Animal Development - Okanagan Mission Secondary -...

Animal DevelopmentCHAPTER 38

Chapter 38 Animal Development

Key Concepts

38.1 Fertilization Activates Development

38.2 Cleavage Creates Building Blocks and Produces a Blastula

38.3 Gastrulation Produces a Second, then a Third Germ Layer

38.4 Gastrulation Sets the Stage for Organogenesis and Neurulation in

Chordates

38.5 Extraembryonic Membranes Protect and Nourish the Embryo

38.6 Development Continues throughout Life

Fertilization Activates Development

How do organisms develop from a single

cell?

Early investigators assumed that a tiny “homunculus” was encapsulated in a

human egg or sperm.

Egg and Sperm make different

contributions to the zygote

Development begins with fertilization, the union of sperm and egg to form

a zygote.

Egg cells are large—they contain the materials and information needed to

initiate and maintain early development.



Many sperm are attracted to an egg, but

only one is allowed entry.

Activation of an egg by a sperm triggers

changes in the egg cell membrane and the

release of ions.

A series of events is triggered:

Entry of additional sperm is blocked

Metabolic rate rises

Protein synthesis is initiated

Cytoplasm is reorganized



The sperm nucleus and centrosome are transferred to the egg.

The centrosome plays a major role in forming the zygote centrosome and organizing the zygote’s microtubules.

When egg and sperm nuclei join, the fertilized egg becomes a diploid

zygote.

Polarity is Established Early in

Development – out of scope

Anterior–posterior and dorsal–ventral polarity of an animal’s body is

established early in development by various mechanisms.

In amphibian eggs, the yolk material is concentrated in the lower half

(vegetal hemisphere). Sperm binding sites are in the upper half (animal

hemisphere), which is pigmented near the surface.

Polarity is Established Early in

Development

Entry of the sperm causes the cortical cytoplasm to shift, forming the gray

crescent. This sets up the polarities.

Early experiments, in which amphibian eggs were bisected, showed that

cytoplasmic determinants in the gray crescent are necessary for

establishing polarity and development of a normal salamander.

Figure 38.1 The Gray Crescent

Cleavage Creates

Building Blocks and

Produces a Blastula38.2

Cleavage and Blastula

Cleavage—rapid series of cell division, but no cell growth

Cells increase in number but get smaller and smaller.

The cells are called blastomeres; the ball of cells is the blastula.

Early stage events are controlled by maternal factors: cytoplasmic

determinants in the egg. mRNA from the mother, already present in the

egg, is used to direct protein synthesis.


The cytoskeleton may

redistribute these factors; cell

divisions also distribute them to

different blastomeres.

At some point, the fate of a

blastomere becomes restricted

or determined.

Experiments in which

blastomeres are separated at

different stages reveal the fates

of each cell. Blastomeres can

also be dyed or labeled in

some way to follow their fates.


As cleavage continues, the zygote becomes divided into progressively

smaller blastomeres:


Complete cleavage: cytoplasm is

completely divided by each cell division

Radial cleavage and spiral cleavage

patterns are seen in eggs with little yolk

and complete cleavage.

The plane of cleavage is determined by

the orientation of the mitotic spindle.

Radial cleavage at the 8-cell stage:


Spiral cleavage at the 8-cell stage:

Specific Blastomeres Generate

Specific Tissues and Organs – Out of Scope

Blastomeres become determined—committed to specific fates—at

different times in different species.

Mosaic development: specific blastomeres give rise to specific tissues and

organs of the adult animal


Specific Tissues and Organs

In many spirally cleaving eggs, a polar

lobe forms at the vegetal pole that

contains yolk and cytoplasmic

determinants.

If the polar lobe is removed, an

abnormal larva develops.

If the two blastomeres are

experimentally separated, two

abnormal larvae develop, each lacking

very specific structures.


Specific Tissues and Organs

In regulative development, cells can compensate for lost cells.

Distribution of cytoplasmic determinants happens later as cell divisions continue.

This type occurs in vertebrates.

The Amount of Yolk Affects Cleavage

The amount of yolk in the egg impacts developmental processes.

In moderately yolky eggs, cleavage is complete, but cells divide more slowly in

the yolky vegetal hemisphere, resulting in an animal hemisphere consisting of

more small blastomeres and a vegetal hemisphere consisting of fewer,

larger blastomeres.

The Amount of Yolk Affects Cleavage

In very yolky eggs, the yolk does not

divide, and the embryo forms as a

small blastodisc of cleaving cells on

the yolk surface.

In some insects such as Drosophila,

the zygote undergoes nuclear

divisions but no cytoplasm divisions.

The early embryo is a syncytium.

As development proceeds, nuclei

migrate to the periphery of the egg,

and cell membranes form.

Cleavage in Placental Mammals is

Unique - Essential

Placental mammals have non-yolky eggs and rotational cleavage. At the

2nd division, blastomeres divide in different planes:


Unique

In mammals, at the 4th cleavage the cells

separate into two groups:

• Inner cell mass will become the embryo

• Surrounding outer cells become a sac

called the trophoblast; at this stage, the

embryo is called a blastocyst

Trophoblast cells secrete fluid, creating

a blastocoel. Later they contribute to

the placenta.


