Chapter 47

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 47Chapter 47

Animal Development


Overview: A Body-Building Plan for Animals

• It is difficult to imagine that each of us began life as a single cell, a zygote

• A human embryo at about 6–8 weeks after conception shows development of distinctive features

LE 47-1

1 mm


• The question of how a zygote becomes an animal has been asked for centuries

• As recently as the 18th century, the prevailing theory was called preformation

• Preformation is the idea that the egg or sperm contains a miniature infant, or “homunculus,” which becomes larger during development



• Development is determined by the zygote’s genome and differences between embryonic cells

• Cell differentiation is the specialization of cells in structure and function

• Morphogenesis is the process by which an animal takes shape


Concept 47.1: After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis

• Important events regulating development occur during fertilization and the three stages that build the animal’s body


Fertilization

• Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote

• The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development


The Acrosomal Reaction

• The acrosomal reaction is triggered when the sperm meets the egg

• This reaction releases hydrolytic enzymes that digest material surrounding the egg

LE 47-3

Sperm-bindingreceptors

Jelly coat

Acrosome

Actin

Spermhead

Basal body(centriole)

Sperm plasmamembrane

SpermnucleusContact

Acrosomalreaction

Acrosomalprocess

Contact and fusionof sperm and eggmembranes Entry of sperm

nucleus

Cortical reaction

Fertilizationenvelope

Egg plasmamembrane

Vitelline layer

Hydrolytic enzymes

Corticalgranule

Fused plasmamembranes

Perivitellinespace

Cortical granulemembrane

EGG CYTOPLASM


• Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy


The Cortical Reaction

• Fusion of egg and sperm also initiates the cortical reaction

• This reaction induces a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg

• These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy

LE 47-4

1 sec beforefertilization

Point ofspermentry

10 sec afterfertilization

Spreading waveof calcium ions

20 sec 30 sec

500 µm


Activation of the Egg

• The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell

• With these rapid changes in metabolism, the egg is said to be activated

• In a sea urchin, a model organism, many events occur in the activated egg

LE 47-5

Binding of sperm to egg

Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)

Increased intracellular calcium level

Cortical reaction begins (slow block to polyspermy)

Formation of fertilization envelope complete

Increased intracellular pH

Fusion of egg and sperm nuclei complete

Increased protein synthesis

Onset of DNA synthesis

First cell division

1

Se

co

nd

s

2

3

68

10

4

20

30

501

2

40

34

10

5

20

3040

9060

Min

ute

s


Fertilization in Mammals

• In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy

LE 47-6

Folliclecell

Acrosomalvesicle

Egg plasmamembrane

Zonapellucida Sperm

nucleus

Corticalganules

Spermbasalbody

EGG CYTOPLASM


Cleavage

• Fertilization is followed by cleavage, a period of rapid cell division without growth

• Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres

LE 47-7

Fertilized egg Four-cell stage Morula Blastula


• The eggs and zygotes of many animals, except mammals, have a definite polarity

• The polarity is defined by distribution of yolk, with the vegetal pole having the most yolk

• The development of body axes in frogs is influenced by the egg’s polarity

LE 47-8

Anterior

Right

Animal pole

Graycrescent

DorsalVentral

Left

Posterior

Body axes Establishing the axes

Futuredorsalside oftadpole

Point ofspermentry

Firstcleavage

Vegetalhemisphere Vegetal pole

Point of sperm entry

Animalhemisphere


• Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles

LE 47-9

Zygote

2-cellstageforming

8-cellstage

4-cellstageforming

Animal pole Blasto-coel

Blastula(crosssection)

Vegetal poleBlastula (at least 128 cells)

0.25 mm

Eight-cell stage (viewedfrom the animal pole)

0.25 mm


• Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds

LE 47-10

Blastocoel

Fertilized egg

BLASTODERM

HypoblastEpiblastYOLK MASS

Cutaway view ofthe blastoderm

Blastoderm

Four-cell stage

Zygote

Disk ofcytoplasm


• Holoblastic cleavage, complete division of the egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs


Gastrulation

• Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut


• The three layers produced by gastrulation are called embryonic germ layers

– The ectoderm forms the outer layer

– The endoderm lines the digestive tract

– The mesoderm partly fills the space between the endoderm and ectoderm

Video: Sea Urchin Embryonic Development

LE 47-11

Animalpole

Blastopore

Filopodiapullingarchenterontip

Archenteron

Mesenchymecells

Blastocoel

Future ectoderm

Vegetalpole

Key

Future mesoderm

Future endoderm

Vegetalplate

Blastocoel

Mesenchymecells

Archenteron

Blastocoel

Mesenchume(mesodermforms futureskeleton)

