Chap 47

Animal Development

Fertilization

• The Acrosomal Reaction.– Acrosomal reaction: when exposed to the jelly

coat the sperm’s acrosome discharges it contents by exocytosis.• Hydrolytic enzymes enable the acrosomal process to

penetrate the egg’s jelly coat.

• The tip of the acrosomal process adheres to the vitelline layer just external to the egg’s plasma membrane.

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– The sperm and egg plasma membranes fuse and a single sperm nucleus enter the egg’s cytoplasm.• Na+ channels in the egg’s plasma membrane opens.

– Na+ flows into the egg and the membrane depolarizes: fast block to polyspermy.

Na+

• The Cortical Reaction.– Fusion of egg and sperm plasma membranes triggers a

signal-transduction pathway. IP3 and DAG are produced. IP3 acts as a second messenger and

opens ligand-gated channels in the ER and the Ca2+ released stimulates the opening of other channels.

• Ca2+ from the eggs ER is released into the cytosol and propagates as a wave across the fertilized egg – cortical granules release contents

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• DAG and IP3 are created when a phospholipase cleaves a membrane phospholipid PIP2.

– Phospholipase may be activated by a G protein or a tyrosine-kinase receptor.

– IP3 activates a gated-calcium channel, releasing Ca2+.

Fig. 11.15

The Cortical Reaction• Fusion of egg and sperm also initiates the

cortical reaction– Inducing a rise in Ca2+ that stimulates cortical

granules to release their contents outside the egg

Figure 47.4

A fluorescent dye that glows when it binds free Ca2+ was injected into unfertilized sea urchin eggs. After sea urchin sperm were added, researchers observed the eggs in a fluorescence microscope.

EXPERIMENT

RESULTS

The release of Ca2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release of more and more Ca2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds.CONCLUSION

30 sec20 sec10 sec afterfertilization

1 sec beforefertilization

Point ofspermentry

Spreading waveof calcium ions

500 m

• The acrosomal reaction

Spermnucleus

Sperm plasmamembrane

Hydrolytic enzymes

Corticalgranule

Cortical granulemembrane

EGG CYTOPLASM

Basal body(centriole)

Spermhead

Acrosomalprocess

Actin

Acrosome

Jelly coatEgg plasmamembrane

Vitelline layer

Fused plasmamembranes

Perivitellinespace

Fertilizationenvelope

Cortical reaction. Fusion of the gamete membranes triggers an increase of Ca2+ in the egg’s cytosol, causing cortical granules in the egg to fuse with the plasma membrane and discharge their contents. This leads to swelling of the perivitelline space, hardening of thevitelline layer, and clipping of sperm-binding receptors. The resulting fertilization envelope is the slow block to polyspermy.

5 Contact and fusion of sperm and egg membranes. A hole is made in the vitelline layer, allowing contact and fusion of both gamete plasma membranes. The membrane becomes depolarized, resulting in the

fast block to polyspermy.

3 Acrosomal reaction. Hydrolytic enzymes released from the acrosome make a hole in the jelly coat, while growing actin filaments form the acrosomal process. This structure protrudes from the sperm head and penetrates the jelly coat, bindingto receptors in the egg cell membrane that extend through the vitelline layer.

2 Contact. The sperm cell contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome.

1

Sperm-bindingreceptors

Entry of sperm nucleus.4

Figure 47.3

1 23

Ca++

• Fertilization in Mammals.• Capacitation, a function of the female reproductive

system, enhances sperm function.– A capacitated

sperm migratesthrough a layerof follicle cellsbefore it reachesthe zona pellucida.

– Binding ofthe sperm cellinduces anacrosomalreaction similarto that seen in thesea urchin.

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Cortical reaction forms slow block

Cortical reaction forms slow block

• Enzymes from the acrosome enable the sperm cell to penetrate the zona pellucida and fuse with the egg’s plasma membrane.– The entire sperm enters the egg.

– The egg membrane depolarizes: functions as a fast block to polyspermy.

– A cortical reaction occurs.

