47 Lecture Presentation PC 2012 (1)

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Overview: A Body-Building Plan

A human embryo at about 7 weeks after

conception shows development of distinctive

features

© 2011 Pearson Education, Inc.



Figure 47.1

1 mm



Development occurs at many points in the life

cycle of an animal

This includes metamorphosis and gamete

production, as well as embryonic development




Figure 47.2

EMBRYONIC DEVELOPMENTSper m

Adultf rog

Egg

Metamorphosis

Larvalstages

Zygote

Blastula

Gastr ula

Tail-budembryo



Although animals display different body plans,

they share many basic mechanisms of

development and use a common set of

regulatory genes Biologists use model organisms to study

development, chosen for the ease with which

they can be studied in the laboratory




Concept 47.1: Fertilization and cleavage

initiate embryonic development

Fer tilization is the formation of a diploid zygote

from a haploid egg and sperm




Fertilization

Molecules and events at the egg surface play a

crucial role in each step of fertilization

± Sperm penetrate the protective layer around the

egg ± Receptors on the egg surface bind to molecules

on the sperm surface

± Changes at the egg surface prevent polyspermy,

the entry of multiple sperm nuclei into the egg




The Acrosomal Reaction

The acrosomal reaction is triggered when the

sperm meets the egg

The acrosome at the tip of the sperm releases

hydrolytic enzymes that digest materialsurrounding the egg




Figure 47.3-5

Basal body

(centriole)

Sper mplasmamembr ane

Sper mnucleus

Sper mhead

Acrosome

Jelly coat

Sper m-bindingreceptors

Fer tilizationenvelope

Cor ticalgr anule

Fusedplasmamembr anes

Hydrolytic enzymes

Vitelline layer

Egg plasma membr ane

Perivitelline

space

EGG CYTOPLASM

Actinf ilament

Acrosomalprocess



Gamete contact and/or fusion depolarizes the egg

cell membrane and sets up a fast block to

polysper my




The Cortical Reaction

Fusion of egg and sperm also initiates the cor tical

reaction

Seconds after the sperm binds to the egg, vesicles

just beneath the egg plasma membrane releasetheir contents and form a fertilization envelope

The fertilization envelope acts as the slow block

to polysper my




The cortical reaction requires a high concentration

of Ca2 ions in the egg

The reaction is triggered by a change in Ca2

concentration Ca2 spread across the egg correlates with the

appearance of the fertilization envelope




Figure 47.4

10 sec after f er tilization

25 sec 35 sec 1 min500 Qm

500 Qm30 sec20 sec10 sec after

f er tilization1 sec bef oref er tilization

Point of sper mnucleusentry

Spreadingwave of Ca2

Fer tilizationenvelope

EXPERIMENT

RESULTS

CONCLUSION



Egg Activation

The rise in Ca2+ in the cytosol increases the rates

of cellular respiration and protein synthesis by the

egg cell

With these rapid changes in metabolism, the eggis said to be activated

The proteins and mRNAs needed for activation

are already present in the egg

The sperm nucleus merges with the egg nucleusand cell division begins




F ertilization in Mammals

Fertilization in mammals and other terrestrial

animals is internal

Secretions in the mammalian female reproductive

tract alter sperm motility and structure This is called capacitation and must occur before

sperm are able to fertilize an egg




Sperm travel through an outer layer of cells to

reach the zona pellucida, the extracellular matrix

of the egg

When the sperm binds a receptor in the zonapellucida, it triggers a slow block to polyspermy

No fast block to polyspermy has been identified in

mammals




Figure 47.5

Zona pellucida

Follicle cell

Sper mbasal body

Sper mnucleus

Cor ticalgr anules



In mammals the first cell division occurs 1236

hours after sperm binding

The diploid nucleus forms after this first division of

the zygote




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 The blastula is a ball of cells with a fluid-filled

cavity called a blastocoel




Figure 47.6

(a) Fer tilized egg (b) Four -cell stage (c) Ear ly blastula (d) Later blastula

50 Qm



In frogs and many other animals, the distribution of

yolk (stored nutrients) is a key factor influencing

the pattern of cleavage

The vegetal pole has more yolk; the animal pole has less yolk

The difference in yolk distribution results in animal

and vegetal hemispheres that differ in appearance


Cleava g e Patterns



The first two cleavage furrows in the frog form

four equally sized blastomeres

The third cleavage is asymmetric, forming

unequally sized blastomeres




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

Meroblastic cleavage, incomplete division of the

egg, occurs in species with yolk-rich eggs, such as

reptiles and birds




Zygote

2-cellstagef or ming

4-cellstagef or ming

8-cellstage

Vegetal pole

Blastula(cross

section)

