Lecture 11: Gastrulation

February 19, 1999 Dr. Thomsen

readings: Chapter 6 p. 209 - 224, 232 - 238.

Gastrulation is the process by which cellular movements cause the embryonic germ layers to become rearranged into their final positions. Several types of characteristic cell movements may be employed to affect gastrulation, depending on the spatial orientation of the germ layers and the type of embryo one is considering. The movements of gastrulation are accompanied by tissue interactions that pattern the body.

Types of movements associated with gastrulation, depending on the animal.
_ Epiboly - movement of epithelial sheets which spread as a unit to enclose deeper layers of the embryo. Fig 6.25. Often associated with `layer thinning' as cells spread out in a perpendicular direction.
_ Invagination - Infolding, or "pocket formation" within a sheet of cells. `Indentation'.
_ Delamination - Splitting of a single sheet into two parallel sheets that separate from each other.
_ Involution - Turning inward by an expanding outer layer. `turning the corner'
_ Ingression - Cells migrate individually from an outer layer to the interior of the embryo.
· Convergence and extension - Cells in a sheet or tube move toward, or converge upon, each other. As that happens the tissue elongates, or extends, outward in a direction perpendicular to the convergent movements. In other words, as cells move to the middle they pile up, but don't have much space, so they move outward (or upward/downward) in a perpendicular direction.

Depending on the species, embryos may use one or more of these types of cell movements to cause gastrulation. EXAMPLES:

Echinoderms: The sea urchin.
Invagination and Convergent extension.
Figures 6.1, 6.2, 6.8, 6.9
of primary mesenchyme cells. Fig 6.5.
extensions of cells promote movement by adherence to substrate.

Fish: Zebrafish
Epiboly of cells pushes them down around yolk
Involution and some Ingression of cells during gastrulation
Convergent extension moves cells to dorsal side
- they form the `embryonic shield' that is the = Spemann Organizer
Figures 6.11, 6.12.

: Xenopus.
Invagination and convergence - extension.
Figures 6.13 through 6.17.
Specialized bottle cells lead the way in gastrulation movements

Birds: Chicken.
Ingression through Henson's node and the primitive streak.
Delamination of cell layers at blastula stages. Figures 6.26, 6.28, 6.29.

Summary of figures illustrating gastrulation:
Read and study the following Figures in these pages first, then read the text.
These figures will illustrate what's written in the text and what we'll cover in lecture.
Figures 6.1 & 6.2: sea urchin early development and gastrulation
Figure 6.4 extracellular matrix (ECM) in the urchin
Figure 6.5 Ingression
Figure 6.8 Invagination
Figures 6.9 & 6.10 Radial extension of the archenteron
Figure 6.12 Convergence - extension in the fish
Figure 6.15 Cell movements in frog gastrulation
Figures 6.16 & 6.17 The frog blastopore lip morphology
Figure 6.22 Sums up the cell movements in a frog gastrula - - look it over. It IS complicated but
it illustrates the basics of convergence - extension and involution.
We will revisit this figure again when we address embryonic patterning and neural induction.
Figure 6.25 illustrates cell layer thinning that occurs in epiboly

Detailed Examples:
Sea Urchin gastrulation.

Urchin gastrulation begins when a cluster of primary mesenchyme cells (PMCs), which form part of the future mesoderm, migrate out of the vegetal plate cell layer by ingression (Fig. 6.5), which involves detachment from the hyaline layer on the outside of the embryo. PMCs enter into the blastocoel.

As the PMCs migrate they extend thin processes called filopodia that make contact with extracellular matrix (ECM) and other cells. The primary mesenchyme cells arrange into a ring, connected by syncytial cables, that link the cells together in the blastocoel. Along these cables the cells secrete calcium carbonate spicules that form the larval skeleton. Figures 6.1 - 6.5.

After PMC ingression, the vegetal plate pushes inward because of forces generated in the hyaline layer on the outside of the vegetal plate cells. Chondroitin sulfate proteoglycan (CSPG) secreted into the lamina swells up with water, forcing shape changes that bend the vegetal plate into the blastocoel. Figure 6.8.

After it bends inward, the vegetal plate cells forms a tube, the archenteron. The archenteron cells move toward each other, narrowing the tube width, and causing it to extend. Those movements are a type known as convergent extension. Fig 6.9.

