Animal Development continued

1. Animal Development continued By the fly hunnys

2. Key Concepts • Morphogenesis in animals involves specific changes in cell shape, position and adhesion • The developmental fate of cells depends on their history and on inductive signals

3. Morphogenesis • Major aspect of development in both animals and plants, but only in animals does it involve the movement of cells (changes in cell shape and position) • Changes in cell shape and position are involved in cleavage, gastrulation, and organogenesis

4. Cytoskeleton • Changes in cell shape involve reorganization of the cytoskeleton. • Cytoskeleton also drives cell migration (active movement of cells from one place to another) Cells crawl within the embryo by using cytoskeletal fibers to extend and retract cellular protrusions. • Changes in cell shape • Invagination (inpocketings) caused by contractions and bending inward • Evaginations (outpocketings) caused by contractions bending outward. • Convergent Extension: type of morphogenic movement in which the cells of a tissue layer rearrange themselves so that the sheet becomes narrower (Converges) while it becomes longer (extends) • Is important in early embryonic dev.

5. Role of the Extracellular Matrix • ECM: The mixture of secreted glycoproteins lying outside the plasma membrane of cells. • Known to help guide cells in many types of morphogenetic movements • ECM Fibers may function as tracks, directing migrating cells along particular routes • Several kinds of extracellular glycoproteins (including fibronectin) promote cell migration by providing specific molecular anchorage for moving cells • Other substances in the ECM keep cells on the correct path by inhibiting migration in certain directions. Depending on which substances they secrete, nonmigratory cells situated along migration pathways may promote or inhibit movement of other cells.

6. Continued • As migrating cells move along pathways, receptor proteins on their surfaces pick up directional cues from the immediate environment. • Such signals from the ECM can direct the orientation of cytoskeletal elements in a way that propels the cell in the proper direction. • CELL ADHESION MOLECULES: glycoproteins which are located on the surface of cells and bind to CAMs on other cells. • Vary in amount, chemical identity, or both from one type of cell to another. These differences help regulate morphogenetic movements and tissue building.

7. Cadherins • Class of CAM that requires calcium ions for proper function. • Many different cadherins and the gene for each cadherin is expressed in specific locations at specific times during embryonic development. • Also involved in the tight adhesion of cells in the mammalian embryo that first occurs at the 8 cell stage (when cadherin production begins).

8. Next Concept • The developmental fate of cells depends on their history and on inductive signals • Development also requires the timely differentiation of many kinds of cells in specific locations

9. 1. General Principle • During early cleavage divisions, embryonic cells must somehow become different from one another

10. Animals • Initial differences between cells result from the uneven distribution of cytoplasmic determinates in the unfertilized egg • By partitioning the heterogeneous cytoplasm of a polarized egg, cleavage parcels out different mRNAs, proteins, and other molecules to blastomeres in a type of asymmetrical cell division. • Resulting differences in the cell’s cytoplasmic composition help specify body axes and influence the expression of genes that affect the developmental fate of the cells.

11. Amniotes • Local environmental differences play the major role in establishing early differences between embryonic cells. • For example-Cells of the inner cell mass are located internally in the early human embryo, whereas trophoblast cells are located on the outside surface of the blastocyst. The different environments of these two groups of cells appear to determine their vey different fates.

12. 2. Major Principle • Once initial cell asymmetrics are set up, subsequent interactions among the embryonic cells influence their fate, usually by causing changes in gene expression. • The mechanism this occurs by is called INDUCTION. • Eventually brings about the differentiation of the many specialized cell types making up an animal. • May be mediated by diffusible chemical signals or, if the cells are in contact, by cell surface interactions.

13. Fate Mapping • FATE MAPS: general territorial diagrams of embryonic development. • How is this done? • Mark an individual blastomere during cleavage then follow the marker as it was distributed to all the mitotic descendents of that cell • Can a cell’s fate be changed by moving it? • 2 important conclusions • 1. In most animals, specific tissues of the older embryo can be attributed to certain early “founder cells” • 2. As development proceeds, a cell’s DEVELOPMENTAL POTENTIAL (the range of structures it can give rise to) becomes restricted.

14. Establishing Cellular asymmetrics • Establishing the basic body plan is a fist step in morphogenesis and is a prerequisite for the development of tissues and organs. • In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization • The animal-vegetal axis indirectly determines the anterior-posterior body axes. • Once any 2 axes are established the third is default • In amniotes the body axes are not fully established until later • Gravity, pH differences

15. Restriction of Cellular Potency • TOTIPOTENT: is capable of developing into all the cell types found in the adult • In many species that have cytoplasmic determinates only the zygote is totipotent • The fates of embryonic cells can be affected not only by the distribution of cytoplasmic determinants but also by how their distribution is affected by the zygote’s characteristic pattern of cleavage • Cells of mammalian embryos remain totipotent until the 16 cell stage • Regardless of how similar or different early embryonic cells are in particular species, the progressive restriction of potency is a general feature in the dev. Of all animals!

16.  Good luck on the quiz!