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 called the 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).

Gastrulation in the 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 the molecular 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 splitting 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 between one 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 extracellular material 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 the blastula 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 and have the potential to develop into many different tissues, including epithelial cells. They have a leading edge with lamellipodia, and a trailing edge. 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 large micromeres 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 occurs with 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 byconvergent 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 to become 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 where to 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 zoneMesoderm = deep zone and deep layer of involuting marginal zoneEndodermEndoderm = vegetal cells and superficial layer of involuting marginal zone = 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 organize the behavior of all other regions.

The dorsal lip of the blastopore appears to be particularly important in regulating the fate of other areas. 2 genes, goosecoid and noggin 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 which elongates anteriorly to become the primitive streak. The center opens to form the primitive groove between 2 primitive ridges. 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 hypoblasthypoblast is the lower layer of epithelial cells. is the lower layer of epithelial cells.

Epiblast cells roll over the primitive ridge and involute into the groove. The cells The cells lose lose contactcontact with with one another and one another and migrate inwards migrate inwards by by ingressioningression = =Mesoderm.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.