Neubüser Organogenese

• developement of mesoderm around endoderm: lateral plate and Splanchnic mesoderm

• the cells are derived from the viceral mesoderm that surrounds the primitive gut tube

• (airway) smooth mucle cells (ASMC), connective tissue

Phenotype:

Hypoplasia (underdeveloped) or agenesis (absence) because its crucial for pancreas progenitor cell formation

Disruption of Endocrine and exocrine cell differentiation and also disruption of the formation of the islets of langerhans

Possibly errors in Glucose uptake -> diabetes

Sox9 (SRY-related HMG-box 9) is a transcription factor:

1. Pancreatic Agenesis or Hypoplasia:

   • Sox9 is involved in the specification and maintenance of pancreatic progenitor cells. A knockout of Sox9 in the pancreas may lead to a reduction in the number of pancreatic progenitor cells, resulting in pancreatic agenesis (absence of the pancreas) or hypoplasia (underdevelopment).

2. Disruption of Endocrine and Exocrine Cell Differentiation:

   • Sox9 is important for the differentiation of both endocrine and exocrine cells in the pancreas. A knockout of Sox9 could lead to defects in the differentiation of insulin-producing beta cells (endocrine) and enzyme-secreting acinar cells (exocrine).

3. Altered Pancreatic Duct Development:

   • Sox9 is involved in the development of pancreatic ducts, which play a crucial role in transporting digestive enzymes. A knockout of Sox9 might lead to abnormalities in pancreatic duct formation or function.

4. Impaired Islet Morphology:

   • Islets of Langerhans are structures within the pancreas that contain endocrine cells, including insulin-secreting beta cells. A Sox9 knockout may result in aberrant islet morphology and compromised function.

5. Glucose Metabolism Abnormalities:

   • Given the role of the pancreas in regulating blood glucose levels, a knockout of Sox9 could potentially lead to glucose metabolism abnormalities, which may manifest as hyperglycemia or impaired glucose tolerance.

6. Failure of Pancreatic Progenitor Expansion:

   • Sox9 is involved in the expansion of pancreatic progenitor cells during development. A knockout could disrupt the normal proliferation and expansion of these cells, impacting overall pancreatic growth.

• developement of mesoderm around endoderm: lateral plate and Splanchnic mesoderm

• the cells are derived from the viceral mesoderm that surrounds the primitive gut tube

• (airway) smooth mucle cells (ASMC), connective tissue

• developement of mesoderm around endoderm: lateral plate and Splanchnic mesoderm

• the cells are derived from the viceral mesoderm that surrounds the primitive gut tube

• (airway) smooth mucle cells (ASMC), connective tissue

• the lungs and the kidneys

• in the lungs the smooth mucle first separate 2 lung buds, then FGF (from the mesodermal cells) triggers apical constriction to form more branching in 3 different modes:

  • Domain branching

  • planar bifurcation

  • orthogonal bifurcation

• in the kidneys the nephric duct developes early after the gonads developed at one point through Ret receptor and GDNF interaction the uretric bud developes to the ureta branching in the metanephric blastema

1. Initiation of Branching:

   • The process often begins with the initiation of a bud or outgrowth from an existing structure. In the case of the kidney, the ureteric bud arises from the Wolffian duct.

2. Cell Proliferation:

   • Rapid cell proliferation occurs at the tip of the bud. This leads to the extension of the bud into the surrounding tissue, creating a branched structure. Cell division is tightly regulated, ensuring proper expansion of the developing organ.

3. Guidance Cues and Signaling Pathways:

   • Signaling pathways play a crucial role in guiding the direction of branching. Various molecular signals, including those from growth factors like FGF (Fibroblast Growth Factor), Wnt (Wingless/integrated), and others, provide cues that direct the cells to migrate and proliferate in specific patterns.

4. Cell-Cell Interactions:

   • Interactions between different cell types contribute to the branching process. For example, in kidney development, there are reciprocal interactions between the ureteric bud and the metanephric mesenchyme. The signals exchanged between these cell populations influence their behavior and drive branching.

