Chapter 47: Animal Chapter 47: Animal DevelopmentDevelopment

Theories of embryonic developmentTheories of embryonic developmentPreformationPreformation

Belief that egg or sperm contain a miniature Belief that egg or sperm contain a miniature embryo/adult formembryo/adult form

EpigenesisEpigenesisEmbryo develops gradually from an eggEmbryo develops gradually from an eggDevelopmental plan influenced byDevelopmental plan influenced by

Zygote genomeZygote genomeMaternal mRNAMaternal mRNACytoplasmic determinantsCytoplasmic determinants

Cell signaling influences gene expressionCell signaling influences gene expression

FertilizationFertilization Activates the eggActivates the egg brings nuclei of egg & sperm togetherbrings nuclei of egg & sperm together begins begins

embryo metabolic reactionsembryo metabolic reactions Acrosomal ReactionAcrosomal Reaction

Acrosome at sperm tip releases hydrolytic Acrosome at sperm tip releases hydrolytic enzymes which digest egg jelly coat/vitelline enzymes which digest egg jelly coat/vitelline membranemembrane

Ion channels open in egg membraneIon channels open in egg membraneNa+ flows Na+ flows in depolarizing membranein depolarizing membrane prevents other prevents other sperm from fusing with egg (fast block sperm from fusing with egg (fast block polyspermy)polyspermy)

Cortical ReactionCortical Reaction Fusion of egg & sperm stimulates a series of Fusion of egg & sperm stimulates a series of

changes in egg cortexchanges in egg cortex Formation of fertilization envelope to function as Formation of fertilization envelope to function as

a slow-block to polyspermya slow-block to polyspermy

Activation of the eggActivation of the egg Sharp rise in CaSharp rise in Ca2+2+ in egg causes: in egg causes:

Increased cell respiration & protein synthesisIncreased cell respiration & protein synthesis Sperm & egg nuclei to fuseSperm & egg nuclei to fuse DNA replication & first cell division DNA replication & first cell division

90 minutes in sea urchins & frogs90 minutes in sea urchins & frogs 12-36 hours in mammals12-36 hours in mammals

Can be stimulated to occur without spermCan be stimulated to occur without sperm

Fertilization in mammalsFertilization in mammals Fertilization is internal (fallopian tubes)Fertilization is internal (fallopian tubes) Secretions of female tract enhance sperm motilitySecretions of female tract enhance sperm motility Sperm migrates into zona pellucida (3D matrix of Sperm migrates into zona pellucida (3D matrix of

egg)egg) Sperm binds to & depolarizes egg to prevent Sperm binds to & depolarizes egg to prevent

polyspermypolyspermy Diploid nuclei form after 1Diploid nuclei form after 1stst cell division (12-36 hours cell division (12-36 hours

after sperm bind)after sperm bind)

CleavageCleavage Partitions the zygote into smaller cellsPartitions the zygote into smaller cells Rapid succession of mitotic cell divisions without G1 & Rapid succession of mitotic cell divisions without G1 &

G2G2 First 5-7 divisions form cluster of cells= morulaFirst 5-7 divisions form cluster of cells= morula Fluid cavity blastocoel forms in morulaFluid cavity blastocoel forms in morula Hollow ball of cells with fluid within= blastulaHollow ball of cells with fluid within= blastula

Polarity & planesPolarity & planes Vegetal poleVegetal pole

high yolk concentrationhigh yolk concentration forms at posterior end of embryoforms at posterior end of embryo

Animal poleAnimal pole low concentration of yolk low concentration of yolk forms the anterior end of embryoforms the anterior end of embryo