Gr ay crescent

Animalpole

Blastocoel

0.25 mm

0.25 mm

8-cell stage (viewedf rom the animal pole)

Blastula (at least 128 cells)

Figure 47.7



Animal embryos complete cleavage when the ratio

of material in the nucleus relative to the cytoplasm

is sufficiently large


Re gulation of Cleava g e



After cleavage, the rate of cell division slows and

the normal cell cycle is restored Morphogenesis, the process by which cells

occupy their appropriate locations, involves

± Gastr ulation, the movement of cells from the

blastula surface to the interior of the embryo

± Organogenesis, the formation of organs


Concept 47.2: Morphogenesis in animals

involves specific changes in cell shape,position, and survival



Gastrulation

Gastr ulation rearranges the cells of a blastula

into a three-layered embryo, called a gastr ula




The three layers produced by gastrulation are

called embryonic ger m layers

± The ectoder m forms the outer layer

± The endoder m lines the digestive tract ± The mesoder m partly fills the space between the

endoderm and ectoderm

Each germ layer contributes to specific structures

in the adult animal




ECTODERM (outer layer of embryo)

MESODERM (middle layer of embryo)

ENDODERM (inner layer of embryo)

Epider mis of skin and its derivatives (including sweat glands,hair f ollicles)

Epithelial lining of digestive tr act and associated organs(liver, pancreas)

Epithelial lining of respir atory, excretory, and reproductive tr actsand ducts

Ger m cells Jaws and teeth Pituitary gland, adrenal medulla Nervous and sensory systems

Skeletal and muscular systems Cir culatory and lymphatic systems Excretory and reproductive systems (except ger m cells) Der mis of skin Adrenal cor tex

Thymus, thyroid, and par athyroid glands

Figure 47.8



Gastrulation begins at the vegetal pole of the

blastula

Mesenchyme cells migrate into the blastocoel

The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through

invagination


G astr ulation in Sea Urchins



The newly formed cavity is called the

ar chenteron

This opens through the blastopore, which will

become the anus




Animalpole

Blastocoel

Mesenchymecells

Vegetal plate Vegetalpole

Blastocoel

Filopodia

Mesenchymecells

Blastopore

Ar chenteron

50 Qm

Ectoder m

Mouth

Mesenchyme(mesoder m f or msfuture skeleton)

Blastopore

Blastocoel

Ar chenteron

Digestive tube (endoder m)

Anus (f rom blastopore)

Key

Future ectoder m

Future mesoder m

Future endoder m

Figure 47.9



Frog gastrulation begins when a group of cells onthe dorsal side of the blastula begins toinvaginate

This forms a crease along the region where the

gray crescent formed

The part above the crease is called the dorsal lipof the blastopore


G astr ulation in F ro g s



Cells continue to move from the embryo surface

into the embryo by involution

These cells become the endoderm and

mesoderm Cells on the embryo surface will form the

ectoderm




Key

Future ectoder m

Future mesoder m

Future endoder m

SURFACE VIEW CROSS SECTION

Animal pole

Vegetal poleEar lygastr ula

Blastocoel

Dorsallip of

blasto-pore

BlastoporeDorsallip of blastopore

Blastocoelshrinking

Ar chenteron

Ar chenteron

Blastocoelremnant

Ectoder mMesoder m

Endoder m

Blastopore

Yolk plugBlastopore

Lategastr ula

3

2

1

Figure 47.10



Prior to gastrulation, the embryo is composed of

an upper and lower layer, the epiblast and

hypoblast, respectively

During gastrulation, epiblast cells move toward themidline of the blastoderm and then into the

embryo toward the yolk


G astr ulation in Chicks



The midline thickens and is called the primitive

streak

The hypoblast cells contribute to the sac that

surrounds the yolk and a connection between theyolk and the embryo, but do not contribute to the

embryo itself




Future ectoder m

Migr atingcells(mesoder m)