Unique

In mammals, fertilization occurs in

the upper oviduct; cleavage takes

place as the zygote travels down

the oviduct.

When the blastocyst arrives in the

uterus, it adheres to the lining

(endometrium) and burrows in

(implantation).

Trophoblast cells send out

projections (chorionic villi) to

increase connection to maternal

blood.

Figure 38.8 A Human Blastocyst at Implantation (A) The mammalian blastocyst consists of an inner cell mass adjacent to a fluid-filled blastocoel and surrounded by trophoblast cells. (B) Molecules and enzymes secreted by trophoblast cells allow the blastocyst to adhere to and burrow into the endometrium. Once the blastocyst is implanted in the uterine wall, the trophoblast cells send out chorionic villi—projections that increase the embryo’s area of contact with the maternal bloodstream.

Gastrulation Produces a

Second, then a Third

Germ Layer38.3

Gastrulation Produces a Second, then

a Third Germ Layer

Gastrulation involves movement of cells in the embryo.

The three germ layers of triploblastic animals are formed; the embryo becomes

a gastrula.


a Third Germ Layer

In sea urchins (little yolk, radial cleavage):

Vegetal pole invaginates and forms the archenteron, or gut; opening is the

blastopore

Outer layer of cells is now ectoderm, and archenteron wall is endoderm and

future mesoderm


a Third Germ Layer

Amphibian eggs (moderate amount of yolk):

Cells in the gray crescent invaginate to form a blastopore.

Cells from the animal pole begin to roll like a sheet over the dorsal lip of the

blastopore and push into the blastocoel.

Animal hemisphere cells grow over the other cells.

Figure 38.10 Gastrulation in a Frog

Embryo

Gastrulation in Birds and Reptiles

Birds and reptiles (eggs with lots of yolk):

Blastopore forms as a groove in the primitive streak, a collection of cells with Hensen’s node at the anterior end.

Cells that will become mesoderm and endoderm migrate inward along the

streak.

Gastrulation

Placental mammals retain the bird gastrulation pattern.

The inner cell mass splits into an upper layer (epiblast) and a lower layer

(hypoblast).

Embryo forms from the epiblast; hypoblast contributes to the

extraembryonic membranes and placenta.

Gastrulation

All three types of gastrulation are found in both protostomes and

deuterostomes.

In protostomes, the blastopore becomes the mouth and the anus forms at

some distance away.

In deuterostomes, the blastopore becomes the anus, and the mouth

breaks through secondarily.

Gastrulation

Gastrulation results in three

germ layers and sets the

stage for formation of the

coelom, a space completely

enclosed in mesoderm.

In deuterostomes, the

mesoderm often forms from

the archenteron wall, and the

coelom develops within in it.

Concept 38.3 Gastrulation Produces a

Second, then a Third Germ Layer

In protostomes, the pattern fits with mosaic development:

Divisions of one blastomere form the mesoderm.

The coelom forms as a split in the prospective mesoderm cells.

Gastrulation Sets the

Stage for Organogenesis

and Neurulation in

Chordates38.4

Organogenesis and Neurulation in

Chordates

Organogenesis: developmental phase when all organs and organ systems

form simultaneously

Neurulation: formation of the nerve cord and brain

The three germ layers give rise to defined tissues and organs.


Chordates

Ectoderm forms epidermis of the

skin and related structures, brain,

and nervous system.

Endoderm forms lining of the

digestive tract, lungs, liver,

pancreas, and gall bladder.

Mesoderm gives rise to the

notochord, heart, blood and blood

vessels, urogenital system, muscles,

bones, and the dermis (inner layer

of the skin).


Chordates

When a cell displays its final specialized characteristics, it is said to be

differentiated.

Differentiation takes place gradually throughout development and often

into adult life.

The Notochord Induces Formation of

the Neural Tube

In chordates, some of the cells that move through the blastopore during

gastrulation form the notochord.

Notochord provides structural support and induces overlying ectoderm to

form the dorsal nerve cord.

Induction: one tissue directs another tissue along a developmental pathway by chemical signaling


the Neural Tube

The ectoderm over the notochord

flattens and rolls up to form the neural

tube.

At the anterior end, three swellings in

the neural tube will become the

hindbrain, midbrain, and forebrain.

The rest of the neural tube will become

the spinal cord.

Figure 38.13 Gastrulation, Mesoderm Formation,

and Neurulation in the Lancelet Amphioxus, a Non-

vertebrate Chordate (Part 1)

Figure 38.13 Gastrulation, Mesoderm Formation,

and Neurulation in the Lancelet Amphioxus, a Non-

vertebrate Chordate (Part 2)


the Neural Tube

The transcription factor β-catenin plays a key role in determining which

cells become the organizer.

