2Embryonic Development Genomes of zygote and differences btwn early embryonic cells determine developmentCytoplasmic Determinants – Uneven distribution of maternal substances in the unfertilized eggDifferences between cells because of their location in the embryoCell Differentiation – specialization of cells form and function, caused by gene expressionMorphogenesis – process by which an embryo takes shape and cells are in the appropriate locations
3Embryonic Stages Fertilization – When Gametes (sperm and egg) unite Cleavage – Rapid Cell divisions after Fertilization. S phase (DNA synthesis) and M phase (mitosis). Skips protein synthesisGastrulation – Morphogenetic phase Drastic rearrangement of the cells of the blastula. Forms a three-layered embryo with a primitive gut.Organogenesis – When regions of the three-layered embryo develop into fundamental organs
4Fertilization VocabAcrosomal Reaction - discharge of a sperm’s acrosome when it is near the eggAcrosome – Vesicle at the tip of sperm, helps sperm penetrate the eggFast Block to Polyspermy – Depolarization of egg membrane after sperm binds to vitelline layer. Prevents more sperm from enteringFertilization Envelope - the changed vitelline layer – prevents other sperm from entering the eggSlow Block to Polyspermy – Formation of fertilization envelope and other changes, opposite of Fast block, lasts longer
5FertilizationFertilize externally – eggs and sperm are released at the same time.Sperm touches egg’s jelly coat – triggering release of acrosome – hole is formed in jellyAcrosomal process forms – protrudes from sperm, penetrates jelly coat, binds to receptors on egg cell – aka acrosomal reactionHole made in vitelline layer – allows contact and fusion of gamete plasma membranes – membranes depolarize forming Fast blockSperm nucleus enters cytoplasm of egg – then slow block forms
7Cleavage Vocab Blastomer – smaller cells that the embryo divides into Morula – cluster of cells after the first 5-7 divisionsBlastocoel – a fluid filled cavityBlastula – hollow ball of cellsYolk – stored nutrients – distributed differently in all embryosVegetal Pole – The pole that the yolk is most concentratedAnimal Pole – Opposite pole, very little yolk
8CleavageAfter fusion of gametes cytoplasm rearranges forming 1 body axis. Other axes form laterFirst 2 divisions are meridional (Vertical) = 4 blastomers of equal sizeThird division is equatorial (Horizontal) = 8 blastomers of unequal size – Animal hemisphere = small cells, Vegetal hemisphere = lager cellsBlastula is located in the Animal Hemisphere
10Gastrulation Vocab Gastrula – 3 layered Embryo Germ Layers – The 3 layers produced.Ectoderm – Outer layerEndoderm – Inner LayerMesoderm – Partly fills space between Ecto and EndoInvagination – When cells fold inwardArchenteron – Primitive GutBlastopore – Opening in the archenteron, develops into the anus.
11GastrulationComplicated mechanics – Large amount of yolk & blastula is more than 1 cell thickBegins on back side of Blastula – cells begin to invaginate in the line along the regionDorsal Lip – The Dorsal side of the blastoporeLip extends and invagination continues until the two ends on the blastopore meet on the ventral sideInvolution – When future endoderm and mesoderm cells on the surface roll over edge of the lip into the interior of the embryo
12GastrulationInside – cells move away from blastopore and become germ layers and blastocoel collapsesYolk Plug – Large food-laden endodermal cells surrounded by blastoporeEnd of Gastrulation, circular lip of blastopore encircles plug, cells on surface becomes the ectodermAnus forms from the blastopore and mouth develops at the opposite end.
14Organogenesis Vocab Notochord – Formed from dorsal mesoderm Neural Tube – when neural plate curves inward – rolling into itselfNeural Crest – band of cells along border of Neural tubeSomites – Paired blocks of mesoderm lateral to notochord
15Organogenesis First organs to take shape – neural tube and notochord Signals from notochord to ectoderm cause ectoderm to become neural plateCells from neural crest migrate to all parts of the body – form peripheral nerves, teeth, skull bonesSome somites become wandering cells – go to new locations.Organogenesis continues – cell differentiation continues to refine organs
17MorphogenesisMajor 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 cellsConvergent Extension – morphogenetic movement – cells of tissue layer rearrange, sheets become narrow (converge) and become longer (extend)
18Extracellular MatrixExtracellular Matrix (ECM) – Mixture of secreted glycoproteins outside plasma membrane of cells – trigger/guide movementSome ECMs promote migration, providing specific molecular anchorage for moving cellsOthers keep cells on correct paths – inhibiting migration – use nonmigratory cellsCell Adhesion Molecules (CAMs) – glycoproteins – help cell migration and stable tissue structureCadherins – important cell-to-cell adhesion molecule.
19Developmental Fate of Cells Development requires a combo of morphogenetic changes and the timely differentiation of cells in specific location2 general principlesEarly cleavage divisions – Embryonic cells must become different from each otherOnce initial cells asymmetries are set up, subsequent interactions among the embryonic cells influence their fate – usually causing changes in gene expression
20A Cell’s FateFate Maps – diagram of embryonic development – reveals future development of individual cells/tissuesA cell’s fate can be changed by moving the cell to a new location2 Important conclusionsSpecific tissues of the older embryo can be attributed to certain early “founder cells”As development proceeds a cell’s developmental Potential becomes restricted
21Establishing Cellular Asymmetries Establishing basic body plan is 1st step in morphogenesis – a prerequisite for the development of tissues/organsTotipotent – describes a cell that can become any part of an organismZygote’s pattern of cleavage affects the fate of cellsProgressive restriction of potency is a feature of development in all animalsThe tissue-specific fates of cells in late gastrula are fixed
22Inductive SignalsCell 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 signalsPositional Information – Molecular cues – control pattern formation
23Limbs 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 developmentApical Ectodermal Ridge (AER) – 1 organizer – thickened area of ectoderm at the tip of the budZone of Polarizing Activity (ZPA) – other organizer – block of mesodermal tissue located underneath ectoderm – posterior side of the bud is attached to body