07 gastrulation Biology

8/10/2019 07 gastrulation Biology

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Gastrulation is the first stage in forming the body plan

Gastrulation occurs by

movement of cells from

the exterior of the

blastula to the interior.

Forms the 3 germ layers

Movement occurs

through a region calledthe blastopore.

Protostomes: in many

animals the blastopore

develops into the mouth

(most animals).

Deuterostomes: in others

it forms the anus and the

mouth develops else

where (vertebrates).

Gastrulat ion in th e sea urchin



How is the characteristic body plan for any organism developed?

Gastrulation: the first step in the process of body formation. It transforms a

complex sphere into 3 basic germ layers from which all other tissues develop.

Ectoderm is the outer layer = forms epidermis and nervous system.

Mesoderm is in the middle and forms a many structures (i.e., heart, muscles).Endoderm is the inner layer and forms the ‘gut’ and related organs.

The first change is to generate the rudiment of the digestive tract, hence the

name gastrulation (gastric = stomach).

2 Questions:1. How does

gastrulation occur

and how does it

differ in different

animals?

2. What are themolecular and

cellular mechanisms

that cause

movement and

rearrangement?



Gastrulation is the first step of morphogenesis

Morphogenesis is the process whereby individual cells undergo complex

movements that generate the organ rudiments. Gastrulation generates the

three basic germ layers from which organs arise.

How do sheets of cells (epithelia) move during gastrulation? 5 methods.

Invagination is the local inward movement of cells from a cavity

Involution is similar, but more dramatic. It is an inward expansion of epithelial

cells around an edge such as the blastpore.



Convergent extension is elongation of an epithelium in one direction while it

shortens in the other direction (stretching taffy). The cells can keep their

relative positions and elongate or they can interdigitate.

Epiboly is spreading movement of an epithelium to a deeper or thinner layer.

Delamination is the s

plitting of one layer into 2 different layers.

Different combinations of these basic movements yield a variety of changes

that characterize gastrulation



How do individual cells move during gastrulation?

Seven basic types of cell movement lead to the changes in epithelial sheets

that characterize gastrulation.

1. Migration is the movement of an individual cell over other cells or a

substrate.

2. Intercalation is wedging of cells between their neighbors. Lateral

intercalation involves lateral movements of cells in the same layer betweenone another = convergent extension. Radial intercalation involves wedging of

2 different layers. This process often leads to epiboly, the surface area of the

epithelium increases while the thickness decreases.



3. Ingression is the movement of individual cells from an epithelium into an

embryonic cavity.

4. Shape changes are coordinated changes in cell shape that cause an

epithelium to invaginate, buckle or undergo convergent extension.

5. Cell division without growth increases the total number of ‘building blocks’

6. Changes in adhesiveness: cells adhere to one another and to extracellularmaterial via different types of specialized attachments. Loss of these

attachments can facilitate cell movement.

7. Programmed cell death (apoptosis): cells undergo death in a programmed

manner in order to sculpt organs into their final appearance (fingers).



What happens during gastrulation in sea urchins?

Sea urchins are studied commonly

because they are transparent

creatures, and it is easy to see

what is happening inside.

Sea urchins develop from a

blastula containing mesomeres,

macromeres, and micromeres.

Each of these layers has a specific

fate during gastrulation and they

always form the same tissue.

Gastrulation begins when theblastula contains about 1000 cells

and has a large fluid filled

blastocoel.



Epithelial cells are well-differentiated. They compose skin and line the body

cavities (ie, the digestive tract). They are polarized. Their apical surface

faces out and their basal surface rests on the basement membrane (extracellular matrix that supports cells). Epithelial cells are closely

connected with adjacent cells by specialized attachments including tight

junctions, gap junctions, and desmosomes.

Embryonic cells are broadly classed as epithelial or mesechymal

Mesenchymal cells are

poorly differentiated andhave the potential to

develop into many

different tissues, including

epithelial cells. They have

a leading edge with

lamellipodia, and a trailingedge. They are not

connected to adjacent

cells but they are in

contact with the

extracellular matrix.



Vegetal plate: the first step in sea urchin gastrulation is formation of the

vegetal plate, a thickening of epithelial cells in the vegetal pole.

Primary mesenchymal cells: these cells

change adhesive properties and the largemicromeres start to migrate into the

blastocoel as free mesenchymal cells

(ingression). Mesenchymal cells are loose

cells that can differentiate into many

different organs.

Archenteron: the primitive gut. The

archenteron is formed in several stages. 1.

the vegetal plate invaginates into the

blastocoel, 2. It elongates by convergent

extension. 3. It hooks up with the front and

is pulled forward, and 4. Involution occurswith movement of cells around the

blastopore and into the archenteron.

What forces drive the process of

gastrulation?



Two models for vegetal plate invagination.

2. Gel swelling model: the

outside surface of the sea urchin

blastula is covered by a hyaline

layer which consists of several

distinct laminations.Cells at the vegetal plate secrete

large amounts of chondroitin

sulfate, a water absorbing

material. The inner hyaline layer

starts to become thickened by

absorption of excess water, and

it causes the vegetal plate to

bend inward.