It triggers a complex series of interactions between transcription factors

and growth factors that control gene expression.


the Neural Tube

Development of the amphibian nervous system

Polypeptides (Noggin and Chordin), released from the notochord, induce

ectoderm cells to become neuronal.

They inhibit activity of the TGF-β bone morphogenetic protein BMP4.

BMP4 induces the ectoderm to form epidermis. In the absence of BMP4,

ectoderm forms neural tissue.


the Neural Tube

Another inducer released by the notochord is the transcription factor Sonic

hedgehog (Shh).

It directs the development of the ventral region of the neural tube into the

ventral structures and circuits of the spinal cord.


the Neural Tube

As the neural tube is closing, certain cells dissociate from it and come to lie

between the neural tube and overlying epidermis.

These neural crest cells are pluripotent—they can differentiate into many cell

types.

They form sensory neurons and major parts of the autonomic nervous

system, most of the skull bones, pigment cells, and many other structures.

Figure 38.16 Neurulation and

Differentiation of Mesoderm in

Vertebrates (Part 1)


the Neural Tube

On either side of the notochord,

three regions of mesoderm can

be recognized—somites,

intermediate mesoderm, and

lateral plate mesoderm.

Body segments are first seen as

the somites. They form bones,

cartilage, skeletal muscle, and

dermis of the skin.


the Neural Tube

Intermediate mesoderm forms the urinary and reproductive system.

Lateral plate mesoderm surrounds the coelom and lines the vertebrate

peritoneal cavity.

Inner layer of lateral plate mesoderm forms part of the peritoneum,

muscles of the digestive tract, most of the circulatory system, including the

heart.

Outer layer forms part of the peritoneum and some muscles of the body

wall.

Done!2 NEXT SECTIONS ARE OUT OF SCOPE!

Extraembryonic

Membranes Protect and

Nourish the Embryo38.5

Concept 38.5 Extraembryonic

Membranes Protect and Nourish the

Embryo

Mammals and reptiles including birds have amniotic eggs, which provide

the embryo with a contained aqueous environment.

Four extraembryonic membranes develop from the germ layers and

surround the embryo.

They function in protection, nutrition, gas exchange, and waste removal.



Embryo

In the chick embryo, four membranes form:

• Yolk sac—encloses yolk; can form blood vessels

Yolk is digested by the endoderm of the yolk sac and transported to the embryo by the blood vessels in the yolk sac wall.

• Allantois—sac for waste storage; also has blood vessels



Embryo

Amnion—surrounds the embryo and secretes fluid into the enclosed cavity, providing a protective, shock-absorbing environment

Chorion—just beneath the egg shell; limits water loss; fuses with the allantois to

form a membrane for exchange of O2 and CO2

Figure 38.18 The Extraembryonic

Membranes of Amniotes (Part 1)



Embryo

In placental mammals, hypoblast cells proliferate to form what in birds

would be the yolk sac.

The allantois and chorion combine, forming the chorioallantoic placenta,

which combines with the endometrium to form the placenta.

The placenta is unique because is contains tissues from two organisms—

the mother and the fetus.

Figure 38.19 The Mammalian

Placenta



Embryo

Human gestation is divided into trimesters of about 12 weeks each.

The first trimester is a time of rapid cell division and tissue differentiation

and the embryo is very sensitive to damage from radiation, drugs,

chemicals, and pathogens.

By the end of the first trimester, most organs have started to form and the

embryo becomes a fetus.



Embryo

Many fish have embryonic yolk sacs.

As the embryo forms, all three germ layers grow around the yolk.

The yolk sac becomes vascularized and materials are carried in the blood

vessels to the embryo.

Figure 38.20 Fish Yolk Sac

Concept 38.6 Development Continues

throughout Life

Development does not stop at birth or hatching.

Some animals stop growing at adulthood, some keep growing throughout

their lives.

Direct development: animal immediately looks like the parent and grows to reach full size

Indirect development: life stages have very different forms

Development Continues

throughout Life38.6


throughout Life

Allometry: change in proportions of body parts relative to one another

due to unequal growth

Isometric growth is a 1:1 size increase.

Allometry is common in direct developers, such as humans.

Human head to trunk ratio changes; the baby’s head is relatively larger.

Figure 38.21 Allometric Growth of a

Human


throughout Life

Indirect development can involve drastic changes in morphology.

Larval forms are often quite unlike adult forms; for example, caterpillars

and butterflies.

Larval and adult stages may have different functions:

Larvae can be the main feeding stage.

Some adult insects do not feed at all and are specialized for reproduction.

Figure 38.22 Larval Forms Are Often

Quite Unlike Their Parents


throughout Life

Larvae can be important in dispersal, especially if adults are sessile or

sedentary.

Example: Corals—new sites are colonized by swimming larvae.


throughout Life

Metamorphosis: major portions of the body may be remodeled or even

discarded

Example: development of amphibian eggs into larvae (tadpoles) and then

adults.

The tadpole undergoes changes in every system—tail and gills are resorbed,

limbs grow, gut changes animal switches from herbivory to carnivory.

Animal Development - Okanagan Mission Secondary -...

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