Download AP Notes Embryology 09

Embryo Development in Animals
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Cleavage:* division without size increase, creates a multicellular embryo
o Radial cleavage: Deuterostome, indeterminate division
o Spiral cleavage: Protostome, determinate division
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Morula: solid ball of cells
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Blastula:* hollow ball of cells (embryo), surrounded by a thin layer of cells
o Blastocoel:* center, fluid filled cavity
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Nourished: by yolk (nutrient rich cytoplasm in the egg)
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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
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Ectoderm: eye, skin (epidermis), nervous system
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Mesoderm: muscles, bones, glands, kidneys, heart
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Endoderm: lines some digestive organs & the respiratory and
reproductive system
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Yolk Plug: site of the blastopore
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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
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Apoptosis* (programmed cell death): suicide of specific cells, essential for normal development
▫ Cell death in hands leave spaces between fingers
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Induction:* causes nearby cells to trigger differentiation of other cells, triggers development
▫ Development of eye; cells touch the optic cup induce lens formation
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Pattern Formation: positional signals determine which master control genes will be expressed &
which body parts form
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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
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Dorsal (upper) lip of blastopore triggers formation of neural tube (move it and neural tube moves)
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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