5. Extracellular Matrix (ECM):

   • The extracellular matrix, which is the network of proteins and carbohydrates surrounding cells, provides structural support and signaling cues for branching. Changes in the composition and organization of the ECM influence cell migration and differentiation during morphogenesis.

6. Apical-Basal Polarity:

   • Cells within developing tissues exhibit apical-basal polarity, meaning they have distinct top and bottom surfaces. This polarity is crucial for proper tissue organization and branching. For example, in the kidney, the epithelial cells lining the tubules have apical surfaces facing the tubular lumen.

7. Remodeling and Maturation:

   • As branching continues, the developing structure undergoes remodeling and maturation. Cells differentiate into specific cell types, and the branched network matures into a functional organ.

8. Feedback Loops:

   • Various feedback loops and regulatory mechanisms control the branching process. For instance, signaling pathways may activate or inhibit each other, ensuring precise spatial and temporal control over the morphogenetic events.

Plasticity phenomenon of cells in early developement

• precursor cells in the foregut region may have common developmental origins

• a specific combination of TFs form a specific organ they overlap at some points that is why an alteration in these TF can reprogram cells to become a different organ

• Signaling pathways from the surrounding microenvironment also influence cell fate decisions. Altering the activity of specific transcription factors may impact how cells respond to signaling cues, leading to a shift in developmental fate.

1. Reduced Organ Size:

   • Premature differentiation may lead to a depletion of the undifferentiated progenitor pool, resulting in a reduced number of cells available for further organ growth. This can lead to an overall decrease in organ size, compromising its structure and function.

2. Impaired Tissue Architecture:

   • Proper tissue architecture and organization rely on the coordinated proliferation and differentiation of precursor cells. Premature differentiation can disrupt the organized patterning of cells, leading to abnormal tissue structures and compromised functionality.

3. Incomplete Organ Formation:

   • Differentiation processes need to be temporally and spatially regulated for the formation of a fully functional organ. Premature differentiation may result in incomplete organ development, with missing or improperly formed structures.

4. Functional Deficits:

   • Differentiated cells often acquire specialized functions necessary for organ function. Premature differentiation may result in cells that are not fully equipped to carry out their intended roles, leading to functional deficits within the organ.

5. Loss of Regenerative Capacity:

   • Undifferentiated progenitor or stem cells often possess regenerative capacity, allowing them to replace damaged or lost cells. Premature differentiation may reduce the pool of regenerative cells, limiting the organ's ability to repair itself in response to injury or normal wear and tear.

6. Cell Fate Imbalance:

   • Differentiation needs to be balanced with proliferation to maintain appropriate cell numbers. Premature differentiation without sufficient proliferation may lead to an imbalance in cell types, disrupting the normal cellular composition of the organ.

7. Altered Signaling Environments:

   • Undifferentiated progenitor cells contribute to the local signaling environment within developing organs. Premature differentiation can alter this environment, affecting the signaling cues that regulate further development and potentially leading to a cascade of abnormal cellular behaviors.

8. Increased Sensitivity to Environmental Insults:

   • Prematurely differentiated cells may be more vulnerable to environmental stresses or insults, as they may lack the protective features or support mechanisms provided by undifferentiated progenitors.

The first stages of lung developement specify the lung precursers: TFs in foregut specification (NKX2.1 and SOX2), WNT26, FGF for bud formation and lung branching)

Nkx2.1 induces seperation from the gut tube -> lung precursor formation

Nkx2.1 accumulates ventrally by upregulation from BMP, FGF and Wnt

Wnt signalling is upregulated by RA

Sox2 inhibited by BMP -> accumulates dorsally

Noggin at dorsal side inhibits BMP -> BMP only ventrally

1. Foregut Specification:

   • During early embryonic development, the foregut region is specified. The foregut gives rise to various organs, including the respiratory system. The foregut endoderm undergoes regionalization, and specific signaling pathways contribute to the formation of the respiratory primordium.

