L5: Glands Flashcards
What are glands?
Epithelial cells or aggregates of epithelial cells that are specialised for secretion of a substance
Define secretion?
The production and release of materials by cells or aggregates of cells
What do glands do?
Regulate human physiology
Receive stimulus from CNS, circulating levels in the body and neighbouring cells
Respond accordingly
What are the different ways in which glands can be classified?
Structure → endocrine or exocrine
How products are released → merocrine, apocrine, holocrine or cytocrine
What is an endocrine gland?
Ductless
Secretes products directly into the blood stream
Release hormones
All cells secrete hormone
What are the different structures of endocrine glands?
Cords → adrenal cortex
Follicles → thyroid
Clusters → pituitary gland
Give examples of where an endocrine gland would be found?
- Pituitary gland of the hypothalamus
- Thyroid gland
- Parathyroid gland
What are the endocrine secretions from the pituitary gland?
Inferior to the hypothalamus
Anterior pituitary
- ACTH → regulates cortisol levels in adrenal cortex
- LH → trigger ovulation and development of corpus leuteum (females) and leydig cells and testosterone (males)
- FSH → development, growth, pubertal maturation, reproductive processes
- TSH → thyroxine (T4) and triiodothyroxine (T3) production
- Prolactin → milk production
- Somatostatin → regulation of growth of body and tissues
Posterior
- Vasopressin (ADH) → prevents water loss
- Oxytocin → uterus contraction in women, milk ejection
What are the endocrine secretions from the thyroid gland?
Produces thyroid hormone T3 and T4 → control metabolism
Calcitonin → Ca2+ homeostasis ↓Ca2+ levels
What are the endocrine secretions from the parathyroid glands?
Parathyroid hormones
Ca2+ homeostasis
↑Ca2+ levels
What is an exocrine gland?
Ducted gland
Secretes into a location or region through a duct
Releases enzyme or lubricants
Only cells at apex secrete
Give an example(s) of exocrine glands and what they release?
- Salivary gland → buccal cavity, saliva, partial digestion of food
- Pancreas → enzymes; amylase (starch and carbs), trypsin (proteins), lipase (fats)
- Mammary → colostrum (babies first drink, antibodies and nutrients), and milk in reponse to oxytocin and prolactin
- Sweat glands → sweat (regulates body temperature)
- Sebaceous glands → sebum onto skin (protection from pathogens, lubricant, waterproof skin)
- Lachrymal glands → eye, water lubricates eye, lysozyme (attacks bacteria)
What are the different structure of the exocrine glands?
Unicellular or multicellular
Unicellular → secretion onto surface epithelium
Multicellular→ ducted system, glandular cells extend from epithelium into connective tissue
Describe the different shapes of gland ducts?
Two types of epithelial cells in exocrine glands 1. Cells lining the ducts 2. Cells that make the secretory products Gives rise to many different gland types Simple: duct not branched 1. Simple tubular 2. Simple branched tubular 3. Simple coiled tubular 4. Simple acinir/ alveolar 5. Simple branched acinir/ alveolar Compound: duct branches 1. Compound tubular 2. Compound acinir/ alveolar 3. Compound tubulacinir/ tubuloalveolar
Using the salivary gland as an example describe how the gland is formed?
Pre bud → initial bud → pseudoglandular bud → canalicular bud → terminal bud
Describe the difference between interlobular duct and intercalated duct?
Interlobular→ between the main duct and where it branches
Intercalated duct → between the duct and the acinir
What are the different types of acinir?
Mucus acinir → produces mucins not a goblet cell
Serous acinir → produces liquid material makes saliva fluid
Serous demilune → ‘half moon’, cell been squeezed out by mucus cells and sits on the top of them forming a cap
What are the different method of secretion?
Merocrine
Apocrine
Holocrine
Cytocrine
What is merocrine secretion?
Form of exocytosis
Vesicles fuse with plasma membrane
Release content into extracellular space
Plasma membrane transiently larger
Membrane returned stabilising cell surface
What is apocrine secretion?
Non membrane bound substrates released
Structure pushes up into plasma membrane
Plasma membrane drops around gets pinched off
Plasma membrane transiently smaller
Membrane added to stabilise cell surface
What is holocrine secretion?