Secondary mesenchyme cells
at the tip of the archenteron extend filopodia that search for a "target" site on the blastocoel roof. They adhere to the target and pull the archenteron to it. The archenteron then fuses with the wall, forming the opening for the mouth, and creating the continuous digestive tube.

Gastrulation in the frog embryo - the essentials:

Gastrulation starts at the position of the Spemann Organizer and is led by bottle cells (Fig 6.16, 6.21, 6.22), which form the dorsal blastopore lip. The dorsal lip invaginates into the blastocoel and leads mesodermal tissues of the marginal zone inside the blastocoel. The invaginating cells migrate as a sheet along the blastocoel roof and come to occupy positions along the anterior-posterior axis
(A -P axis). Fig 6.15.

Bottle cells end up in the head endoderm, which forms the pharynx. Cells behind the bottle cells form head mesoderm, also known as prechordal mesoderm. Further to the posterior, along the trunk of the embryo, other mesodermal cells form the axial mesoderm (chordamesoderm) that includes the notochord (the primitive backbone).

The movements of gastrulation begin at the organizer, but gradually the movements spread laterally and ventrally until all of the mesoderm invaginates. See Fig 6.17. Dorsal mesoderm migrates much further than the ventral mesoderm, and it is dorsal mesoderm that directs elongation of the A-P axis.

The driving force in amphibian gastrulation is convergence - extension of cells located in the Involuting Marginal Zone (or IMZ). Figure 6.22 shows that these IMZ cells pull toward each other in the lateral direction. This "pulling" at each other has the effect of moving cells outward toward the anterior. The IMZ cells migrate along the roof of the blastocoel. They carry the bottle cells ahead of them and drag the superficial cells along for the ride.

Rearrangement of the germ layers in vertebrate gastrulation is easily visualized in amphibia by putting nile blue dye marks onto the surface of the blastula and watching where they end up (= fate mapping) Figure 6.13. Cells on the dorsal side move furthest anterior, while cells in more lateral positions move toward the embryo's mid-line by convergence

Like in the sea urchin, convergence-extension of the involuting mesoderm and the surface layer drives the archenteron forward toward the anterior, where it fuses with the surface to form the mouth.

Here's an analogy for convergent extension: Imagine that all of the students in our lecture hall decided to move toward the middle of the room. As you converge toward the middle of the room there would not be enough space to accommodate everyone unless you also move toward the front of the room - that's the extension part. It's like trying to put everyone in the room (or the dorsal mesoderm cells in the embryo) on a narrow line down the middle - - To do that you have to move along a line extends in the A-P direction.

A nice, simple illustration of convergent extension is shown in figure 6.12 for the Zebrafish, which gastrulates in a similar way as the frog. The major difference between frog and fish gastrulation is that there is only a very narrow blastocoel in the fish (because of the yolk), and there is a bit more mixing of cells between germ layers. Compare figure 6.22 (frog) to 6.11 (fish). Don't worry about the details of the fish, just appreciate that the fundamentals of the process are about the same in both organisms.

Epiboly - the thinning of ectodermal and mesodermal tissue layers by cell intercalation.
It causes ectoderm and mesoderm to move down over yolky vegetal hemisphere, covering the yolk plug

, an extacellular matrix molecule, is required for gastrulation movements to occur normally in amphibians. Fibronectin fibers line the blastocoel and the involuting mesoderm cells migrate along these fibers. Migration can be blocked by a synthetic molecule (`RGD peptide'), that mimics fibronectin. Figure 6.23 on page 230 shows what happens.

Bird Gastrulation in outline - read p 233 to 238, top paragraph.

discoidal cleavage produces the epiblast.
The hypoblast forms beneath the epiblast by delamination and migration of posterior cells.
Posterior thickening of the epiblast defines Koller's sickle, and this cell thickening (due to
migration) moves anteriorly along the midline, forming the primitive streak.
Cells of the epiblast migrate into the blastocoel by passing through the primitive streak.
It is analogous to the frog blastopore.
At the anterior end of the streak is Henson's node, nearly equivalent to the frog dorsal
blastopore lip.
All three germ layers are derived from cells of the epiblast. The hypoblast provides inductive
signals and contributes to extra-embryonic structures, such as the yolk sac.