2. Expression of Transcription Factors:

   • Specific transcription factors play a crucial role in the specification of lung precursors. Notable examples include:

     • NKX2.1 (also known as TTF-1)

     • SOX2

3. BMP and Wnt Signaling Pathways:

   • Signaling pathways, such as BMP (Bone Morphogenetic Protein) and Wnt, play important roles in foregut specification and lung development. These pathways contribute to the patterning of the foregut endoderm and influence the expression of key transcription factors.

4. Mesodermal Interactions (Cardiogenic mesoderm):

   • Interactions with adjacent mesodermal tissues, such as the mesenchyme surrounding the developing foregut, are crucial for lung specification. Signaling molecules and cell-cell interactions between the endoderm and mesoderm contribute to the commitment of endodermal cells to a lung fate.

5. Bud Formation:

   • As lung specification progresses, the respiratory primordium undergoes branching morphogenesis to form the lung buds. The outgrowth of these buds is regulated by complex signaling interactions, including those involving FGF (Fibroblast Growth Factor) and SHH (Sonic Hedgehog).

6. Proximal-Distal Patterning:

   • The developing lung undergoes proximal-distal patterning, resulting in the differentiation of different cell types along the respiratory tract. Proximal regions give rise to conducting airways, while distal regions give rise to gas-exchange regions (alveoli).

7. Endodermal Cell Fate Decisions:

   • Within the respiratory primordium, endodermal cells make cell fate decisions leading to the development of specific cell types, such as ciliated cells, secretory cells, and alveolar cells. This process is tightly regulated by various signaling pathways and transcription factors.

FGF signaling —> FGF10 from mesenchyme activates FGF receptor (Fgfr2) in lung epithelium and promotes branching, it interacts with BMP and WNT pathways

WNT signaling —> WNT ligands from mesenchyme interact with WNT receptors in branching of lung. it interacts with FGF and Shh

Shh signal —> inhibits FGF together with BMP inbetween the branching “peaks” to prevent overbranching

1. Fibroblast Growth Factor (FGF) Signaling:

   • FGF signaling is crucial for the initiation and progression of lung branching.

   • FGF ligands, particularly FGF10, secreted by the mesenchyme, activate the FGF receptor (FGFR2b) in the epithelium, promoting cell proliferation and branching.

   • The FGF pathway interacts with other pathways to coordinate branching, including Wnt and BMP pathways.

2. Wnt Signaling:

   • Wnt signaling is involved in lung branching and proximal-distal patterning.

   • Wnt ligands from the mesenchyme interact with Wnt receptors in the epithelium, contributing to branching morphogenesis.

   • Wnt signaling interacts with FGF and Shh pathways to coordinate branching.

3. Sonic Hedgehog (Shh) Signaling:

   • Shh signaling plays a role in lung development, including branching morphogenesis.

   • Shh ligands are expressed in the distal lung epithelium, and their receptors (Patched and Smoothened) are present in the mesenchyme.

   • Shh signaling interacts with FGF and BMP pathways to control lung branching.

4. Bone Morphogenetic Protein (BMP) Signaling:

   • BMP signaling is involved in lung development, including branching morphogenesis and epithelial differentiation.

   • BMP ligands from the mesenchyme activate BMP receptors in the epithelium, influencing branching and differentiation.

   • BMP signaling interacts with FGF and Wnt pathways in regulating lung branching.

5. Notch Signaling:

   • Notch signaling is involved in cell fate decisions and influences branching morphogenesis.

   • Notch receptors and ligands are expressed in both the epithelium and mesenchyme.

   • Notch signaling interacts with FGF and other pathways to regulate branching and differentiation.

