Week 3

Gastrulation (from bilaminar to trilaminar disc) + notochord development

We'll bring in a slightly more detailed version of the bilaminar disc now.

The primitive streak at right, leading to the primitive node.

Day 15

The prochordal plate, at the cranial end of the hypoblast.

At the cranial end of the embryo there is a patch of columnar cells known as the prochordal plate. At the cranial end of the primitive streak, cells are burrowing thickly, forming this circular depression known as the primitive node.

Cranial: head end

Caudal: tail end

Day 16

The mass migration of epiblast cells produces change in its macroscopic conformation.

By now there are thousands of cells in the epiblast layer alone.

It is the mass movement of these cells that produces these changes in the shape of the epiblast.

The cells that burrow through the primitive streak and node differentiate into cell populations known as germ cell layers. These are progenitor cells, which will differentiate into the bulk of our bodily constituents.

The germ layers are

• endoderm, which displace and replace cells of the hypoblast

• mesoderm, which remain in between the layers of the former bilaminar disc

• ectoderm, which epiblast cells differentiate into.

Some mesoderm spreads to the edges to meet the extra-embryonic mesoderm.

Germ: meaning ‘bud, seed, embryo’

Endoderm: meaning ‘internal skin’. The translation ‘skin’ makes sense given they are thin layers to begin with: they look like skins.

Mesoderm: meaning ’middle skin’

Ectoderm: meaning ’outer skin’

From top to bottom: ectoderm, mesoderm and endoderm.

Day 17

The path of epiblast cells to become the notochordal process.

By this stage, cells burrowing down the primitive node have started to form a hollow rod, called the notochordal process.

Notochordal: from noto, ‘back’ and chord, ‘cord’, because the ‘back-cord’ later serves as integral structural support for the early embryo.

In pink, the notochordal process. Remember this is a hollow rod.

Having made a coronal cut through the epiblast and notochordal process.

Day 18

On day 18, ectoderm between the primitive node and the cranial end of the embryo thickens, and is known as the neural plate. This 'thickening' occurs via increased mitotic division of cells in the area.

The dark blue portion has thickened to become the neural plate.

Day 19

The ectoderm and notochordal plate, with mesoderm and endoderm removed.

The notochord partly fuses with the endoderm, and is known as the notochordal plate. A temporary passage thus exists between the amniotic cavity and yolk sac. This is known as the neurenteric canal, and is thought to be important in equilibrating pressure between these two spaces.

With the endoderm in place, we can see the connection of the notochordal plate.

I'll add some images now that show the passage in better detail.

In this image we've introduced sagittal sections of the amniotic cavity and yolk sac.

In these images, the ectoderm and endoderm are also cut in the sagittal plane.

Here we are in the amniotic cavity. We see the primitive node, and are now in the neurenteric canal. Down below is the yolk sac.

Neurenteric: from ‘nervous system’ and ‘enteric system (or GIT)’.

Day 20

Neural plate begins to fold inward.

The neural plate has began buckling in the midline. We call this dip the neural groove, and the mounds either side, the neural folds.

Neural groove through the midline, neural folds either side.

Day 21

Neural folds come together in the midline.

These folds progress toward each other, and begin to fuse around day 21. Where they have fused, they form a hollow cylinder, which is the neural tube - the very beginning of the central nervous system.

Forming a hollow tube.

The notochordal plate has by now lengthened, closed its inferior opening, and has become a solid rod, the notochord.

Just beneath the developing neural tube lies this structural backbone of the embryo.

Either side, cells of the mesoderm have started forming chunks known as somites.

And that is it for the first three weeks. Thanks for joining me!

The ectoderm with neural tube, directly above the notochord.

Mesoderm either side, and endoderm at the base of the 3-week-old embryo.