Blastocoel

Epiblast

YOLK

Endoder mHypoblast

Primitive streak

Fer tilized eggPrimitivestreak

Embryo

Yolk

Figure 47.11



Human eggs have very little yolk

A blastocyst is the human equivalent of the

blastula

The inner cell mass is a cluster of cells at oneend of the blastocyst

The trophoblast is the outer epithelial layer of the

blastocyst and does not contribute to the embryo,

but instead initiates implantation


G astr ulation in H umans



Following implantation, the trophoblast

continues to expand and a set of

extr aembryonic membr anes is formed

These enclose specialized structures outside of the embryo

Gastrulation involves the inward movement from

the epiblast, through a primitive streak, similar

to the chick embryo




Blastocyst reaches uter us.1

2

3

4

Blastocyst implants(7 days after f er tilization).

Extr aembryonic membr anesstar t to f or m (10±11 days),and gastr ulation begins(13 days).

Gastr ulation has produced athree-layered embryo withf our extr aembryonicmembr anes.

Uter us

Mater nalbloodvessel

Endometrial epithelium(uterine lining)

Inner cell mass

Trophoblast

Blastocoel

Expanding region of trophoblast

Epiblast

Hypoblast

Trophoblast

Expanding region of

trophoblastAmniotic cavity

Epiblast

Hypoblast

Yolk sac (f rom hypoblast)

Extr aembryonic mesoder m cells(f rom epiblast)

Chorion (f rom trophoblast)

Amnion

Chorion

Ectoder m

Mesoder m

Endoder m

Yolk sac

Extr aembryonic mesoder m

Allantois

Figure 47.12



Developmental Adaptations of Amniotes

The colonization of land by vertebrates was made

possible only after the evolution of

± The shelled egg of birds and other reptiles as well

as monotremes (egg-laying mammals)

± The uterus of marsupial and eutherian mammals




In both adaptations, embryos are surrounded

by fluid in a sac called the amnion

This protects the embryo from desiccation and

allows reproduction on dry land Mammals and reptiles including birds are

called amniotes for this reason




The four extraembryonic membranes that formaround the embryo

± The chorion functions in gas exchange

± The amnion encloses the amniotic fluid ± The yolk sac encloses the yolk

± The allantois disposes of waste products and

contributes to gas exchange




Organogenesis

During organogenesis, various regions of the

germ layers develop into rudimentary organs

Early in vertebrate organogenesis, the notochor d

forms from mesoderm, and the neural plate formsfrom ectoderm


Fi 4 13



Figure 47.13

Neur al f olds

1 mm

Neur al

f old

Neur al

plate

Notochor d

Ectoder m

Mesoder m

Endoder m

Ar chenteron

(a) Neur al plate f or mation

(b) Neur al tube f or mation

(c) Somites

Neur alf old

Neur al plate

Neur alcrest cells

Outer layer of ectoder m

Neur alcrest cells

Neur altube

Eye Somites Tail bud

SEM

Neur al tube

Notochor dCoelom

Neur alcrest

cells

Somite

Ar chenteron(digestive

cavity)

1 mm



The neural plate soon curves inward, forming theneur al tube

The neural tube will become the central nervous

system (brain and spinal cord)




Neur al crest cells develop along the neural tubeof vertebrates and form various parts of the

embryo (nerves, parts of teeth, skull bones, and so

on)

Mesoderm lateral to the notochord forms blocks

called somites

Lateral to the somites, the mesoderm splits to form

the coelom (body cavity)




Organogenesis in the chick is quite similar to thatin the frog


Figure 47 14



Figure 47.14

Neur al tube

Notochor d

Ar chenteron

Later alf old

These layersf or m extr aembryonicmembr anes.

