Lecture 16: branching morphogenesis Flashcards
What is branching morphogenesis?
Branching morphogenesis = enables physiological functions of different tissues, especially where a high SA:V ratio is needed in gas, fluid or solute exchange, secretion or excretion. In all of the structures where it occurs there is a branch epithelium and a mesenchyme, including lung, kidney, prostate and pancreas.
Where does branching morphogenesis occur?
The mammalian lung = lots of branching for high SA:V ratio.
Kidney = dense network of epithelial tubes – mesenchyme clouds around it.
Rat mammary gland = epithelial secretory ducts suspended in mesenchyme and fat.
Drosophila larval tract (like mammal lung) – develops from placodes starting as a placode of epithelium.
Describe the process of branching morphogenesis in the fly tracheal system.
Process of tubular outgrowth from larval epithelium is driven by interactions between two cells:
- Epidermal cells produce branchless (FGF-like ligand), secreted in patches.
- Epithelial cells produce breathless (FGF-like receptor)
Leader cells closest to the branchless source have activated Breathless triggering uni-cellular sprouting. This produces a tree of fingerlike projections (filopodia) from within each leader cell to form terminal unicellular branches and maximise SA:V ratio for gas exchange.
The epithelium moves towards the branchless source as it becomes a projection - the filopodia extrude the cells behind them from the flat epithelium.
Cells undergo intercalation which allows the tube to become long and narrow. Cell migration coupled with intercalation = looking for high concentrations of the branchless ligand. Once they reach position, the leader cells signal to the cells beneath by inducing sprouty.
Sprouty is an inhibitor of RTK signalling and blocks the ability of cell beneath to respond to the branchless ligand => branching only occurs in leader cells.
Leader cells undergo terminal branching and are specified by notch-delta signalling. Cells below are inhibited by the notch signal from adopting leader cell fate.
Cells that lack sprout => can’t inhibit RTK signalling. The tracheal system is hyper branched => hyper profusion.
Describe the process of branching morphogenesis in the mammalian kidney.
The kidney develops in the trunk of the early embryo.
The intermediate mesoderm gives rise to the uteric bud epithelium and the metanephrogenic mesenchyme of the kidney.
Three major phases of kidney development in mammals.
1 – Pronephros formation = all low vertebrates – acquires filtration functions.
- Starts as a tubular epithelium called the nephric duct which elongates
- Mesenchyme converted into lateral branches of the ureteric bud.
2 – Mesonephros formation = functions in fish in birds. Transitional in mammals.
- Duct gets longer
- Pronephric components degenerate
- Massive expansion of metanephrogenic mesenchyme
- Extension of nephric duct which makes contact with the bladder.
3 - Metanephros formation = definitive kidney of mammals.
- Mesenchyme sends a signal to the nephric duct to induce its transformation into the ureteric bud – start of kidney development.
Induction of uteric bud branching by metanephrogenic mesenchyme
Large clump of metanephrogenic mesoderm in the posterior.
- Ureteric bud induced
- Branching process begins regulated by bud and mesenchyme interactions
- Glial-derived neurotrophic factor (GDNF) from mesenchyme causes proliferation and outgrowth of RET receptor (an RTK) in the bud tip cells
- Cells proliferate and the bud grows out
- Leading-edge tip cells receive a signal to arrest proliferation
- The cells underneath do not recognise the signal so the bud grows laterally producing a T junction
- Tip bud cells secrete BMP2 and FGF7 to promote mesenchyme apoptosis in the middle of the junction
- The tip cells, still surrounded by mesenchyme undergoes further branching at the two ends of the tip junction
Sprouty = inhibitor of FGF signalling in branching of tracheal system of the fly. The mammalian ortholog has a similar role in the kidney.
LOF sprouty = excessive branching => sprouty limits branching like in the drosophila tranchea
In flies it acts on FGF (RTK) but in the mammalian ureteric bud it blocks the function of the Ret receptor, an RTK, for GDNF. This is an example of combos of similar signalling molecules to produce similar results.
Mesenchyme to epithelial transition induced by Wnt signalling in the ureteric bud.
- Ureteric bud undergoes multiple rounds of branching through activities of GDNF, ret and Sprouty.
- Wnt signalling molecules released by the ureteric bud epithelium induces the characteristics of the mesenchyme – aggregation, cavitation and epithelization to form the renal tubule and Bowman’s capsule (epithelial to mesenchyme transition)
- Epithelium fuses with the ureteric bud which goes on to develop into the collecting duct and ureter.
- Proximally the renal tubule fuses with the collective duct-forming ureteric bud
- Distally, the renal tubule attracts epithelial capillaries that form the glomerulus
The Wnt signalling from the ureteric bud is essential for kidney development shown through Wnt11/9b knockout mice which don’t develop kidneys.
Branching morphogenesis fills the kidney with nephron-collecting duct units that all connect to a single ureter.
Explain the role process of branching morphogenesis in the human lung.
Involves the endoderm and mesoderm.
Endoderm -> epithelial lining of trachea, larynx, bronchi, bronchioles and alveoli. Contains the respiratory diverticulum located ventrally under the foregut which becomes the trachea.
Mesoderm -> cartilage, muscle and connective tissue.
The lung bud undergoes branching surrounded by a sack of mesoderm.
FGF, SHH and BMP Signalling
- The bud epithelium is close to the mesenchyme which expresses FGF10 in patches.
- FGF10 upregulates the expression of FGF2B receptor in the epithelium
- Epithelium protrusions grow out towards the source of FGF10.
- FGF10 also induces Sprouty expression in the bud epithelium which suppresses proliferation of cells below the tip = negative feedback loop because stops the cells from responding the FGF signals
- Further outgrow is restricted only to the cells at the tips
- Tip cells express and secrete both SHH and BMP in response to sustained FGFR2B activation by FGF10
- SHH suppresses FGF10 in the mesenchyme, splitting the expression into two separated domains. BMP4 acts on the tip cells themselves to stop their proliferation = negative feedback
- Epithelial cells beneath the tip do not express BMP4 so continue to grow laterally towards the 2 separate FGF10 sources
- Branching starts again
Identify similarities between these processes.
Drosophila tracheal systemMammalian LungMammalian kidneyEpidermal cells produce and FGF-like ligand (branchless) and the epithelium expresses an FGF-like receptor (breathless).Mesenchyme expresses FGF10 which signals to the FGF2B receptor on epithelium at the tip of the lung bud.Mesenchyme produces GDNF which signals to epithelial RET receptor (RTK) in the tip cells.Epithelial cells grow out towards branchless source in filopodia.Epithelium cells out towards FGF10 in protrusions.Epithelial cells grow out towards GDNF.Sprouty expression induced by leader cells supresses outgrowth of the cells beneath by inhibiting breathless receptor in themSprouty expression induced in epithelium suppresses proliferation of the cells beneath by inhibiting FGF2B receptor in them.Sprouty expression induced in epithelium suppresses proliferation of cells beneath by inhibiting RET receptor in them.
Tip cells express and secrete BMP4 and Shh in response to sustained FGFBR activation.
Shh suppresses FGF10 in the mesenchyme.
BMP inhibits FGFBR activation.
Tip cells express and secrete BMP7 and FGF to induce mesenchymal apoptosis in the middle of the T junction.