Disintegration of cell
Releases contents
Discharge whole cell into extracellular space
What is cytocrine secretion?
Very rare
Whole cell released as secretion
e.g. Spermatid in the seminiferous tubules
What is the difference between constitutive and regulated secretion?
Constitutive→ continuously released from the cell
Regulated→ released upon stimulation, require Ca2+ ions to work
What is constitutive merocrine secretion?
- Secretory products packaged into vesicles not concentrated into granules
- Continuously released from the cell surface
- Repopulate the plasma membrane with plasma proteins
What is regulated merocrine secretion?
Active process→ uses energy
- Specific Ca2+ signal
- Vesicle moves towards plasma membrane using microtubules
- Presence of Ca2+, vesicle membrane fuses with plasma membrane
- Cargo released into extracellular space
Give an example of where regulated merocrine secretion is used?
Secretion of insulin from ß cells in the pancreas
- Glucose bind to GLUT2 transporter and enters the cell
- Glucose phosphorylate to glucose-6-phosphate and metabolised (krebs and OP) to produce ATP
- ATP activate ATP senstive K+ channel causing it to close
- Prevents K+ exiting resulting in depolarisation
- VOCC open allowing Ca2+ into the cell
- Vesicles containing insulin fuse with the plasma membrane
- Release insulin into the bloodstream
What is the clinical relevance of regulated merocrine secretion in diabetes?
Type 1→ not enough insulin produced
Give sulphonylurea→ binds to receptor Sulphonylurea receptor causing ATP sensitive K+ channels to close …
Give an example of where holocrine secretion is used?
Sebaceous glands
- The secretory cell gradually fills up with secretory granules
- The cell organelles degenerate
- The cells die
- The plasma membrane breaks and the contents (secretum) empties
- Dead cells are replaced by mitotic division of the basal cells
What role does the golgi apparatus plat in secretion?
Recieves protein-containing vesicles from the ER
The golgi chemically modifies, sorts, and packages proteins ready for secretion
Targets them to the correct address
Enters the cis side exits the trans side
Define glycosylation?
Covalent attachment of sugars by enzymes to proteins and lipids to form glycoproteins and glycolipids
50% of all cellular proteins undergo glycosylation
What is the function of glycosylation?
- Aid proteins folding
- Prevent protein digestion by intracellular proteases
- Prevent lipid digestion by intracellular lipases
- Cell recognition
- Role on cell to extracellular matrix attachment
Define endocytosis?
Engulfing of molecules inside the cell via vesicle formation
Define exocytosis?
Secretion of molecules outside the cell via vesicle fusing to a membrane
Define phagocytosis?
Process by which cells; phagocytes, envelop or engulf other ells or particles
Mainly immune system cells
Define pinocytosis?
Process by which liquid droplets are ingested by cells
What is receptor-mediated endocytosis?
Molecule binds to receptor on the surface of a cell and get endocytoses
Vesicle often labelled so can be transported to specific place in the cell
Used to transport drugs through tissue into the circulation
What is transepithelial transport/ transcytosis?
Movement of molecule from one side of the cell to another by different methods
What are the different methods of transcytosis?
- Paracellular transport → Molecules passively diffuse through the aqueous channels between the cells (intracellular junctions)
- Transcellular transport→ Lipid soluble molecules pass through the cell membrane, travel across the cytoplasm and across the other membrane
- Counter-transport system → Carrier proteins allow one molecule to pass one way and another to pass the other way
- Endocytosis/ exocytosis system → engulfed into vesicle one side (endocytsosed), transported across the cell and released the other side (exocytosed)
What are the types of glandular control?
Humoral stimulus → feedback loops in response to changes in extracellular fluids (mostly endocrine glands)
Neural stimulus → nerve stimulates the gland
Hormonal stimulus → hormones released from one gland has an effect on another gland
What is an example of neurocrine communication?
Hypothalamo-hypophyseal portal system
Nerve cells in hypothalamus release hormones into the portal system into anterior pituitary
Acts on target cells
Release there contents into second set of capillaries for distribution to rest of the body
Only other one is the hepatic portal system
