5 - Glands Flashcards
What is a gland?
An aggregate of epithelial cells specialised for the secretion of a substance
What are the two ways that glands can be classified?
- By their structure (endocrine/exocrine)
- By the mechanism of product release (merocrine, apocrine, holocrine, cytocrine)
What is an endocrine gland? Give some examples.
Secrete hormones into the blood, which act at distant parts of the body
E.g.
- Pituitary gland - ACTH, LH, FSH, TSH, prolactin, somatotrophin, vasopressin, oxytocin
- Thyroid gland - T3/T4 in metabolism & calcitonin (Ca2+ homeostasis)
- Parathyroid gland - parathyroid hormone (Ca2+ homeostasis)
What is an exocrine gland? Give some examples.
- Secrete enzymes or lubricants through ducts
E.g. - Salivary gland
- Pancreas - amylase, trypsin, lipase
- Mammary
- Sweat glands - secrete sweat onto skin
- Sebaceous glands - secretes sebum onto the skin to protect from pathogens
- Lachrymal glands - moisten the eye and makes lysozyme
How are glands developed in utero?
- Epithelial cells proliferate in response to growth signal and produce extracellular protein degradation enzymes
- Epithelial cells invade the space they create
Exocrine glands - central cells die off to make a duct (canalicularisatrion)
Endocrine glands - angiogenesis around the epithelial cells, link to ‘mother’ cells is broken by apoptosis (independent from epithelial surface)
What is the difference between simple and compound duct structure?
Simple - duct does not branch
Compound - duct branches
What are the different shapes of gland ducts? Give an example for each.
- Simple tubular - intestinal glands
- Simple branched tubular - stomach (gastric) glands
- Simple alveolar - none in humans
- Simple branched alveolar - sebaceous glands
- Compound tubular - duodenal glands of small intestine
- Compound alveolar - mammary glands
- Compound tubuloalveolar - salivary glands
What are the two functions of cells in exocrine glands?
- Cells lining the ducts
2. Cells making secretory products
What is a myoepithelial cell of a duct?
- Cells at secretory ends of ducts with features of both epithelial and smooth muscle cells
- Eject secretions from ducts
What are the two types of acini in salivary glands?
- Serous
- Mucous
What is merocrine secretion?
Fusion of vesicles with apical membrane (form of exocytosis)
What is apocrine secretion?
Partial loss of the cytoplasm
- E.g. lactating mammary gland, sweat glands in axilla
What is holocrine secretion?
Complete loss of cytoplasm or cell
E.g. sebaceous gland in skin and tarsal glands in eyelid
What is cytocrine secretion?
Cells released as a secretion
E.g. spermatozoa in the testis
What are the 2 types of merocrine secretion?
- Regulated - vesicles released on stimulation
- Constitutive - vesicles continuously released
How does regulated merocrine secretion occur? Give a clinically relevant example of this.
- vesicle migrates to cell surface along microtubules
- in presence of Ca2+ ions, vesicle fuses with plasma membrane due to SNARE complex and synaptotagmin
- cargo released by exocytosis
E.g. release of insulin from pancreatic B-cells
Give an example of apocrine secretion.
In the breast:
- fats are secreted by apocrine secretion in neonatal period
- during lactation both fats and proteins are released by apocrine secretion
Give an example of holocrine secretion.
Sebaceous glands:
- secretory cell gradually fills with secretory granules
- cell dies and plasma membrane ruptures, releasing contents
- Dead cells replaced by mitotic division of those below
What is the role of the golgi apparatus in secretion?
- Newly synthesised proteins are transferred from the RER to the golgi in vesicles
- The golgi chemically modifies them and targets them to the correct locations
What are the cis and trans faces of the golgi apparatus?
Cis - faces the RER
Trans - faces the plasma membrane
Material flows cis –> trans
What is glycosylation? How is it different from glycation?
Covalent attachment of sugars BY ENZYMES to proteins/lipids, forms glycoproteins/glycolipids
Glycation - same but without the presence of enzymes
What is the role of glycosylation?
- Aids protein folding
- Prevents protein/lipid digestion
- Cell recognition (e.g. blood groups)
- Cell-ECM attachment
What is phagocytosis? Name some cells that use this process.
Cells engulf other cells or particles.
E.g. macrophages, dendritic cells, neutrophils, basophils and eosinophils
What is pinocytosis? What cells use this most often?
Liquid droplets are ingested by cells - sampling the environment
- Used by all cells but especially in smooth muscle
What are the 4 pathways of transcytosis (transport across an epithelium)?
Give an example of each.
- move through aqueous channels between cells (paracellular transport) - e.g. amino acids
- move through lipid cell membranes (transcellular transport) - e.g. steroid hormones
- transported by carrier proteins - e.g. thyroxine across a follicular cell
- endocytosis and exocytosis - cholesterol transport
What are the different pathways for control of an endocrine gland?
- Humoral - feedback due to concentration in the blood
- Neural - under neural control
- Hormonal - most prevalent in endocrine glands (e.g. HPA axis)
What specialised form of neural communication occurs between the hypothalamus and anterior pituitary?
Neurocrine communication:
- Neurones in hypothalamus release trophic hormones into the portal vessels when excited
- trophic hormones carried to endocrine cells of anterior pituitary
