Introduction to the Endocrine System

Introduction to the Endocrine System

• The endocrine system uses hormones to communicate between cells and organs.
• Hormones are chemical messengers that affect target cells or organs in different ways depending on the type and receptors involved.
• There are three main types of hormones: protein, steroid, and amine hormones.

Learning Objectives

• Define a hormone and compare endocrine signaling with other cell-to-cell communication.
• Identify the three main types of hormones and understand how their type affects synthesis, secretion, and transport.
• Define a receptor and discuss major hormone receptor types.
• Identify and describe the role of the hypothalamus and pituitary in the endocrine system.

Hormones

• Hormones are produced and secreted by cells.
• Hormones travel through the bloodstream to reach target cells
• Hormones bind to receptors on target cells, triggering a response.
• Hormones are present in extremely low concentrations in the bloodstream.

Types of Hormones

• Hormones can be classified by their function or the type of molecule they are made of.
  • System/Organ: hormones can be categorized by the gland or organ that produces them (ex. Pituitary gland, thyroid gland, adrenal glands, ovaries and testes).
  • Type of Molecules: hormones can be categorized by the molecules they are made of (ex. protein/peptide, steroid, and amine).
• Examples of hormones: testosterone, cortisol, aldosterone, triiodothyronine, and epinephrine

Protein Hormones

• Synthesized like other proteins.
• Synthesis is highly controlled.
• Hydrophilic (water-loving); cannot pass through the plasma membrane.
• Stored in cells until secreted.
• Do not require transport proteins in the blood.
• Bind to transmembrane receptors on the cell surface.

Protein Hormones - Transport

• Enter blood vessels at capillary beds.
• Do not need carrier proteins in blood.
• Exit blood vessels at capillary beds.

Protein Hormones - Receptors

• Cannot enter cells.
• Bind to transmembrane receptors.
• Activate intracellular signaling cascades.

Amine Hormones

• Small and hydrophilic.
• Stored and actively secreted.
• Examples include thyroid hormones and adrenergic hormones.
• Examples: triiodothyronine (T3), epinephrine.

Amine Hormone Synthesis

• Thyroid hormone synthesis involves taking thyroglobulin into thyroid cells, iodination, conjugation, and proteolysis.
• Catecholamines (epinephrine, norepinephrine, and dopamine) are synthesized in various steps from tyrosine, employing enzymes like tyrosine hydroxylase.

Amine Hormone Receptors

• Thyroid hormones bind to intracellular receptors.
• Catecholamines bind to transmembrane receptors.

Steroid Hormones

• Lipophilic (fat-loving); cannot be stored inside cells.
• Need carrier proteins in the blood.
• Different types include glucocorticoids, mineralocorticoids, and sex steroids
• Examples include cortisol, estradiol.

Steroid Hormones - Transport

• Corticosteroid-binding globulin (CBG): binds to cortisol and progesterone.
• Sex hormone-binding globulin: binds to testosterone and estrogen.
• Transcortin: binds to aldosterone
• Albumin: binds to all steroids, though with lower affinity than other carrier proteins

Hormone Receptors

• No receptor results in no direct effect.
• Cells integrate signals from multiple hormones.

Types of Hormone Receptors

• Intracellular receptors: located inside the cell.
• Examples include receptors for steroids and thyroid hormones.
• Require hormones to enter cell.
• Response occurs in hours to days.
• Transmembrane receptors: span the cell membrane.
• Examples include receptors for catecholamines.
• Respond in seconds to minutes.

Intracellular (Nuclear) Receptors

• Receptors for steroids and thyroid hormones.
• Steroid hormones travel through plasma membrane; bind to carrier proteins.
• Regulation of transcription and production of new proteins

Transmembrane Receptors

• Initiate cascades inside cells, leading to changes in protein activity and production.

Hypothalamus & Pituitary

• The hypothalamus and pituitary gland work together to regulate many bodily functions.
• The hypothalamus is a region of the brain.
• The pituitary is a gland located in the brain that works with the hypothalamus, which signals which hormones to release.

The Pituitary

• Divided into anterior and posterior lobes.

Circumventricular Organs

• Structures involved in the regulation of hormones

Posterior Pituitary

• Made of neural tissue.
• Secretion of oxytocin and antidiuretic hormone (ADH).

Anterior Pituitary

• Made of glandular tissue.
• Connected to hypothalamus via a portal system.
• Secretion of hormones in response to hypothalamic hormones.

Portal Systems

• Specialized blood vessel system connecting the hypothalamus and the anterior pituitary.
• Carry hormones from hypothalamus to anterior pituitary.

Anterior Pituitary - Hormones

• Growth hormone (GH)
• Thyroid-stimulating hormone (TSH)
• Adrenocorticotropic hormone (ACTH)
• Prolactin
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)

In Summary

• Endocrine system releases hormones into blood to affect cells.
• Hormones categorized by system regulated or their source (molecules).
• Receptors bind to specific hormones and initiate intracellular signaling pathways.

This quiz covers the fundamental aspects of the endocrine system, including the definition and types of hormones, endocrine signaling, and the roles of the hypothalamus and pituitary gland. Understand how hormones function as chemical messengers and the significance of receptors in this complex communication system.