5. Organogenesis Flashcards
Define mature organ
Mature organ - carefully organised collection of cells with specialised functions to sustain vital bodily functions
Explain kidney anatomy (organogenesis studied example)
Nephron (metanephric mesenchyme MM):
- proximal tubules
- glomerulus
- loop of Henle
- distal convulated tubules
PraGaLaDalsia
Collecting duct (ureteric bud UB)
What germ layer gives rise to kidneys? Which part of the germ layer?
From mesoderm - intermediate mesoderm -> kidneys
How kidney differentiation is started from intermediate mesoderm?
Early kidney development - by gene expression (TFs): Lim-1, Pax-2, Pax-8:
- paracrine signal (morphogen: bone morphogenetic protein 2 - BMP2) + signal from paraxial mesoderm to intermediate mesoderm - induces TFs transcription (when paraxial and intermediate mesoderm separated - no kidney) - medium level of BMP2 needed for kidney development (Lim-1, Pax-2)
What is the sequence of genetic signalling to induce kidney differentiation in intermediate mesoderm?
BMP2 from external sources
Which cell types are the primary source for kidney development?
Metanephric mesenchyme (MM) -> nephron
Ureteric bud (UB) -> collecting duct
Local reciprocal inductive interactions between UB and MM drives kidney development (internal communication between tissues)
Explain reciprocal inductive interactions
Reciprocal inductive interactions - a two-way dialogue - both cell populations are signalling and targeted (inducers + responders) - responder must be competent to sense the signal - appropriate receptor
What are reciprocal inductive interactions in kidney development?
- Different tissues can grow and develop in coordinated way - development is proportionate - self organisation of the kidney
- MM and UB use reciproval inductive interactions for growth and branching of UB + mesenchymal-to-epithelial transition (MET) in MM (nephron formation)
Explain transfilter induction assay in kidney development
- Kidney rudiment dissected - MM and UB separated -> in isolation cells did not develop into kidney
- Kidney rudiment dissected - MM and UB left together - UB branching + nephron formation in vitro
=> close proximity MM and UB needed for kidney development - communicate between each other
Explain what is Ret/GDNF signalling used for
Ret/GDNF signalling - outgrowth of UB:
signal: glial cell line-derived neurotrophic factor (GDNF) active in MM
receptor: Ret in nephric duct and tips of UB branches
Signalling induces UB growth into MM - induces branching to produce ureteric tree
How was it determined that GDNF needed for outgrowth of UB? Removal of component experiment
GDNF mutant mice / Ret inhibited - no UB branching => GDNF signalling is necessary for budding/branching
How was it determined that GDNF needed for outgrowth of UB? Providing extra ectopic signal
Is GDNF sufficient for UB branching - extra GDNF provided -> extra (ectopic) branching => GDNF is both necessary and sufficient for UB budding/branching
Ret/GDNF signalling in UB branching disrupted by Ret mutation
Cells with highest Ret activity branch if different activity is induced
How do cells in UB physically branch in kidney development?
UB tips swell - bifurcate into two new branches - extend and bifurcate again
What cellular behaviours drive UB branching in kidney development?
- differential cell proliferation
- cell rearrangement
- localised remodelling of ECM
What is differential proliferation?
Differential proliferation - different potential of cell populations to proliferate under specific conditions
Where is differential proliferation observed branching of UB in kidney development?
Differential proliferation in branching between tip and trunk of UB:
tip cells proliferate more than trunk - correlation in proliferation and branching - driven by Ret/GDNF signalling
How does cell rearangement occur in branching of UB in kidney development?
Ret/GDNF signalling mediates rearrangement of epithelial cells before budding to form UB tip
Where does ECM remodelling occur in UB branching in kidney development?
Remodelling of ECM:
different components of ECM between UB tip/trunk - collagen 18 in trunk but not tip - because GDNF represses collagen 18 in tips
=> collagen 18 pattern influences trunk branching
Explain how mesenchymal-to-epithelial transition (MET) is induced and occurs in kidney development
- MM induced for MET by Wnt morphogens (family of genes) - Wnt9B secreted from UB (paracrine signal) - induce MET to form nephrons
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Wnt9B induces Wnt4 expression - autocrine signalling - propagates mesenchymal aggregation: renal vesicle grows by recruiting more MM until Wnt4 threshold reached - renal vesicle begins to differentiate
=> ensured that renal vesicle is of appropriate size
What experiment could be used to determine if WntB9 is needed for kidney development?
Wnt9B mutants - failed to undergo MET in MM - no kidney formed => Wnt9B crucial for kidney development
What is quorum sensing?
Quorum sensing - for of cell communication which allows cells to share info about cell density - regulate of gene expression responding to fluctuations in cell density
What is the evidence for quorum sensing in kidney development?
Explain what epithelial character is developed in MET in nephron development in the kidneys
- tubular shape is developed - lumen formed (single-cell layer, non-startified tubular epithelium)
- adhesion junctions developed (ex cadherins)
- apicobasal polarity established (different transport proteins on membranes)
- occluding junctions developed (to ensure tubes are not leaky + stops passing unwanted solutes)
What are the functions of nephron parts?
Explain th patterning process in the nephron
Patterning process:
- patterning starts in renal vesicle (proximo-distal axis): initial RV -> comma shaped RV -> S shaped RV -> mature nephron
- patterning controlled by morphogens
What kind of differentiation events occur after patterning in nephron?
Differential gene expression:
- ion channels, transporters, specialised cell architectures
Explain Bartter syndrome
Bartter syndrome:
- ascending limb of loop of Henle doesn’t differentiate properly - dysfunction in Na-K-Cl ion transporters
=> low K levels, increased blood pH
Explain how the glomerulus capillaries develop in kidney development
Endothelial cells migrate into the developing glomerulus - migrate into cleft of S shaped body - sit **on top **of podocyte precursors -> development -> glomerular capillary surrounded by podocytes inside Bowmann’s capsule
What is the function of glomerulus?
Glomerulus filters blood - mostly small molecules like water
What is chemotaxis?
Chemotaxis - movement of cell in response to a increasing / decreasing gradient of a chemical signal
Chemoattractants / chemorepellents
How do endothelial cells know that they have to migrate into the S shaped body cleft in kidney development?
Endothelial cells respond to a signal - chemotaxis - chemoattractant vascular endothelial growth factor VEGF - cells move towards the higher conc of signal in S shaped body
How was is the importance of VEGF in kidney development tested?
VEGF mutants - no endothelial cell migration - capillaries did not develop
How is blood delivery and filtration matched?
Blood delivery (capillaries)
Blood filtration (podocytes)
Capillaries and podocytes grow and develop proportionately -> proportionate development
Explain polycystic kidney disease
Polycystic kidney disease (PKD):
- autosomal dominant (ADPKD)
- autosomal recessive (ARPKD)
Progressive formation of large fluid-filled cysts - increase tubular diameter, nephron loss, kidney failure
How is a tubule formed in organogenesis?
- regulated orientation of cell divisions (anisotropic growth)
- organ shape sculted by cell rearrangement
Explain how the cell divisions are oriented in renal tube formation
Tubules only increase in length but not diameter - oriented cell division - anisotropic growth - measured mitotic angle (mitosis orientation 95% along long axis)
BUT in polycystic mutant - mitotic alignment is random
Explain how the cells are rearranged in renal tube formation
Renal tubes sculpted by cell rearrangement - renal tube diameter reduces in embryonic development - rearrangement driven by cell intercalation - increase tubule length - decrease diameter => convergent extension