50 µm

Mouth

Ectoderm

Blastopore

Digestive tube (endoderm)

Anus (from blastopore)


• The mechanics of gastrulation in a frog are more complicated than in a sea urchin

LE 47-12

Future ectoderm

Key

Future mesoderm

Future endoderm

Archenteron

Blastocoelremnant

Ectoderm

MesodermEndoderm

Yolk plugYolk plugGastrula

Blastocoelshrinking

Blastocoel

Dorsal tip of blastopore

CROSS SECTION

Animal pole

Dorsal lipof blastopore

Vegetal pole Blastula

SURFACE VIEW


• Gastrulation in the chick and frog is similar, with cells moving from the embryo’s surface to an interior location

• During gastrulation, some epiblast cells move toward the blastoderm’s midline and then detach and move inward toward the yolk

LE 47-13

Futureectoderm

Epiblast

Migratingcells(mesoderm)

YOLK

HypoblastEndoderm

Primitivestreak


Organogenesis

• During organogenesis, various regions of the germ layers develop into rudimentary organs


• Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm

Video: Frog Embryo Development

LE 47-14a

Neural folds

Neuralplate

LM1 mm

Neuralfold

Notochord

Archenteron

Neural plate formation

Endoderm

Mesoderm

Ectoderm


• The neural plate soon curves inward, forming the neural tube

LE 47-14b

Neuralfold

Neural plate

Neural tube

Formation of the neural tube

Neural crest

Outer layerof ectoderm

Neural crest


• Mesoderm lateral to the notochord forms blocks called somites

• Lateral to the somites, the mesoderm splits to form the coelom

LE 47-14c

1 mm

Notochord

Archenteron(digestive cavity)

Neural tube

Neural crest

Eye Somites Tail bud

SEM

Coelom Somite

Somites


• Organogenesis in the chick is quite similar to that in the frog

LE 47-15

Notochord

ArchenteronEndoderm

Mesoderm

Ectoderm

Neural tube

Eye

Coelom

Somite

Somites

Neural tube

Lateral fold

Yolk stalkYOLK

Form extraembryonicmembranes

Yolk sac

Early organogenesis

Forebrain

Heart

Bloodvessels

Late organogenesis


• Many structures are derived from the three embryonic germ layers during organogenesis



Developmental Adaptations of Amniotes

• Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus

• Organisms with these adaptations are called amniotes

• In these organisms, the three germ layers also give rise to the four membranes that surround the embryo

LE 47-17

Embryo

Amnioticcavitywithamnioticfluid

AllantoisAmnion

Albumen

Yolk(nutrients)

Yolk sacChorion

Shell


Mammalian Development

• The eggs of placental mammals

– Are small and store few nutrients

– Exhibit holoblastic cleavage

– Show no obvious polarity

• Gastrulation and organogenesis resemble the processes in birds and other reptiles

• Early cleavage is relatively slow in humans and other mammals


• At completion of cleavage, the blastocyst forms

• The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells

• As implantation is completed, gastrulation begins

• The extraembryonic membranes begin to form

• By the end of gastrulation, the embryonic germ layers have formed

LE 47-18a

Blastocystreaches uterus.

Endometrium(uterine lining)

Maternalbloodvessel

Blastocystimplants.

Inner cell mass

Trophoblast

Blastocoel

Hypoblast

Trophoblast

Epiblast

Expandingregion oftrophoblast

LE 47-18b

Hypoblast

Chorion (fromtrophoblast

Epiblast

Amnioticcavity

Amnion

Yolk sac (fromhypoblast)

Extraembryonic mesoderm cells(from epiblast)

Extraembryonicmembranes startto form andgastrulationbegins.

Amnion

Chorion

Endoderm

Mesoderm

Ectoderm

Yolk sac

Extraembryonicmesoderm

Gastrulation has produced a three-layered embryo with fourextraembryonic membranes.

