Transcript Cleavage
Early Development in Selected Invertebrates Gastrulation in Molluscs Chapter 5 1 Early Development in Selected Invertebrates • Fertilization – initiating stage in development • Zygote – genetically fused single cell (2n) • Cleavage – stage of rapid cell division, dividing the cytoplasm of a fertilized egg into numerous cells • Gastrulation – process whereby cells move to different parts of the embryo and acquire new neighbor cells 2 Cleavage • Cleavage – Process of developing multicellular organisms – Series of mitotic division – Blastomeres – cleavage stage cells – Most species (except mammals) control this division with oocytes storing up proteins and mRNAs (maternal factors) • Later controlled by nuclear genome • Cytoplasmic volume does not increase 3 Cleavage • Cleavage – 1st division into half …. 2nd division into quarters … 3rd eighths…..4th so on….. – Division without cytoplasm increasing volume • Accomplished by abolishing gap periods (G1 & G2) – Nuclear division occurs rapidly • Frog egg – 37,000 cells/43hrs • Drosophila – 50,000 cells/12hrs 4 Early Development Overview 5 Early Development Overview 6 Cleavage • From fertilization to cleavage – MPF (mitosis-promoting factor) • Causes transition from fertilization to cleavage • Responsible for resuming mitotic division – Biphasic cell cycle (2 steps: M-to-S) • M (mitosis) – MPF factors • S (DNA synthesis) – MPF factors • Shift between M to S – MPF factors degraded 7 Fertilization to Cleavage • From fertilization to cleavage – What causes MPF levels to cycle? • MPF is made up of 2 subunits – Cyclin B – large subunit • Shows cyclical behavior (key to mitotic regulation) • Accumulation in S phase • Degraded after cells have reached the M phase – Cyclin dependent kinase (cdc2) – small • Activates kinase by phosphorylating target proteins • i.e. histones, nuclear envelope lamin proteins, and cytoplasmic myosin subunits 8 Fertilization to Cleavage 5.2 Cell cycles of somatic cells and early blastomeres 9 Fertilization to Cleavage • Mid-blastula transition – Point where cytoplasmic components are used up and the nucleus begins to synthesize new proteins 1. G1 – G2 (gap stages) are now added back to cell cycle • • 2. Synchronicity of cell division is lost • 3. Frog – at 12th cleavage cycle Drosophila – G2 after 14th, G1 after 17th Different cells synthesize different regulators of MPF New mRNAs are transcribed • Many are necessary for gastrulation even though the embryo is not able to initiate gastrulation 10 Cytoskeleton Mechanisms of Mitosis • Cytoskeleton mechanisms of mitosis – 2 coordinated processes 1. Karyokinesis – Mitotic division of the cell’s nucleus – Uses Mitotic spindles made up of microtubules (tubulin) 2. Cytokinesis – – Contractile rings made up of microfilaments (actin) Cleavage furrow uses perpendicular tightening microfilament ring 11 Cytoskeleton Mechanisms of Mitosis 12 Cytoskeleton Mechanisms of Mitosis 5.2 Role of microtubules and microfilaments in cell division 13 Patterns of Embryonic Cleavage • Patterns of Embryonic Cleavage – Patterns are species specific – Determined by 2 major parameters 1. Amount and distribution of yolk protein within the cytoplasm 2. Factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation 14 Patterns of Embryonic Cleavage • Amount and distribution of yolk – Determines where cleavage can occur – Determines relative size of blastomeres – One pole – yolk free • Called Animal Pole • Cell division fast – Other pole – yolk rich • Called Vegetal Pole • Inhibits cleavage 15 Patterns of Embryonic Cleavage • Classification of cleavage symmetry and patterns 5.3 Summary of the main patterns of cleavage 16 Patterns of Embryonic Cleavage • Holoblastic Cleavage – cleavage furrow through the entire egg – Isolecithal – equally spaced yolk • • Cleavage furrows through the entire egg 4 major patterns of cleavage: – – – – – Radial Spiral Bilateral Rotational Mesolecithal • • moderate vegetal yolk distribution Displaced radial 17 Patterns of Cleavage 18 Patterns of Embryonic Cleavage • Meroblastic cleavage – Portion of cytoplasm cleaves over large yolk accumulation – Cleavage does not penetrate the yolky portion • Impedes membrane formation – Telolecithal – free of yolk • Bilateral • Discoidal • Birds, fishes, reptiles – Centrolecithal – yolk in center • insects 19 Gastrulation • Remember blastula consists of numerous cells, their position was established in cleavage • Cells are now given new positions/neighbors • Will form: – Endoderm – Mesoderm – Ectoderm • 3 germ layers 20 Gastrulation • Involves combinations of cell movement – Organisms and patterns vary – Basic type of movement common: • Invagination – infolding • Involution – inturning/inward movement of expanding outer layer becomes mesenchymal • Ingression – migration of individual cells from the surface to the interior 5.4 Types of cell movements during gastrulation 21 Gastrulation – (cont) Basic type of movement common: • Delamination – splitting of one cellular sheet into 2 parallel sheets • Epiboly – movement of epithelial sheet (usually ectoderm) as a unit to enclose a deeper layer 5.4 Types of cell movements during gastrulation 22 Cell Specification and Axis Formation • Cell fate specified by cell-to-cell interaction • Asymmetric distribution of transcribed molecules: – Molecules bound to cytoplasm – given to cell that bonds to that part – Molecules are actively transported along the cytoskeleton to a particular cell – Molecule becomes associated with specific centrosome 23 Cell Specification and Axis Formation • Once asymmetry occurs – cells communicate to neighboring cells (Paracrine, Juxtacrine interactions) • Must develop 3 crucial axes – Anterior-posterior axis – Dorsal-ventral axis – Right-left axis • Some organs not symmetrical – heart, liver…. 24 Cell Specification and Axis Formation 5.5 Axes of a bilaterally symmetrical animal 25