Unit 16: The Endocrine System – Structure, Function, and Histology

Unit 16: The Endocrine System

Overview of 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. The system includes both primary endocrine organs and neuroendocrine organs that interact closely with the nervous system.

• Endocrine glands: Ductless glands that release hormones into the blood.

• Hormones: Chemical messengers that act on target cells to elicit specific responses.

• Neuroendocrine organs: Structures like the hypothalamus that integrate neural and endocrine functions.

Major Endocrine Organs and Their Hormones

Each endocrine gland produces specific hormones that regulate distinct physiological functions. The following table summarizes the main glands, their hormones, and primary actions:

Gland	Hormone(s)	Main Function(s)
Hypothalamus	Releasing & inhibiting hormones, Oxytocin, ADH	Regulates pituitary; water balance; uterine contractions
Pituitary (Anterior)	TSH, GH, Prolactin, ACTH, LH, FSH	Stimulates thyroid, growth, lactation, adrenal cortex, gonads
Pituitary (Posterior)	Stores Oxytocin, ADH	Does not produce hormones; releases those made by hypothalamus
Pineal	Melatonin	Regulates sleep-wake cycles
Thyroid	Thyroxine (T4), Triiodothyronine (T3), Calcitonin	Metabolic rate, calcium homeostasis
Parathyroid	PTH	Increases blood calcium
Thymus	Thymosin, Thymopoietin	T cell development
Adrenal Cortex	Mineralocorticoids, Glucocorticoids, Gonadocorticoids	Electrolyte balance, stress response, sex hormones
Adrenal Medulla	Epinephrine, Norepinephrine	Fight or flight response
Pancreas	Insulin, Glucagon	Blood glucose regulation

Hormonal Regulation and Feedback Loops

Hormone secretion is tightly regulated by negative feedback mechanisms to maintain homeostasis. A stimulus triggers hormone release, which acts on target cells to correct the disturbance. Once balance is restored, hormone secretion decreases.

• Negative feedback: The end product inhibits its own production by suppressing the initial stimulus.

• Example: Blood glucose regulation by insulin and glucagon.

Endocrine Histology

Pituitary Gland

The pituitary gland is divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes. The anterior pituitary contains hormone-secreting cells, while the posterior stores and releases hormones produced by the hypothalamus.

• Anterior pituitary: Densely packed cells, stains darker.

• Posterior pituitary: Contains nerve fibers, stains lighter.

Thyroid Gland

The thyroid gland is composed of spherical follicles filled with colloid. Follicle cells produce thyroid hormones, while parafollicular cells secrete calcitonin.

• Thyroid follicles: Spheres lined by follicle cells, filled with colloid (thyroglobulin).

• Parafollicular cells: Located between follicles, secrete calcitonin.

Adrenal Gland

The adrenal gland consists of an outer cortex and inner medulla. The cortex is divided into three zones, each producing different steroid hormones.

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

• Zona fasciculata: Produces glucocorticoids (e.g., cortisol).

• Zona reticularis: Produces gonadocorticoids (sex hormones).

• Medulla: Produces catecholamines (epinephrine, norepinephrine).

Pancreas

The pancreas contains both exocrine (acinar cells) and endocrine (islets of Langerhans) tissue. The islets secrete insulin and glucagon to regulate blood glucose.

• Acinar cells: Secrete digestive enzymes (exocrine function).

• Pancreatic islets: Contain alpha cells (glucagon) and beta cells (insulin).

Thymus

The thymus is prominent in children and is the site of T cell maturation. It contains lobules with an outer cortex and inner medulla.

• Cortex: Densely packed with developing T cells.

• Medulla: Contains fewer, more mature T cells.

Summary Table: Key Endocrine Glands and Hormones

Gland	Hormone(s)	Main Target(s)	Main Effect(s)
Hypothalamus	Releasing/inhibiting hormones, ADH, Oxytocin	Pituitary, kidneys, uterus	Pituitary regulation, water retention, uterine contraction
Pituitary (Ant.)	TSH, GH, ACTH, LH, FSH, Prolactin	Thyroid, bones, adrenals, gonads, mammary glands	Growth, metabolism, stress, reproduction, lactation
Pituitary (Post.)	ADH, Oxytocin	Kidneys, uterus, mammary glands	Water retention, labor, milk ejection
Thyroid	T3, T4, Calcitonin	All cells, bones	Metabolism, lowers blood calcium
Parathyroid	PTH	Bones, kidneys, intestines	Raises blood calcium
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Kidneys, all cells	Electrolyte balance, stress response, sex traits
Adrenal Medulla	Epinephrine, Norepinephrine	Many tissues	Fight or flight response
Pancreas	Insulin, Glucagon	Liver, muscle, fat	Blood glucose regulation
Thymus	Thymosin, Thymopoietin	T cells	T cell maturation

Additional info: The pineal gland (melatonin), ovaries (estrogen, progesterone), and testes (testosterone) are also important endocrine organs but are not the main focus of this unit.