• the ends of the branches grow further (directed by the mesenchymal cells that send FGF signaling) the cells outside of the FGF / BMP/Shh / WNT threshhold (more proximal cells) already start to differentiate

1. Proximal-Distal Patterning:

   • As the lung buds elongate and branch, proximal-distal patterning occurs. This process involves the differentiation of cells along the proximal (airway) to distal (alveolar) axis. Signaling molecules, such as FGFs and BMPs (Bone Morphogenetic Proteins), contribute to this patterning, influencing the specification of different cell types along the respiratory tract.

2. Cell Differentiation along the Airways:

   • As branching progresses, cells along the airways differentiate into various cell types, including ciliated cells, goblet cells (mucus-producing), and basal cells. The differentiation of these cell types is controlled by specific transcriptional programs and signaling pathways.

3. Alveolar Development:

   • Distal regions of the lung buds give rise to alveoli, which are the primary sites for gas exchange. The differentiation of alveolar cells, including type I and type II pneumocytes, is tightly regulated. Surfactant production by type II pneumocytes is essential for proper lung function.

• the injured cells spread ?

• inflammatory response removal of damages cells by macrophages

• damaged cells send WNT signal to ASMC they send FGF10 signal to variant club cells to divide into the damaged epithilium and differentiate again

• Activated progenitor cells and resident epithelial cells undergo proliferation to replace the lost or damaged cells. This is a critical step in regenerating the distal airway epithelium (guided by WNT and FGF10).

The pancreas develops through fusion of a dorsal and a ventral bud

Signals from cardiac (and septum transversum) mesoderm induce the ventral pancreas

Signals from the notochord allow dorsal pancreas development by suppressing endodermal Shh expression

Signals from the aorta and vitellin veins further support pancreas development

Proliferation and maintenance of the pancreas progenitor cells requires FGF10 from the surrounding mesoderm

Cell types and architecture of the adult pancreas:

1. Exocrine pancreas: acinar tissue (digestive enzymes) and ducts

2. Endocrine pancreas: islets of langerhans with alpha-cells: Glukagon, beta-cells: Insulin; delta-cells: Somatostatin, PP-cells und Epsilon-cells: Ghrelin

Pancreas morphogenesis: Development of the branched pancreas ductal system

Cell type specification in the pancreas (delta - notch)

1. Specification of Pancreatic Progenitors:

   • Early in embryonic development, cells in the endodermal layer of the gut tube undergo specification to become pancreatic progenitor cells. This process is influenced by signaling molecules, including fibroblast growth factors (FGFs(10) —> upregulate to inhibit Shh), bone morphogenetic proteins (BMPs), and sonic hedgehog (Shh —> inhibited for pancreas).

2. Formation of the Dorsal and Ventral Pancreatic Buds:

   • Pancreatic progenitors give rise to two buds, the dorsal and ventral pancreatic buds, which will fuse to form the mature pancreas. The transcription factor Pdx1 (Pancreatic and duodenal homeobox 1) plays a crucial role in the specification and maintenance of pancreatic progenitors.

   • Signals from the aorta and vitellin veins further support pancreas development

3. Fusion of Pancreatic Buds:

   • The dorsal and ventral pancreatic buds fuse to form a single organ. This process is mediated by signaling centers and involves the coordinated expression of various genes, including Ptf1a (Pancreas transcription factor 1a).

4. Endocrine Cell Differentiation:

   • Within the developing pancreas, some cells differentiate into endocrine cells, which will later form the islets of Langerhans. Neurogenin 3 (Ngn3) is a key transcription factor involved in the specification of endocrine progenitors. These cells then differentiate into specific endocrine cell types, such as insulin-producing beta cells and glucagon-producing alpha cells.

5. Exocrine Cell Development:

   • Other cells in the pancreas differentiate into exocrine cells, primarily acinar cells responsible for producing digestive enzymes. Transcription factors such as Ptf1a and Rbpjl (Recombination signal-binding protein for immunoglobulin kappa J region-like) play essential roles in exocrine cell development.

6. Ductal Cell Formation:

   • Ductal cells, which form the pancreatic ducts, are also essential for proper pancreas function. Transcription factors such as Sox9 (SRY-Box Transcription Factor 9) are involved in ductal cell specification.

