Unit 2b Cells and Developmental Biology What we ll cover We ll only cover the minimum amount of cell biology we need to study embryos We won t cover many areas of cell biology These are best left for another class like Zoo 570 We will study cell biology concepts that allow us to a understand how cells associate with one another cell cell adhesion b how cells associate with the outside world cell extracellular matrix adhesion c how cells move themselves and their components cell motility and d how cells signal one another signal transduction The Cell Surface Sorting Out Gilbert 10e Fig 3 1 The Cell Surface Sorting Out Gilbert 10e Fig 3 2 The Cell Surface Sorting Out Conclusion There are surface differences between differentiated cells in an embryo that can lead to segregation of tissues Holtfreter didn t know what these were Gilbert 10e Fig 3 2 Cadherins and Sorting Out We now know that cell surface differences are in part accounted for by cell adhesion proteins One class of such proteins are cadherins short for calciumdependent adherence proteins Gilbert 10e Fig 3 5 Cadherins and the Cytoskeleton Gilbert 10e Fig 3 5 Cadherins are transmembrane proteins In the cytoplasm they connect to actin via b and a catenin Cadherins and Sorting Out P cadherin E cadherin Overlay courtesy M Takeichi Conclusion Differences in amount and type of specific cell cell adhesion proteins can lead to tissue segregation Intercellular Junctions Adhesion molecules assemble at specific locations into elaborate structures in sheets of cells epithelia These structures are called cell cell intercellular junctions Adherens junctions contain cadherins and confer adhesion Tight junctions form a permeability barrier Gap junctions allow passage of small molecules between cells Desmosomes form mechanically strong spot welds between cells Hardin et al 8e Fig 17 7 Adherens junctions Tight junctions Gap junctions Desmosomes The Extracellular Matrix Collagen is a scaffold for adhesion proteins such as fibronectin and laminin not shown Integrins allow cells to bind to ECM proteins and to the cytoskeleton Proteoglycans are space filling molecules that swell in contact with water From Campbell et al 6e Fig 7 29 The Extracellular Matrix Collagen Collagen fibrils http www visualphotos com image 1x7467776 scanning electron micrograph sem of human collagen The Extracellular Matrix Fibronectin Fibrils Fibronectin fibrils in an amphibian gastrula The Extracellular Matrix Glycosaminoglycans GAGs Proteoglycans GAGs and proteoglycans are huge space filling molecules that swell in water Adapted from Alberts et al 3e Fig 19 34 Epithelia and the ECM The ECM associated with the inner side of epithelia often contains laminin and it is often called a basal lamina Hemidesmosomes allow cells in epithelia to attach to the ECM They contain specialized integrins Hardin et al 8e Fig 17 7 Hemidesmosomes basal lamina contains laminin The Extracellular Matrix Basal Lamina Gilbert 10e Fig 3 36 Microtubules Tubulin polymers The Cytoskeleton Microfilaments Actin polymers Intermediate filaments IF subunits The cytoskeleton allows cells to move and to move internal components There are three main types of cytoskeletal filaments These are polymers that form from subunits Hardin et al 8e Table 15 1 Drugs that perturb the cytoskeleton Polymer Depolymerizing agents Excess polymerization actin cytochalasins latrunculin phalloidin microtubules nocodazole colcemid colchicine taxol D2O Molecular Motors dynein dynein or kinesin From Campbell et al 6e Fig 7 21 Molecular motor proteins attach to actin myosins or MTs dyneins and kinesins They couple ATP hydrolysis to movement along cytoskeletal filaments Molecular Motors Dyneins Kinesins Dyneins move toward the minus ends of MTs Kinesins move towards the plus ends Hardin et al 8e Fig 16 2 Actin and Cell Crawling actin Actin polymerizes at the leading edge as cells crawl to help form protrusions Thin protrusions are called filopodia broad flat protrusions are called lamellipodia adapted from Hardin et al 8e Fig 16 26 Actin and Cell Crawling Actin polymerizes at the leading edge as cells crawl to help form protrusions Thin protrusions are called filopodia broad flat protrusions are called lamellipodia Hardin et al 8e Fig 16 25 The Extracellular Matrix Integrins Integrins attach cells to the ECM e g fibronectin Their cytoplasmic tails bind to F actin via linker proteins Clusters of integrins form focal adhesions Gilbert 10e Fig 3 37 Cadherins and the Cytoskeleton Gilbert 10e Fig 3 5 Cadherins are transmembrane proteins In the cytoplasm they connect to actin via b and a catenin Epithelial Mesenchymal Transition EMT Gilbert 10e Fig 3 39