Lecture 2.2: Glands Flashcards
How are glands classified?
Destination of secretion: Exocrine, Endocrine (some are mixed)
Method of secretion: Merocrine/Eccrine, Apocrine, Holocrine
Nature of secretion: Serous, Mucous
Organisation/Structure: Simple, Compound, Uni/Multicellular
Destination of secretion: Exocrine
These glands secrete their products through ducts
Destination of secretion: Endocrine
Ductless
Directly into blood
Method of secretion: Merocrine/Eccrine
Exocrine Glands only
Most glands are merocrine
The secretions of that cell are excreted via exocytosis from secretory cells into an epithelial-walled duct or ducts and then onto a bodily surface or into the lumen.
Producing a secretion that does not contain cellular components and is discharged without major damage to the secreting cell, no part of the gland is lost or damaged
Method of secretion: Apocrine
Exocrine Glands only
Mammary glands, anogenital region and axillae.
They bud their secretions off through the plasma membrane producing extracellular membrane-bound vesicles.
It loses part of its cytoplasm in their secretions.
Method of secretion: Holocrine
Exocrine Glands only
Sebaceous glands of the skin, Meibomian glands of the eyelid
Secretions are produced in the cytoplasm of the cell and released by the rupture of the plasma membrane, which destroys the cell and results in the secretion of the product into the lumen.
Nature of secretion: Serous
Produce a watery (protein rich) secretion
Stain intensely with H&E
Sweat Glands, Mammary Glands
Nature of secretion: Mucous
Mucous glands secrete a protein called mucin (highly glycosylated polypeptides) , which with water forms the substance known as mucus
Mucus is washed way during H&E staining, so mucous glands stain more poorly, leaving a ‘foamy’ appearance
Along whole digestive tracts
Organisation/Structure: Simple (Ducts)
Unbranched
Single duct
Organisation/Structure: Compound (Ducts)
Branched ducts
Acinar
Pear-like secretory portions with wide base and little inner free space
Tubular
Tube-like secretory portions
Goblet Cells
Unicellular Glands
Contain large Golgi in which new carbohydrates are added to newly-synthesised protein to create mucin
Colonic Crypts
Simple Tubular Glands
Resorption of water and electrolytes
Eccrine Sweat Glands
Simple coiled tubular glands
Secretory portions are surrounded by contractile “myoepithelial cells”
Their contraction transports luminal contents towards the ducts
Simple branched acinar galnds
Simple: Duct
Branched Acinar: Secretory Portion
Complex Glands: Features + Examples (acinar/tubular)
Branching Ducts so compound ductal sections
Glands of Brunner in the Duodenum
Exocrine Pancreas (compound tubular)
Salivary Glands (compound acinar)
Mammary Glands (compound tubuloacinar)
Intercalated Ducts
Join acini with larger collecting ducts
What is an acinus?
An acinus is any cluster of cells that resembles a berry
Exocrine Pancreas
Complex Acinar Gland
The simple squamous epithelium of the intercalated ducts penetrates the pancreatic exocrine acini
The Pancreas Gland Type
Mixed
Exocrine/Endocrine Gland
Cystic Fibrosis and Sweat Ducts
In CF, the absence of the CFTR in the apical membranes of the epithelial cells of the sweat duct results in poor reabsorption of Cl-
This suppresses Na+ reabsorption
Cystic fibrosis and sweat testing for abnormally can confirm a diagnosis of CF
Why was Cystic Fibrosis previously known as ‘Fibrocystic Disease of the Pancreas’?
In CF, exocrine secretions contain too little water
They become thickened, and block ducts
The exocrine pancreas becomes painfully inflamed (pancreatitis) and fibrotic
The gut receives insufficient pancreatic digestive enzymes, and malabsorption results
When do symptoms of insufficient secretion of pancreatic digestive enzymes appear in CF?
