Cellular rearrangements during morphogenesis Instructor: Dr. Shahzad A. Mufti Advisor Department of Biosciences

Cellular Rearrangements During Morphogenesis Differential or selective affinities of cells Towns & Holtfreter experiments(1955). Dissociation and re-aggregation of amphibian late gastrulae cells. Steinberg(1964) experiments on adult tissues & cells (e.g. heart. Cartilage, liver or retina). Re-aggregation by types as well as position in normal embryo.

Cellular Rearrangements During Morphogenesis CADHERINS: cell adhesion molecules, Ca++ dependant, anchored into cells by catenins proteins, which bind to actin (cytoskeleton). Several types, E, P, N-cadherins. Bind with own kind (homophilic). Amount related to adhesion.

Cellular Rearrangements During Morphogenesis Timing and duration of their appearance important in morphogenesis. Morphogenetic movements: microtubules and microfilaments; colchicine, vinblastin sulfate and cytochalazin B experiments.

Cellular Rearrangements During Morphogenesis Thermodynamic model of cell interaction (by Steinberg): adhesivity property of availability of free energy on surface. Aggregation results in availability of least (smallest) amount of free energy. Example: oil & water model Davis et. al. (1997) actual measurements.

Change in cellular shape during morphogenesis Microtubules help elongate the cells of the neural plate. 1 Pinching off of the neural plate forms the neural tube. 4 Ectoderm Neural plate Microfilaments at the dorsal end of the cells may then contract, deforming the cells into wedge shapes. Cell wedging in the opposite direction causes the ectoderm to form a “hinge.” 2 3

Convergent extension of a sheet of cells Convergence Extension convergent extension, a type of morphogenetic movement in which the cells of a tissue layer rearrange themselves so the sheet converges and extends, becoming narrower but longer. Convergent extension allows the archenteron to elongate in the sea urchin and frog and is responsible for the change in shape of a frog embryo from spherical to submarine shaped.

Does fibronectin promote cell migration? EXPERIMENT Researchers placed a strip of fibronectin on an artificial underlayer. After positioning migratory neural crest cells at one end of the strip, the researchers observed the movement of the cells in a light microscope. CONCLUSION RESULTS In this micrograph, the dashed lines indicate the edges of the fibronectin layer. Note that cells are migrating along the strip, not off of it. Fibronectin helps promote cell migration, possibly by providing anchorage for the migrating cells. Direction of migration 50 µm Fibronetin is an ECM extracellular matrix (ECM), a mixture of secreted glycoproteins lying outside the plasma membrane. Cadherin is an CAM Cell adhesion molecules (CAMs), located on cell surfaces, bind to CAMs on other cells. regulate morphogenetic movement and tissue binding

Is cadherin required for development of the blastula? CONCLUSION EXPERIMENT Researchers injected frog eggs with nucleic acid complementary to the mRNA encoding a cadherin known as EP cadherin. This “antisense” nucleic acid leads to destruction of the mRNA for normal EP cadherin, so no EP cadherin protein is produced. Frog sperm were then added to control (noninjected) eggs and to experimental (injected) eggs. The control and experimental embryos that developed were observed in a light microscope. RESULTS As shown in these micrographs, fertilized control eggs developed into normal blastulas, but fertilized experimental eggs did not. In the absence of EP cadherin, the blastocoel did not form properly, and the cells were arranged in a disorganized fashion. Control embryo Experimental embryo Proper blastula formation in the frog requires EP cadherin.

Reaggregation of cells from amphibian neurulae Reaggregation of cells from amphibian neurulae. Presumptive epidermal cells from pigmented embryos and neural plate cells from unpigmented embryos are dissociated and mixed together. The cells reaggregate so that one type (here, the presumptive epidermis) covers the other. (After Townes and Holtfreter 1955.)

Figure 3.27. Sorting out and reconstruction of spatial relationships in aggregates of embryonic amphibian cells. (After Townes and Holtfreter 1955.)

Schematic representation of cadherin-mediated cell adhesion Schematic representation of cadherin-mediated cell adhesion. Cadherins are associated with three types of catenins. The catenins can become associated with the actin microfilament system within the cell. (After Takeichi 1991.)

Hierarchy of cell sorting in order of decreasing surface tensions Hierarchy of cell sorting in order of decreasing surface tensions. The equilibrium configuration reflected the strength of cell cohesion, with the cell types having the more cell cohesion segregating inside the cells with less cohesion. The images were obtained by sectioning the aggregates and assigning colors to the cell types by computer. The black areas represent cells whose signal was edited out in the program of image optimization. (From Foty et al. 1996; photograph courtesy of M. S. Steinberg and R. A. Foty.)