CHAPTER 8

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CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8

Principles of Principles of DevelopmentDevelopmentPrinciples of Principles of DevelopmentDevelopment

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Organizing cells during development


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Development Development Series of Series of progressive changesprogressive changes in an in an

individual from its individual from its beginning to maturitybeginning to maturity Begins when a fertilized egg divides Begins when a fertilized egg divides

mitoticallymitotically Specialization occurs as a Specialization occurs as a hierarchy of hierarchy of

developmental “decisions”developmental “decisions”

DevelopmentDevelopment


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Fertilization and ActivationFertilization and Activation A century of research has been conducted A century of research has been conducted

on on marine invertebratesmarine invertebrates Especially Especially sea urchinssea urchins

Contact and Recognition Between Egg Contact and Recognition Between Egg and Spermand Sperm Marine organisms Marine organisms

release enormous numbers of sperm in release enormous numbers of sperm in the ocean to fertilize eggsthe ocean to fertilize eggs

Many eggs release a Many eggs release a chemical moleculechemical molecule Attract sperm of the same speciesAttract sperm of the same species

FertilizationFertilization


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Sea urchin spermSea urchin sperm Penetrate a Penetrate a jelly layerjelly layer surrounding egg surrounding egg Next, contacts the Next, contacts the vitelline envelopevitelline envelope

Thin membrane above the egg plasma membraneThin membrane above the egg plasma membrane Egg-recognition proteinsEgg-recognition proteins bind to bind to species-specific species-specific

sperm receptorssperm receptors on the vitelline envelope on the vitelline envelope Ensures an egg recognizes only sperm of the same Ensures an egg recognizes only sperm of the same

speciesspecies In the marine environmentIn the marine environment

Many species may be spawning at the same timeMany species may be spawning at the same time Similar recognition proteins are found on sperm of Similar recognition proteins are found on sperm of

vertebrate speciesvertebrate species



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Prevention of PolyspermyPrevention of Polyspermy Fertilization coneFertilization cone forms where the sperm forms where the sperm

contacts the contacts the vitelline membranevitelline membrane Sperm head drawn in and fuses with egg Sperm head drawn in and fuses with egg

plasma membraneplasma membrane Important changes in the egg surface block Important changes in the egg surface block

entrance to any additional spermentrance to any additional sperm Polyspermy,Polyspermy, the entry of more than one the entry of more than one

spermsperm In the sea urchin, an In the sea urchin, an electrical potential electrical potential

rapidly spreads across the membranerapidly spreads across the membrane “ “fast block” fast block”



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The The cortical reaction cortical reaction followsfollows Fusion of thousands of Fusion of thousands of enzyme-rich enzyme-rich

cortical granules cortical granules with the egg membrane with the egg membrane Cortical granules release contents Cortical granules release contents

between the membrane and vitelline between the membrane and vitelline envelopeenvelope

Creates an Creates an osmotic gradientosmotic gradient Water rushes into space Water rushes into space Elevates the envelope Elevates the envelope Lifts away all bound sperm except the Lifts away all bound sperm except the

one spermone sperm that has successfully fused that has successfully fused with the egg plasma membranewith the egg plasma membrane



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One cortical granule enzyme One cortical granule enzyme Causes the vitelline envelope to Causes the vitelline envelope to

hardenharden Now called the Now called the fertilization fertilization

membranemembrane Block to polyspermy is now completeBlock to polyspermy is now complete

Similar process occurs in Similar process occurs in mammals mammals



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Binding Sperm to Sea Urchin Egg


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After sperm and egg membranes fuseAfter sperm and egg membranes fuse Sperm loses its flagellumSperm loses its flagellum

Enlarged sperm nucleusEnlarged sperm nucleus migrates inward to migrates inward to contact the contact the female nucleusfemale nucleus

Fusion of male and female nuclei forms a Fusion of male and female nuclei forms a diploid zygote nucleusdiploid zygote nucleus



