Download Ch.47 Animal Development

Ch.47 Animal Development
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Theories:
1.Preformation­
enlargement of embryo, contains smaller embryos within a larger embryo
­embryo is a "minature infant"
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2. epigenesis­ "form of an animal emerges gradually from a relatively formless egg"
http://www.uoguelph.ca/zoology/devobio/210labs/34frogwm.htm
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How development happens­
­genetic programming due to genome
­cytoplasmic determinants ­ proteins and RNAs
as zygote divides, get uneven distribution of these determinants and cell signals
­cell differentiation
­morphogenesis ­ process by which an organism takes shape and appropriate cells go to their location
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What happens when an egg is fertilized?
function of fertilization
1.combine haploid sets of chromosomes
(one from male, one from female)
2. activation of the egg­ sperm contact starts metabolic reactions in egg to produce embryo
use sea urchins for study because they are deuterostomes and early development is similar to vertebrates
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The acrosomal and cortical reactions during sea urchin fertilization
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Acrosomal Reaction in sea urchin
­sperm has acrosomal head that by exocytosis releases hydrolytic enzymes that allow acrosomal process to penetrate the jellied outside of egg
­acrosomal process has protein that binds to receptor molecules under jelly coat (vitelline layer)­ only recognizes same species
­only one sperm is allowed in, in 1­3 seconds causes a chain reaction
­sodium ion channels open get membrane depolarization
called fast block to polyspermy (common to animals)
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Cortical Reaction
= changes in the outer zone of egg cytoplasm to further prevent sperm from entering via a signal­transduction pathway
endoplasmic reticulum releases Ca ions into cytosol
second messenger IP3 and DAG
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IP3 opens ligand gated channels
Ca ions triggers release of other channels
High concentration of Ca ions
cortical granules (just under plasma membrane) fuse with plasma membrane and contents go to perivitelline layer
separate vitelline and plasma layers and mucopolysaccharides make an osmotic gradient, pulling water into perivitelline layer
fertilization envelope
= slow block to polyspermy
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Activation of egg
­Ca ions also starts metabolic reactions
­increases cellular respiration
­increases protein synthesis
­DAG also activates the transportation of H+ ions out of egg, so egg is alkaline
­sperm don't directly donate materials for activation
­activation can be induced by injecting Ca ions to eggs or heat shock, even when no nucleus present (proves mRNA is inactive in cytoplasm)
­nucleus of sperm cell enlarges
­within 20 mins. fuses with egg nucleus = diploid zygote
­DNA synthesis begins first cell division
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timeline for fertilization
in sea urchin eggs
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Differences in fertilization of mammals
­internal fertilization
­capacitation­when secretions of female reproductive tract alter molecules on surface of sperm, increase motility
­egg is surrounded by follicular cells and under that is the zona pellucida­ extracellular matrix ­zona pellucida = three glycoproteins filaments making a network
ZP3 is a sperm receptor
­depolarization and cortical reaction are same
­egg cell extensions take sperm into egg
­basal body of sperm flagellum divides to form 2 centrosomes (will make mitotic spindles)
­nucleus of sperm and eggs do not fuse quickly
­chromosomes of two gametes share a common spindle apparatus in first mitotic cell division, then two sets of chromosomes come together
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How is cleavage involved?
= succession of rapid cell divisions after fertilization
­do the S (DNA synthesis) and M (mitosis) stages ­skip G1 and G2 phases
­no enlargement of embryo
­makes blastomeres, each with own nucleus
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Sea urchin development from single cell to larva
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animal end becomes anterior of embryo melanin=dark gray
vegetal = yolk= yellow
gray crescent forms opposite where sperm enters
first cleavage bisects gray crescent
gray crescent = dorsal side
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Frog embryo
­first two divisions are vertical (also sea urchins)
­third division = horizontal (eight cells)
­continued cleavage = morula
­blastula= hollow ball with blastocoel inside (fluid filled cavity)
­in sea urchin, blastocoel is in center
­in frog, blastocoel is on animal side due to uneven cell division
­ bird eggs, yolk = ovum, egg white provides nutrients
meroblastic cleavage = incomplete division of yolk­rich egg (bird eggs)
holoblastic cleavage = complete division of eggs with little yolk (frog eggs)
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Cleavage of frog embryo
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Gastrulation
= rearrangement of cells of the blastula
­change in cell motility
­change in cell shape
­change in cell adhesion to other cells and to molecules of extracellular matrix
frog embryo development
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in sea urchin
­begins at vegetal pole
­migration of mesenchyme cells into blastocoel
­vegetal plate invaginates, mesenchyme cells form filopodia (extensions)
­endocells form from endoderm, mesenchyme cells form bridges between tip of archenteron and endoderm
­filopodia contract and pull archenteron to ectoderm
­digestive tube then formed (mouth and anus­blastopore)
­some mesenchyme cells secrete calcium carbonate to form "skeleton"
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*by end of gastrulation should have three embryonic germ layers: a. endoderm ­ becomes the digestive tract, liver pancreas
b. mesoderm­ fills area between endoderm and ectoderm (kidney, heart, muscles)
c. ectoderm­becomes outer layer (skin, nervous system)
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Frog Gastrulation
­more than one cell thick
­starts with small tuck called dorsal lip of the blastopore, cells push in from surface
­additional cells go in over dorsal lip and move to interior (involution)
­cells of animal pole spread over outer surface
­three germ layers start to form inside, blastocoel shrinks
­circular blastopore surrounds plug of yolk cells (food cells)
­by end, all layers are in place
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How do organs form?
