1 / 44

Cleavage and Gastrulation - Sea Urchin and Frog

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

cookwanda
Download Presentation

Cleavage and Gastrulation - Sea Urchin and Frog

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. Amphibian Cleavage & Gastrulation • Large eggs, rapid development • Fell out of favor - can’t do genetic manipulations • New techniques brought them back into favor

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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. . . .)

  16. 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)

  17. 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)

  18. 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

  19. Epiboly of the Ectoderm - formation of endoderm

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

  21. 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 :)

More Related