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FertilizationFertilization Sets in motion important changes in the egg Sets in motion important changes in the egg

cytoplasm cytoplasm Fertilized eggFertilized egg called a called a zygotezygote

Zygote now enters Zygote now enters cleavagecleavage



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Cleavage and Early DevelopmentCleavage and Early Development

CleavageCleavage Embryo divides repeatedlyEmbryo divides repeatedly Large cytoplasmic mass converted into small Large cytoplasmic mass converted into small

maneuverable cells: maneuverable cells: blastomeresblastomeres No cell growthNo cell growth occurs, only subdivision until cells occurs, only subdivision until cells

reach regular somatic cell sizereach regular somatic cell size At the end of cleavageAt the end of cleavage

Zygote has been divided into many hundreds or Zygote has been divided into many hundreds or thousands of cells thousands of cells

BlastulaBlastula is formed is formed


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Types of Cleavage is Determined by Yolk


Cleavage Types

Holoblastic Cleavage extends entire length of egg Egg does not contain a lot of yolk, so cleavage

occurs throughout egg Example: mammals, sea stars, worms

Meroblastic Cells divide sitting on top of yolk Too much yolk and yolk can’t divide Examples: birds, reptiles, fish


Egg Types and Cleavage

Isolecithal Very little yolk, evenly distibuted Use Holoblastic cleavage- full cleavage

Mesolecithal Moderate yolk Use Holoblastic - full cleavage

Telolecithal Have an abundance of yolk Use Meroblastic cleavage - partial cleavage


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Development of Sea Urchin


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An Overview of Development Following CleavageAn Overview of Development Following Cleavage

BlastulationBlastulation Cleavage creates a cluster of cells called the Cleavage creates a cluster of cells called the

blastulablastula Blastula stage typically consists of a Blastula stage typically consists of a few hundred to few hundred to

several thousand cellsseveral thousand cells During blastula stage, first germ layer formsDuring blastula stage, first germ layer forms In most animalsIn most animals

Cells are arranged around a fluid-filled cavity called the Cells are arranged around a fluid-filled cavity called the blastocoel (blas-to-seal)blastocoel (blas-to-seal)


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Gastrulation and Formation of Two Germ LayersGastrulation and Formation of Two Germ Layers GastrulationGastrulation

Results in the formation of a Results in the formation of a second germ second germ layerlayer

Involves an Involves an invaginationinvagination of one side of of one side of blastulablastula

Forms a new internal cavityForms a new internal cavity gastrocoelgastrocoel Opening into the cavity: Opening into the cavity: BlastoporeBlastopore

Gastrula has an outer layer of Gastrula has an outer layer of ectodermectoderm and an inner layer of and an inner layer of endodermendoderm



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Incomplete/Blind Gut

Complete Gut

Generalized Development showing germ layers

Blastopore (Opening)

Gastrocoel (Cavity)


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The only opening into embryonic gut is The only opening into embryonic gut is the the blastoporeblastopore

Blind or incomplete gutBlind or incomplete gut Some animals retain the Some animals retain the blind gut - blind gut - the the

opening does not fully extend to other opening does not fully extend to other sideside (flatworms, sea anemones) (flatworms, sea anemones)

Most develop a Most develop a complete gutcomplete gut - in which - in which the opening extends and produces a the opening extends and produces a second opening, the anussecond opening, the anus



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Incomplete/Blind Gut

Complete Gut

Generalized Development showing germ layers


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Formation of MesodermFormation of Mesoderm Animals with Animals with two germ layerstwo germ layers

Diploblastic (Endoderm and Ectoderm)Diploblastic (Endoderm and Ectoderm) Most animals add a Most animals add a 33rdrd germ layer germ layer

TriploblasticTriploblastic MesodermMesoderm

33rdrd germ layer germ layer Forms Forms betweenbetween the endoderm and the the endoderm and the

ectodermectoderm Mesoderm Mesoderm arises from endodermarises from endoderm



Germ Layer Outcomes:Germ Layer Outcomes:

