General structure and excretory pathways
Anatomically, the gland consists of 15–20 lobes. Their hemispherical shape and volume are supported by a developed layer of adipose tissue. The morphological hierarchy is structured according to the principle: lobe $\rightarrow$ lobule $\rightarrow$ terminal secretory unit.
The secretion outflow system is a complex network of tubes. Intralobular ducts merge into interlobular ones, which unite into a single lobe duct. All lobar ducts head towards the nipple. Before entering the nipple, they expand, forming lactiferous sinuses—local reservoirs for storing milk before feeding. At the apex of the nipple, the ducts narrow and open outward through lactiferous openings. It is important to note that there are 15–20 ducts, but only 8–15 openings, as some pathways merge distally to the sinuses.
As the duct enlarges, the epithelium lining it changes: from single-layered cuboidal in small branches, through columnar and double-layered, to stratified squamous in the nipple region.
Histophysiology at rest and during lactation
In the non-lactating mammary gland, secretory terminal units are poorly developed and represented by blind tubes—lactiferous alveolar ducts. Due to the abundant branching of the excretory pathways, the gland is classified as compound.
By the end of pregnancy, under the influence of high concentrations of progesterone, the gland transforms. The lactiferous ducts proliferate, forming new fully functional terminal structures—alveoli (acini). These are hollow sacs whose walls consist of two types of cells:
- Lactocytes (glandular cells): lie in a single layer on the basement membrane, are cuboidal in shape, with round nuclei and microvilli. They contain intracellular canaliculi for protein excretion.
- Myoepithelial cells: surround the alveolus from the outside like a basket. They are characterized by typical rod-shaped nuclei. Their task is to contract and squeeze the secretion into the ducts.
Features of the nipple and areola skin
Histologically, the nipple is a skin thickening penetrated by a system of lactiferous ducts. The epidermis and dermis in this zone contain a large number of melanocytes, which determines the specific pigmentation.
In the papillary layer of the dermis, there are numerous sensory nerve endings. Their stimulation during the act of suckling forms the afferent link of the neurohumoral milk ejection reflex. The reticular layer of the dermis contains bundles of smooth myocytes that circularly encompass the excretory ducts and help empty them.
Hair follicles are absent on the nipple, but there are sebaceous glands whose ducts may open directly into the milk pathways. Sweat glands are also present in the areola zone.
Mechanisms of cellular milk secretion
Lactocytes use different strategies to secrete milk components, which is clearly visible on histological preparations:
- Merocrine type (for proteins, lactose, and salts).
The synthesis of the main milk protein—casein—occurs in the developed granular ER. In the Golgi apparatus, it is packaged into granules. The granules approach the apical pole of the lactocyte, their membrane fuses with the plasmalemma, and the contents are expelled outward without damaging the cell itself (via exocytosis).
- Apocrine type (for lipids).
Large droplets of triglycerides accumulate at the apex of the cell. A droplet pushes the plasmalemma outward, the envelope encompasses it, after which the droplet detaches along with a fragment of the cell cytoplasm and membrane. Because of this, the apex of the lactocyte is temporarily destroyed.
Hierarchy of hormonal regulation
The control of mammary gland function proceeds in a cascade along the axis "Hypothalamus $\rightarrow$ Pituitary $\rightarrow$ Target organ":
- Central link: The hypothalamus releases thyroliberin (stimulates the pituitary) and dopamine (inhibits it).
- Prolactin (LTH): A hormone of the anterior pituitary gland. It has a direct effect on lactocytes, triggering intensive synthesis of milk components. Indirectly, it also maintains the corpus luteum, which produces progesterone.
- Oxytocin: Synthesized in the hypothalamus, but stored and released from the posterior pituitary gland. It is responsible for milk ejection: it binds to receptors on myoepithelial cells, forcing them to contract.
- Sex steroids: Estrogens stimulate the proliferation and branching of excretory ducts, while progesterone is necessary for the growth of secretory alveoli.
To avoid confusing the functions of sex hormones, remember: Estrogens are for Excretory pathways, Progesterone is for Pouches (terminal alveoli).
Class and exam questions
How does human milk fundamentally differ from cow's milk?
Human milk contains significantly less protein (1.0–1.5% vs. 3.0–4.0% in cows) and mineral salts (0.2% vs. 0.75%), but a higher carbohydrate content—lactose (up to 7.5%). The content of finely dispersed fats is approximately comparable, and water makes up about 87%.
Why is the non-lactating mammary gland classified as a compound gland?
The classification is based on the structure of the duct system. In the non-lactating gland, the secretory units themselves are poorly developed, but the system of intralobular and interlobular excretory ducts is highly branched, which is a sign of a compound gland.
What is the essence of the neurohumoral milk ejection reflex?
During suckling, the receptors of the nipple are stimulated. The nerve impulse goes to the hypothalamus, which leads to the immediate release of oxytocin from the pituitary gland into the blood. Oxytocin causes myoepithelial cells to contract, pushing milk from the alveoli into the lactiferous sinuses.
What else to study on this topic
- Cytophysiology of merocrine and apocrine secretion at the electron microscopic level
- Change of epithelial lining types throughout the milk pathways
- Comparative morphology of lactating and non-lactating gland lobules
- Direct and indirect action of the lactotropic hormone (prolactin)
- Interaction of myoepitheliocytes and lactocytes in the alveoli
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Full study guide on "Mammary glands" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the "Female reproductive system" section
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.