Lecture 9 Blastulation - Developmental Biology Lecture Notes

Blastulation, partitioning, and MBT Lecture 9 Weismann’s theory of determinative development 1880s started as a Lamarckian, became a Darwinist Famous for elucidation of germ plasm theory – cleavage partitions different ingredients to different daughter...

Blastulation, partitioning, and MBT Lecture 9 Weismann’s theory of determinative development 1880s started as a Lamarckian, became a Darwinist Famous for elucidation of germ plasm theory – cleavage partitions different ingredients to different daughter cells. Roux’s evidence for mosaic development conclusion that frog cleavage divisions are strictly mosaic – NOT! If you tie a loop of baby hair and tighten slowly to separate blastomeres, both develop normally, but ½ regular size Driesch’s evidence for regulative development (1890s-1910) Conclusion that sea urchin cleavage is regulative But half regular size – entelechy – vitalist perspective – some life force divided Why the discrepancy? One possibility is that sea urchins and frogs use different strategies for cell fate determination Other possibility is that observed differences are due to differences in experimental procedure TH Morgan repeated Dreisch’s sea urchin experiment – but this time on frog  both blastomeres developed into complete embryos Mosaic vs regulative development Regulative – cell fate is established later, via cell – cell interactions predominates in late cleavage divisions in most animals Mosaic or determinative – cell fate is established/restricted at each cell division C. elegans Most organisms develop by a mix of both strategies. C. elegans lineage restriction (mosaic development) Mid-blastula transition Transition from maternal control of development to zygotic control Most evident in animals that have large eggs (birds, fish, fruit flies, amphibians) transition from early blastula Cleavage divisions that are rapid, lack gap phases Absence of transcription mRNAs and proteins deposited in egg during oogenesis carry out all cellular processes … to late blastula Slowing of cell cycle, introduction of gap phases, G1, G2 Asynchronous divisions Zygotic transcription Many maternal mRNAs are actively destroyed MBT in fly, frog, mouse Mechanism of MBT What controls timing of MBT? counting cleavage divisions? counting time from first cleavage division? Diminution of critical stores of cytoplasmic or nuclear materials? Experiment #1 Remove cytoplasm from egg but leave nucleus: Smaller embryo undergoes normal cleavage divisions but enters MBT early (fewer cell divisions and cell number) Experiment #2 Addition of cytoplasm only Larger embryo undergoes normal cleavage, but MBT occurs later (more cell divisions, and consequently more cells) Experiment #3: Create a haploid frog embryo Cleavage divisions occur as normal even though each cell has half the normal amount of DNA MBT occurs later with more cells Experiment #4 Inject surplus of non-frog DNA Cleavage divisions occur as normal even though each cell more DNA than normal MBT occurs earlier with more cells Mechanism of MBT Studies in Xenopus and Drosophila  MBT triggered when Nuclear/Cytoplasmic ratio reaches critical point A possible model: A critical factor is deposited into the egg maternally and is “used up” by dividing nuclei Drosophila MBT – best understood at molecular level 1st 14 embryonic cell cycles are within a common cytoplasm - 8 minutes/ cycle - nuclear divisions start slowing down at ~ cycle 10 - MBT at cycle 14 dramatic slowing of cell cycle zygotic transcription activated, maternal mRNAs destroyed Exhaustion of Cellular Stores Cell cycle depends on Cyclin dependent kinases (Cdk) -activated by cyclin binding - cyclin is degraded after each cell division -activated by Cdc25 Probably involves consumption of histone stores as well Mitotic cyclins are present in egg before fertilization Each mitosis in embryo, overall mitotic cyclin levels go down due to being destroyed after use. By cycle 11 overall cyclin levels drop enough to get slowing of cell cycle  longer gap between cell divisions  some genes able to be transcribed  Transcription of genes that promote destruction of Cdc25  even longer gap in cell division  allows time for most genes to be transcribed  these genes include genes involved in maternal mRNA degradation, genes required for gastrulation Compaction in mammals In mammals – transition from blastula to morula High level expression of E-cadherin – cell-cell adhesion molecule  all cells begin to associate tightly with each other  cells become polarized – outer surface = apical contains microvilli  compaction results in outer and inner cells - cells on inside will contribute to embryo - outer cells will contribute to placenta Compaction begins Inner vs outer cells have distinct fates after compaction Completion of the cleavage divisions fish blastoderm mouse blastocyst frog blastula Blastula stage Also known as blastoderm or blastocyst depending on organism Completion of cleavage divisions, just before gastrulation In most cases the result is an asymmetric embryo Cells at different locations will give rise to different cell types eg. mammals inner cell mass  embryo proper + some extra-embryonic trophoectoderm  extra-embryonic Cavitation to create fluid filled centre – the bastocoel