endocrine

• Endocrine System


  System that uses hormones secreted into the bloodstream to regulate the activity of target cells
• Endocrine System


  • Acts via hormones secreted into blood to control activities that require duration rather than speed
  • Essential for survival
• Nervous System


  • Acts via electrical signals to control rapid responses of the body
• Nervous System and Endocrine System


  Both systems work together to maintain homeostasis in the body
• Endocrine cells

  Secrete hormones into the bloodstream that carry them to target cells often distant from the endocrine cells
• Hormones


  Chemical messengers that regulate the activity of target cells
• Endocrine cells


  • Can be single scattered cells or clumped together into glands
• Paracrine and autocrine secretions


  Influence the activity of adjacent or self cells, acting locally and not generally entering the bloodstream
• Paracrine cell


  • Mast cell releasing histamine
• Types of substances secreted by specialized neurons (neurosecretory/neuroendocrine cells)


  • Hypophysiotropic hormones (peptides: such as TRH, CRH, GnRH)
  • Posterior pituitary hormones (peptides: vasopressin, oxytocin)
  • Adrenal medulla hormones (catecholamines)
• Neurohormones


  Substances secreted by specialized neurons (neurosecretory/neuroendocrine cells) into the bloodstream (may also serve as neurotransmitters or as autocrine or paracrine messengers)
• Endocrine gland development

  • Ingrowth of cord of cells from surface epithelium
  • Attachment to surface lost
  • Cells release secretion into capillaries
• Not every organ that could be considered endocrine in its function is necessarily epithelial in nature. The brain is also considered 'endocrine' but is not an epithelial structure.

  
• Many organs that we don't think of as 'glands' also have endocrine functions, including the kidney, the intestine, skin and adipose.

  
• General endocrine gland structure


  • Clumps and strands of cells
  • Supported by reticular fibres and minimal connective tissue cells
  • Minimal barriers between cells and capillaries
  • Fenestrated capillaries and sinusoids
  • No duct system

  
• Endocrine and exocrine cells are found within the pancreas

  
• Main types of hormones


  • Proteins and glycoproteins (e.g., growth hormone, insulin)
  • Small peptides (e.g., thyroid stimulating hormone (TSH), oxytocin)
  • Amino acid derivatives, including amines (e.g., adrenaline, thyroxine)
  • Steroids (e.g., cortisol, testosterone, progesterone)
• Actions of water soluble (hydrophilic) hormones


  • Hormone binds to receptor in cell membrane
  • Binding activates 2nd messengers, e.g., cAMP
  • Second messengers in turn activate enzymes that produce a specific physiological response
  • Act primarily at a translational level
• Actions of fat soluble (lipophilic) hormones


  • Receptor in cytoplasm or nucleus
  • Act at transcription level
  • Generally slower in triggering physiological changes in their target cells compared with non-steroid hormones
• Typical amine, polypeptide, protein & glycoprotein hormone producing cells


  • Secrete water soluble (lipophobic) hormones
  • Have many small membrane bound secretory granules
  • Polarity is not as obvious as exocrine cells
  • Contain small to moderate amounts of RER and Golgi (typically do not secrete large volume of hormone)
• Typical steroid hormone producing cell


  • Cholesterol stored in lipid droplets
  • Cholesterol converted to steroid hormone in SER and mitochondria
  • Abundance of SER and mitochondria
  • Lack of secretory granules
  • Some have a long life; hence contain an abundance of lysosomes and lipofuscin (pigmented residue of lysosomal digestion)