Development of the Endocrine System

Development of the Endocrine System

• 3rd week: Pituitary gland begins to develop from two different regions of ectoderm.
• 4th week: Thyroid gland develops as a midventral outgrowth of endoderm; parathyroid glands develop from endoderm from the 3rd and 4th pharyngeal pouches.
• 5th week: Adrenal cortex develops from the same region of mesoderm that produces the gonads, while the adrenal medulla develops from ectoderm from neural crest cells; the thymus develops from endoderm.
• 5th to 7th weeks: Pancreas develops from two outgrowths of endoderm.
• 7th week: Pineal gland develops from ectoderm.

Comparison of Nervous and Endocrine Systems

• Mediator molecules:
  • Nervous system: Neurotransmitters released locally in response to nerve impulses.
  • Endocrine system: Hormones delivered to tissues throughout the body by blood.
• Site of mediator action:
  • Nervous system: Close to site of release, at synapse; binds to receptors in postsynaptic membrane.
  • Endocrine system: Far from site of release (usually); binds to receptors on or in target cells.
• Types of target cells:
  • Nervous system: Muscle (smooth, cardiac, and skeletal) cells, gland cells, other neurons.
  • Endocrine system: Cells throughout the body.
• Time to onset of action:
  • Nervous system: Typically within milliseconds (thousandths of a second).
  • Endocrine system: Seconds to hours or days.
• Duration of action:
  • Nervous system: Generally briefer (milliseconds).
  • Endocrine system: Generally longer (seconds to days).

Endocrine Glands

• Endocrine glands: Pituitary, thyroid, parathyroid, adrenal, and pineal glands.
• Other organs and tissues with hormone-secreting cells: Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.

Hormone Activity

• Hormone receptors: Protein receptors that bind hormones.
• Down-regulation: Decrease in target-cell receptors if a hormone is present in excess.
• Up-regulation: Increase in target-cell receptors if a hormone is deficient.
• Circulating and local hormones:
  • Circulating hormones: Hormones that pass from secretory cells into interstitial fluid and then into the bloodstream.
  • Local hormones: Hormones that act locally on neighboring cells or on the same cell that secreted them without entering the bloodstream.
• Paracrines: Local hormones that act on neighboring cells.
• Autocrines: Local hormones that act on the same cell.

Chemical Classes of Hormones

• Lipid-soluble hormones: Steroid hormones, thyroid hormones (T3 and T4), and nitric oxide.
• Water-soluble hormones: Amine hormones, peptide hormones, protein hormones, and eicosanoid hormones.

Mechanisms of Hormone Action

• Hormonal interactions:
  • Permissive effect
  • Synergistic effect
  • Antagonist effect
• The hypothalamus is the major integrating link between the nervous and endocrine systems.
• The hypothalamus and the pituitary gland regulate virtually all aspects of growth, development, metabolism, and homeostasis.