EctodermEctoderm Epithelium and nervous systemEpithelium and nervous system

Endoderm Endoderm Epithelial lining of the digestive and respiratory Epithelial lining of the digestive and respiratory

tract, liver, pancreas, tract, liver, pancreas, MesodermMesoderm

Muscular system, reproductive system, bone, Muscular system, reproductive system, bone, kidneys, bloodkidneys, blood

Developmental CharacteristicsDevelopmental Characteristics


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Formation of the Coelom (see-lom)Formation of the Coelom (see-lom) Coelom Coelom

Body cavity surrounded by mesodermBody cavity surrounded by mesoderm The method by which the coelom forms is The method by which the coelom forms is

an an inherited characterinherited character Important in grouping organisms based on Important in grouping organisms based on

developmental charactersdevelopmental characters Upon completion of coelom formation Upon completion of coelom formation

Body has Body has 3 tissue layers3 tissue layers and and 2 cavities2 cavities Animals Without a Coelom are called Animals Without a Coelom are called

Acoelomates (Ex. flatworms)Acoelomates (Ex. flatworms)




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Developmental CharacteristicsDevelopmental Characteristics

Two major groups of Two major groups of triploblastic animals triploblastic animals (animals with 3 germ layers)(animals with 3 germ layers) ProtostomesProtostomes and and deuterostomesdeuterostomes

The groups are identified by four The groups are identified by four developmental charactersdevelopmental characters Cleavage Patterns (radial or spiral)Cleavage Patterns (radial or spiral) Fate of Blastopore (mouth or anus)Fate of Blastopore (mouth or anus) Coelom Formation (split mesoderm or Coelom Formation (split mesoderm or

outpocketing mesoderm)outpocketing mesoderm) Embryo Type (Regulative or Mosaic)Embryo Type (Regulative or Mosaic)


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Fate of BlastoporeFate of Blastopore Deuterostome embryos Deuterostome embryos

Develop a complete gutDevelop a complete gut Blastopore Blastopore becomes the becomes the anusanus Second openingSecond opening becomes the becomes the mouthmouth

Protosome embryosProtosome embryos BlastoporeBlastopore becomes the becomes the mouthmouth AnusAnus forms from a second opening forms from a second opening

ProtostomesProtostomes and and DeuterostomesDeuterostomes Blastopore Fate Blastopore Fate


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EnterocoelyEnterocoely Mesoderm sides push Mesoderm sides push

outward and expand into outward and expand into a a pouch-like coelomic pouch-like coelomic compartmentcompartment

Pouch-like compartment Pouch-like compartment pinches off and forms a pinches off and forms a mesoderm bound space mesoderm bound space surrounding the gutsurrounding the gut

Occurs in Occurs in Deuterostomes ( Sea Deuterostomes ( Sea stars, fish, frogs, etc.)stars, fish, frogs, etc.)

Coelom Formation - mesoderm movementCoelom Formation - mesoderm movement

SchizocoelySchizocoely Coelom forms fromCoelom forms from

Endodermal cells move Endodermal cells move to blastopore and to blastopore and develop into mesodermdevelop into mesoderm

Mesoderm seperates or Mesoderm seperates or splits to form cavity splits to form cavity (coelom)(coelom)

Occurs in Protostome Occurs in Protostome (Earthworms, snails)(Earthworms, snails)



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Blastula and GastrulaOf Embryos


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Vertebrate DevelopmentVertebrate Development

The Common Vertebrate HeritageThe Common Vertebrate Heritage All vertebrate embryos share chordate All vertebrate embryos share chordate

hallmarkshallmarks Dorsal neural tubeDorsal neural tube NotochordNotochord Pharyngeal gill pouches with aortic Pharyngeal gill pouches with aortic

archesarches Ventral heart Ventral heart Postanal tailPostanal tail


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Amniotes and the Amniotic EggAmniotes and the Amniotic Egg Reptiles, birds, and mammalsReptiles, birds, and mammals Embryos develop within the Embryos develop within the amnionamnion

