Functional Anatomy of Endocrine Glands: Structure, Hormones, and Histology

Introduction to the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control growth, metabolism, reproduction, and homeostasis. Understanding the structure and function of each gland is essential for comprehending how the body maintains internal balance.

• Major Endocrine Glands: Hypothalamus, Pituitary gland, Thyroid gland, Parathyroid glands, Adrenal glands, Pancreas, Ovaries, Testes

• Key Concepts: Identification of glands, hormones produced, major functions, and target organs/cells

Pituitary Gland (Hypophysis)

Structure and Relationship to Hypothalamus

The pituitary gland, often called the "master gland," is located at the base of the brain and is connected to the hypothalamus. It consists of two main lobes: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The hypothalamus regulates pituitary function through releasing and inhibiting hormones.

• Anterior Pituitary (Adenohypophysis): Produces tropic and general hormones

• Posterior Pituitary (Neurohypophysis): Stores and releases hormones produced by the hypothalamus

Histological Features

• Anterior Pituitary: Appears more cellular and granular due to hormone-producing cells

• Posterior Pituitary: Contains more nerve fibers and appears lighter

Pituitary Hormones, Targets, and Effects

The pituitary gland secretes several hormones, each with specific targets and physiological effects. Tropic hormones primarily regulate other endocrine glands, while general hormones affect various tissues.

Hormone	Stimulus for Release	Target	Effects
Thyroid-stimulating hormone (TSH)	Thyrotropin-releasing hormone (TRH)	Thyroid gland	Stimulates secretion of thyroid hormones
Follicle-stimulating hormone (FSH)	Gonadotropin-releasing hormone (GnRH)	Ovaries and testes	Females: ovarian follicle maturation and estrogen; Males: sperm production
Luteinizing hormone (LH)	Gonadotropin-releasing hormone (GnRH)	Ovaries and testes	Females: ovulation and estrogen/progesterone; Males: testosterone production
Adrenocorticotropic hormone (ACTH)	Corticotropin-releasing hormone (CRH)	Adrenal cortex	Stimulates release of glucocorticoids and androgens
Growth hormone (GH)	Growth hormone-releasing hormone (GHRH)	Liver, muscle, bone, cartilage	Stimulates growth and protein synthesis
Prolactin (PRL)	A decrease in prolactin-inhibiting hormone	Mammary glands	Stimulates milk production

Hormone	Stimulus for Release	Target	Effects
Oxytocin (OXT)	Nerve impulses from hypothalamus	Uterus, mammary glands	Stimulates uterine contractions and milk ejection
Antidiuretic hormone (ADH)	Nerve impulses from hypothalamus	Kidneys	Stimulates water reabsorption, reduces urine output

Thyroid and Parathyroid Glands

Structure and Identification

The thyroid gland is located in the anterior neck, consisting of two lobes connected by an isthmus. The parathyroid glands are small, oval glands embedded on the posterior surface of the thyroid. Histologically, the thyroid contains spherical follicles lined by follicular cells, while the parathyroid contains principal (chief) cells.

• Thyroid Follicles: Spheres lined by simple cuboidal epithelium (follicular cells) that produce thyroid hormones (T3 and T4)

• Parathyroid Glands: Principal cells produce parathyroid hormone (PTH)

Hormones, Targets, and Effects

Hormone(s)	Stimulus for Release	Target	Effects
Thyroxine (T4), Triiodothyronine (T3)	Thyroid-stimulating hormone (TSH)	Most cells of the body	Increase basal metabolic rate (BMR), regulate growth and development
Calcitonin	High blood calcium	Bones	Lowers blood calcium (pharmacological effect)
Parathyroid hormone (PTH)	Low blood calcium	Bones, kidneys	Increases blood calcium by stimulating osteoclasts and kidney reabsorption

Adrenal Glands

Structure and Histology

The adrenal glands are located atop the kidneys and consist of an outer cortex and an inner medulla. The cortex is divided into three zones, each producing different hormones, while the medulla produces catecholamines.

• Zona Glomerulosa (outer): Produces mineralocorticoids (e.g., aldosterone)

• Zona Fasciculata (middle): Produces glucocorticoids (e.g., cortisol, cortisone)

• Zona Reticularis (inner): Produces androgens

• Medulla: Produces epinephrine and norepinephrine

Pancreas

Structure and Endocrine Function

The pancreas is a mixed gland with both exocrine and endocrine functions. The endocrine portion consists of clusters called pancreatic islets (Islets of Langerhans), which contain alpha and beta cells.

• Alpha (α) cells: Produce glucagon, which raises blood glucose

• Beta (β) cells: Produce insulin, which lowers blood glucose

Note: Only the endocrine function is covered here; exocrine function is discussed in the digestive system chapter.

Gonads: Testes and Ovaries

Testes

• Produce sperm and androgens (e.g., testosterone)

Ovaries

• Produce eggs, estrogens, and progesterone

Note: Gonads are covered in detail in the reproductive system chapter; here, focus is on their endocrine function.

Sample Lab Practical Questions

1. Which portion of the pituitary gland is highlighted? Answer: Posterior pituitary

2. Which structures are highlighted? Answer: Thyroid follicles

3. Which of the following hormones is produced from the highlighted organ? Answer: Glucagon

4. What is the result of secretions from the highlighted region? Answer: Increase blood pressure and heart rate