Endocrine System Overview

Endocrine System

• The endocrine system comprises various organs and tissues secreting hormones into interstitial fluid.
• Circulating hormones diffuse into capillaries, transported throughout the body.
• Local hormones affect neighboring cells or the secreting cell, inactivated quickly.
• Circulating hormones active for minutes to hours.
• Hormones only affect target cells with specific receptors.
• Receptors found inside or on the plasma membrane of target cells.
• Hormones lead to metabolic and growth changes in target cells.
• Endocrine glands studied: pituitary, thyroid, parathyroid, suprarenal (adrenal), and pineal glands.
• Other organs also contain cells/tissues with endocrine function (hypothalamus, pancreas, ovaries, testes, and thymus).

Studying Tissues/Organs

• Before placing slides under the microscope, examine them against a white background.
• Identify large structures and compare to known anatomy.
• Observe tissue appearance (lobes or solid), color changes, or patterns.
• Start at low magnification (4x/10x) for large structures, then increase to 40x for details.
• Match observable structures on the slide to photomicrographs and drawings.
• Notice cell groupings, connective tissue presence, and vascularization (location of capillaries/blood vessels).

Hypothalamus and Pituitary Gland

• The hypothalamus is in the brain, synthesizes several hormones.
• The pituitary gland (sella turcica in sphenoid bone) releases these hormones.
• The hypophyseal portal system carries hormones directly to the anterior pituitary.
• The posterior pituitary stores hormones.
• Seven hormones released from the anterior pituitary enter general circulation.
• Oxytocin and antidiuretic hormone (ADH) are packaged into posterior pituitary vesicles.

Pituitary Gland Histology

• Distinguish anterior (darker stained) and posterior (lighter stained)pituitary glands.
• Anterior pituitary composed of glandular epithelium (cuboidal).
• Posterior pituitary stains light pink, an extension of the brain (neurons).

Hormones Released by Pituitary

• Growth hormone (GH): target tissues: liver, bone, muscle, and cartilage, stimulates growth and cell reproduction.
• Thyroid-stimulating hormone (TSH): stimulates the thyroid to secrete T3 and T4.
• Follicle-stimulating hormone (FSH): stimulates egg or sperm development.
• Luteinizing hormone (LH): stimulates ovulation or testosterone production.
• Prolactin stimulates milk production.
• Adrenocorticotropic hormone (ACTH): stimulates the release of cortisol from adrenal cortex.
• Melanocyte-stimulating hormone (MSH): regulates melanin production.
• Oxytocin: stimulates uterine contractions and milk ejection during lactation.
• Antidiuretic hormone (ADH): increases water reabsorption and constricts blood vessels.

Thyroid and Parathyroid Glands

• Thyroid gland is inferior to the larynx.
• Parathyroid glands are posterior to thyroid.
• Thyroid glands store hormones in follicles.
• Follicular cells synthesize thyroglobulin and iodine.
• Parafollicular cells produce calcitonin.
• Parathyroid glands have principal and oxyphil cells.
• Principal cells produce parathyroid hormone (PTH) to regulate calcium levels.

Thyroid and Parathyroid Histology

• Thyroid gland examined at 10x. Identifies follicles (large oval sacs), follicular cells, and parafollicular cells.
• Parathyroid glands viewed at 10x, showing a dense mass of cells.
• Differences in structure between thyroid and parathyroid tissues must be accounted for as well.

Suprarenal (Adrenal) Glands

• Suprarenal glands located retroperitoneally above the kidneys.
• Cortex (outer) layer has three zones.
• The zones secrete different hormones: mineralocorticoids (aldosterone), glucocorticoids (cortisol), androgens (DHEA).
• Medulla (inner) produces epinephrine and norepinephrine.

Pancreatic Islets

• Pancreas is retroperitoneal, has exocrine (digestive enzymes) and endocrine (insulin and glucagon) functions.
• Pancreatic islets (Islets of Langerhans) contain alpha cells (glucagon), and beta cells (insulin).

Pancreatic Histology

• Pancreatic tissue examined at 4x and 10x shows lobules separated by connective tissue and blood vessels.
• Pancreatic acini, the groups of cells, are visible.
• Pancreatic islets (lighter-staining) are scattered throughout the acini.

Gonads (Ovaries and Testes)

• Ovaries and testes produce gametes and hormones.
• Ovarian follicles develop, releasing eggs and hormones (estrogens, progesterone, inhibin).
• Corpus luteum develops after ovulation, secreting hormones.
• Testes produce sperm and hormones (testosterone, inhibin).
• Leydig cells within the testes secrete testosterone.

Pineal Gland

• The pineal gland is in the brain, produces melatonin.

Thymus Gland

• Thymus is in the mediastinum.
• Lobes are divided by connective tissue.
• Produces thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin.

Endocrine Disorders

• Hyposecretion (inadequate hormone levels) or hypersecretion (excessive hormone levels) lead to various conditions (e.g., pituitary dwarfism, gigantism, acromegaly, diabetes insipidus, hypo/hyperthyroidism).
• Imbalances in hormone production can significantly impact body function and health.