Gray crescentGray crescent near equatornear equator

Types of cleavageTypes of cleavage Meroblastic- incomplete division; found in yolk-rich eggsMeroblastic- incomplete division; found in yolk-rich eggs Holoblastic- complete division; found in eggs with little or Holoblastic- complete division; found in eggs with little or

moderate amounts of yolkmoderate amounts of yolk

GastrulationGastrulation Rearranges the cells in the blastulaRearranges the cells in the blastula

forms a 3 layered embryo with a primitive gut (archenteron)forms a 3 layered embryo with a primitive gut (archenteron) Invagination distributes the cells into layersInvagination distributes the cells into layers

open end of archenteron= blastopore (future anus)open end of archenteron= blastopore (future anus) Germ layersGerm layers

EctodermEctoderm outermost layer outermost layer forms nervous system, skin, hair, nails, eye lensforms nervous system, skin, hair, nails, eye lens

EndodermEndoderm inner layerinner layer gives rise to digestive lining, pancreas, liver, thyroid, lungs, bladdergives rise to digestive lining, pancreas, liver, thyroid, lungs, bladder

MesodermMesoderm middle layermiddle layer gives rise to muscles, skeleton, gonads, excretory & circulatory systemsgives rise to muscles, skeleton, gonads, excretory & circulatory systems

3 layer embryo= gastrula3 layer embryo= gastrulaOrganogenesisOrganogenesis Formation of organs from germ layersFormation of organs from germ layers Localized morphological changes in tissue & cell shapeLocalized morphological changes in tissue & cell shape Neural tube & notochord first to appear & elongate in chordate Neural tube & notochord first to appear & elongate in chordate

embryoembryo Blocks of somites from which vertebrae & muscles will form appear Blocks of somites from which vertebrae & muscles will form appear

along the notochordalong the notochord

AmniotesAmniotes Embryos develop in a fluid-filled sac within a Embryos develop in a fluid-filled sac within a

shell or uterusshell or uterus Formation of extraembryonic membranes (life Formation of extraembryonic membranes (life

support system for embryo); occurs support system for embryo); occurs simultaneously with gastrulationsimultaneously with gastrulation AmnionAmnion

fluid protecting the embryo from temperature & movement fluid protecting the embryo from temperature & movement shocksshocks

ChorionChorion gas exchangegas exchange

AllantoisAllantois gas exchange & metabolic waste storagegas exchange & metabolic waste storage incorporated into umbilical cordincorporated into umbilical cord

Yolk sacYolk sac blood vesselsblood vessels carry food to the embryocarry food to the embryo

Avian developmentAvian development Yolk contains food supplyYolk contains food supply Blastodisc= embryo forming portion of egg Blastodisc= embryo forming portion of egg

on upper surfaceon upper surface Primitive streak- longitudinally thickens along Primitive streak- longitudinally thickens along

blastodiscblastodisc Extraembryonic membranesExtraembryonic membranes

Mammalian developmentMammalian development Fertilization & cleavage occurs in oviductsFertilization & cleavage occurs in oviducts Blastocyst reaches uterus & implants at day 7Blastocyst reaches uterus & implants at day 7 Extra-embryonic membranesExtra-embryonic membranes Organogenesis begins with formation of Organogenesis begins with formation of

neural tube, notochord, & somitesneural tube, notochord, & somites

Morphogenesis Morphogenesis Involves specific changes in cell shape, Involves specific changes in cell shape,

position, & adhesion; reorganization of position, & adhesion; reorganization of the cytoskeletonthe cytoskeleton Morphogenetic movements partially guided Morphogenetic movements partially guided

by extra-cellular matrixby extra-cellular matrix Cell Adhesion Molecules (CAM) contribute to Cell Adhesion Molecules (CAM) contribute to

selective association of cells with one anotherselective association of cells with one another Cytoplasmic determinants help establish Cytoplasmic determinants help establish

body axes & differences among cells of the body axes & differences among cells of the early embryoearly embryo

Inductive signals drive differentiation & Inductive signals drive differentiation & pattern formation in vertebratespattern formation in vertebrates

The “organizer” influences & induces the The “organizer” influences & induces the development of other cellsdevelopment of other cells

Inducers turn some genes on & other offInducers turn some genes on & other off