Cleavage and Gastrulation - Sea Urchin and Frog

1. Cleavage and Gastrulation - Sea Urchin and Frog Gilbert - Chapter 8 pp. 217-228 & 10 pp. 291 - 299

2. Today�s Goals Become familiar with the concepts of Cleavage, Gastrulation and Axis Determination Become familiar with the types of cell movements in the embryo Describe the processes of Cleavage and Gastrulation in Sea Urchin and Xenopus embryos

3. Sea Urchin Cleavage Radial Holoblastic Cleavage First two divisions Meridional, perpendicular to each other Third division Equitorial, perpendicular to first 2 Divides into animal half, vegetal half

5. Cleavage in Sea Urchins (cont.) Fourth cleavage Animal half divides into 4 equal mesomeres Vegetal half divides into 2 smaller micromeres and 2 larger macromeres Regular cleavages continue through the 128 cell stage (then become less regular)

7. Blastula Formation At 128-cell stage blastula forms Cells form a hollow sphere (blastocoel) Cells have become the same size Every cell contacts fluid in center As growth continues, cells remain a single epithelial layer of cells Held together by tight junctions

8. Blastula Formation Cells develop cilia Begins to rotate inside fertilization envelope At this point the cells are specified* What does this mean? Cells at vegetal pole begin to thicken Forms the vegetal plate Cells at animal pole secrete a hatching enzyme Embryo hatches

10. Gastrulation to Pluteus Larva Step 1: Ingression of Primary Mesenchyme Cluster of cells in vegetal plate extend filipodia (long, thin processes) Cells dissociate from epithelium Migrate into blastocoel Fate mapping: these cells form skeleton of larva

14. Gastrulation Step 2: Archenteron Invagination Cells remaining in the vegetal plate begin to bend inward and invaginate into the blastocoel This forms the archenteron which is the primitive gut of the animal The opening caused by this invagination is called the blastopore

16. The archenteron extends, forming a long thin gut tube Cells become longer and flatter and intermix with each other (convergent extension) Secondary mesenchyme cells form at the tip of the archenteron Secondary mesenchyme cells will disperse into the blastocoel and form the mesodermal organs The germ layers begin to differentiate into primitive organs of the larval stages

20. Amphibian Cleavage & Gastrulation Large eggs, rapid development Amphibians such as Xenopus laevis and Rana pipiens were used frequently in early embryology Fell out of favor - can�t do genetic manipulations New techniques brought them back into favor

22. Amphibian Cleavage Radially symmetrical, holoblastic - but unlike sea urchin, mesolecithal egg Yolk is concentrated in vegetal pole Cell divisions are slower in the vegetal hemisphere First cleavage bisects the grey crescent Second cleavage begins in animal pole, while first cleavage is not yet complete in vegetal pole As in sea urchins, there are no Gap phases in the cell cycle to allow for rapid divisions

23. Amphibian Cleavage First & Second cleavage Meridional Third cleavage Equatorial (but not actually at the equator) Divides the embryo into 4 small micromeres, 4 large macromeres As cleavage continues: animal pole packed with many small cells vegetal pole has fewer large yolk-laden cells

24. SEM cleavages 1,2, 4SEM cleavages 1,2, 4

25. 1st two cleavages meridional. 3rd cleavage equatorial (but in animal pole)1st two cleavages meridional. 3rd cleavage equatorial (but in animal pole)

26. Amphibian Cleavage At 16-64 cells, embryo is called a morula Solid ball of cells At 128 cell stage, embryo is a blastula Open cavity called blastocoel has appeared in animal pole Permits cell migration during gastrulation Prevents cells below from interacting with the cells above prematurely**** (next lesson. . . .)

28. Amphibian Gastrulation Different in different species Goals Bring endoderm cells to the inside of the embryo Allow ectoderm cells to coat the outside of the embryo Position mesoderm cells in between

29. Fate-maps Fate-mapping of blastula stage embryos has provided some insight Using vital dyes to mark cells Superficial layers of embryo form ectoderm and endoderm Mesoderm lie mostly in the deeper layers of cells Discuss fate mapping and vital dyesDiscuss fate mapping and vital dyes

31. Cell Movements in Amphibian Gastrulation Gastrulation begins on dorsal side Below the equator, in region of grey crescent Cells invaginate to form a slender blastopore Dorsal lip of blastopore will become important organizing region of embryo (Spemann organizer) Cells become elongated as they contact the inner surface (Bottle cells)

33. Cell Movements in Amphibian Gastrulation Next steps: Involution of the cells at the marginal zone (outer sheet spreads over inner sheet) Cells from Animal pole undergo epiboly Converge at the blastopore When reach blastopore, travel inward Bottle cells continue to migrate, form leading edge of archenteron (primitive gut)

34. Mass of yolk left by surrounding blastopore = yolk plugMass of yolk left by surrounding blastopore = yolk plug

36. Amphibian Gastrulation Cells from the dorsal lip (the first cells that migrated inward) become prechordal plate (will form head mesoderm) Next cells that involute form chordamesoderm (will become notochord) Important for patterning the nervous system

40. Neuralation begins Neuralation begins

41. Cells that become notochord induce overlying ectoderm to round up and form a tube - neural tube Precursor to the entire nervous systemCells that become notochord induce overlying ectoderm to round up and form a tube - neural tube Precursor to the entire nervous system

43. Next Lesson We�ll look more closely at gastrulation in Frog Cell movements Spemann organizer Molecular control and signaling

44. Lab Activity - 30 points On a sheet of paper Put your name Examine prepared slides of Xenopus Draw: Cleavage, early and late gastrulation Examine �Whole-mount� specimens of Xenopus Draw: Cleavage, early and late gastrulation Be sure to label any structures that you see that we have discussed :)