7. Mature Pancreas Formation:

   • The coordinated development and differentiation of endocrine, exocrine, and ductal cells lead to the formation of the mature pancreas. This process involves intricate signaling pathways, including Notch signaling, which plays a role in cell fate decisions.

8. Islet Maturation and Functional Integration:

   • Following the initial differentiation of endocrine cells, further maturation occurs as these cells integrate into functional islets. The maturation process involves the expression of additional transcription factors, such as Nkx6.1 and MafA, which are crucial for beta cell function.

Notochord Ablation or Notochord tissue transplantation to different localization:

• Surgical removal/ablation or relocalization of the notochord (tissue) in chick embryos can be performed

• compare pancreas developement between normal and operated chick

Inhibition of Notochordal Signaling

• pathway inhibitors or blocking antibodies specific to notochord-derived signaling molecules

Tissue Recombination Experiments:

• isolate the Pdx1 positive endoderm and combine it with different mesodermal tissues

Removal or transplantation of the surrounding mesodermal structures and compare the effects on the mouse

genetic or chemical blocking of Signaling Pathways from the mesoderm

Live imaging techniques

Context-Dependent Signaling:

• in different tissues FGF10 has different co-factors/downstream effectors that influence the signal

Singnaling network

• FGF10 does not work alone all involved molecules can crosstalk in the signaling pathway and influence the gene regulation

Different sets of activated transcription factors downstream of the FGF signaling pathway

—> the lung tissue has different TF: NKX2.1 or FOXF1

—> the pancreas has: PDX1 or SOX9 or HNF

• defects in organ growth, disruption of duct morphogenesis, altered exo and endocrine cell populations

• Disruption of the feed forward loop of FGF10 —> FGFR2B —> Sox9 leads to respecification of the pancreas progenitors into liver progenitors

• Neurogenin 3 (Ngn3) is a key transcription factor involved in the specification of endocrine progenitors. These cells then differentiate into specific endocrine cell types, such as insulin-producing beta cells and glucagon-producing alpha cells.

knock out phenotypes would have:

• Loss of Endocrine Cell Differentiation

• Decreased Beta Cell Mass (insulin producers —> type 1 diabetes) and alpha cell mass (glucagon)

• Islets of Langerhans altered, which contain clusters of endocrine cells, may exhibit altered morphology

Lineage Tracing:

• Utilize lineage tracing techniques to mark and track the descendants of Ngn3-positive cells over time —> genetic reporter system under control of Ngn3 promotor

Generate transgenic mouse lines that express reporter genes (e.g., green fluorescent protein, LacZ) under the control of the Ngn3 promoter

Specific marker such as immunohistochemistry or immunofluorescence on the Ngn3 pos cells

knock out mice —> inhibit or delete Ngn3 and see which cell do not deveope later

• Development of 3 different transient kidneys that develop from intermediate mesoderm: Pronephros ( no function), Mesonephros (embryonic kidney), Metanephros (permanent kidney)

• Formation of the nephric duct which elongates through proliferation -> Chemotaxis towards FGF8 produced in tail bud region

Metanephros development:

• GDNF binds to Ret receptors on nephric duct. Binding initiates branching -> Uretric duct formation

• Negative Regulators in adjacent cells of uretric bud inhibit GDNF (FOXC1) & Ret receptor expression (SPRY1)

• GDNF also controls further branching morphogenesis

• Wnt signals essential for nephron formation & branching morphogenesis of collecting ducts ( WNT11 tip positive feedback loop GDNF & RET, WNT9b acts on underlying mesenchyme -> mesenchym condensation -> WNT9b (stalk) -> ß-Catenin-> FGF8 -> WNT4. 3 Progenitor pools exposure to different concentrations of WNT9b in the stalk -> differentiation -> fusion with duct -> Nephron patterning with Glomerulus

1. Ureteric Bud Induction:

• Molecular Regulation:

  • GDNF (Glial cell-derived neurotrophic factor): Secreted by the metanephric mesenchyme, GDNF binds to its receptor Ret on the Wolffian duct, initiating ureteric bud formation.

