Mesoderm induction is the first differentiation of the germ layers and is therefore very important, however it occurs in quite different ways in different species. Here we will discuss Xenopus mesoderm induction as it is by far the best studied.
(Image 3 - A diagram of mesoderm induction) (Image 4 - The organizing centers in Xenopus)
In the late blastula stage, Vg1, (a growth factor) and VegT (a transcription factor) are maternally localized in the vegetal pole, whilst beta-catenin is localized dorsally. The overlapping portion containing beta-catenin and Veg-T induces expression of Nodal (in Xenopus the homologs are Xenopus-nodal-related proteins - Xnrs). Nodal is secreted along a dorsal-ventral gradient, and the area with the highest concentration of Nodal becomes the Nieuwkoop center. The lowest concentration of Nodal induces ventral mesoderm.
The Nieuwkoop center induces the Xenopus organizer - the Spemann organizer - to form in the dorsal mesoderm. Beta-catenin binds to another transcription factor - Tcf3 - and induces expression of Siamois, which when combined with Nodal induces the expression of the goosecoid gene. Goosecoid, a transcription factor, activates the Spemann organizer's function, by inducing expression of specific genes which induce dorsal mesoderm:
-BMP inhibitors - Noggin, Chordin, and Follistatin - which all inhibit the action of Bone morphogen protein (BMP) which is secreted ventrally, and this induces dorsal mesoderm formation.
-Wnt inhibitors - Dickkopf and Frisbee act in the same way by inhibiting Wnt.
The Cerebrus protein inhibits both Wnt and BMP, and induces head formation. The combination of these proteins - 1. Induces lateral mesoderm around the organizer 2. Induces the ectoderm above it into a neural fate 3. Later induces the neural plate to undergo neurulation. It also induces gastrulation.
Is where the blastula is rearranged to form the 3 germ layers of the body. Mesoderm and endoderm precursors are brought inside the embryo with mesoderm forming the middle layer. Gastrulation is important when discussing the neural tube as it positions the notochord, (what the organizer develops into) underneath the neural plate, a structure vital for neural development.
There are several movements involved in gastrulation -
Invagination - the infolding of a smooth surface.
Involution - an expanding outer layer turning in on itself and spreading into an inner layer.
Ingression - individual cells migrating from the surface into the inside of the embryo.
Delamination - the movement of cells out of a sheet of cells.
Epiboly - the co-ordinated movement of an epithelial sheet of cells to form an inside layer. Usually performed by ectoderm.
Convergent extension - change in tissue shape causing tissue to narrow in one axis and elongates along another axis.
As in mesoderm induction, Xenopus is still the best studied so we will continue with this model system. In Xenopus gastrulation mainly involves involution at the blastopore of the mesoderm and endoderm, convergent extension of the mesoderm, and epiboly of the ectoderm during involution.
Gastrulation begins when cells in the dorsal marginal zone (dorsal mesoderm) change shape. These cells become what is called ‘Bottle cells’ and invaginate, forming a thin invagination - the blastopore - just below the equator. This paves the way for involution to occur, i.e. for mesoderm and ectoderm to move inside the blastocoel, whilst the ectoderm undergoes epiboly over the top of the embryo. The first mesoderm to involute is the deep mesoderm in the involuting marginal zone, or IMZ. Next is the mesoderm above the blastopore - the Spemann organizerwhich goes on to form the notochord and other dorsal mesoderm structures. The mesoderm surrounding the Spemann organizer then involutes, and forms the somites. The endoderm involutes during this as well, and forms the lining of the roof or the primitive gut.