layers:
  • inner luminal epithelial cells (secretory in lactation);
  • outer myoepithelial cells, which contract and keep a basement membrane. Their loss marks invasion in carcinoma.
• Stroma: loose, hormone-responsive stroma inside lobules; dense fibrous tissue and fat between lobules.
• Male and prepubertal breast: ducts only, with no lobules.

Hormones of breast development

Prolactin and milk production

• Prolactin is a polypeptide from the lactotroph cells of the anterior pituitary.
• Control is mainly inhibitory: the hypothalamus releases dopamine (prolactin-inhibiting hormone) from the arcuate nucleus into the portal vessels. Cutting the stalk or blocking dopamine (for example with antipsychotics, metoclopramide, methyldopa) raises prolactin. TRH and oestrogen stimulate prolactin release.
• In pregnancy, prolactin rises from early pregnancy to about 10-20 times the non-pregnant level at term.
• During pregnancy the high placental oestrogen and progesterone block milk secretion, so only small amounts of colostrum leak.
• After delivery of the placenta, oestrogen and progesterone fall sharply, prolactin acts unopposed and milk "comes in" over the first few days.
• Suckling sends nerve signals from the nipple to the hypothalamus, cutting dopamine release. Each feed gives a surge of prolactin that peaks within about 45 minutes and keeps the next supply going. If suckling stops, milk production fails within about a week.
• Prolactin also suppresses pulsatile GnRH and LH, giving lactational amenorrhoea and a contraceptive effect that is not fully reliable.

Clinical links

• Sheehan syndrome: pituitary infarction after severe postpartum haemorrhage; failure of lactation is usually the earliest sign, as prolactin production stops.
• Suppression of lactation: dopamine agonists (bromocriptine, cabergoline) lower prolactin, but cause drowsiness, low blood pressure, headache and stomach upset; Ten Teachers prefers a firm bra and fluid restriction where suppression is wanted.
• Hyperprolactinaemia (prolactinoma, drugs, hypothyroidism) gives galactorrhoea and amenorrhoea.

Oxytocin and the let-down (milk ejection) reflex

• Oxytocin is made by neurons whose cell bodies lie in the hypothalamic supraoptic and paraventricular nuclei; it is released from the posterior pituitary in pulses during suckling.
• It contracts the myoepithelial cells around the alveoli and along the ducts, squeezing milk to the nipple within a minute of the baby latching.
• The reflex can be conditioned by seeing, hearing or smelling the baby (crying) and is inhibited by stress, pain and anxiety.
• It also contracts the uterus, causing "after-pains" during feeds and helping involution.

Colostrum and breast milk

Colostrum

• A yellow fluid that can be expressed from about 16 weeks of pregnancy; replaced by milk within the first few days after birth.
• More protein (mainly globulins, especially IgA), less sugar and fat than mature milk.
• Protects against infection and has a laxative effect that helps pass meconium.

Breast milk versus cow's milk

Breastfeeding in practice

• Good attachment: the baby takes the nipple and much of the areola, drawing a teat back to the junction of the hard and soft palate; the tongue milks it against the palate. A correctly attached feed should be painless.
• Feed on demand; avoid fixed schedules and supplementary water, glucose or formula, which reduce milk supply.
• The commonest reasons women stop are "not enough milk" and sore, cracked nipples, both usually due to poor positioning.
• Exclusive breastfeeding is promoted in Pakistan as in WHO guidance; check the current national guideline for the recommended duration.

Lactation problems

Quick revision

• 2nd-6th rib; 15-20 lobes; one duct per lobe; Cooper's ligaments.
• Lymph: lateral → axillary; medial → internal thoracic.
• Milk line: axilla to groin; extra nipples along it.
• TDLU = functional unit; myoepithelial outer layer.
• Oestrogen → ducts; progesterone → alveoli.
• Prolactin makes milk; dopamine inhibits it; placental steroids block it until delivery.
• Oxytocin ejects milk (myoepithelium); stress blocks it.
• Colostrum: protein and IgA; breast milk lacks vitamin K.

Written by The House Job from standard references (Obstetrics by Ten Teachers; Guyton and Hall Textbook of Medical Physiology; Snell's Clinical Anatomy).

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