Unit 6 Cleavage What Does Cleavage Accomplish Multicellularity Allows for new cell cell interactions induction Allows for cell movement after divisions slow Allows for partitioning of localized determinants to different cells Allows for regionalization different areas of the embryo can become different Cleavage rate slows prior to cell movements There is an antagonism between rapid cell division and cell movement Why Gilbert 7e Fig 8 1 The cleavage division cycle is highly abbreviated Somatic cell cycle G1 0 4 S 8 G2 12 Early embryonic cell cycle S M The two gap phases G1 and G2 are essentially absent M 16 The cleavage division cycle is simplified MPF can also mean mitosis promoting factor cyclins are resynthesized at each cell division cdc2 is a Cdk Gilbert 10e Fig 5 12a The anaphase promoting complex APC leads to cyclin degradation The somatic cell division cycle is more complex Gilbert 10e Fig 5 2b In somatic cells different cyclins assemble with different Cdks to control different phases of the cell cycle Cyclins are synthesized and destroyed at each cell cycle Cleavage index Cyclin level mitotic cyclin Cyclin B synthesis precedes entry into mitosis 50 Cyclin level Cleavage index 75 25 0 0 based on Gilbert 3e Fig 3 39 1 Time hrs 2 Early embryonic cleavages are reductive Gilbert 10e Fig 1 2 Cleavage cytoskeletal proteins Asters Spindle adapted from Gilbert 9e Fig 5 2 Cleavage spindle and asters microtubules DNA Courtesy of Jon Holy Polyspermy disrupts cleavage Multiple centrosomes lead to a multipolar spindle DNA microtubules Gilbert 10e Fig 4 16 Astral microtubules and the spindle midzone are required for positioning the contractile ring astral microtubules Hardin et al World of the Cell 8e Fig 19 29A The contractile ring is dominated by actin Gilbert 7e Fig 8 3 Rho activity precedes actin polymerization at the contractile ring sea urchins active Rho Courtesy of Bill Bement Blocking Rho blocks cleavage multiple nuclei but no cleavage courtesy of George von Dassow embryos injected with C3 transferase which blocks Rho Cleavage creates a surface area volume challenge Volume is constant Surface area increases rapidly The increase requires extra membrane adapted from Campbell et al 5e Fig 7 5 New membrane is inserted at the furrow frogs new membrane is not pigmented Browder 3e Fig 5 8 Cleavage Patterns Where is the yolk lecithal Telolecithal lots at one end e g birds reptiles Centrolecithal middle e g insects Isolecithal uniform not a lot Mesolecithal moderate often more on one end e g frogs Cleavage Patterns cont How complete Holoblastic complete Meroblastic incomplete Cleavage Patterns cont Orientation Radial Rotational Spiral Cleavage patterns Gilbert 10e Fig 5 4 Cleavage patterns Gilbert 10e Fig 5 4 Cleavage orientation can be radial or spiral radial modified from Gilbert 10e Fig 5 34 spiral radial spiral Cleavage orientation correlates in some case with phylogenetic grouping Deuterostome mouth second Cleavage radial Gilbert 8e Fig 2 16 see 10e Fig 5 1 Cleavage orientation correlates in some case with phylogenetic grouping Cleavage spiral Protostome mouth first Gilbert 8e Fig 2 16 see 10e Fig 5 1 Triploblastic 3 primary germ layers Bilateral symmetry Cleavage sea urchins Blastocoel a fluid filled space with different ionic balance from the embryo s surroundings Gilbert 10e Fig 7 3 blastocoel Cleavage can be unequal sea urchins Blastomere is another word for an early embryonic cell Micromeres are small blastomeres Macromeres are large blastomeres Mesomeres are intermediate in size Cleavage orientation is determined by spindle positioning Courtesy George Von Dassow microtubules actin Cleavage Amphibians Animal hemisphere cells contain less yolk than vegetal hemisphere cells which are much larger and divide more slowly Gilbert 10e Fig 8 3 Cleavage amphibian Browder et al 3e Fig 5 17 Blastula structure is influenced by yolk content Ectoderm Blastocoel Sea urchin blastocoel has an eccentric noncentered location adapted from Purves et al 4e Fig 17 4 Frog Cleavage Frog blastula Gilbert 10e Fig 1 2 Cell cell adhesion is required for blastula formation Control Antisense EP cadherin Anti sense nucleotides corresponding to EP cadherin were added to maturing oocytes the fertilized oocytes lack EP cadherin Gilbert 10e Fig 8 5 Junctions form during the blastula stage blastocoel tight junction gap junction Tight junctions or septate junction in invertebrates are important for sealing cells off allowing the blastocoel to acquire different ionic content Gap junctions can connect small groups of cells Alberts et al 3e Fig 21 4 Cleavage Birds Embryos from meroblastic eggs must create a cellular floor Blastoderm Blastocoel Yolk Chick Purves et al 4e Fig 17 4 Cleavage leads to cytoplasmic partitioning myoplasm is segregated into specific blastomeres that eventually give rise to muscle Gilbert 10e Fig 7 20 Germ plasm can be segregated by cleavage Gilbert 10e Fig 17 6