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Fertilization, Part 3 Early Development - Overview (Cleavage and Gastrulation) Gilbert - Chapter 7 pp. 193-195 Chapter 8 pp. 211-216 Today’s Goals • Finish Fertilization (see last lesson goals) • Define the processes of Cleavage and Gastrulation and compare the unique ways in which these processes happen in various model organisms. • Discuss how the amount of yolk in the egg affects the type of cleavage and gastrulation that occur and that the amount of yolk in the egg relates to the evolution of the organism. • Describe specific types of cellular movements that occur during Gastrulation including: epiboly, convergent extension, involution Fertilization: 4 major events • Sperm and other as the same species • ONE (and only one) sperm enters egg make contact and must recognize each egg • Fusion of the genetic material • Activation of egg to begin development Fusion of genetic material (Sea Urchin model) • Sperm nucleus and centriole enter egg, but mitochondria and flagella disintegrate • Thus, mitochondrial DNA is solely from maternal contribution • The sperm centriole will provide part of the mitotic spindle (all species but mouse) Fusion of the Genetic Material (Sea Urchin Model) • Haploid female pronucleus* in egg • Sperm nucleus decondenses to the male pronucleus – Includes breakdown of nuclear envelope and decondensation of chromatin • Sperm pronucleus then rotates 180º and positions the centriole between sperm and egg pronuclei *Mammals - must undergo 2nd meiotic division • Sperm centriole then acts as a microtubule organizing center (MTOC) which extends microtubules to form an aster • Microtubules extend to female pronucleus and pull the two pronuclei together • Fusion of these pronuclei forms the diploid zygote nucleus Pronuclear movements during human fertilization Sperm Enters on left B - Egg completes Meiosis 2 C - 15 hours later, nuclei fuse Fertilization: 4 major events • Sperm and egg make contact and must recognize each other as the same species • ONE (and only one) sperm enters egg • Fusion of the genetic material • Activation of egg to begin development Activation of egg metabolism • Occurs as a result of sperm entry – “Early” responses: within seconds of cortical granule reaction – “Late” responses: several minutes after fertilization occurs Early Responses • Ca2+ is also required for the egg to reenter the cell cycle, and activate protein synthesis • This triggers several metabolic events – Increase in lipid biosynthesis – Increase in oxygen use (respiration) Late Responses • Activation of DNA synthesis – Increase in pH after sperm entry (sea urchin) and Calcium activation of MAP kinase allow DNA synthesis to resume • Activation of protein synthesis – Occurs several minutes after sperm entry – Not dependent on transcription of new mRNA’s (uses maternal mRNA stored in egg) – Maternal mRNA’s include histones, tubulins, actins, morphogenetic factors Rearrangement of the Egg Cytoplasm • Fertilization can cause distinct changes in the arrangement of the egg cytoplasm • This is especially important and easily viewed in amphibian eggs • Amphibian eggs have an animal pole and a vegetal pole – Animal pole has dark pigment – Egg is radially symmetrical around the A-V axis • Sperm can enter anywhere on animal half Formation of the Grey Crescent Amphibians • Once sperm enters, the darkly pigmented cortical (outer) cytoplasm rotates relative to the clear inner cytoplasm (about 30°) • This exposes some of the more diffuse pigment granules in the animal half, which appear grey - “grey crescent” – 180° from the point of sperm entry • As the frog develops, this area will mark the place where gastrulation begins Cleavage and Gastrulation • Cleavage – Rapid cell division – Little or no cell growth • Gastrulation – Rearrangement of cells – Formation of 3 germ layers • Axis formation – Dorsoventral, anterior-posterior, left-right – Depending on species, determined during either fertilization, cleavage or gastrulation Cleavage • Zygote becomes a multicellular organism: Blastula (often becomes a hollow ball of cells) • Egg cytoplasm is divided into smaller nucleated cells called blastomeres • Cell divisions are synchronous • Cells do not grow - volume of the embryo stays the same • Rate and pattern of cell division is controlled by maternal mRNAs (except in mammals) • Rapid!! – Frog can divide into 37,000 cells in 43 hours (see next slide – Rana pipiens Synchronous divisions Blastula does not increase in size Cell Cycle During Cleavage • Rapid – Due to the lack of G1 and G2 phases of cell cycle • Stores of Cyclin B from maternal mRNAs allow cells to proceed from M to S phase without G phases • When the stores of Cyclin B become used up, the transcription of zygotic cyclins begins • Mid-blastula transition – Embryonic genes begin to become expressed – Gap phases return to the cell cycle – Cells no longer divide synchronously Cells in cleavage: NO Interphase Normal Somatic Cell Patterns of Embryonic Cleavage • Determined by – The amount and distribution of yolk in the TODAY cytoplasm NEXT – Factors in the egg cytoplasm that influence WEEK the location of the mitotic spindle Yolk and Embryonic Cleavage • Yolk inhibits cleavage – If one pole of egg has less yolk than the other pole, cleavage will occur unequally – Amphibians • Animal pole: little yolk • Vegetal pole: yolk-rich • Yolk determines where blastocoel (cavity) can form Holoblastic (complete) Cleavage • Ex. Sea urchin, snail, mammal • Isolecithal yolk distribution – Evenly distributed – Very Little Yolk (why??) • Results in holoblastic cleavage – Cleavage furrow extends throughout entire egg – Many patterns can occur depending on cleavage planes • Radial, Spiral, Bilateral, Rotational cleavage Holoblastic Cleavage • Mesolecithal eggs can also undergo holoblastic cleavage – Eggs have uneven yolk disposition – Animal and vegetal pole (like amphibians) – Result is cleavage occurs through whole egg, but after first few divisions, animal pole divides faster than vegetal pole Meroblastic (Incomplete) Cleavage • Occurs in Telolecithal eggs – Dense yolk throughout most of the egg (why?) – Ex. Birds, fish, reptiles, molluscs • Occurs in Centrolecithal eggs – Yolk in the center of egg – Ex. Insects • Only a portion of the cytoplasm is cleaved • Cleavage furrow does not penetrate through the whole egg – Discoidal cleavage, bilateral cleavage, superficial cleavage Gastrulation • Rearrangement of the cells of the blastula • Establishes 3 “germ layers” of the embryo – Ectoderm, mesoderm and endoderm • Involves the whole embryo and must be coordinated – If cells migrate in one location, other cells may need to move simultaneously • Occurs very differently in various species – Depends on yolk, cleavage, gene expression • Uses several kinds of cell movements Sea Urchin Amphibian Fish Human Chick Cell Movements in Gastrulation • Invagination: Infolding of cells into embryo Cell Movements in Gastrulation • Involution: Inturning of outer cell sheet – Outer cell sheet spreads over internal surface Cell Movements in Gastrulation • Ingression: Migration of individual cells into the interior – Cells that were epithelial become mesenchyme – Migrate independently Cell Movements in Gastrulation • Delamination: Splitting of one cellular sheet into 2 sheets Cell Movements in Gastrulation • Epiboly: Movement of epithelial sheets of cells – Cells spread as a unit – Can occur by cells dividing or changing shapes Axis Formation • 3 crucial axes must be developed – Anterior-posterior (head to tail) – Dorso-ventral (back to belly) – Left-right (lateral symmetry) • HOW? Sample Mechanisms:` – Site of gastrulation – Maternal mRNA distribution – Cleavage planes