Endocrine_Glands

Endocrine System Overview

• Endocrine System: Composed of glands that secrete hormones directly into the bloodstream to regulate various bodily functions.

• Endocrine Glands: Secrete signaling molecules (hormones) without the use of ducts, differing from exocrine glands.

  • Characteristics: Typically formed from epithelial cells arranged in clusters or cords.

  • Examples of Specialized Organs/Tissues with Endocrine Cells:

    • Heart

    • Thymus

    • Gut

    • Kidneys

    • Testes

    • Ovaries

Major Endocrine Glands

• Pituitary Gland:

  • Anterior Pituitary secretes:

    • Adrenocorticotropic hormone (ACTH)

    • Follicle-stimulating hormone (FSH)

    • Growth hormone (GH)

    • Luteinizing hormone (LH)

    • Melanocyte-stimulating hormone (MSH)

    • Prolactin (PRL)

    • Thyroid-stimulating hormone (TSH)

  • Posterior Pituitary releases:

    • Antidiuretic hormone (ADH)

    • Oxytocin (OT)

• Pineal Gland: Produces melatonin.

• Thyroid Gland: Secretes:

  • Calcitonin (CT)

  • Thyroid hormones (T3 and T4)

• Parathyroid Glands: Secretes parathyroid hormone (PTH).

• Adrenal Glands:

  • Cortex: Secretes corticosteroids.

  • Medulla: Secretes epinephrine (E) and norepinephrine (NE).

Hormone Classification

• Hormones:

  • Hydrophilic:

    • Usually proteins, glycoproteins, peptides, or modified amino acids.

    • Have receptors on the surface of target cells and circulate freely in the bloodstream.

  • Hydrophobic:

    • Primarily steroids and thyroid hormones.

    • Usually bind to transport proteins (like albumin) but can diffuse through cell membranes to activate intracellular receptors.

Modes of Hormone Delivery in Endocrine Glands

1. Paracrine Secretion: Localized effects in interstitial fluid or short blood vessel loops (e.g., Gastrin from G cells).

2. Juxtacrine Secretion: Signaling molecules remain on cell surfaces and affect adjacent cells upon contact (important in embryonic and regenerative tissues).

3. Autocrine Secretion: Hormones act on the secreting cell or similar cells.

• Feedback Mechanism: Glands respond to hormones from other glands to regulate hormonal levels in the blood.

Feedback Mechanisms

• Physiological Regulation: A system that returns the body to its normal state (homeostasis).

  • Positive Feedback: Increases output in the same direction (important in processes like childbirth).

  • Negative Feedback: Reduces output in the opposite direction (common in hormone regulation).

• Example Mechanism:

  1. Stimulus (e.g., low temperature) causes hypothalamus to secrete TRH.

  2. TRH stimulates anterior pituitary to release TSH.

  3. TSH prompts thyroid hormone (TH) production.

  4. Increased TH levels inhibit TRH and TSH secretion, controlling metabolism and temperature.

Hypothalamus Overview

• Location: Base of the brain, connected to the pituitary gland via the pituitary stalk.

• Function: Acts as a control center, influencing both the autonomic nervous system and endocrine system, crucial for maintaining homeostasis.

  • Neurohormones Produced:

    • Thyrotropin-Releasing Hormone (TRH)

    • Corticotrophin-Releasing Hormone (CRH)

    • Gonadotropin-Releasing Hormone (GnRH)

    • Growth Hormone-Releasing Hormone (GHRH)

    • Somatostatin (Growth Hormone Inhibiting Hormone)

    • Dopamine (Prolactin Inhibiting Hormone)

    • Oxytocin

    • Vasopressin (Anti-Diuretic Hormone).

• Each of these hormones regulates specific pituitary hormone releases affecting target organs throughout the body.

Specific Neurohormones and Their Functions

• TRH: Stimulates TSH and PRL release from the anterior pituitary.

• CRH: Stimulates ACTH release affecting adrenal hormones.

• GnRH: Stimulates release of gonadotropins (FSH and LH) affecting reproductive functions.

• GHRH: Stimulates growth hormone release impacting growth and metabolism.

• Somatostatin: Inhibits GH and TSH release.

• Dopamine: Inhibits prolactin release and regulates sexual arousal.

• Oxytocin: Promotes uterine contractions and lactation.

• Vasopressin: Increases water reabsorption in kidneys, regulates blood volume.

Pituitary Gland Details

• Structure: Composed of anterior (Adenohypophysis) and posterior (Neurohypophysis) parts. Anterior pituitary regulates stress, growth, and reproduction.

• Anterior Pituitary:

  • Divided into three parts:

    • Pars distalis (75% of adenohypophysis)

    • Pars intermedia

    • Pars tuberalis

  • Hormonal Products Influencing Gonads, Metabolism, and Milk Production:

    • Growth Hormone (GH)

    • Thyroid Stimulating Hormone (TSH)

    • Adrenocorticotropic Hormone (ACTH)

    • Prolactin (PRL)

    • Follicle Stimulating Hormone (FSH)

    • Luteinizing Hormone (LH).

• Posterior Pituitary: Stores and releases hormones produced in the hypothalamus (ADH and OT).

Adrenal Glands Structure and Function

• Location: Near the kidneys, consist of medulla and cortex:

  • Cortex: Produces mineralocorticoids (Aldosterone), glucocorticoids (Cortisol), and weak androgens.

  • Medulla: Produces catecholamines (Epinephrine and Norepinephrine). Both parts regulated by ACTH in a feedback loop.

Pancreatic Islets Overview

• Structure: Endocrine clusters embedded in the pancreas.

  • Cell Types:

    • Alpha cells: Secrete glucagon

    • Beta cells: Produce insulin

    • Delta cells: Secrete somatostatin

    • PP cells: Secrete pancreatic polypeptide.

• Function: Regulate blood glucose levels and digestive processes.

Thyroid Gland Structure and Function

• Anatomy: Consists of two lobes connected by an isthmus, synthesizes T3, T4, and calcitonin.

• Function of T3 and T4: Regulate metabolism, brain development, heart rate, and thermal regulation.

• Calcitonin: Lowers blood calcium levels, counteracting parathyroid hormone effects by promoting calcium deposit in bones.

Parathyroid Glands Function

• Principal cells produce parathyroid hormone (PTH):

  • Targets bone, kidneys, and intestines to increase ca2+ levels in blood.

Pineal Gland Characteristics

• Structure: Small cone-shaped gland, produces melatonin regulating circadian rhythms.

• Release influenced by light, promoting sleep and daily activity cycles.