Developmental Biology Biology 4361 Early Development in Invertebrates October 25, 2006
Early Development Overview Cleavage rapid cell divisions divisions of fertilized egg into many cells Gastrulation cells undergo displacement cells move to different parts of the embyro cells acquire new neighbors; new inductive relationships Establishment of body axes anterior posterior dorsal ventral left right Establishment of cell fate
Cleavage Stage Cell Cycle Mitosis Promoting Factor (MPF) stimulates the cell cycle; i.e. MPF initiates cell division during mitosis and meiosis division stops in the absence of MPF Initially, factors stored in the egg controls cleavage (the cell cycle); e.g. stored proteins stored mrnas.. initiate the cell division Later, the embryo takes over control (necessitating new proteins)
Cleavage Stage Cell Cycle MPF mitosis promoting factor: Cyclin B controls cdc2 activity cdc2 = cyclin dependent kinase CDK phosphorylates histones, etc. cyclin B Cyclin B degrades; cell division stops mitosis mitosis mitosis
Cleavage Stage Cell Cycle Cyclin B presence/degradation controlled by egg cytoplasmic proteins early cleavage pre programmed ; maternally controlled after maternal stores used up: embryonic control of cell cycle Mid Blastula Transition (MBT)
Post Cleavage Cell Cycle Post MBT: cell cycle adds two G phases cell division becomes asynchronous new mrna transcription cyclin B G1 S G2 mitosis
Cytoskeletal Mechanisms Note orientation Karyokinesis Cytokinesis
Cleavage Patterns
Yolk classification Holoblastic (complete cleavage) Species echinoderms, amphioxis Cleavage Radial Isolecithal annelids, molluscs, flatworms Spiral tunicates Bilateral mammals, nematode Rotational Mesolecithal Telolecithal amphibians Meroblastic (incomplete cleavage) cephalopod molluscs fish, reptiles, birds Displaced radial Bilateral Discoidal Centrolethical most insects Superficial
Cell Movements Invagination Ingression infolding region of cells Involution migration of individual cells the internal surface layer into the interior of the embryo cells become mesenchymal, migrate independently in turning or inward movement of an expanding outer layer spreads over the internal surface of the external cell layer
Cell Movements Delamination splitting one cellular sheet into two parallel sheets (resembles ingression, but results in a new layer of cells) Epiboly epithelial sheets (usually ectoderm) spread as a unit to enclose deeper layers of the embryo occurs by cells dividing, changing shape, or by several layers of cells intercalating into fewer layers Intercalation
Sea Urchin Cleavage holoblastic radial cleavage
inner = middle = outer = endoderm mesoderm ectoderm Zygote
2 nd meridional 1 st cleavage meridional 3 rd equatorial 1 st 2 nd pronuclei 3 rd 4 th 4 th unequal cleavage between animal and vegetal hemispheres
Sea Urchin Cleavage 5 th 6 th 7 th +: at 128 cells, divisions become less regular; forms Blastula 5 th blastocoel proteinaceous fluid fills cavity osmotic pressure expands blastocoel Blastula (128+) one cell thick tight junctions in epithelium cells adhere to: hyaline layer (outside) basal membrane (inside) neighboring cells cilia develop embryo rotates hatching initiated at animal pole
Cells are specified by either: cell cell interactions Cell Fate or asymmetric distribution of patterning molecules into particular cells Mechanisms for establishing asymmetry: 1. Patterning molecules bound to egg cytoskeleton 2. Molecules actively transported along the cytoskeleton 3. Molecules become associated with one centrosome, and then follow that centrosome into one of the two mitotic sister cells Once asymmetry is established, one cell can specify another (and participate in reciprocal inductions)
Sea Urchin Cell Fate mesenchyme loosely packed, unconnected; mesoderm, neural crest origin coelom internal body wall; mesoderm origin
Sea Urchin Cell Fate 60 cell stage specified not committed blastomeres undergo conditional specification (mostly..)
Blastomere Specification Micromeres are the only blastomeres to undergo autonomous specification All other cell types undergo conditional specification most induction events triggered by micromeres
Micromere Specification β catenin plays the major role in micromere specification β catenin transcription factor often activated by Wnt signal transduction pathway β catenin β catenin also accumulates in cells fated to become endoderm and mesoderm
Endoderm & Mesoderm Specification β catenin in veg 2 cells biases them to become endomesoderm NOTE Lithium chloride (LiCl) causes accumulation of β catenin in every cell; transforms presumptive ectoderm into endoderm Micromere signal early veg 2 (?) amplifies endomesoderm specification β catenin: endomesoderm endomesoderm early veg :
Secondary Mesenchyme Specification Micromere Delta protein activates Notch pathway in adjacent veg 2 cells micromere Notch pathway activation results in secondary mesenchyme fate (rather than endoderm) veg 2 Notch Delta
Specification of Endoderm Wnt8 is made by micromeres and endoderm cells (i.e. endomesoderm cells not receiving Delta signal) Wnt8 acts as an autocrine boosts specification of both veg 2 endoderm and micromeres Wnt8
Axis Formation
Urchin Axes Jean Marie Cavanihac Jean Marie Cavanihac Jean Marie Cavanihac
Axis Specification Anterior Aboral Oral Posterior
Anterior posterior Axis Specification Cell fates line up along an animal vegetal axis established in the egg cytoplasm prior to fertilization. animal vegetal axis appears to structure future anterior posterior axis the vegetal region sequesters maternal components necessary for posterior development Anterior Dorsal ventral Dorsal ventral (oral aboral) axis established after fertilization approximately 45 clockwise from the 1 st cleavage plane oral fates promoted by Nodal aboral fates promoted by BMP2/4 Left right Left right axis established after oral aboral asymmetric expression of a Nodal gene Aboral Posterior * Oral
Urchin Gastrulation ingression skeletogenic mesenchyme
Larval Stages prism pluteus
adhesion Primary Mesenchyme Cell Ingression
syncytial cables Syncitial Cables
Invagination of the Vegetal Plate
Convergent Extension
Mid Gastrula secondary mesenchyme filopodial extensions
Imaginal Rudiment 5 fold radial symmetry