• Enzymes from cortical granules catalyze alterations to the zona pellucida: functions as a slow block to polyspermy.

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– The envelopes of both the egg and sperm nuclei disperse.• The chromosomes from the two gametes share a common

spindle apparatus during the first mitotic division of the zygote.

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• Cleavage follows fertilization.– The zygote is partitioned into blastomeres.

• Each blastomere contains different regions of the undivided cytoplasm and thus different cytoplasmic determinants.

3. Cleavage partitions the zygote into many smaller cells

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

Figure 47.7a–d

Fertilized egg. Shown here is thezygote shortly before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center.

(a) Four-cell stage. Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division.

(b) Morula. After further cleavage divisions, the embryo is a multicellular ball that is stillsurrounded by the fertilization envelope. The blastocoel cavityhas begun to form.

(c) Blastula. A single layer of cells surrounds a large blastocoel cavity. Although not visible here, the fertilization envelope is still present; the embryo will soon hatch from it and begin swimming.

(d)

– Except for mammals, most animals have both eggs and zygotes with a definite polarity.• Thus, the planes of division follow a specific pattern

relative to the poles of the zygote.

• Polarity is defined by the heterogeneous distribution of substances such as mRNA, proteins, and yolk.– Yolk is most concentrated at the vegetal pole and least

concentrated at the animal pole.

• In some animals, the animal pole defines the anterior end of the animal.

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• In amphibians a rearrangement of the egg cytoplasm occurs at the time of fertilization.

• The plasma membraneand cortex rotatetoward the pointof sperm entry.– The gray crescent

is exposed and marksthe dorsal surfaceof the embryo.

• Cleavage occurs morerapidly in the animalpole than in thevegetal pole.

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Fig. 47.7

Forms dorsal side of animal

Forms dorsal side of animal

Sperm’s basal body turns into centrosome

Sperm’s basal body turns into centrosome

First cleavage is perpendicular through gray crescent area

First cleavage is perpendicular through gray crescent area

Axes of body established before first cleavage

Axes of body established before first cleavage

• In both sea urchins and frogs first two cleavages are vertical.

• The third division is horizontal.

• The result is an eight-celled embryo with two tiers of four cells.

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Fig. 47.8a

• Continued cleavage produces the morula.

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Fig. 47.8b

• A blastocoel forms within the morula blastula

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Fig. 47.8d

• In animals with less yolk there is complete division of the egg: holoblastic cleavage.

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Yolk impedes cleavage establishing vegetal pole

Yolk impedes cleavage establishing vegetal pole

• Gastrulation in a sea urchin– Produces an

embryo with a primitive gut and three germ layers

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

Figure 47.12

SURFACE VIEW CROSS SECTIONAnimal pole

Blastocoel

Dorsal lipof blastopore

Dorsal lipof blastoporeVegetal pole Blastula

Blastocoelshrinking

Archenteron

Blastocoelremnant

EctodermMesoderm

Endoderm

GastrulaYolk plugYolk plug

Key

Future ectoderm

Future mesoderm

Future endoderm

Gastrulation begins when a small indented crease, the dorsal lip of the blastopore, appears on one side of the blastula. The crease is formed by cellschanging shape and pushing inward from the surface (invagination). Additional cells then rollinward over the dorsal lip (involution) and move intothe interior, where they will form endoderm andmesoderm. Meanwhile, cells of the animal pole, the future ectoderm, change shape and begin spreading over the outer surface.

The blastopore lip grows on both sides of the embryo, as more cells invaginate. When the sides of the lip meet, the blastopore forms a circle thatbecomes smaller as ectoderm spreads downward over the surface. Internally, continued involutionexpands the endoderm and mesoderm, and the archenteron begins to form; as a result, the blastocoel becomes smaller.

1

2

3 Late in gastrulation, the endoderm-lined archenteron has completely replaced the blastocoel and the three germ layers are in place. The circular blastopore surrounds a plug of yolk-filled cells.

