Lecture 9 - Glands Flashcards
What is the definition of a ‘gland’
An aggregation of epithelial cells that are specialised for secretion of a substance.
Describe the functions and differences between endocrine and exocrine glands. Give some examples for each.
Endocrine = ductless, secretions are hormones and are released directly into the blood to distant parts of the body. E.g.: pituitary gland (ACTH, FH, LSH in anterior oxytocin, VP in posterior), thyroid gland and parathyroid gland.
Exocrine = Secretions are mostly enzymes and lubricants and are secreted through a duct. Cells of the apex of the duct only secrete the products. E.g.: salivary glands, pancreas, sweat glands, mammary glands.
Describe merocrine, apocrine and holocrine modes of secretion by glands.
Merocrine = Product produced by golgi apparatus, packaged into secretory vesicles then release in EC space upon arrival of stimulus - e.g.: acinar and endocrine glands of pancreas
Apocrine = Product produced in same way, but released from the apical membrane via pinching off/loss of top portion of the cell - e.g.: sweat glands, lactating mammary glands
Holocrine = Mature cell dies off an becomes secretory product itself, e.g.: sebaceous gland in skin.
Describe the 2 possible ways in which exocytosis (merocrine) solution can occur, explain why the 2nd method requires ATP and give an example of each.
1) Constitutive secretion - Secretory product packaged into small vesicles and released continuously to cell surface, e.g.: repopulation of plasma proteins in plasma membrane.
2) Regulated secretion - Secretory granules accumulate in large vesicles, released by exocytosis upon stimulation . This requires Ca2+ ions once they’ve been released, ATP is required to move the vesicles to the cell surface along microtubules. Example = insulin release from B-cells of pancreas upon Ca2+ ion stimulus.
Give a key example of apocrine and holocrine secretion
Apocrine = From lactating mammary glands, fats and proteins released via apocrine secretion (portion of cell pinched off)
Holocrine = From sebaceous glands, cells die and contents in granules (secretum) is emptied.
How is the Golgi apparatus important in both constitutive and regulatory secretion?
- If protein for secretion is not glycosylated (has sugars added to it), the Golgi will package protein into vesicle ready for fusion with PM (constitutive pathway)
- If protein for secretion is glycosylated, golgi puts it into secretory vesicle and positions it close to membrane ready for signal (hormone or NT) to mobilise calcium for release (regulatory pathway)
What is glycosylation of proteins/lipids and what are its roles?
- Covalent attachment of sugars by enzymes to protein/lipids to produce glycoproteins/lipids.
- Roles = aids protein folding, prevents protein/lipid digestion by intracellular proteases/lipases and is important in cell recognition.
Approx 50% of all cell proteins are glycosylated.
Define exocytosis and endocytosis
Exocytosis = Secretion of molecules to outside of cell via vesicle fusing with membrane
Endocytosis = Engulfing of molecules inside the cell via vesicle formation
What are the 2 main forms of endocytosis and what cells use them?
1) Phagocytosis = when phagocytes engulf other cells or particles, usually by immune cells, e.g.: macrophages, dendritic cells, neutrophils, basophils and eosinophils.
2) Pinocytosis = when liquid droplets are ingested by cells, done by all cells, particularly smooth muscle cells.
Describe the 4 ways in which transepithelial transport of molecules can occur.
1) Molecules may move via passive diffusion through aqueous channels in intracellular junction (paracellular transport)
2) Molecules may move through lipid cell membranes (transcellular transport)
3) Molecules with correct characteristics may be transported by transport proteins
4) Molecule impermeable to membrane may bind to receptors, be engulfed by membrane (endocytosis) then expelled again via exocytosis (in and out via vesicles)
What are the 3 mechanisms used to control glandular secretions?
What type of feedback mechanism are glandular secretions usually controlled by?
1) Humoral control
2) Neural control
3) Hormonal control
- Negative feedback mechanisms - especially for humoral, neural and hormonal control.
Define a portal blood system and explain what the hypothalamo-hypophyseal portal system is.
- A portal system is when blood from a capillary bed drains into another capillary bed before going back to the heart - via a connection called a “portal” vein. This allows direct transport of substances from one organ to another.
- The hypothalamo-hypophyseal portal system allows connection of the hypothalamus and pituitary gland.
- The main portal system is the hepatic portal system - venous drainage from the GI tract pools into the portal vein to reach the liver.
