1. Selected Invertebrates
Early Development
Selected Invertebrates

2. Cleavage What characterizes this process?
rapid cell division; no overall growth; no increase in volume

3. Cleavage
How does the cell cycle of blastomeres compare with that of somatic cells?
biphasic versus multiphasic
MPF responsible for biphasic cycle; controlled by presence of cyclin B
chromosome condensation, nuclear envelop breakdown and spindle formation all depend on kinase activity

4. Cleavage
What happens to the embryo during the mid-blastula transition?
When does this occur?
G1 and G2 are added to cell cycle; varies with organism (12-17 divisions); loss of cell synchrony

5. Cleavage
What is karyokinesis and cytokinesis and how do they compare?

6. Cleavage
What influences the pattern of cleavage in a particular organism?
amount of yolk and position and timing of spindle formation

7. Cleavage
inherited pattern of cell division also influences pattern

8. Cleavage

9. Cleavage How are cell fates specified during cleavage?
cell-cell interactions
asymmetric distribution of morphogenic determinants
morphogenic determinants – transcription factors- cytoskeleton plays a role in distribution; also centrosome

10. Gastrulation What characterizes gastrulation in organisms?
What kinds of movements occur?
invagination – sea urchin endoderm
involution – amphibian mesoderm
ingression – sea urchin mesoderm
delamination – mammalian hypoblast formation
epiboly – ectoderm formation in amphibians

11. Axis Formation
What are the three major body axes?

12. Sea Urchin Development
What does the cleavage pattern look like?

13. Sea Urchin Development
What characterizes the blastula stage?
expanding blastocoel,cilla develops, embryo rotates in fertilization membrane, formation of vegetal plate, formation of hatching enzyme

14. Sea Urchin Development
At what stage are the fates of individual cells determined?
60-cell fate map; most fates by conditional specification; micromeres via autonomous
transplanted micromeres will alter fate of adjacent cells causing them to gastrulate

15. Sea Urchin Development
What role does β-catenin play in specification?
specifies micromeres (accumulates in nuclei of cells fated to become mesoderm and endoderm) and vegetal half of embryo
lithium chloride causes accumulation of catenin in every cell; needed for inductive effects of micromeres
β-catenin works through Pmar I genef
delta-notch pathway also involved in specification of vegetal cells to become secondary mesenchyme

16. Sea Urchin Development

17. Sea Urchin Development
When does axis specification occur?
animal-vegetal axis established before fertilization
anterior-posterior axis determined by a-v axis
vegetal – determinants for posterior development
dorsal-ventral and left-right after fertilization
d-v by first cleavage plane

18. Sea Urchin Development
How does gastrulation begin?
ingression of primary mesenchyme (descendents of micromeres) – formation of filapodia

19. Sea Urchin Development
What appears to be responsible for the ingression of primary mesenchyme?
detachment from hyaline layer and adhersion to fibronectin and laminin associated with extracellular matrix
primary mesenchyme form spicules

20. Sea Urchin Development
What appears to be responsible for the initial invagination that occurs during gastrulation?
shape changes in vegetal plate cells and in extracellular matrix

21. Sea Urchin Development
secretory granules secrete CSPG into inner lamina of hyaline layer – absorbs water, swells – buckling of hyaline layer
foregut, midgut and hindgut
What is the fate of these vegetal plate cells?

22. Sea Urchin Development
What happens during later stages of invagination?
convergent extension

23. Sea Urchin Development
secondary mesenchyme and filapodia; dispersion of secondary mesenchyme

24. Snail Development
What kind of cleavage pattern characterizes these animals?
spiral holoblastic – no blastocoel
after second cleavage get micromeres in animal pole – displaced to right or left of macromere

25. Snail Development
Orientation of cleavage plane determines right or left coiling snails
determined by cytoplasmic factors in oocyte
direction of shell coiling is controlled by single pair of genes and right is dominant to left
direction of cleavage is determined by genotype of snail’s mother not its own genotype – factors come from egg cytoplasm

26. Snail Development
What appears to be responsible for the mosaic development seen in molluscs?
in situ hybridization shows accumulation of mRNA for BMP- paracrine factor dpp in B and C
distribution of transcription factors to certain cells

