Development

• patterning is how cells get their identity in space and time
• • Founder Cells: Give rise to differentiated cells downstream
• Epiboly: Sheet of cells flatten and spread (occurs in ectoderm in frogs). Thinner cell layer, larger surface area, increased cell-substratum adhesion
• Intercalation: Cells intermix (so different layers intermix) and spread out to form a single sheet (originally chunky individual sheets). Changes in cell-cell adhesions  thinner layer – facilitates gut forming in frogs as ectoderm did this too (to create via thin layer)
• Convergent extension: Tissue mass elongates due to extensive and coordinated intercalation of cells – change in cell-cell and cell-substratum adhesion. Creates stretched out tissue structures (e.g. mesoderm in frogs – can elongate axes + presumptive endoderm cells in sea urchin and frogs repack via this to form primitive gut, ectoderm also does this to help form thinner layer)
• Invagination: Localised areas of cell constriction causing contracting and bending of the sheet (like a beach ball poked in middle)

  Contraction of bottle cells in frogs, invagination in sea urchins to form gut
• Involution: Folding of the cell layer after invagination (can be extensive folding – with multiple folds)
• Migration: Cells move away from the edges of a coherent mass (loss of cell-cell adhesion). Mesoderm does this in sea urchins, can go to a very different microenvironment (impact differentiation)
• Ingression: Cells detach from epithelial layer and migrate into basal extracellular matrix – changes in cell-cell and cell-matrix adhesion

  Moving out of the epithelial layer to extracellular matrix different microenvironment = affects differentiation
• Proliferation: Cell multiplication expands cell layer (so cell divisions). Can lead to localised growth at an edge (so many cell divisions at one point) or folding and buckling (if cannot extent further)
• in both sea urchins and frogs gastrulation is basically: Ball of cells, flips itself inside out to create a hollow structure with mouth on one end, anus on the other and gut in the middle (or can be seen as tube itself)