Development of salivary glands Flashcards
What are the 2 main cells in a salivary gland?
Acinar cells and ductal cells.
Give all details on the structure of a salivary gland
Acini transfer water from blood vessels and direct it into the lumen of the duct via osmosis.
Acini cells put their own protein into saliva.
Rich dual innervation of sympathetic and parasympathetic nerves controlling flow rate and which proteins are secreted into saliva.
Ductal cells - modify the ionic components of saliva.
Intercalated duct cells - sit between the acini and duct cells. They are cuboidal cells and are the precursor for two acini and duct cells in adult glands.
Myoepithelial cells (blue cells in picture) have actin filaments to form a contraction of the gland. They support the acini during the secretion process by holding the acini.
In-between all cells are tight junctions (red dots) to maintain the osmotic difference between out and inside the duct.
Where do major and minor salivary glands develop from?
Major from oral ectoderm.
Minor from oral ectoderm and endoderm (This is because minor salivary glands are still present in the endoderm).
What interactions do salivary glands develop from?
Epithelial-mesenchymal interactions.
Summarise the stages of salivary gland development
We start with the initial bud where we have development from the oral ectoderm.
Then get a pseudo-glandular structure.
Canicular stage where the lumen of the ducts are being formed.
Then have the terminal bud where we see the development of cell types.
Most of the development is mostly completed by birth in humans.
Explain the processes that occur in the initial bud stage
Occurs around 6 weeks of embryo development.
Thickening of the oral cavity ectoderm to form epithelial placode.
Condensing mesenchyme as plasode expands.
Minor cells beginning around the initial bud. Causes the mesenchymal cells to squash even closer.
Lots of growth factors are released from the mesenchyme and the epithelium. Factors from the epithelium and mesenchyme work on each other. This causes the development.
One of the big growth factors of the salvary glands include the FGF10 and FGFr2b. FGF = fibroblast growth factor but doesn’t only come from fibroblasts.
These come form the epithelium which interact with receptors on the mesenchyme.
What happens in the pseudo-glandular stage?
Further proliferation of the initial bud forms the pseduo-glandular stage.
These are solid groups of cells.
Still have crucial epithelium-mesenchymal interactions but there are more factors which are important.
BMPs become important (bone morphogenic proteins). No bone here but just help interactions,
MMPs (matrix metalloprotinases) create space for the epithelium to expand into by breaking down mesenchymal cells.
Shh (sonic hedgehog) is important in epithelium expansion, along with EPFR (epithelial group factor).
These growth factors are important in the branching stage to decide where branching of the duct will occur, the mesenchyme will only influence the shape of the branching. The mesenchyme does not affect the growth factors as each salivary gland has its own collection of proteins that it expresses.
What does experimental data tell us about growth factors?
Evidence of which factors are important in branching.
Salivary glands extracted from embryos and separated from mesenchyme. Indiduval growth factors added on in different concentrations.
Compared to having no growth factors, if you have FGF1, start to get elongation of the buds.
FGF2 will give proliferation of just the end part of the salivary gland.
Different factors give different patterns of proliferation.
BMPs tend to inhibit proliferation.
If you add a few different growth factors, you start to get all the different branching effects.
Overall, FGFs promote proliferation by BMPs inhibit it. Important in forming the separation between the cell types. Want more branching to enhance surface area.
What occurs in the canalicular stage?
After branching morphogenesis stage, we start to see the canicular stage.
Around 10th week in the parotid gland.
Start to see formation of the lumens that were originally solid stalks of cells.
Usually involves apoptosis to form the lumen.
Failure of salivary gland development at this stage can cause cysts and large swellings under the mucosa.
BMP7, retinoid acid receptors and grain-head transcription factors are all important here in cause the canalicular stage.
What cells do we get in the terminal bus stage?
16 weeks in embryo
Type 1 cells - go onto form the striated duct
Type 3 cells - go onto form the acini cells
These are not fully functional cell types yet.
We start to see the myo-epithelial cells.
What happens in the perinatal stage?
36 weeks in embryo development.
Last stage shows integration of nerves that were originally growing on the outside of the duct. Nerves start to attach to the acini.
Starts off with the parasympathetic neves and then the sympathetic nerves that integrate.
We needs the nerves attaching to start causing salivary secretion.
Salivary proteins that the glands are making are changing at this stage.
What protein is only present in adults?
Amylase
What are the influences on adult salivary glands?
Tooth eruption as in the periodontal ligament, there are many mechanoreceptors picking up senses causes more salivary production
Weaning (liquid to solid diet), more chewing needed, more innervation from periodontal ligament to salivary glands
Puberty as hormones affect
saliva production and gland development
Age has no influence on salivary glands so salivary function should be present until death
Medications can cause xerostomia so tends to be in older people on more medication
Answer these:
When does branching morphogenesis occur?
When do nerves connect into acinar cells?
From which tissue do myoepithelial cells develop?
Is amylase an embryonic or adult salivary protein?
Which signalling molecule opposes FGF-signalling?
Pseudo-glandular stage
Perinatal stage
Epithelium
Adult
BMPs