• Gastrulation in the chick– Is affected by the large amounts of yolk in the egg

Figure 47.13

Epiblast

Futureectoderm

Migratingcells(mesoderm)

Endoderm

Hypoblast

YOLK

Primitivestreak

Organogenesis

• Various regions of the three embryonic germ layers– Develop into the rudiments of organs during

the process of organogenesis

• Early in vertebrate organogenesis– The notochord forms from mesoderm and the

neural plate forms from ectoderm

Figure 47.14a

Neural plate formation. By the timeshown here, the notochord has developed from dorsal mesoderm, and the dorsal ectoderm hasthickened, forming the neural plate, in response to signals from thenotochord. The neural folds arethe two ridges that form the lateral edges of the neural plate. These are visible in the light micrographof a whole embryo.

Neural folds

1 mm

Neuralfold

Neuralplate

Notochord

Ectoderm

Mesoderm

Endoderm

Archenteron

(a)

LM

• The neural plate soon curves inward– Forming the neural tube

Figure 47.14b

Formation of the neural tube. Infolding and pinching off of the neural plate generates the neural tube. Note the neural crest cells, which will migrate and give rise to numerousstructures.

Neuralfold

Neural plate

Neural crest

Outer layer of ectoderm

Neural crest

Neural tube

(b)

Organogenesis involves folds, splits, and dense clustering of cellsOrganogenesis involves folds, splits, and dense clustering of cells

• Mesoderm lateral to the notochord– Forms blocks called somites

• Lateral to the somites– The mesoderm splits to form the coelom

Figure 47.14c

Somites. The drawing shows an embryoafter completion of the neural tube. By this time, the lateral mesoderm hasbegun to separate into the two tissuelayers that line the coelom; the somites, formed from mesoderm, flank thenotochord. In the scanning electron micrograph, a side view of a whole embryo at the tail-bud stage, part of the ectoderm has been removed, revealingthe somites, which will give rise to segmental structures such as vertebrae and skeletal muscle.

Eye SomitesTail bud

1 mmNeural tube

Notochord Neuralcrest

Somite

Archenteron(digestive cavity)

Coelom

(c)

SEM

• Organogenesis in the chick– Is quite similar to that in the frog

Figure 47.15a, b

Neural tube

Notochord

Archenteron

Lateral fold

Form extraembryonicmembranes

YOLKYolk stalk

Somite

Coelom

Endoderm

Mesoderm

Ectoderm

Yolk sac

Eye

Forebrain

Heart

Blood vessels

Somites

Neural tube

Early organogenesis. The archenteron forms when lateral folds pinch the embryo away from the yolk. The embryo remains opento the yolk, attached by the yolk stalk, about midway along its length,as shown in this cross section. The notochord, neural tube, and somites subsequently form much as they do in the frog.

(a) Late organogenesis. Rudiments of most major organs have already formed in this chick embryo, which is about 56 hours old and about 2–3 mm long (LM).

(b)

Meroblastic cleavage due to yolk

Meroblastic cleavage due to yolk

In birds the yolk is so plentiful that it restricts cleavage to the animal pole: meroblastic cleavage.

• Many different structures– Are derived from the three embryonic germ

layers during organogenesis

Figure 47.16

ECTODERM MESODERM ENDODERM

• Epidermis of skin and itsderivatives (including sweatglands, hair follicles)

• Epithelial lining of mouthand rectum

• Sense receptors inepidermis

• Cornea and lens of eye• Nervous system• Adrenal medulla• Tooth enamel• Epithelium or pineal and

pituitary glands

• Notochord• Skeletal system• Muscular system• Muscular layer of stomach, intestine, etc.• Excretory system• Circulatory and lymphatic

systems• Reproductive system

(except germ cells)• Dermis of skin• Lining of body cavity• Adrenal cortex

• Epithelial lining ofdigestive tract

• Epithelial lining ofrespiratory system

• Lining of urethra, urinarybladder, and reproductivesystem

• Liver• Pancreas• Thymus• Thyroid and parathyroid

glands

Develops into embryo

Will form fetal portion of placenta

Forms yolk sacForms yolk sac

The Cytoskeleton, Cell Motility, and Convergent Extension

• Changes in the shape of a cell– Usually involve reorganization of the

cytoskeleton

• The formation of the neural tube– Is affected by microtubules and microfilaments

Figure 47.19

Microtubules help elongatethe cells of the neural plate.1

Pinching off of the neural plate forms the neural tube.4

Ectoderm

Neuralplate

Microfilaments at the dorsal end of the cells may then contract,deforming the cells into wedge shapes.