Usually in the first year of life in 90% of cases
Major Salivary Glands: Parotid Glands
Largest of the 3 main salivary glands
Complex Tubuloacinar Glands
Exclusively serous acini producing α-amylase
Ingestion of food stimulates the release of saliva via parasympathetic innervation
Strong sympathetic innervation leads to a dry mouth (xerostomia)
Striated ducts are important in Na+ resorption
Major Salivary Glands: Submandibular Gland
2nd largest of the 3 main salivary glands
Mixed serous/ mucous gland
Striated ducts
Saliva is hypotonic, and striated ducts have membrane folds bearing transporters in their basal aspect to facilitate electrolyte resorption
In addition to producing a-amylase, serous cells produce lysozyme
Major Salivary Glands: Sublingual Gland
Smallest of the 3 main salivary glands
Contains mainly mucous acini
Mammary Gland: Structure + Functionality
Compound Tubuloacinar Gland
Myoepithelialcells assist secretion of milk from acini
Contraction of myoepithelial cells under control of oxytocin is experienced as ‘let-down’
Mammary Gland: Apocrine Secretion
Non-membrane bound lipid pushes through the plasmalemma, covering droplets with membrane and a thin layer of apical cytoplasm
Membrane becomes transiently smaller, requiring addition of extra membrane
Mammary Gland: Merocrine Secretion
Other milk constituents (e.g. lactose, proteins and minerals) are released by merocrine secretion
Apocrine Sweat Glands
Whereas eccrine sweat glands are widely distributed, apocrine sweat glands develop at puberty
In the axillae, areolae of nipples, and genital and perianal regions
Confusingly, despite their name, these cells use merocrine secretion
Thyroid Gland
Endocrine gland located anterior and inferior to the larynx
Active thyrocytes form a cuboidal epithelium that produces thyroxine (T4) and triiodothyronine (T3)
These hormones regulate basal metabolic rate
Parafollicular (or C cells) produce calcitonin (which slows bone turnover to reduce plasma calcium)
Control of Thyroid Hormones
Release of thyroid hormones is under the endocrine control of thyroid stimulating hormone (TSH)
This is produced by the anterior pituitary
Synthesis of Thyroid Hormones Brief
The tyrosine-rich glycoprotein thyroglobulin is synthesised by thyrocytes (thyroid follicular cells) and released by exocytosis into the colloid
Synthesis of Thyroid Hormones
Tyrosine residues within the thyroglobulin molecule undergo iodination at one
or two positions
A diiodinated tyrosine residue is conjugated with a monoiodinated tyrosine
residue to create a pre-T3.
A diiodinated tyrosine residue is conjugated with another diiodinated tyrosine to create a pre-T4
The modified thyroglobulin molecule is endocytosed following TSH stimulation
Then undergoes proteolytic cleavage in lysosomes to release mature T3
and T4 molecules from the polypeptide chain
Thyroid hormones are released from the basal aspect of the cell by exocytosis and enter the bloodstream
What is the most abundant Thyroid Hormone?
T4 is the most abundant and stable thyroid hormone
What is the most metabolically active Thyroid Hormone?
T3
How can T4 be converted to T3?
T4 can be converted to T3 in tissues by the removal of an iodine molecule via deiodinase action
What happens when iodine is removed from T3?
Removal of a further iodine molecule creates inactive forms rT3 or T2
Parathyroid Gland
Parathyroid hormone (PTH) secreted by principal cells stimulates bone resorption by osteoclasts.
This raises blood calcium levels
Adrenal (Suprarenal) Glands
Adrenal glands are embedded in perirenal adipose near the superior poles
of each kidney
The adrenal cortex has three layers and secretes corticosteroid hormones
The medulla secretes adrenaline and noradrenaline
Blood Supply to the Adrenal Gland
The adrenal medulla has a dual blood supply
It receives arterial blood from the medullary arterioles and venous blood from the capillaries of the cortex
Sinusoids allow maximum exchange of macromolecules
Outer Cortex of the Adrenal Gland
The outer fibrous adrenal capsule provides protection
The zona glomerulosa produces mineralocorticoids such as aldosterone
The zona fasciculata produces glucocorticoids such as cortisol
Inner Cortex of the Adrenal Gland
The zona reticularis produces weak androgens
The medulla produces adrenaline and noradrenaline
Adrenal Secretions (4)
Mineralocorticoids: aldosterone regulates Na+ uptake by renal tubules
Glucocorticoids: cortisol regulates carbohydrate metabolism
Gonadocorticoids: weak androgens
Catecholamines: stress responses