Both methods may contribute to

invagination of the vegetal plate.

1. Apical constriction model: the epithelial cells of the vegetal plate change

morphology. The apical surface constricts, causing the vegetal plate to bend

inward.



The second phase of gastrulation is caused by

convergent extension of cells into the blastocoel

The invaginated cells of the vegetal plate extend to form a long thin

archenteron. It is unclear as to how this occurs.

Stretching model: the extension could result from the cells changing shape tobecome long and thin.

Cell movement model: invaginated cells could move to extend the length of

the tube.



How does the developing archenteron know which way to grow?

Secondary mesenchymal cells: cells at the tip of the archenteron guide the

progress. They send filopodia (thin extensions) to find the correct area of the

roof. The roof cells send back other filopodia to direct the archenteron whereto go.

This allows the mouth to hook up with the gut.



Gastrulation in Amphibians

Amphibian gastrulation is much more complex than sea urchins, due in part

to the large amount of yolk. There are also more layers in the blastula

Different areas of the gastrula show distinct behavior:Animal cap: about 3 layers deep and derived from the animal hemisphere of

the egg. It expands by epiboly until it covers about half of the surface of the

gastrula.

Non involuting marginal zone: a wide belt of cells 4 to 5 layers deep. It

expands by convergent extension to cover the lower part of the gastrula



Involuting marginal zone: involutes during gastrulation and comes to lie in

the interior. The limit of involution reaches the edge of the blastopore by

the end of gastrulation.

The superficial layer of the involuting zone forms the roof of the

archenteron (the lining of the future gut).

The deep layer of the involuting zone forms the future trunk mesoderm

(muscles of the body)



Deep zone: a ring of cells that migrate towards the animal pole. Deep zone

cells become head mesoderm and heart.

Vegetal base: formed by the large yolky cells in the vegetal hemisphere.

During gastrulation this base is tilted and displaced ventrally. It will form

the floor of the archenteron (gut).

Ectoderm = animal cap and non involuting marginal zone

Mesoderm = deep zone and deep layer of involuting marginal zone

Endoderm = vegetal cells and superficial layer of involuting marginal zone



What causes gastrulation?

What factors are responsible for all of the complex and orchestrated

movements that occur during gastrulation? There are at least 2 hypotheses

and they are not mutually exclusive.

1. Cellular behaviors observed in each gastrula region are caused by local

accumulations of gene products that are already present in the fertilized

oocyte

2. One region of the

blastula could be

determined to organizethe behavior of all other

regions.

The dorsal lip of the

blastopore appears to be

particularly important inregulating the fate of

other areas. 2 genes,

goosecoid and nogg in

seem to mediate this

effect.



Chickens gastrulate differently: they develop from

an ingression of cells from the blastoderm

Blastoderm: a thin layer of blastomeres that are located on top of the yolk.

Area pellucida: the central portion is clear as it is raised from the yolk.Area opaca: the outer area is opaque because it rests on the yolk.

Gastrulation starts in

the rear of the area

pellucida. Cells move

into a mound whichelongates anteriorly to

become the primitive

streak. The center

opens to form the

primitive groove

between 2 primitiveridges. The primitive

pit is the most anterior

region and it is

surrounded by cells

called Henson’s node.



The primitive groove and pit are the site of gastrulation in birds

Crossection of blastoderm (blastula in birds).

Epiblast is the upper layer of epithelial cells, blastocoel is the space below the

epiblast, and hypoblast is the lower layer of epithelial cells.

Epiblast cells

roll over the

primitive ridge

and involuteinto the groove.

The cells lose

contact with

one another and

migrate inwards

by ingression =Mesoderm.

3 germ layers

are established



Germ layers spread to form extra embryonic membranes

Extra embryonic ectoderm: ectoderm spreads by epiboly to form a wide sheet

that surrounds the entire yolk mass.

Extra embryonic mesoderm spreads out below the ectoderm to form blood

vessels

As development continues, the embryo proper is pinched off from the extra

embryonic membranes.



How does gastrulation occur in humans?

Mammalian embryos have not been studied as extensively as others because

they are difficult to maintain in culture after the blastula stage (this is because

they implant in the uterus).

Surprisingly, mammals undergo gastrulation very similar to birds!!

After about 5 days the

blastula appears as an

inner cell mass and a

tropoblast



12 day old human embryo

This is very similar to a blastula in birds before primitive streak formation.

The blastoderm is very small (only 0.2 mm in diameter).



Gastrulation in a 16 day old human embryo

• The primitive streak and Henson’s node form just as in birds.

• The cell movements are similar

• Cells roll over the primitive ridge and into the groove

•

They ingress individually and move out to form discs with the 3 germ layers• The 3 layers also move laterally to form the extra embryonic endoderm and

mesoderm even though there is no yolk to digest. This is surprising

because mammalian embryos could gastrulate easily by invagination as sea

urchins (they have a placenta and no yolk).

•

Instead, gastrulation appears to recapitulate a pattern established by bird-like ancestors and reptiles.

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07 gastrulation Biology

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