Cell wedging in the opposite direction causes the ectoderm to form a “hinge.”

2

3

• The cytoskeleton also drives cell migration, or cell crawling– The active movement of cells from one place to

another

• In gastrulation, tissue invagination – Is caused by changes in both cell shape and cell

migration

• Cell crawling is also involved in convergent extension– A type of morphogenetic movement in which

the cells of a tissue become narrower and longer

Figure 47.20Conve

rgence

Extension

Convergent Extension

cell crawling results in morphogenic movement

may involve the ECM (extracellular matrix) which is the mixture of glycoproteins outside the plasma membrane

ECM fibers may acts as tracks that act as routes for migrating cells in morphogenic movement or they inhibit migration in certain directions

Convergent Extension

cell crawling results in morphogenic movement

may involve the ECM (extracellular matrix) which is the mixture of glycoproteins outside the plasma membrane

ECM fibers may acts as tracks that act as routes for migrating cells in morphogenic movement or they inhibit migration in certain directions

Example

fibronectin lines the the roof of the blastocoel and acts as a guide to a sheet of migrating mesoderm cells.

Example

fibronectin lines the the roof of the blastocoel and acts as a guide to a sheet of migrating mesoderm cells.

Roles of the Extracellular Matrix and Cell Adhesion Molecules

• Fibers of the extracellular matrix– May function as tracks, directing migrating

cells along particular routes

• Several kinds of glycoproteins, including fibronectin– Promote cell migration by providing specific

molecular anchorage for moving cells

Figure 47.21

EXPERIMENT Researchers placed a strip of fibronectin on an artificial underlayer. After positioning migratory neural crest cells at one end of the strip, the researchers observed the movement of the cells in a light microscope.

CONCLUSION

RESULTS In this micrograph, the dashed lines indicate the edges of the fibronectin layer. Note that cells are migrating along the strip, not off of it.

Fibronectin helps promote cell migration, possibly by providing anchorage for the migrating cells.

Direction of migration50 µm

• Cell adhesion molecules – Also contribute to cell migration and stable

tissue structure

• One important class of cell-to-cell adhesion molecule is the cadherins– Which are important in the formation of the

frog blastula

Figure 47.22 CONCLUSION

EXPERIMENT Researchers injected frog eggs with nucleic acid complementary to the mRNA encoding a cadherin known as EP cadherin. This “antisense” nucleic acid leads to destruction of the mRNA for normal EP cadherin, so no EP cadherin protein is produced. Frog sperm were then added to control (noninjected) eggs and to experimental (injected) eggs. The control and experimental embryos that developed were observed in a light microscope.

RESULTS As shown in these micrographs, fertilized control eggs developed into normal blastulas, but fertilized experimental eggs did not. In the absence of EP cadherin, the blastocoel did not form properly, and the cells were arranged in a disorganized fashion.

Control embryo

Experimental embryo

Proper blastula formation in the frog requires EP cadherin.

