Endocrinology: Overview of Endocrine Systems Flashcards
Way to split endocrine glands (by location)
Central and peripheral endocrine glands
Where are central endocrine glands?
In the brain
Examples of central endocrine glands
In the brain
- Pineal
- Hypothalamus*
- Posterior and anterior pituitary*
What are peripheral endocrine glands?
All other endocrine glands, i.e., those not in the brain
Examples of peripheral endocrine glands
Endocrine glands not in the brain
- Thyroid*
- Parathyroid*
- Thymus
- Pancreas
- Adrenal*
-Kidney
- Testes/ ovaries
- = vital glands
Role of hypothalamus
important integration of endocrine and nervous systems; efferent signals bring about changes that help to re-establish normal values
Role of posterior and anterior pituitary glands and hypothalamus, in homeostasis
Maintain critical roles in maintaining homeostasis (endocrine = chemical signals, work with nervous system = electrical signals)
Hypothalamus = linkage between endocrine and nervous system
How do the endocrine and nervous systems work together? Why is this important?
The endocrine system works alongside the nervous system (integrated control systems)
Nervous system acts via electrical signals to control rapid responses of the body
Endocrine system acts via hormones secreted into blood to control activities that require duration rather than speed (chemical signals)
Integrated control systems essential for survival of body cells, and hence systems to work together to achieve body homeostasis
Endocrine cells
Secrete hormones into the blood stream that carries them to target cells often distant from the endocrine cells
Where are endocrine cells found?
Single scattered cells, e..g, enteroendocrine cells in epithelium lining of GIT
Clumped together into glands e.g., adrenal glands
What do target cells for hormones have?
Appropriate receptors for endocrine signals, making them specific
Hormones
Released from endocrine cells
Chemical messengers
Regulate activity of target cells
Neurohormones
Substances secreted by specialised neurons (neurosecretory/ neuroendocrine cells, rather than glandular epithelial cells) into the bloodstream
Can also serve as a neurotransmitter or as an autocrine/ paracrine messenger
Meet definition of a hormone (enter bloodstream, act on target cells
Example of a neurohormone
ADH, catecholamines from the adrenal medulla (adrenal cells are essentially modified neurons
Paracrine secretions
Influence activity of adjacent cells
E.g., mast cells (paracrine cell) release histamine which act on nearby blood vessels to increase vasodilation and permeability
Autocrine secretions
Influence the activity of self-cells (act locally, generally don’t enter bloodstream)
Main types of hormones
- Proteins and glycoproteins
- Small peptides
- Amino acid derivatives, including amines
- Steroids
E.g., of proteins and glycoproteins hormones
Growth hormone, insulin
E.g., of small peptide hormones
Thyroid stimulating hormone (TSH), oxytocin
E.g., of amino acid derivative hormones
Adrenaline, thyroxine
E.g., of steroid hormones
Cortisol, testosterone, progesterone
Three broad types of hormones. How do these differ?
proteins, amines and steroids
differ in way they’re synthesised, transported, stored, released and regulated
Actions of water-soluble (hydrophilic) hormones
- Peptides and most amine hormones are hydrophilic = lipophobic, hence cannot pass through lipid bilayer of cell membrane
- 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
- Calcium ions serve as important messengers within cells
Actions of fat soluble (lipophilic) hormones
- Steroid hormones and the thyroid hormones are hydrophobic = lipophilic
- Receptor is in the cytoplasm or nucleus, as they can pass through the cell membrane
- Acts at transcription level
- Steroid hormones are generally slower in triggering physiological changes in their target cells compared with non-steroid hormones
How does a typical endocrine gland develop?
- Ingrowth of cord of cells from surface epithelial cells during development
- Attachment to surface lost
- Cells release secretion into capillaries
General structure of endocrine glands
- Clumps and strands of cells
- Supported by reticular fibres and minimal CT cells
- Minimal barriers between cells releasing their secretions and adjacent capillaries
- Fenestrated capillaries and sinusoids (gaps and spaces) to facilitate diffusion of hormones into bloodstream
- No duct system
Features of typical amine, polypeptide, protein and glycoprotein hormone producing cells
- Secrete water soluble (lipophobic) hormones
- Have many small membrane-bound secretory granules (where the hormones are stored)
- Polarity is not as obvious as exocrine cells (no significant difference in structure between the top and bottom of the cell)
- Contain small to moderate amounts of RER and Golgi (typically don’t secrete large volume of hormone
Features of typical steroid hormone producing cells
- cholesterol stored in lipid droplets (often identifiable under the microscope)
- 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 (help destroy invading organisms and bacteria) and lipofuscin (pigmented residue of lysosomal digesetion)
- if beyond repair, lysosomes help cell self-destruct via apoptosis
- lipid hormones not stored in vesicles as they are lipid soluble
