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Embryo Development in Animals Cleavage:* division without size increase, creates a multicellular embryo o Radial cleavage: Deuterostome, indeterminate division o Spiral cleavage: Protostome, determinate division Morula: solid ball of cells Blastula:* hollow ball of cells (embryo), surrounded by a thin layer of cells o Blastocoel:* center, fluid filled cavity Nourished: by yolk (nutrient rich cytoplasm in the egg) Establish Polarity http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FrogEmbryology.html o Difference in cytoplasm lead to different fates for cells o Determined by where sperm enters the cell = ventral side o Cytoplasm shifts using cytoskeleton Vegetal Pole: lower region, are yolk rich, posterior Animals Pole: upper region, anterior Grey Crescent: forms opposite of sperm entry, dorsal side st ▫ 1 division: through both poles, bisects grey crescent, 2nd division through poles 90º from 1st ▫ Blastopore forms where crescent center once was Gastrulation:* 2nd phase, sorts cells into a 3 layered embryo called a Gastrula* o Blastopore:* small groove (hole), endoderm moves inward into cavity of blastula (invagination) o Archenteron:* endoderm forms a primitive gut o Completion: 3 layered embryo Ectoderm: eye, skin (epidermis), nervous system Mesoderm: muscles, bones, glands, kidneys, heart Endoderm: lines some digestive organs & the respiratory and reproductive system Yolk Plug: site of the blastopore Organogenesis: organ formation o Begins after gastrulation Notochord: forms from mesoderm, provides support for developing tissue, develops into the backbone Neural Tube formation: ▫ Neural Folds & Plate : formed from ectoderm ▫ Neural plate rolls up & forms the neural tube ▫ Neural tube sinks into the embryo, covered by ectoderm – will become the brain & spinal cord ▫ Somites:* blocks of mesoderm, give rise to segmented structures (vertebrae) o Embryo Development Cell migration: cells migrate to a specific destination, bind to similar cells, differentiate Apoptosis* (programmed cell death): suicide of specific cells, essential for normal development ▫ Cell death in hands leave spaces between fingers Induction:* causes nearby cells to trigger differentiation of other cells, triggers development ▫ Development of eye; cells touch the optic cup induce lens formation Pattern Formation: positional signals determine which master control genes will be expressed & which body parts form Control of Growth o Cell-cell signaling can give positional information (near gets lots-responds, far gets little-ignore) o Activates or inactivates genes to trigger differentiation o Sequential inductions drive the development of the embryo Dorsal (upper) lip of blastopore triggers formation of neural tube (move it and neural tube moves) In limbs: apical ectodermal ridge at tip of limb bud and zone of polarizing activity under limb at body joint determine polarity in limbs by secreting morphogens In roundworms, gonad anchor cell causes development of vulva (egg laying hole) o Homeobox: DNA sequence in front of many genes (Hox genes) responsible for determining identity of body parts the resulting proteins bind to specific DNA to affect expression of other genes – master switches probably respond to morphogens in induction o Totipotent: cells that are not fated, can become anything