  • Wnt11: Wnt11 signaling is also involved in the initiation of the ureteric bud.

2. Branching Morphogenesis of the Ureteric Bud:

• Molecular Regulation:

  • Retinoic Acid (RA) Signaling: RA is essential for branching morphogenesis. It is produced by the mesenchyme and acts on the ureteric bud.

  • FGF (Fibroblast Growth Factor) Signaling: FGFs, particularly FGF2 and FGF7, play key roles in ureteric bud branching.

3. Formation of the Metanephric Mesenchyme:

• Molecular Regulation:

  • GDNF: Continues to be crucial for the maintenance and expansion of the metanephric mesenchyme.

  • BMP (Bone Morphogenetic Protein) Signaling: BMPs are involved in the condensation of mesenchymal cells.

4. Epithelialization of the Metanephric Mesenchyme:

• Molecular Regulation:

  • Pax2 and HNF1β: Both transcription factors are expressed in the metanephric mesenchyme and are crucial for promoting the epithelialization process.

5. Conversion of Renal Vesicles into Comma-Shaped Bodies and S-Shaped Bodies:

• Molecular Regulation:

  • WT1 (Wilms' Tumor 1): WT1 is expressed in developing nephrons and is involved in patterning renal vesicles into comma-shaped and S-shaped bodies.

  • Pax2: Continues to play a role in the differentiation of nephron structures.

6. Formation of Glomeruli and Tubules:

• Molecular Regulation:

  • Notch Signaling: Mediates the differentiation of podocytes within the glomerulus.

  • Wnt Signaling: Regulates the differentiation of tubular structures within nephrons.

  • Growth Factors (e.g., EGF, FGF): Contribute to the overall differentiation and growth of nephron components.

7. Maturation and Vascularization:

• Molecular Regulation:

  • VEGF (Vascular Endothelial Growth Factor): Critical for the invasion of blood vessels into the developing kidney.

  • Angiopoietins: Cooperate with VEGF in the formation and maturation of the renal vasculature.

  • Hox Genes: Transcription factors that contribute to nephron segmentation and maturation.

8. Connecting Nephrons to the Ureteric Bud:

• Molecular Regulation:

  • Wnt Signaling: Maintains communication between the ureteric bud and nephrons.

  • Growth Factors (e.g., FGFs): Contribute to the coordination of nephron and collecting duct development.

These molecular mechanisms are tightly regulated spatially and temporally to ensure the proper formation and function of the mammalian kidney. The intricate interplay between signaling pathways, transcription factors, and growth factors orchestrates the sequential events leading to the formation of a functional and mature kidney.

GDNF functions at different times of development:

• Prior to Ureteric Bud Formation: Initiation of Bud fails and no collecting duct system is formed

• During Ureteric Bud Growth: Branching of uretric bud -> Inactivation would cause reduced collecting duct complexity with limited numbers of branches

• After Bud Branching: Further Growth and Maturation compromised

  Experiments: Fusion to gene switch (adding of substrate -> gene switched off), adding GDNF inhibitors during different stages, Knockout of GDNF gene during different stages of development

Canonical WNT pathway: Wnt -> frz -> dishevelled -> destruction complex inactive -> beta catenin into nucleus (transcription)

Non canonical pathway: —> not primarily involve the stabilization and nuclear translocation of β-catenin but Ca2+ pathway —> IP3 leads to release of Ca2+ and Planar Cell Polarity (PCP) Pathway —> activation of small GTPases, such as Rho and Rac, which regulate the actin cytoskeleton

• Overexpression/Inhibition of Wnt ligands (or ß-catenin)

  • Adv: distinct phenotype

  • Disadv: wnt signalling is everywhere -> off target effect limited specificity -> can effect multiple tissues & pathways

