Human Biology & Histology: The Mammary Gland and Endocrine System

The Mammary Gland

Structure and Function

The mammary gland is a specialized organ in mammals responsible for the production and secretion of milk. It is composed of glandular tissue, ducts, and supporting structures.

• Pectoral fascia: Connective tissue covering the pectoralis major muscle.

• Pectoralis major muscle: Major chest muscle underlying the gland.

• Intercostal muscles: Muscles between the ribs, providing structural support.

• Suspensory ligaments (Cooper's ligaments): Fibrous bands supporting the breast tissue.

• Lactiferous sinus: Dilated portion of the duct where milk accumulates before release.

• Lactiferous duct: Channels that transport milk from lobules to the nipple.

• Gland lobules: Clusters of alveoli where milk is produced.

• Fat: Adipose tissue providing shape and protection.

• Ribs and lung: Underlying structures supporting the breast.

Example: During lactation, the glandular tissue becomes more prominent, and the ducts expand to facilitate milk flow.

Inactive vs. Active Mammary Gland

• Inactive mammary gland: Characterized by small ducts and limited glandular tissue; seen in non-lactating individuals.

• Active mammary gland: Enlarged ducts and lobules, increased secretory activity; seen during pregnancy and lactation.

Example: Histological sections show more prominent alveoli and ductules in the active gland.

Hormonal Control of Mammary Gland Development

The development and function of the mammary gland are regulated by several hormones:

• Estrogen: Stimulates ductal growth and branching.

• Progesterone: Promotes development of glandular tissue and lobules.

• Prolactin: Stimulates milk production during lactation.

Example: During pregnancy, high levels of estrogen and progesterone prepare the gland for milk production, while prolactin initiates lactation after childbirth.

Modes of Milk Secretion

• Merocrine secretion: Water, proteins, lactose, minerals, and antibodies are secreted via exocytosis without loss of cytoplasm.

• Apocrine secretion: Lipids are secreted with a portion of the cell membrane, involving partial loss of cytoplasm.

Example: Casein (a milk protein) is secreted by merocrine mechanism, while milk fat globules are released by apocrine secretion.

The Endocrine System

Overview and Hormone Secretion

The endocrine system consists of glands and cells that secrete hormones directly into the bloodstream to regulate various physiological processes.

• Endocrine glands: Organs with exclusive endocrine function (e.g., thyroid gland).

• Mixed-function organs: Organs with both endocrine and exocrine functions (e.g., pancreas).

• Dispersed endocrine cells: Single hormone-secreting cells within other tissues (e.g., digestive tract epithelium).

Definition: Hormones are signaling molecules secreted by endocrine cells, distributed via the blood, and acting on distant target cells at very low concentrations (picomolar range).

Types of Hormonal Regulation

• Endocrine: Hormones travel through the bloodstream to distant targets.

• Paracrine: Hormones act on neighboring cells by diffusion.

• Autocrine: Hormones act on the same cell that secreted them.

Chemical Classification of Hormones

Class	Examples	Properties
Amines	Adrenaline (epinephrine)	Derived from amino acids
Peptides/Proteins	Insulin	Chains of amino acids
Steroids	Cortisol	Derived from cholesterol
Fatty acid derivatives	Prostaglandins	Derived from fatty acids

Hormone Receptors and Signal Transduction

• Membrane-bound receptors: For peptides and most amines; hormones cannot cross the cell membrane and bind to extracellular receptors, activating intracellular second messengers (e.g., cyclic AMP, DAG, calcium).

• Intracellular receptors: For steroids and fatty acid derivatives; hormones cross the membrane, bind cytoplasmic receptors, and the complex translocates to the nucleus to regulate gene expression.

Example: Cortisol (a steroid) binds to its receptor in the cytoplasm, and the complex enters the nucleus to modulate transcription.

The Pituitary Gland (Hypophysis)

Anatomy and Structure

The pituitary gland is a small endocrine organ located at the base of the brain, divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

• Anterior lobe (adenohypophysis): Produces and secretes several hormones.

• Posterior lobe (neurohypophysis): Stores and releases hormones produced by the hypothalamus.

Histology of the Anterior Lobe

• Chromophobes: Cells with little staining; function less clear.

• Acidophils: Stain with acidic dyes; include somatotropes (GH) and mammotropes (PRL).