Yolk stalk

YOLK

Yolk sac

Somite

Coelom

Endoder m

Mesoder m

Ectoder m

(a) Ear ly organogenesis (b) Late organogenesis

Eye

Forebr ain

Hear t

Bloodvessels

Somites

Neur altube



The mechanisms of organogenesis ininvertebrates are similar, but the body plan is very

different

For example, the neural tube develops along theventral side of the embryo in invertebrates, rather

than dorsally as occurs in vertebrates




Mechanisms of Morphogenesis

Morphogenesis in animals but not plants involvesmovement of cells




Reorganization of the cytoskeleton is a major forcein changing cell shape during development

For example, in neurulation, microtubules oriented

from dorsal to ventral in a sheet of ectodermalcells help lengthen the cells along that axis

© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

The Cytoskeleton in Morpho g enesis

Figure 47 15-5E t d



Figure 47.15 5Ectoder m

Neur alplate

Microtubules

Actinf ilaments

Neur al tube



The cytoskeleton promotes elongation of thearchenteron in the sea urchin embryo

This is convergent extension, the

rearrangement of cells of a tissue that cause it tobecome narrower (converge) and longer (extend)

Convergent extension occurs in other developmental processes

The cytoskeleton also directs cell migration


Figure 47 16



Figure 47.16



Programmed cell death is also called apoptosis

At various times during development, individualcells, sets of cells, or whole tissues stopdeveloping and are engulfed by neighboring cells

For example, many more neurons are produced indeveloping embryos than will be needed

Extra neurons are removed by apoptosis


Pro g rammed Cell Death



Concept 47.3: Cytoplasmic determinants

and inductive signals contribute to cellfate specification

Deter mination is the term used to describe the

process by which a cell or group of cells becomescommitted to a particular fate

Diff erentiation refers to the resulting

specialization in structure and function




Cells in a multicellular organism share the samegenome

Differences in cell types are the result of the

expression of different sets of genes




Fate Mapping

Fate maps are diagrams showing organs andother structures that arise from each region of an

embryo

Classic studies using frogs indicated that cell

lineage in germ layers is traceable to blastula cells


Epidermis EpidermisFigure 47.17



Epider mis Epider misCentr alnervoussystem

Notochor d

Mesoder m

Endoder m

Blastula Neur al tube stage(tr ansverse section)

(a) Fate map of a f rog embryo

64-cell embryos

Blastomeres

injected with dye

Larvae

(b) Cell lineage analysis in a tunicate



Later studies of C. elegans used the ablation(destruction) of single cells to determine the

structures that normally arise from each cell

The researchers were able to determine the

lineage of each of the 959 somatic cells in the

worm


ZygoteFigure 47.18



First cell division

Zygote

Hatching T i m e

a f t e r f e r t i l i z a

t i o n ( h o u r s )

Intestine

Intestine

Mouth

Eggs Vulva

Anus

1.2 mmANTERIOR POSTERIOR

Nervous

system,outer skin,

muscula-

ture

Muscula-

ture, gonads

Outer skin,

nervous system

Ger m line

(futuregametes)

Musculature

10

0



Germ cells are the specialized cells that give riseto sperm or eggs

Complexes of RNA and protein are involved in the

specification of germ cell fate

In C. elegans, such complexes are called P

granules, persist throughout development, and

can be detected in germ cells of the adult worm


Figure 47.19



100 Qm



P granules are distributed throughout the newlyfertilized egg and move to the posterior end before

the first cleavage division

With each subsequent cleavage, the P granules

are partitioned into the posterior-most cells

P granules act as cytoplasmic determinants, fixing

germ cell fate at the earliest stage of development


Figure 47.20 20 Qm



Newly f er tilized egg

Zygote prior to f irst division

Two-cell embryo

Four -cell embryo

Q

2

1

3

4



Axis F ormation

A body plan with bilateral symmetry is foundacross a range of animals

This body plan exhibits asymmetry across the

dorsal-ventral and anterior-posterior axes

The right-left axis is largely symmetrical




The anterior-posterior axis of the frog embryo isdetermined during oogenesis

The animal-vegetal asymmetry indicates where

the anterior-posterior axis forms

The dorsal-ventral axis is not determined until

fertilization




Upon fusion of the egg and sperm, the eggsurface rotates with respect to the inner cytoplasm