• Tissue specific knockout models —> disadvantage from above solved

• Fusionprotein with gene switch -> makes Wnts visible + modulating amount

• β-Catenin Staining

• fluorescence in situ hybridization

• immunohistochemistry: antibody staining

• reportergene with responsive molecule e.g. beta-galactosidase

• GDNF Staining

• GDNF Reporter mice/construct

• fluorescence in situ hybridization

• immunohistochemistry: antibody staining

• reportergene with responsive molecule e.g. beta-galactosidase

• Patterning of endoderm (high nodal = endo / low nodal = meso) and interactions with mesoderm (FGF / WNT) specifies the site of lung development —> gradients of different signals and their combination of triggered transcription factors determine location of lung (or pancreas or liver …) developement along the gut tube (

• Branching morphogenesis (FGF) establishes the shape of the lung and heavily depends on tissue interactions through multiple pathways; it is an iterative (repeating) process with self-limiting feedback loops (BMP and sonic hedgehog [Shh] inhibiting FGF —> no overbranching and dirctional growth). Coupling of epithelial (BMP, SOX) and mesenchymal development.

• Patterning of the growing organ and cell differentiation are precisely controlled and coupled to coordinate the maintenance of enough undifferentiated progenitors for continued growth and morphogenesis and the formation of differentiate cell types.

• During development stem cell populations for adult homeostasis and injury repair are established.

• Stem cell niches often contains key regulators (FGF) of embryonic development.

• The pancreas forms from Pdx1-positive endoderm in the posterior foregut region.

• The pancreas forms from two primordia that are independently induced in the dorsal and ventral foregut.

• The ventral pancreas primordium is induced through interactions with the cardiac and septum transversum mesoderm (FGF)

• The dorsal pancreas primordium requires signals from the notochord.

• Further interactions involve the aorta and vitellin veins.

• A feed forward loop involving FGF10 from the mesoderm of the pancreas primordium stimulates endodermal cell proliferation and maintains pancreas identity.

• Development of the ramified ductal system of the pancreas differs from branching morphogenesis observed in the lung and involves the coalescence of micro-lumina and a subsequent remodeling of an early plexus thus formed.

• Cell type specification in the pancreas occurs in a stepwise manner: early pancreas progenitors give rise to trunk and tip progenitor cells. Tip cells give rise to the acinar cell of the exocrine pancreas. Trunk cells give rise to duct cells of the exocrine pancreas and Ngn3-positive endocrine precursors.

• Endocrine precursors delaminate and form the islets of Langerhans.

• Endoderm induction through high levels of Nodal

• Initial patterning of the endoderm through morphogen gradients at gastrulation and early somite stages

• Further patterning through interactions with surrounding mesodermal structures (splanchnic mesoderm, and others) —> spacially restricted, overlapping expression domains of transcription factors in the endoderm and surrounding mesoderm

• Further refinement of the patterning through sequential reciprocal interactions between endoderm and mesoderm

• Combinatorial transcriptions factor „code“ specifies organ specific endodermal precursor populations

• Organ morphogenesis and cell differentiation requires further interactions of the endodermal organ precursors with the corresponding mesodermal cells

• Cell differentiation and spatial arrangement of endoderm derived cells in the organs also involves interactions of different cell types within the epithelium.

• The urogenital system develops from the intermediate mesoderm.

• Kidney development begins with the formation of the nephric duct, which elongates through proliferation and chemotaxis towards FGF8 produced in the tail bud region.

• Pax2/8 are essential for renal fate specification.

• The ureteric bud is induced by GDNF from metanephrogenic mesenchyme.

• The metanephros forms through reciprocal interactions between the ureteric bud and the metanephrogenic mesenchyme. Key signals are GDNF and members of the WNT family (Wnt9b, Wnt11, Wnt4), which coordinate branching morphogenesis of the collecting ducts and nephron formation

• A tight regulation of the balance between nephron progenitor proliferation and differentiation is essential for normal kidney development.

• Proximal distal patterning of the kidney tubules is initiated already during induction of the renal vesicles.