Organogenesis
­first organ formed in chordates are the neural tube and notochord
­notochord from dorsal mesoderm above archenteron
­neural tube­from dorsal ectoderm above notochord, becomes spinal cord and brain
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somites form vertebral skeleton and muscles attached to axial skeleton
­ found on both sides of the notochord ­ as organs develop they get more specialized
­also have neural crest in vertebrate embryos, becomes pigment cells of skin, some bones and muscles of skull, teeth, medulla of kidney and sensory and sympathetic ganglia of nervous system
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What is so special about amniotes?
­allows organisms to lay shelled eggs on land or in utero
­birds, reptiles and mammals
­embryos are surrounded by amnion (fluid filled sac)
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amniota.html
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Avian Development
1. cleavage is meroblastic, produces a blastodisc
­blastodisc becomes epiblast and hypoblast that surround blastocoel
­Gastrulation ­ some cells of epiblast migrate through primitive streak
­some move laterally to form mesoderm
­some form endoderm
­organogenesis
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eye
limb buds
chick embryo
vertebral column
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tissue layers outside the embryo develop into four extraembryonic membranes
­yolk sac­digests yolk and nourishes embryo
­amnion­encloses embryo in fluid­filled sac, protects embryo from drying out
­chorion­helps cushion embryo against mechanical shock
­allantois­disposal sac for uric acid
­chorion and allantois form respiratory organ
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Development of extraembryonic membranes of chick
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Mammalian Development
­egg is small compared to birds and reptiles
­cleavage is holoblastic (slow)
1.at 7 days = have blastocyst, contains inner cell mass
­trophoblast = outer epithelium, forms fetal placenta
2.trophoblast releases enzymes so blastocyst implants
­trophoblasts extends projections into uterine tissue
­upper layer of cells becomes epiblast and hypoblast
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3. extraembryonic membranes start to develop
­amnion and chorion start to form
­mesodermal cells (make placenta) form from epiblast
4. Gastrulation
­cells of epiblast move through primitive streak
­chorion surrounds embryo
­amnion begins to dome
­yolk sac forms (makes early blood cells)
­allantois becomes part of umbilical cord
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Morphogenesis
cells change shape ex. formation of neural tube
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cells can change position as they mature
­cytoskeleton moves cells
­some cells are pulled (like amoeba movement) by filopodia extensions
­cell crawling involves convergent extension
­sheet of cells becomes narrower and longer
­occurs during archenteron elongation in sea urchins
­in frog involution
­do not know what triggers it, might involve glycoproteins
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convergent extension of a sheet of cells
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cell adhesion molecules (CAMs)
­glycoproteins on surface of cells
­bind to other CAMs on other cells
­help with cell migration
­cadherins
­important in frog blastula formation
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How do cells become specialized?
­early scientists did fate mapping, added dyes and saw where they ended up in embryo
­discovered two things:
1. "founder cells" generate specific tissues in older embryo
2. a cell's range of structures that it can give rise to becomes restricted
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fate mapping
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Polarity and Basic body plan
bilateral animal­ anterior/posterior, dorsal/ventral, right/left
­in mammals may be related to where sperm enters egg
­in frog have animal and vegetal hemispheres
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Restriction of cellular potency
fates of embryonic cells can be affected by zygote's characteristic cleavage and cytoplasmic determinants
totipotent­ the ability of a cell to form all parts of an organism
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induction ­ when one group of cells influences the development of neighboring cells
­usually caused by switching on genes
Primary organizer­crucial to early development
ex. dorsal lip ­ if grafted onto another embryo, will start the process of making a neural tube and notochord ­ so another embryo attached to the first
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Pattern Formation
=development of animal's spatial organization of organs and tissues
­caused by induction
­positional information ­ tell a cell where it is in respect to the body's axes
ex. limb development in chick
­begin a bumps (limb buds) of mesodermal tissue covered by ectoderm
two key areas
1. apical ectodermal ridge­ on tip of bud
­produces fibroblast growth factor that promotes outgrowth of limb
­also involved in dorsal ­ventral axis
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Organizer regions in vertebrate limb development
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2. zone of polarizing activity­posterior side of bud ­needed for proper pattern formation along anterior/posterior axis of limb
­closest to posterior form "little finger"
­closest to anterior form "thumb"
­cells of ZPA secrete "sonic hedgehog" growth factor (if implanted, forms extra digits)
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HOX genes are very important in developing a limb
­turn other genes on
"Various pathways of pattern formation occur in all the different parts of the developing embryo, eventually producing the final set of differentiated structures in the fully formed animal"
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