Endocrine Glands and Hormones: Structure, Function, and Key Hormones

Endocrine Glands and Their Cellular Structure

Overview of Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream, regulating various physiological processes. Each gland contains specialized cells responsible for hormone production and release.

• Pituitary Gland

  • Anterior Pituitary (Adenohypophysis): Produces several tropic hormones.

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

  • Infundibulum: Connects the pituitary to the hypothalamus.

• Adrenal Gland

  • Capsule: Outer covering.

  • Medulla: Produces catecholamines.

  • Cortex: Divided into three zones:

    • Zona Glomerulosa: Produces mineralocorticoids.

    • Zona Fasciculata: Produces glucocorticoids.

    • Zona Reticularis: Produces androgens.

• Thyroid Gland

  • Follicles: Spherical structures containing colloid.

  • Thyroglobulin: Precursor for thyroid hormones.

  • Follicular Cells (Principal Cells): Produce thyroid hormones.

  • Parafollicular Cells: Produce calcitonin.

• Parathyroid Gland

  • Parathyroid Cells (Chief Cells): Secrete parathyroid hormone.

• Pancreas

  • Acinar Cells: Exocrine function (digestive enzymes).

  • Pancreatic Islets (Islets of Langerhans):

    • Alpha Cells: Secrete glucagon.

    • Beta Cells: Secrete insulin.

• Thymus

  • Medulla

  • Cortex

• Pineal Gland (Pineal Body)

  • Pineal Sand: Calcified deposits, marker of gland activity.

• Ovary

  • Primordial Follicles

  • Primary Follicles

  • Secondary Follicles

  • Graafian Follicles (Tertiary/Antral Follicles)

  • Oocytes

• Testes (plural) / Testis (singular)

  • Seminiferous Tubules: Site of sperm production.

  • Interstitial Cells: Produce testosterone.

Endocrine Gland Hormones

Major Hormones and Their Functions

Endocrine glands secrete hormones that regulate growth, metabolism, reproduction, and homeostasis. Understanding the source, target cells, and effects of each hormone is essential for recognizing clinical conditions related to hormone imbalances.

Gland	Hormone	Main Function
Anterior Pituitary (Adenohypophysis)	Adrenocorticotropic Hormone (ACTH)	Stimulates adrenal cortex to release corticosteroids
Anterior Pituitary	Growth Hormone (GH)	Promotes growth and metabolism
Anterior Pituitary	Thyroid-Stimulating Hormone (TSH)	Stimulates thyroid hormone production
Anterior Pituitary	Follicle-Stimulating Hormone (FSH)	Stimulates gamete production in ovaries/testes
Anterior Pituitary	Luteinizing Hormone (LH)	Stimulates sex hormone production
Anterior Pituitary	Prolactin (PRL)	Promotes milk production
Adrenal Medulla	Epinephrine, Norepinephrine, Dopamine	Fight-or-flight response, increases heart rate and blood pressure
Adrenal Cortex	Aldosterone	Regulates sodium and potassium balance
Adrenal Cortex	Cortisol	Regulates metabolism, stress response
Adrenal Cortex	Androgens	Sex hormone precursors
Thyroid Gland	Calcitonin	Lowers blood calcium levels
Thyroid Gland	Thyroid Hormone (TH), Thyroxine (T4), Triiodothyronine (T3)	Regulates metabolism
Parathyroid Gland	Parathyroid Hormone (PTH)	Raises blood calcium levels
Pancreas (Islets of Langerhans)	Glucagon	Raises blood glucose
Pancreas (Islets of Langerhans)	Insulin	Lowers blood glucose
Thymus	Thymosin	Promotes T-cell maturation
Pineal Gland	Melatonin	Regulates circadian rhythms
Ovary	Estrogen, Progesterone	Regulate female reproductive cycle
Testes	Testosterone	Regulates male reproductive functions

Hypothalamic Hormones

The hypothalamus controls the pituitary gland by releasing and inhibiting hormones. These hormones regulate the secretion of anterior pituitary hormones.

• Growth hormone-releasing hormone (GHRH): Stimulates release of growth hormone.

• Growth hormone-inhibiting hormone (GHIH): Inhibits release of growth hormone.

• Thyrotropin-releasing hormone (TRH): Stimulates release of thyroid-stimulating hormone.

• Corticotropin-releasing hormone (CRH): Stimulates release of adrenocorticotropic hormone.

Hormone Regulation and Clinical Relevance

Hormone concentrations are tightly regulated. Excess or deficiency can lead to clinical conditions:

• Hypersecretion: Excess hormone production (e.g., hyperthyroidism).

• Hyposecretion: Insufficient hormone production (e.g., diabetes mellitus).

• Target Cell Response: Hormones act on specific target cells, producing physiological effects.

Key Terms and Definitions

• Hormone: Chemical messenger secreted by endocrine glands.

• Endocrine gland: Ductless gland that releases hormones into the bloodstream.

• Target cell: Cell with specific receptors for a hormone.

• Homeostasis: Maintenance of stable internal environment.

Example: Regulation of Blood Glucose

The pancreas regulates blood glucose through the antagonistic actions of insulin and glucagon:

• Insulin: Lowers blood glucose by promoting cellular uptake.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown.

Formula:

$\text{Blood Glucose Regulation:} \\ \text{Insulin} \rightarrow \text{Glucose uptake} \rightarrow \text{Lower blood glucose} \\ \text{Glucagon} \rightarrow \text{Glycogen breakdown} \rightarrow \text{Raise blood glucose}$

Additional info: The notes above include inferred details about gland structure and hormone function to provide a complete, self-contained study guide for Anatomy & Physiology students.