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Chapter 47 Animal Development Nicole Gallup Embryonic Development • Genomes of zygote and differences btwn early embryonic cells determine development • Cytoplasmic Determinants – Uneven distribution of maternal substances in the unfertilized egg • Differences between cells because of their location in the embryo • Cell Differentiation – specialization of cells form and function, caused by gene expression • Morphogenesis – process by which an embryo takes shape and cells are in the appropriate locations Embryonic Stages • Fertilization – When Gametes (sperm and egg) unite • Cleavage – Rapid Cell divisions after Fertilization. S phase (DNA synthesis) and M phase (mitosis). Skips protein synthesis • Gastrulation – Morphogenetic phase Drastic rearrangement of the cells of the blastula. Forms a threelayered embryo with a primitive gut. • Organogenesis – When regions of the three-layered embryo develop into fundamental organs Fertilization Vocab • Acrosomal Reaction - discharge of a sperm’s acrosome when it is near the egg • Acrosome – Vesicle at the tip of sperm, helps sperm penetrate the egg • Fast Block to Polyspermy – Depolarization of egg membrane after sperm binds to vitelline layer. Prevents more sperm from entering • Fertilization Envelope - the changed vitelline layer – prevents other sperm from entering the egg • Slow Block to Polyspermy – Formation of fertilization envelope and other changes, opposite of Fast block, lasts longer Fertilization • Fertilize externally – eggs and sperm are released at the same time. • Sperm touches egg’s jelly coat – triggering release of acrosome – hole is formed in jelly • Acrosomal process forms – protrudes from sperm, penetrates jelly coat, binds to receptors on egg cell – aka acrosomal reaction • Hole made in vitelline layer – allows contact and fusion of gamete plasma membranes – membranes depolarize forming Fast block • Sperm nucleus enters cytoplasm of egg – then slow block forms Cleavage Vocab • Blastomer – smaller cells that the embryo divides into • Morula – cluster of cells after the first 5-7 divisions • Blastocoel – a fluid filled cavity • Blastula – hollow ball of cells • Yolk – stored nutrients – distributed differently in all embryos • Vegetal Pole – The pole that the yolk is most concentrated • Animal Pole – Opposite pole, very little yolk Cleavage • After fusion of gametes cytoplasm rearranges forming 1 body axis. Other axes form later • First 2 divisions are meridional (Vertical) = 4 blastomers of equal size • Third division is equatorial (Horizontal) = 8 blastomers of unequal size – Animal hemisphere = small cells, Vegetal hemisphere = lager cells • Blastula is located in the Animal Hemisphere Gastrulation Vocab • Gastrula – 3 layered Embryo • Germ Layers – The 3 layers produced. • Ectoderm – Outer layer • Endoderm – Inner Layer • Mesoderm – Partly fills space between Ecto and Endo • Invagination – When cells fold inward • Archenteron – Primitive Gut • Blastopore – Opening in the archenteron, develops into the anus. Gastrulation • Complicated mechanics – Large amount of yolk & blastula is more than 1 cell thick • Begins on back side of Blastula – cells begin to invaginate in the line along the region • Dorsal Lip – The Dorsal side of the blastopore • Lip extends and invagination continues until the two ends on the blastopore meet on the ventral side • Involution – When future endoderm and mesoderm cells on the surface roll over edge of the lip into the interior of the embryo Gastrulation • Inside – cells move away from blastopore and become germ layers and blastocoel collapses • Yolk Plug – Large food-laden endodermal cells surrounded by blastopore • End of Gastrulation, circular lip of blastopore encircles plug, cells on surface becomes the ectoderm • Anus forms from the blastopore and mouth develops at the opposite end. Organogenesis Vocab • Notochord – Formed from dorsal mesoderm • Neural Tube – when neural plate curves inward – rolling into itself • Neural Crest – band of cells along border of Neural tube • Somites – Paired blocks of mesoderm lateral to notochord Organogenesis • First organs to take shape – neural tube and notochord • Signals from notochord to ectoderm cause ectoderm to become neural plate • Cells from neural crest migrate to all parts of the body – form peripheral nerves, teeth, skull bones • Some somites become wandering cells – go to new locations. • Organogenesis continues – cell differentiation continues to refine organs Neural Plate formation Neural Tube Formation Somites Morphogenesis • Major aspect of development in animals – involves movement of cells. • Changes in shape involve reorganization of the cytoskeleton. Cytoskeleton drives cell migration. • Cells that move 1st drag others behind them – directs movement of a sheet if cells • Convergent Extension – morphogenetic movement – cells of tissue layer rearrange, sheets become narrow (converge) and become longer (extend) Extracellular Matrix • Extracellular Matrix (ECM) – Mixture of secreted glycoproteins outside plasma membrane of cells – trigger/guide movement • Some ECMs promote migration, providing specific molecular anchorage for moving cells • Others keep cells on correct paths – inhibiting migration – use nonmigratory cells • Cell Adhesion Molecules (CAMs) – glycoproteins – help cell migration and stable tissue structure • Cadherins – important cell-to-cell adhesion molecule. Developmental Fate of Cells • Development requires a combo of morphogenetic changes and the timely differentiation of cells in specific location • 2 general principles – Early cleavage divisions – Embryonic cells must become different from each other – Once initial cells asymmetries are set up, subsequent interactions among the embryonic cells influence their fate – usually causing changes in gene expression A Cell’s Fate • Fate Maps – diagram of embryonic development – reveals future development of individual cells/tissues • A cell’s fate can be changed by moving the cell to a new location • 2 Important conclusions – Specific tissues of the older embryo can be attributed to certain early “founder cells” – As development proceeds a cell’s developmental Potential becomes restricted Establishing Cellular Asymmetries • Establishing basic body plan is 1st step in morphogenesis – a prerequisite for the development of tissues/organs • Totipotent – describes a cell that can become any part of an organism • Zygote’s pattern of cleavage affects the fate of cells • Progressive restriction of potency is a feature of development in all animals • The tissue-specific fates of cells in late gastrula are fixed Inductive Signals • Cell division creates cells that differ from each other the cells then influence each other’s fate (induction) • Pattern Formation – development of an animal’s spatial organization, arrangement of organs/tissues – influenced by inductive signals • Positional Information – Molecular cues – control pattern formation Limbs • Limbs begin as bumps of tissue called Limb buds • Buds – consist of a core of mesoderm tissue covered by a layer of ectoderm – 2 organizer locations affect limb’s development • Apical Ectodermal Ridge (AER) – 1 organizer – thickened area of ectoderm at the tip of the bud • Zone of Polarizing Activity (ZPA) – other organizer – block of mesodermal tissue located underneath ectoderm – posterior side of the bud is attached to body Citations • http://www.vcharkarn.com/uploads/0/80.jpg • http://3.bp.blogspot.com/_NDw_XebDkYI/S7ApTP1gibI/AAAAAAAAAO4/a YitNrMkyWo/s1600/cleavage.jpg • http://bio1152.nicerweb.com/doc/class/bio1152/Locked/media/ch47/47_ 12FrogGastrulation.jpg • http://bio1151.nicerweb.com/Locked/media/ch47/47_14FrogOrganogene sis_CL.jpg