27. Snail Development What is the polar lobe and why is it important?
Removing polar lobe produces abnormal larvae – no endoderm, lacks mesodermal organs and eyes
lobe contains endodermal and mesodermal determinants – localization established just after fertilization and
associated with cytoskeleton or cortex

28. Snail Development
Why does removal of the D blastomere or its first or second derivatives result in incomplete larvae?
If D blastomeres don’t directly contribute cells to formation of many structures why are they so important to the formation of the same structures?
receives determinant from polar lobe
involved in inductive interactions

29. Snail Development
What other aspect of snail development does the polar lobe influence?
How do we know this to be true?
dorsal-ventral axis formation; exp. that force lobe material into AB as well as CD blastomere

30. Snail Development How does gastrulation take place in snails?
epiboly of micromeres from animal pole overgrows embryo

31. Tunicate Development What makes these organisms rather unique?
What type of cleavage pattern do they have?
invertebrate chordates because of larval form
bilateral holoblastic; first cleavage plane establishes symmetry

32. Tunicate Development
In what way does the pigmentation in Styela partita provide developmental information?
cell fate information; A)before fertilization B)5 min after fert c)migration of sperm pronucleus
clear cytoplasm = ectoderm; yellow cytoplasm = mesoderm; dark-gray = endoderm; light gray = neural tube and notocord

33. Tunicate Development
What evidence is there of autonomous specification in tunicate blastomeres?
transplant experiments
RNA hybridization experiments
altering β-catenin levels in cells
transplants of larval nuclei into enucleated egg fragments produces cell characteristic of cytoplasm not nucleus
Macho-1 mRNA found in vegetal hemisphere of unfertilized egg, migrates with yellow crescent into posterior vegetal region and eventually only in blastomeres giving rise to muscle cells
inhibition of B-catenin results in loss of endoderm and increased B-catenin caused ectoderm to develop as endoderm

34. Tunicate Development
What evidence is there for conditional specifiction?
BMP signal from endoderm induces anterior cell to become notocord
works through activation of Brachury gene
FGF signal induces posterior cell to become mesenchyme

35. Tunicate Development When are the embryonic axes established?
dorsal-ventral – prior to first cleavage
anterior-posterior – prior to first cleavage
left-right – first cleavage
cap of cytoplasm at vegetal pole – future dorsal side – initiation of gastrulation
posterior vegetal region (yellow crescent) – posterior side

36. Tunicate Development What is gastrulation like in these organisms?
invagination, involution, and epiboly

37. Caenorhabditis elegans
What does C. elegans look like?
959 cells; embryonic development in 16 hours; 1 mm long

38. C. elegans What pattern of cleavage in seen in this nematode?
rotational holoblastic – each division produces founder cell and stem cell

39. C. elegans

40. C. elegans What determines anterior-posterior axis?
What is the importance of the P-granules?
elongated axis of egg and sperm pronucleus enters posterior
P-granules specify germ cells

41. C. elegans When is dorsal-ventral and left-right axes established?
d-v from division of AB cell; Abp becomes dorsal side; EMS becomes ventral side
l-r at 12cell stage

42. C. elegans
In what way is autonomous specification demonstrated in this organism?
P1 develops without presence of AB cell
In what way is conditional specification demonstrated?
AB cell requires cell-cell interactions
EMS requires signal from P2
maternal proteins acting as transcription factors include SKN-1, PAL-1, and PIE-1 determine fates of cells descended from P1.
SKN-1 needed for fates of EMS cells (pharyngeal mesoderm and endoderm)
PAL-1 needed for somatic cell derivatives and PIE-1 needed for germ cell derivatives
AB alone only develops posterior parts of embryo

43. C. elegans cell-cell signaling APX-1 –delta; GLP-1 –notch
MOM-2 wnt; MOM-5 frizzled

44. C. elegans
Where in this organism is autonomous and conditional specification integrated?
pharynx development

45. C. elegans When does gastrulation begin in this organism?
just after formation pf P4 cell – 24 cell embryo