Lecture 9+11 Early Development in Sea Urchins.pdf

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University of Toronto St. George
Cell and Systems Biology
William Navarre

CSB328H1 © Lisa| Page 113 L E C T U R E 9 & 1 1 : E A R L Y D E V E L O P M E N T I N S E A U R C H I N S Reading: Gilbert Chapter 7 (sea urchin section only) Optional reading: Oliveri P. et al. PNAS 2008;105:5955-5962 SEA URCHIN CLEAVAGE  radial holoblastic cleavage (Fig 7.2, Fig 7.3)  Fig 7.3A black arrow marks site of sperm entry, white arrow marks vegetal pole  first 7 cleavages are stereotypic – same pattern for every indiv of the same species st nd 1. 1 & 2 cleavages are meridional & pass through animal & vegetal poles 3. 3 cleavage is equatorial – separates animal & vegetal th 4. 4 cleavage: (producing 16-cells)  animal tier cells divide meridionally into 8 equal blastomeres (mesomeres)  vegetal tier cells undergo unequal equatorial cleavage into 4 large macromeres & 4 small micromeres 5. 5 cleavage:  8 animal mesomeres divide equatorially into 2 tiers, an & a1 (stag2ered above each other)  macromeres divide meridionally, forming an 8-cell tier below an 2  thcromeres divide unequally into a cluster of 4 small micromeres at the tip of the vegetal pole 6. 6 cleavage:  animal cells divide meridionally  vegetal cells divide equatorially  the small micromeres divide once more, then stop dividing until the larval stage th 7. 7 division: (120-cell)  pattern reversed from 6 cleavage  blastocoel formed  division patterns then become less regular BLASTULA FORMATION  all cells are the same size at the blastula stage  every cell is in contact w the blastocoel fluid on the inside & the hyaline layer on the outside  blastomeres are connected in an epithelial sheet around the blastocoel by tight junctions  the blastula remains one cell layer thick as the cells continue dividing – rapid & invariant cell cleavages until the 9 & 10 cleavages (cells have become differentiated)  blastomeres are adhered to the hyaline later  blastocoel is expanded by the influx of water  each cell then becomes ciliated on the cell membrane farthest the blastocoel – apical (outside)-basal (inside) polarity  PAR proteins distinguish the basal membrane  the ciliated blastula rotates w/in the fertilization envelope  cells at the vegetal pole begin to thicken – forms vegetal plate  cells at the animal pole synthesize & secrete a hatching enzyme – digests the fertilization envelope  embryo is now a free-swimming hatched blastula Page 2|CSB328H1 © Lisa Zhao 2013 FATE MAPS & THE DETERMINATION OF SEA URCHIN BLASTOMERES  by the 60-cell stage, most cell fates are specified, but the cells are not irreversibly committed  animal hemisphere  ectoderm  larval skin & neurons  veg layer  ectodermal/endodermal organs 1  veg2layer  cells that populate the endoderm, coelom (internal mesodermal body wall), non- skeletogenic (2°) mesenchyme (generates pigment cells, immunocytes, muscle cells)  first-tier (large) micromeres  skeletogenic (1°) mesenchyme  skeleton  second-tier (small) micromeres have no role in embryonic dvpmt but contribute cells to the coelom where adult tissues are derived during metamorphosis & the germline  the fates of the diff cell layers are determined in 2 steps: 1. the large micromeres are autonomously specified by maternal determinants at the vegetal pole to become skeletogenic mesenchyme cells which migrate along the blastocoel wall & differentiate into larval skeleton (EMT, first cells to move in [ingress] during gastrulation)  the skeletogenic micromeres don’t need any other signals to generate their skeletal fates 2. the large micromeres can now produce paracrine & juxtacrine factors that conditionally specify the fates of neighbouring cells – signals the cells above them to become endoderm, induces them to invaginate into the embryo  micromeres induce presumptive ectodermal cells to acquire other fates o if the micromeres are removed and placed on an isolated animal cap (the top 2 tiers that become ectoderm), the animal cap cells will generate endoderm & form a normal-looking larva (Fig 7.5C) – animal cells are competent, but will only generate endoderm when induced by micromeres (usu don’t bc are not in contact w them) o if they are transplanted into the animal region, their descendants will form skeletal spicules (autonomously generated) AND induce the fates of nearby cells to form a secondary site for gastrulation – presumptive ectodermal skin cells will be respecified as endoderm & produce a secondary gut (Fig 7.6) CSB328H1 © Lis| Page 3013 GENE REGULATORY NETWORK OF SKELETOGENIC MESENCHYME SPECIFICATION  skeletogenic mesenchyme derived from large micromeres are generated during early cleavage  GRN in this context refers to time from the birth of the large micromeres until they ingress  specification process: acquisition of a specific regulatory state – expression of genes which confer fate on the cells  not a one step process 1) maternal cues translated into regulatory (zygotic) gene expression – at mid-blastula transition when blastula begins transcribing its own genes 2) initial state is transient must be expanded & stabilized – through signalling 3) endpoint is differentiation of genes expressed to confer cell fate  specification & differentiation of the skeletongenic micromere lineage: 1. initial identity from localized maternal determinants lead to transcription of specific genes 2. expression of particular regulatory genes via a double-negative gate 3. feedback subcircuit used to dynamically stabilize the regulatory state 4. other regulatory states are prevented 5. signals are produced which are required for the development of neighboring lineages  regulatory specificity & specification functions of the skeletogenic micromere lineage (Figure)  signals produced by skeletogenic micromere lineages (blue)  transcriptional regulatory fx (black) th  as soon as the micromeres are formed at the 4 cleavage, they inherit 3 transcription factors: Disheveled, β-catenin, Otx 1. Disheveled & β-catenin  during oogenesis, Disheveled becomes located in the vegetal cortex (Fig 7.7A, arrows) where it prevents the degradation of β-catenin in the micromeres & veg macromeres 2  β-catenin then enters the nucleus & combines w the TCF transcription factor to activate gene expression – specifies the micromeres  β-catenin accumulates in the nuclei of presumptive endoderm & mesoderm cells (Fig 7.7B)  levels of nuclear β-catenin determines the mesodermal &
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