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 micromeresand 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

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 PrimaryMesenchyme • Cluster of cells in vegetal plate extend filipodia (long, thin processes) • Cells leave epithelium (INGRESSION) • Migrate into blastocoel • Fate mapping: these cells form skeleton of larva

14. Gastrulation • Step 2: ArchenteronInvagination • Cells remaining in the vegetal plate begin to bend inward and invaginateinto 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 • Secondarymesenchyme 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 • Fell out of favor - can’t do genetic manipulations • New techniques brought them back into favor

21. Rearrangement of the Egg Cytoplasm • Fertilization can cause distinct changes in the arrangement of the egg cytoplasm • This is especially important and easily viewed in amphibian eggs • Amphibian eggs have an animal pole and a vegetal pole • Animal pole has dark pigment • Egg is radially symmetrical around the A-V axis • Sperm can enter anywhere on animal half

22. Formation of the Grey Crescent - Amphibians • Once sperm enters, the darkly pigmented cortical (outer) cytoplasm rotates relative to the clear inner cytoplasm (about 30°) • This exposes some of the more diffuse pigment granules in the animal half, which appear grey - “grey crescent” • 180° from the point of sperm entry • As the frog develops, this area will mark the place where gastrulation begins

25. 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

26. 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

29. 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. . . .)

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 into the cavity (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)

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

38. Epiboly of the Ectoderm - formation of endoderm

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 :)