Embryology for Vertebrate Paleontologists

A couple of days ago, I wrote a post about what makes vertebrates distinct from other animals. I alluded to the fact that our closest living relatives that aren’t chordates are the echinoderms, sea stars, sea urchins, sea cucumbers, sea lilies and apparently any organism for which the common name begins with ‘sea’ (except for sea horses – those are fish).

But how do we know this? The answer is in our embryos.

Most of us know that animals begin as a female gamete (the egg) meets the male gamete (the sperm). This fertilized egg, now called a zygote begins to divide. It becomes a ball of cells. It’s about here when common knowledge quits. We only know that somehow this ball of cells becomes a new animal. A baby.

Here, I’ll fill in the blanks a bit. We’ll consider what happens as chordate zygotes develop and compare that with what happens when most other invertebrate zygotes develop (except for echinoderms – we’ll get to them later).

As the cells divide to form the ‘ball of cells,’ they either divide one on top of the other (radial), or with a little twist (spiral). Chordates all divide radially; everything else divides spirally.

Here’s an interesting thing that happens as well. In humans, to get identical twins, the ball of cells breaks apart early on when there are only two or four cells so far. The two (or four) cells have the exact same genetic information and can develop separately into two (or four) identical individuals. Any one of the cells then can develop into a complete individual. We know humans can do this, because we know of identical twins. As it happens all chordates can do this. This is called an indeterminate cell fate.

All the other animals (excluding echinoderms) exhibit determinate cell fate. That means that if the cells are separated at the two-cell stage, each cell develops into only half an animal.

Let’s move further in the development of the embryo and look at more differences.

The ball of cells grows and becomes a hollow ball, now called a blastula. This ball begins to fold in on itself, like if you push your finger against an inflated balloon until it touches the opposite wall. The end result is a two-layered ball of cells with an opening on one side (where your finger was in the balloon example). This two-layered ball is called a gastrula, and the opening is called the blastopore.

The two layers of the gastrula become different parts of the animal. The outer layer (ectoderm) forms the skin and outer covering of the animal. The inner layer (endoderm) becomes the gut. The blastopore is the only entrance to the gut, and for some organisms that’s all that’s needed. For most organisms though, the gut has two openings, the mouth on one end, and the anus on the other.

In chordates, the blastopore becomes the anus. A mouth opens on the other side of the embryo later in development. Because the mouth comes second, chordates are called deuterostomes.

In most non-chordates, the blastopore becomes the mouth. These organisms are called protostomes.

But wait! There’s more!

Think about your own body. We have skin on the outside, and guts on the inside. The guts sit and can move around in a big body cavity. Your heart and lungs are not embedded in your flesh, but are inside your chest cavity, free to move, if only a little. This body cavity is called the coelom (pronounced see-loam). The coelom is not unique to chordates, but is also not present in all animals.

If we go back and look at our gastrula, there is no coelom. The gut (endoderm) presses right against the outer wall (ectoderm) with no gap in between. There are two ways to get from the gastrula to having a coelom. One way is to have pockets of the gut bubble out. These pockets enlarge and seal themselves off from the rest of the gut, forming a big pocket that separates the endoderm from the ectoderm. This type of coelom formation is termed enterocoelous.

Another way to make a coelom is the thicken the layer of cells between the endoderm and the ectoderm, then split it open making a body cavity that separates the gut from the outer cells. This type of coelom formation is called schizocoelous.

Chordates have an enterocoelous coelom; most of the rest of animals (that have coeloms) are schizocoelous.

One of the interesting things that happens during the formation of the coelom is the development of another embryonic layer, the mesoderm. The mesoderm is important in chordates because much of our bones and muscles originate there.

To summarize: Chordates (and vertebrates) are deuterostomes, thus had radial cleavage, indeterminate cell fate, and enterocoelous coelom formation. All other animals (except echinoderms) are protostomes, with radial cleavage, determinate cell fate, and schizocoelous coelom formation.

So what about the echinoderms?

They’re deuterostomes, just like us. They lack bones and heads, but as embryos are fundamentally more like us than like other invertebrates. Thus, we know them to be our closest living non-chordate relatives.

Yup, starfish are our cousins.

One last thing: Brains.

The three embryonic layers, endoderm, mesoderm, and ectoderm are called germ layers. Each of the germ layers provides cells that go on to form important organs and tissues in the body. The endoderm forms the majority of our organs (except from the brain, spinal cord and nerves). Mesoderm is the real workhorse of the germ layers because most of our bones and muscles come from here. Mesoderm also lines the coelom and covers all the organs. Ectoderm forms the outermost layer of our skin, but also our entire nervous system, plus our teeth, some bones of our skulls, and the scales of fish.

Yes, you read that right. Our brains are made of ectoderm. The dorsal hollow nerve chord that makes chordates what they are is derived from the ectoderm.

It works like this:

Part of the ectoderm along the top surface of the embryo begins to thicken into a plate. This thickening is called the neural crest. The neural crest begins to fold up onto itself, making a tube of neural crest along the top of the embryo. This is the dorsal hollow nerve chord. Other ectodermal cells cover the neural tube and the tube itself develops into the brain and spinal cord.