This cortical rotation brings molecules from one

area of the inner cytoplasm of the animal

hemisphere to interact with molecules in the

vegetal cortex

This leads to expression of dorsal- and ventral-

specific gene expression


DorsalFigure 47.21



Right

Anterior Posterior

Ventr al

Left

(a) The three axes of the fully developed embryo

(b) Establishing the axes

Animalhemisphere

Vegetalhemisphere

Animal pole

Vegetal pole

Point of sper mnucleusentry

Gr ay crescent

Pigmentedcor tex

Future

dorsalside

Firstcleavage



In chicks, gravity is involved in establishing theanterior-posterior axis

Later, pH differences between the two sides of the

blastoderm establish the dorsal-ventral axis

In mammals, experiments suggest that orientation

of the egg and sperm nuclei before fusion may

help establish embryonic axes




Restrictin g Developmental Potential

Hans Spemann performed experiments todetermine a cell¶s developmental potential (range

of structures to which it can give rise)

Embryonic fates are affected by distribution of

determinants and the pattern of cleavage

The first two blastomeres of the frog embryo are

totipotent (can develop into all the possible cell

types)


EXPERIMENTFigure 47.22-2



Control egg(dorsal view)

2

1a 1b

Gr aycrescent

Control

group

Experimental

group

Experimental egg(side view)

Gr aycrescent

Thread

Nor malNor malBelly piece

EXPERIMENT

RESULTS



In mammals, embryonic cells remain totipotentuntil the 8-cell stage, much longer than other

organisms

Progressive restriction of developmental potential

is a general feature of development in all animals

In general tissue-specific fates of cells are fixed by

the late gastrula stage




Cell Fate Determination and Pattern

Formation by Inductive Signals As embryonic cells acquire distinct fates, they

influence each other¶s fates by induction




The ³Or g anizer´ of Spemann and Man g old

Spemann and Mangold transplanted tissuesbetween early gastrulas and found that the

transplanted dorsal lip triggered a second

gastrulation in the host

The dorsal lip functions as an organizer of the

embryo body plan, inducing changes in

surrounding tissues to form notochord, neural

tube, and so on


Figure 47.23



Dorsal lip of blastopore

Pigmentedgastr ula(donor embryo)

Nonpigmentedgastr ula

(recipient embryo)

Primary embryo

Secondary

(induced) embryoPrimary str uctures:

Neur al tubeNotochor d

Secondary str uctures:

Notochor d (pigmented cells)

Neur al tube(mostly nonpigmented cells)

EXPERIMENT RESULTS



F ormation of the Vertebrate Limb

Inductive signals play a major role in patter nf or mation, development of spatial organization

The molecular cues that control pattern formation

are called positional inf or mation

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 vertebratelimbs, begin as bumps of tissue called limb buds


Figure 47.24



Limb buds

50 Qm

Anterior

Limb bud

AER

ZPA

Posterior

Apicalectoder malridge (AER)

(a) Organizer regions (b) Wing of chick embryo

Digits

Anterior

Proximal

Dorsal

Posterior

Ventr al

Distal

2

34



The embryonic cells in a limb bud respond topositional information indicating location along

three axes

± Proximal-distal axis

± Anterior-posterior axis

± Dorsal-ventral axis




One limb bud±regulating region is the apicalectoder mal ridge (AER)

The AER is thickened ectoderm at the bud¶s tip

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

The ZP A is mesodermal tissue under the

ectoderm where the posterior side of the bud is

attached to the body




Tissue transplantation experiments support thehypothesis that the ZP A produces an inductive

signal that conveys positional information

indicating ³posterior´


Figure 47.25

A t i

EXPERIMENT



Donor limbbud

Hostlimbbud

ZPA

Anterior

Posterior

New ZPA

4

4

3

3

22

RESULTS



Sonic hedgehog is an inductive signal for limbdevelopment

Hox genes also play roles during limb pattern

formation

© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.



Cilia and Cell F ate

Ciliary function is essential for proper specificationof cell fate in the human embryo

Motile cilia play roles in left-right specification

Monocilia (nonmotile cilia) play roles in normalkidney development


Figure 47.26



Lungs

Hear t

Liver

Spleen

Stomach

Large intestine

Nor mal locationof inter nal organs

Location insitus inversus

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47 Lecture Presentation PC 2012 (1)