Developmental Bio Final

Somatic vs Splanchnic Mesoderm

Somatic mesoderm + ectoderm form the body wall; splanchnic mesoderm + endoderm form gut-associated tissues and the heart

Somatopleure

Tissue layer of somatic mesoderm + ectoderm forming limbs and body wall

Splanchnopleure

Tissue layer of splanchnic mesoderm + endoderm forming heart tube surfaces and gut

Coelom Formation

Fusion of left and right coeloms to form pleural, pericardial, and peritoneal cavities

Cardiogenic Mesoderm Location

Located in splanchnic mesoderm of LPM; forms heart fields

Single Progenitor for Cardiovascular System

One precursor population produces cardiomyocytes, endocardium, valves, conduction system, and blood cells

Anterior Endoderm Functions

Specifies cardiogenic mesoderm and guides bilateral heart fields to midline

Experiment: Removing Endoderm

Prevents heart field migration and produces cardia bifida (two hearts)

Cardia Bifida Cause

Loss of fibronectin-guided migration cues prevents fusion of heart fields

Primary Heart Field Fate

Forms the left ventricle

Secondary Heart Field Fate

Forms right ventricle, atria, and outflow tract

Heart Looping Purpose

Positions atria above ventricles for correct adult heart orientation

Hemangioblasts

Progenitor cells that generate blood vessels and blood cells

Vasculogenesis vs Angiogenesis

Vasculogenesis creates vessels de novo; angiogenesis creates vessels from existing ones

VEGF Function

Promotes endothelial differentiation, tube formation, and vascular sprouting

Hypothetical: Block VEGF Early

Embryo fails to form vessel network and dies

Hypothetical: Overexpress VEGF in One Limb

Produces a hypervascular limb with excess sprouting

Paraxial Mesoderm Specifies IM

Removing paraxial mesoderm prevents intermediate mesoderm formation

Nephric Duct Function

Induces pronephros, mesonephros, and ureteric bud formation

Pronephros Fate

First kidney; degenerates in mammals but functional in fish and amphibians

Mesonephros Fate

Temporary kidney in mammals; forms parts of male reproductive system

Metanephros (Adult Kidney)

Forms via reciprocal signaling between metanephric mesenchyme and ureteric bud

Reciprocal Induction Logic

Mesenchyme induces ureteric bud via GDNF; bud prevents mesenchyme apoptosis via FGF and BMP

Experimental: Remove GDNF

No ureteric bud forms and kidneys fail to develop

Experimental: Overexpress GDNF

Multiple ureteric buds form and kidneys may be duplicated

Wnt Exposure Timing Rule

Early/short Wnt makes ureteric epithelium; late/long Wnt makes nephron mesenchyme

Branching Morphogenesis Cause

Driven by GDNF-RET signaling causing iterative branching of ureteric bud

Mesenchyme-to-Epithelium Transition in Kidney

Nephron precursors undergo MET to form renal tubules and Bowman’s capsule

Nephron vs Collecting Duct Origins

Nephron arises from mesenchyme; collecting duct arises from ureteric bud

Wnt9b Role

Required for converting mesenchyme to tubular epithelium; mutants lack kidneys

Hypothetical: Inhibit RET Receptor

Ureteric bud cannot respond to GDNF, leading to kidney agenesis

Bladder Development Note

Nephric duct connects to cloaca; ureteric bud migrates separately into bladder

Somites

Repeating segmented blocks of paraxial mesoderm forming vertebrae, ribs, dermis, and muscle

Somite Compartments

Sclerotome forms bone; dermamyotome forms muscle and dermis

Somitogenesis Direction

Somites form anterior to posterior via MET

Segmentation Clock Discovery

Hairy1 oscillations in presomitic mesoderm reveal timing mechanism for somite formation

Hairy1 Oscillatory Pattern

Begins posterior, moves anterior, stabilizes at new somite boundary

Notch Pathway Role in Somites

NICD induces hairy and Lfng, which then suppress Notch to create oscillations

Determination Front

Region where low FGF and high retinoic acid allow somite formation

Clock-and-Wavefront Model

Clock defines when somites form; wavefront defines where they form

Experiment: Increase FGF Gradient

Wavefront shifts posterior, producing fewer large somites

Experiment: Decrease FGF

Somites form prematurely and produce many small somites

Mesp Function

Turns on MET to establish a new somite at the determination front

Species Differences in Segmentation Clock

Zebrafish 30 min; chick 90 min; mouse 120 min; human 5 hours

Notch Pathway Mutants

Mutations in clock genes produce fused or disorganized vertebrae

Human Congenital Scoliosis Cause

Segmentation clock defects result in irregular somite boundaries

Hypothetical: Inhibit Notch Oscillation

Somite periodicity is lost, causing fused or malformed vertebrae

Limb patterning axes

Limbs are patterned along anterior–posterior, dorsal–ventral, and proximal–distal axes.

Proximal–distal limb regions

Stylopod (upper limb), zeugopod (forearm), autopod (hand/foot).

Source of limb muscles

Limb muscles come from paraxial mesoderm (somites).

Source of limb bones

Limb bones come from lateral plate mesoderm.

Limb bud definition

Early outgrowth of mesoderm covered by ectoderm that becomes the limb.

Proximal–distal signaling

RA from the flank specifies proximal fates; FGFs/Wnts from the AER specify distal fates.

Role of retinoic acid

Promotes proximal limb identity like stylopod formation.

Role of FGFs/Wnts

Promote distal limb identity and drive elongation.

Hox genes function

Pattern the limb skeleton and define stylopod, zeugopod, and autopod identities.

Hox11 mutant phenotype

Missing zeugopod bones (radius, ulna, tibia, fibula).

Hox13 mutant phenotype

Autopod defects: malformed digits, fusions.

Apical Ectodermal Ridge (AER)

Distal ectodermal thickening that signals for limb outgrowth.

AER removal result

Limb development stops; distal structures fail to form.

Extra AER result

Causes duplicated limb structures.

AER signals

Secretes FGFs (especially Fgf8) to maintain limb growth.

FGF bead rescue

An FGF-soaked bead can replace the AER and restore limb development.

Progress zone definition

Mesenchyme beneath the AER kept proliferative and undifferentiated.

Arm vs. leg mesenchyme swap

Mesenchyme determines limb type; arm mesenchyme makes arm, leg mesenchyme makes leg.

Non-limb mesenchyme swap

Causes AER regression and stops limb development.

ZPA definition

Posterior limb region controlling anterior–posterior patterning.

ZPA transplant result

Causes mirror-image digit duplication.

Shh location

Expressed in posterior limb mesenchyme, same region as the ZPA.

Shh gain-of-function

Anterior Shh causes duplicated digits like a ZPA transplant.

Digit identity + Shh

No Shh = digit 1; Shh concentration = digits 2–3; Shh duration = digits 4–5.

Dorsal vs. ventral limb

Dorsal = nails/knuckles; ventral = palm.

DV patterning genes

Wnt7a and Lmx1b specify dorsal identity; BMP/En1 specify ventral.

Lmx1b mutant phenotype

Dorsal limb becomes ventralized.

Human LMX1B condition

Nail-patella syndrome (nail/patella defects).

Interdigital webbing removal

Webbing is removed through apoptosis in interdigital tissue.

Species differences in webbing

Ducks and bats keep webbing because apoptosis isn’t induced.

BMP role in webbing

BMP triggers apoptosis in interdigital cells.

Gremlin function

Gremlin inhibits BMP, preventing apoptosis.

Gremlin bead experiment

Adding Gremlin reduces apoptosis and makes chick digits more duck-like.