Fluid-filled sac that encloses the embryoFluid-filled sac that encloses the embryo Provides an aqueous environment in which Provides an aqueous environment in which

the embryo floatsthe embryo floats Protection from mechanical shock Protection from mechanical shock

Amniotic egg contains Amniotic egg contains 4 extraembryonic 4 extraembryonic membranesmembranes including the amnion including the amnion



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In the shelled amniotic egg:In the shelled amniotic egg: Yolk sacYolk sac

Stores yolk Stores yolk Allantois Allantois

Storage of metabolic wastes during Storage of metabolic wastes during developmentdevelopment

Respiratory surface for gas exchangeRespiratory surface for gas exchange



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Chorion Chorion Lies beneath the eggshellLies beneath the eggshell Encloses the embryo and other Encloses the embryo and other

extraembryonic membraneextraembryonic membrane As embryo growsAs embryo grows

Need for oxygen increasesNeed for oxygen increases Allantois and chorion fuse to form a Allantois and chorion fuse to form a

respiratory surface, the chorioallantoic respiratory surface, the chorioallantoic membrane membrane



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Chick Embryo


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A. Fish Larvae - 1 day old, has large yolk sac

B. 10 day old fish larva, developed mouth, yolk sac smaller


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The Mammalian Placenta and Early The Mammalian Placenta and Early Mammalian Development Mammalian Development Most mammalian embryos do not develop Most mammalian embryos do not develop

within an egg shellwithin an egg shell Develop within the mother’s body Develop within the mother’s body Most retained in the mother’s bodyMost retained in the mother’s body

Monotremes Monotremes Primitive mammals that Primitive mammals that lay eggslay eggs Large yolky eggs resembling bird eggsLarge yolky eggs resembling bird eggs Duck-billed platypus and spiny anteaterDuck-billed platypus and spiny anteater



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MarsupialsMarsupials Embryos born at an early stage of Embryos born at an early stage of

development and development and Continue development in Continue development in abdominal pouchabdominal pouch

of motherof mother Placental MammalsPlacental Mammals

Represent Represent 94%94% of the class Mammalia of the class Mammalia Evolution of the placentaEvolution of the placenta

Required Required reconstruction of reconstruction of extraembryonic membranesextraembryonic membranes

Modification of oviductModification of oviduct Expanded region formed a Expanded region formed a uterusuterus



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Extraembryonic membranes of a mammal


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Early Stages of Mammalian Development (Human)Early Stages of Mammalian Development (Human)Germinal Period (1Germinal Period (1stst two weeks) two weeks) Blastocyst Blastocyst transported by oviduct to the uterustransported by oviduct to the uterus

Propelled by Propelled by ciliary action ciliary action Around 6Around 6thth day day

Blastocyst = Blastocyst = 100 cells100 cells Contacts uterusContacts uterus

By the twelfth dayBy the twelfth day Implantation is completeImplantation is complete Embryo surrounded by pool of maternal bloodEmbryo surrounded by pool of maternal blood Chorion thickens, sends out tiny Chorion thickens, sends out tiny fingerlike projectionsfingerlike projections

Chorionic villiChorionic villi



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Early Development of the human embryo


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Amnion Amnion Remains unchangedRemains unchanged Surrounds embryoSurrounds embryo Secretes fluid in which embryo floatsSecretes fluid in which embryo floats

Yolk sacYolk sac Contains no yolk Contains no yolk Source of Source of stem cellsstem cells that give rise to that give rise to blood and blood and

lymphoid cellslymphoid cells Stem cells migrate to into the developing embryoStem cells migrate to into the developing embryo



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Allantois Allantois Not needed to store wastesNot needed to store wastes Contributes to the formation of the Contributes to the formation of the

umbilical cordumbilical cord Chorion Chorion

Forms most of the Forms most of the placentaplacenta



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Human embryo showing somites - They will differentiate into skeletal muscle and the axial skeleton

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CHAPTER 8