Allantois

Expandingregion oftrophoblast


• The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way


Concept 47.2: Morphogenesis in animals involves specific changes in cell shape, position, and adhesion

• Morphogenesis is a major aspect of development in plants and animals

• But only in animals does it involve the movement of cells


The Cytoskeleton, Cell Motility, and Convergent Extension

• Changes in cell shape usually involve reorganization of the cytoskeleton

• Microtubules and microfilaments affect formation of the neural tube

LE 47-19Ectoderm

Neuralplate


• The cytoskeleton also drives cell migration, or cell crawling, the active movement of cells

• In gastrulation, tissue invagination is caused by changes in cell shape and migration

• Cell crawling is involved in convergent extension, a morphogenetic movement in which cells of a tissue become narrower and longer

LE 47-20

ConvergenceExtension


Roles of the Extracellular Matrix and Cell Adhesion Molecules

• Fibers of the extracellular matrix may function as tracks, directing migrating cells along routes

• Several kinds of glycoproteins, including fibronectin, promote cell migration by providing molecular anchorage for moving cells

LE 47-21

Direction of migration50 µm


• Cell adhesion molecules contribute to cell migration and stable tissue structure

• One class of cell-to-cell adhesion molecule is the cadherins, which are important in formation of the frog blastula

LE 47-22

Control embryo

Experimental embryo


Concept 47.3: The developmental fate of cells depends on their history and on inductive signals

• Coupled with morphogenetic changes, development requires timely differentiation of cells at specific locations

• Two general principles underlie differentiation:

– During early cleavage divisions, embryonic cells must become different from one another

– After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression


Fate Mapping

• Fate maps are general territorial diagrams of embryonic development

• Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells

LE 47-23a

Fate map of a frog embryo

Epidermis Centralnervoussystem

Blastula

Epidermis

Neural tube stage(transverse section)

Endoderm

Mesoderm

Notochord


• Techniques in later studies marked an individual blastomere during cleavage and followed it through development

LE 47-23b

Cell lineage analysis in a tunicate


Establishing Cellular Asymmetries

• To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established


The Axes of the Basic Body Plan

• In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization

• In amniotes, local environmental differences play the major role in establishing initial differences between cells and, later, the body axes


Restriction of Cellular Potency

• In many species that have cytoplasmic determinants, only the zygote is totipotent

• That is, only the zygote can develop into all the cell types in the adult


• Unevenly distributed cytoplasmic determinants in the egg cell help establish the body axes

• These determinants set up differences in blastomeres resulting from cleavage

LE 47-24

Left (control): Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres.

Right (experimental): Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere.

Gray crescent

Gray crescent

Normal Bellypiece

Normal

The two blastomeres were then separated and allowed to develop.


• As embryonic development proceeds, potency of cells becomes more limited


Cell Fate Determination and Pattern Formation by Inductive Signals

• After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction


The “Organizer” of Spemann and Mangold

• Based on their famous experiment, Spemann and Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo

• The organizer initiates inductions that result in formation of the notochord, neural tube, and other organs

LE 47-25a

Nonpigmented gastrula(recipient embryo)

Pigmented gastrula(donor embryo)

Dorsal lip ofblastopore

LE 47-25b

Secondary (induced) embryo

Primarystructures:

Secondarystructures:

Neural tube

Notochord

Notochord (pigmented cells)

Neural tube (mostly nonpigmented cells)

Primary embryo


Formation of the Vertebrate Limb

• Inductive signals play a major role in pattern formation, development of spatial organization


• The molecular cues that control pattern formation are called positional information

• This information tells a cell where it is with respect to the body axes

• It determines how the cell and its descendents respond to future molecular signals


• The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds

LE 47-26a

Anterior

Organizer regions

Limb bud

PosteriorZPA

AER

50 µm

Apicalectodermalridge


• The embryonic cells in a limb bud respond to positional information indicating location along three axes

LE 47-26b

Digits

AnteriorVentral

Distal

Posterior

Proximal

Dorsal

Wing of chick embryo


• One limb-bud organizer region is the apical ectodermal ridge (AER)

• The AER is thickened ectoderm at the bud’s tip

• The second region is the zone of polarizing activity (ZPA)

• The ZPA is mesodermal tissue under the ectoderm where the posterior side of the bud is attached to the body


• Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating “posterior”

LE 47-27Anterior

Posterior

New ZPA

Hostlimbbud

ZPA

Donorlimbbud


• Signal molecules produced by inducing cells influence gene expression in cells receiving them

• Signal molecules lead to differentiation and the development of particular structures

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Chapter 47