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.

• Glands: Hypothalamus, anterior pituitary & posterior pituitary, thyroid, parathyroid, thymus, adrenal cortex & adrenal medulla, pancreas, ovaries, testes.

• Related Structures: Infundibulum of pituitary gland, sphenoid bone, sella turcica, capsules of adrenal gland, thyroid follicles, parafollicular cells, chief cells, acinar cells.

Major Endocrine Glands and Their Functions

Each gland in the endocrine system has a specific role and releases distinct hormones that affect target organs and tissues.

• Hypothalamus: Regulates pituitary gland activity via releasing and inhibiting hormones.

• Pituitary Gland: Often called the "master gland"; controls other endocrine glands. Divided into anterior and posterior lobes.

• Thyroid Gland: Produces hormones that regulate metabolism, growth, and development.

• Parathyroid Glands: Regulate calcium and phosphate balance.

• Thymus: Important for immune system development, especially in children.

• Adrenal Glands: Consist of cortex (produces corticosteroids) and medulla (produces catecholamines).

• Pancreas: Has both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.

• Ovaries/Testes: Produce sex hormones (estrogen, progesterone, testosterone).

Microscopic Structures of Endocrine Tissues

Endocrine glands are composed of specialized cells that secrete hormones. Microscopic examination reveals distinct cell types and tissue organization.

• Thyroid Follicles: Spherical structures filled with colloid, lined by follicular cells that produce thyroid hormones.

• Parafollicular Cells (C cells): Located in the thyroid, secrete calcitonin.

• Chief Cells: Found in parathyroid glands, secrete parathyroid hormone (PTH).

• Acinar Cells: Found in the pancreas, secrete digestive enzymes (exocrine function).

• Capsules: Connective tissue surrounding glands, such as the adrenal gland.

Hormones Released by Major Endocrine Glands

Each gland releases specific hormones that regulate body functions. Knowing the hormone and its source is essential for understanding endocrine physiology.

• Anterior Pituitary: Growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin.

• Posterior Pituitary: Antidiuretic hormone (ADH), oxytocin.

• Thyroid: Thyroxine (T4), triiodothyronine (T3), calcitonin (from parafollicular cells).

• Parathyroid: Parathyroid hormone (PTH).

• Adrenal Cortex: Cortisol, aldosterone, androgens.

• Adrenal Medulla: Epinephrine, norepinephrine.

• Pancreas: Insulin (beta cells), glucagon (alpha cells), somatostatin (delta cells).

• Ovaries: Estrogen, progesterone.

• Testes: Testosterone.

Zones of the Adrenal Cortex

The adrenal cortex is divided into three zones, each responsible for producing different types of corticosteroids.

Key Cell Types and Their Secretions

Understanding the function of specific cell types within endocrine glands is crucial for identifying hormone sources.

• Follicular Cells (Thyroid): Secrete T3 and T4.

• Parafollicular Cells (Thyroid): Secrete calcitonin.

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

• Alpha Cells (Pancreas): Secrete glucagon.

• Beta Cells (Pancreas): Secrete insulin.

• Delta Cells (Pancreas): Secrete somatostatin.

• Acinar Cells (Pancreas): Secrete digestive enzymes (exocrine function).

Examples and Applications

• Example: The thyroid gland regulates metabolism by releasing T3 and T4, which increase the metabolic rate of cells throughout the body.

• Application: Disorders such as hypothyroidism (low thyroid hormone) and hyperthyroidism (excess thyroid hormone) can be diagnosed by measuring hormone levels and observing gland structure.

Additional info: The notes reference the use of lab models and histology images for identification and practice, which is a common approach in Anatomy & Physiology courses. The ability to name glands, related structures, and hormones is essential for lab practicals and exams.