CLEAVAGE

1. CLEAVAGE 1

2. Ciona intestinalis http://www.mun.ca/biology/scarr/Garstang_Hypothesis.htm http://biodev.obs-vlfr.fr/recherche/biomarcell/ascidies/Anim-fromegg.htm 2 http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.07EmbIt9ELpT-YFA1rDieIO4AMQFlmlIwWj

4. E. G. Conklin, 1905 - In Styela partida, cells that contain yellow crescent pigment form muscle (mesoderm) http://sdb.bio.purdue.edu/AboutThisSite/conklin.article.html Swalla & Jeffery, 1995 - showed that a maternally derived mRNA was segregated into cells containing yellow crescent pigment. 4

5. Clear cytoplasm = ectoderm Yellow cytoplasm = mesoderm Dark gray cytoplasm = endoderm Light gray cytoplasm = neural tube and notochord. http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.0f0icUJoQpTbPzZ-do7pAF-n3I2E18sYrYZ 5

6. Frog 6

7. L Ant Point of sperm fusion with egg determines: Dor R • Position of grey crescent • Major axes of embryo • Position of dorsal lip • Position of first cleavage furrow Ven Pos Wilhelm Roux, 1885 Second polar body 15-30 min after fertilization Frog 5-10 min after fertilization Pronuclei will fuse 1.5 - 2 hr after fertilization 7 http://www.md.huji.ac.il/~yisraeli/embryology/embryo.html

8. What do these experiments tell us? • In some species, the cytoplasm of the zygote is very organized. • That maternally derived cytoplasmic information is sequestered into specific cytoplasmic compartments. • The end result - maternal mRNAs end up in specific groups of cells - determines those cells eventual fate • the cells containing the yellow pigment cytoplasm of Stylea’s embryos form muscle • the cells of the frog embryo that contain gray crescent cytoplasm become the chordamesoderm and eventually the notochord. 8

9. Purposes of cleavage • Decrease cell size • Increase cell number - allows for formation of blastocoel in many species by a process called cavitation. • In some cases, distribution of maternal cytoplasmic information to specific groups of cells • Beginning of Differentiation - as cleavage proceeds, you reach a point where the blastomeres are no longer totipotent • Synthesis of embryo derived mRNA for direction of future development begins during cleavage. Mammals - starts after a few cleavage divisions; Frogs - starts during late blastula stage 9

11. V IV III II I STARFISH CLEAVAGE - EQUAL HOLOBLASTIC I A ~32 cell stage A Meiosis complete at ovulation Isolecithal, oligolecithal, microlecithal egg Sagittal view V View of animal pole View of vegetal pole V 11

12. V IV III I UNEQUAL HOLOBLASTIC (may be meroblastic cleavage, but not always) Amphibians - meroblastic,unequal starting with 3rd cleavage division 32 cell stage 4 cell stage 8 cell stage II I 2 cell stage 16 cell stage A Sagittal view View of animal pole View of vegetal pole Ovulated as primary oocyte. First meiotic division completed in oviduct, second after fertilization. A V Mesolecithal vs telolecithal Frog blastula 18 hr 1st - 3.5 hr 2nd - 4.5 hr 3rd - 5.5 hr V http://www.luc.edu/depts/biology/dev/stages.htm http://www.its.caltech.edu/~bi12/2003/movies/xclvgtop.mov 12

13. Blastodisc Structure of the chicken (bird) shelled egg Polylecithal or megalecithal egg (huge amount of yolk) 13

14. I II III IV V I AVIAN CLEAVAGE - DISCOIDAL Ovulated as primary oocyte. Meiosis I completed almost immediately after ovulation. Meiosis 2 completed after fertilization. 1st - 3 hr 2nd - 3.5 hr 3rd - 4 hr 4th - 4.5 hr 5th - 5 hr Looking down on the animal pole. Yolk A Discoidal cleavage is a type of meroblastic cleavage Yolk 14

15. A 15

16. Delamination Movie of delamination in digital lab manual Polyinvagination 16

17. SUPERFICIAL CLEAVAGE - INSECTS Superficial cleavage is a type of meroblastic cleavage pole plasm 17 http://www.luc.edu/depts/biology/dev/insect.htm

19. Human embryo cleavage and cavitation. 19 http://www.erin.utoronto.ca/~w3bio380/lecture/Lect11/L11.htm