CAM or cell adhesion molecules are glycoproteins that help regulate morphogenic movements and tissue building

CAM or cell adhesion molecules are glycoproteins that help regulate morphogenic movements and tissue building

Cadherin is a CAM that is produced at specific times and locations during development

EP calhedrin can interfere with the blastula formation in frogs

Cadherin is a CAM that is produced at specific times and locations during development

EP calhedrin can interfere with the blastula formation in frogs

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

• Coupled with morphogenetic changes– Development also requires the timely differentiation

of many kinds of cells at specific locations

Two general principlesUnderlie differentiation during embryonic

development

• First, during early cleavage divisions– Embryonic cells must somehow become different

from one another

• Second, once initial cell asymmetries are set up– Subsequent interactions among the embryonic cells

influence their fate, usually by causing changes in gene expression

Somites will give rise to segmental structures such as vertebrae and serially arranged skeletal muscles

Somites will give rise to segmental structures such as vertebrae and serially arranged skeletal muscles

Name the 3 different germ layers

Establishing Cellular Asymmetries

• To understand at the molecular level how embryonic cells acquire their fates– It is helpful to think first about how the 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, capable of

developing into all the cell types found in the adult

• Unevenly distributed cytoplasmic determinants in the egg cell– Are important in establishing the body axes – Set up differences in blastomeres resulting from

cleavage

Blastomeres that receive half or all of the gray crescent develop into normal embryos, but a blastomere that receives none of the gray crescent gives rise to an abnormal embryo without dorsal structures. Spemann called it a “belly piece.”

EXPERIMENT

RESULTS

CONCLUSION The totipotency of the two blastomeres normally formed during the first cleavage division depends on cytoplasmic determinants localized in the gray crescent.

Left (control):Fertilizedsalamander eggswere allowed todivide normally,resulting in thegray crescent beingevenly dividedbetween the twoblastomeres.

Right (experimental):Fertilized eggs wereconstricted by athread so that thefirst cleavage planerestricted the graycrescent to oneblastomere.

Graycrescent

The two blastomeres werethen separated andallowed to develop.

Graycrescent

Normal

Bellypiece Normal

1

2

Figure 47.24

• As embryonic development proceeds– The potency of cells becomes progressively

more limited in all species

Cell Fate Determination and Pattern Formation by Inductive Signals

• Once 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 the results of their most famous experiment– Spemann and Mangold concluded that the

dorsal lip of the blastopore functions as an organizer of the embryo

• The organizer initiates a chain of inductions– That results in the formation of the notochord,

the neural tube, and other organs

Figure 47.25

EXPERIMENT

RESULTS

CONCLUSION

Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the ventral side of the early gastrula of a nonpigmented newt.

During subsequent development, the recipient embryo formed a second notochord and neural tube in the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryorevealed that the secondary structures were formed in part from host tissue.

The transplanted dorsal lip was able to induce cells in a different region of the recipient to form structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo.

Pigmented gastrula(donor embryo)

Dorsal lip ofblastopore

Nonpigmented gastrula(recipient embryo)

Primary embryo

Secondary (induced) embryoPrimarystructures:

Neural tube

Notochord

Secondarystructures:

Notochord (pigmented cells)

Neural tube (mostly nonpigmented cells)

In molluscs and annelids, the first cleavage is determinate, separating vital cytoplasmic constituents

In molluscs and annelids, the first cleavage is determinate, separating vital cytoplasmic constituents

Indeterminant cleavage

Formation of the Vertebrate Limb

• Inductive signals play a major role in pattern formation– The development of an animal’s spatial

organization

• The molecular cues that control pattern formation, called positional information – Tell a cell where it is with respect to the

animal’s body axes– Determine 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

Figure 47.26a

Limb bud

Anterior

AER

ZPAPosterior

Organizer regions. Vertebrate limbs develop fromprotrusions called limb buds, each consisting of mesoderm cells covered by a layer of ectoderm. Two regions, termed the apical ectodermal ridge (AER, shown in this SEM) and the zone of polarizing activity (ZPA), play key organizer roles in limb pattern formation.

(a)

Apicalectodermal

ridge

50 µm

• The embryonic cells within a limb bud – Respond to positional information indicating

location along three axes

Figure 47.26b

Digits

Anterior

Ventral

DistalProximal

DorsalPosterior

Wing of chick embryo. As the bud develops into alimb, a specific pattern of tissues emerges. In the chick wing, for example, the three digits are always present in the arrangement shown here. Pattern formation requires each embryonic cell to receive some kind of positional information indicating location along the three axes of the limb. The AERand ZPA secrete molecules that help provide thisinformation.