• Basophils: Stain with basic dyes; include corticotropes (ACTH), thyrotropes (TSH), and gonadotropes (FSH, LH).

Hormones of the Anterior Lobe

Cell Type	Hormone	Function
Mammotropes (acidophils)	Prolactin (PRL)	Stimulates mammary gland growth and lactation
Somatotropes (acidophils)	Growth hormone (GH)	Stimulates bone and muscle growth
Corticotropes (basophils)	ACTH	Stimulates adrenal cortex hormone secretion
Thyrotropes (basophils)	TSH	Stimulates thyroid hormone secretion
Gonadotropes (basophils)	FSH, LH	Regulate gonadal function
Melanotropes (basophils, pars intermedia)	MSH	Stimulates melanocytes (function in humans unclear)

Regulation of Pituitary Hormone Secretion

• The hypothalamus controls the pituitary via releasing (RH) and inhibiting (IH) hormones.

• These hormones reach the anterior pituitary through the hypophyseal portal system.

• Feedback loops regulate hormone levels.

Histology and Function of the Posterior Lobe

• Contains axons from hypothalamic neurons and pituicytes (supporting cells).

• Stores and releases hormones produced in the hypothalamus.

Hormones of the Posterior Lobe

• Antidiuretic hormone (ADH, vasopressin): Stimulates water reabsorption in the kidneys via aquaporins; high doses cause vasoconstriction.

• Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding; also has psychotropic effects.

Example: Diabetes insipidus results from defects in ADH production, secretion, or receptor function, leading to excessive water loss.

The Adrenal Gland

Localization and Structure

The adrenal glands are paired organs located above the kidneys, each consisting of a cortex and medulla.

• Weight: 4-10 g each.

• Cortex: Outer region, subdivided into three zones.

• Medulla: Inner region, contains chromaffin cells.

Zones of the Adrenal Cortex

Zone	Main Hormones	Function
Zona glomerulosa	Mineralocorticoids (aldosterone)	Regulates sodium reabsorption in kidneys
Zona fasciculata	Glucocorticoids (cortisol)	Stress response, glucose metabolism, immune suppression
Zona reticularis	Androgens (androstenedione)	Weak masculinizing effects

Cells of the Adrenal Medulla

• Chromaffin cells: Modified postganglionic sympathetic neurons; secrete epinephrine (adrenaline) and norepinephrine (noradrenaline).

• Ganglion cells: Less numerous, involved in autonomic regulation.

Effects of Adrenal Medullary Hormones:

• β1-receptors: Increase heart rate.

• β2-receptors: Dilate coronary and skeletal muscle arteries.

• α-receptors: Vasoconstriction in abdominal organs, lungs, kidneys, and skin; stimulate glycogenolysis and lipolysis.

The Thyroid Gland

Structure and Topography

• Weight: 15-40 g.

• Lobes: Right and left lobes connected by an isthmus; sometimes a pyramidal lobe is present.

Histology of the Thyroid Gland

• Follicles: Spherical units lined by simple epithelium, filled with colloid (storage form of thyroid hormones).

• Colloid: Contains thyroglobulin, a precursor to thyroid hormones (T3, T4).

Hormones of the Thyroid Gland

• T3 (triiodothyronine) and T4 (thyroxine): Secreted by follicular cells; regulate metabolism, growth, and development.

• Calcitonin: Secreted by parafollicular (C) cells; lowers blood calcium levels.

Example: T4 is the main hormone secreted, but is converted to the more active T3 in peripheral tissues.

Disorders of the Thyroid Gland

• Struma (goiter): Enlargement of the thyroid gland, often due to iodine deficiency or hormonal imbalance.

The Parathyroid Glands

Structure and Function

• Small glands located on the posterior surface of the thyroid gland.

• Secrete parathyroid hormone (PTH), which increases blood calcium levels.

Regulation of Blood Calcium

Hormone	Source	Effect on Blood Calcium	Mechanism
Calcitonin	Thyroid (C cells)	Decreases	Inhibits osteoclasts, increases renal excretion
PTH	Parathyroid	Increases	Stimulates osteoclasts, increases renal reabsorption, activates vitamin D

Example: PTH and calcitonin act as antagonists to maintain calcium homeostasis.