(b)

Inductive signals causes pattern formation

Inductive signals causes pattern formation

Positional information in the chick limb bud responds to molecular cues from organizier regions

Positional information in the chick limb bud responds to molecular cues from organizier regions

AER apical

ZPA

• One limb-bud organizer region is the apical ectodermal ridge (AER)– A thickened area of ectoderm at the tip of the bud

• The second major limb-bud organizer region is the zone of polarizing activity (ZPA)– A block of mesodermal tissue located underneath the

ectoderm where the posterior side of the bud is attached to the body

If AER is placed onto other region that is almost completed with development

If AER is placed onto other region that is almost completed with development

• Tissue transplantation experiments– Support the hypothesis that the ZPA produces

some sort of inductive signal that conveys positional information indicating “posterior”

Figure 47.27

EXPERIMENT

RESULTS

CONCLUSION

ZPA tissue from a donor chick embryo was transplanted under the ectoderm in the anterior margin of a recipient chick limb bud.

Anterior

Donorlimbbud

Hostlimbbud

Posterior

ZPA

The mirror-image duplication observed in this experiment suggests that ZPA cells secrete a signal that diffuses from its source and conveys positional information indicating “posterior.” As the distance from the ZPA increases, the signal concentration decreases and hence more anterior digits develop.

New ZPA

In the grafted host limb bud, extra digits developed from host tissue in a mirror-image arrangement to the normal digits, which also formed (see Figure 47.26b for a diagram of a normal chick wing).

ZPA zone of polarizing activity determines anterior-posterior axis

ZPA zone of polarizing activity determines anterior-posterior axisZPA

transplanted

• Signal molecules produced by inducing cells– Influence gene expression in the cells that

receive them– Lead to differentiation and the development of

particular structures

Cells from a patient's bladder are grown in a Petri dish then layered onto a bladder-shaped mold.

Tuesday, April 4, 2006; Posted:

9:37 a.m. EDT (13:37 GMT) Doctors grow organs from patients' own cells

Humans Could Regenerate Tissue Like Newts Do By Switching Off a Single Gene By Clay Dillow Posted 03.16.2010                                                                                                 Regrowing Tissues Like Newts By shutting off a single gene, researchers think humans could regenerate damaged tissue just as newts do. J. Carmichael Scientists have long been stymied by human regenerative healing -- that is, wholesale regrowth of, say, a severed limb -- an ability inherent in some species but lost on humans. But new research suggests the ability to regenerate isn't based on something newts and flatworms have that we don't; rather, it's something we do have that's keeping us from regenerating tissues. Researchers think a gene called p21 may control regenerative healing, and that by switching it off, humans could perform our own regeneration.The new research suggests that the potential to heal without scarring -- or possibly even to regrow a limb, albeit in a limited manner -- may lie dormant in human cells, kept in check by the p21 gene. A group of lab mice engineered to lack p21 were able to regenerate surgically removed tissue to the point that no evidence of the surgery remained. Holes punched in their ears -- a standard procedure for tagging lab animals -- also healed perfectly, leaving behind no traces of scar tissue or previous damage.

Essentially, switching off the p21 gene allows adult cells to behave like pluripotent stem cells, reorienting themselves into whatever kind of tissue they need to be. But naturally there is a give-and-take; p21 is closely intertwined with another gene, p53, a cell-division regulator that, if allowed to run amok, can lead to many types of cancers. The p21 gene acts as a safety valve for p53, stopping cell division in the case of DNA damage. So switching off p21 can allow cells to engage in regenerative healing, but the risks of doing so include rampant cell division (read: cancer). However, in the p21-free lab mice there was no cancer surge as one might expect, but rather an increase in apoptosis, or cell suicide, which directs damaged cells to destroy themselves. So it would appear that by striking some kind of controlled balance between allowing regenerative cells to work, while letting apoptosis regulate out-of-control cell division, could lead to regenerative treatments for humans somewhere down the road.