Making The Vertebrate Body Cavity – #365papers – 2018 – 33

Minot, 1890, The Mesoderm and the Coelom of Vertebrates: The American Naturalist, v. 24, p. 877-898.

What’s it about?

At the start of every vertebrate’s life, they are merely a ball of cells. Then a hollow ball (blastula). Then a ball that’s been punched in on one side (gastrula) with a hole (blastopore). At the gastrula stage, the early embryo is composed of two simple layers. On the outside is the ectoderm (sometimes called entoderm), and on the inside is the endoderm.

A third layer, called the mesoderm, arises in vertebrates and later opens to form the body cavity or coelom (pronounced SEA-lome). The coelom is the open space in which our organs essentially hang. Our stomach, intestines, bladder, heart, lungs, liver, pancreas, etc. are not embedded in our body walls, but float and move relatively freely in the coelom, with some connections (mesentaries) to the body wall to keep everything from falling to the bottom. It’s this arrangement that makes things like laparoscopic surgery possible.

Anyway… At the gastrula ‘hollow ball of cells’ stage of an embryo, somehow mesoderm forms between the ectoderm and the endoderm, then divides to form the coelom and the lining thereof.

This paper looks at embryos of many different vertebrates, from sharks to mammals, to explore the origins of the mesoderm and the coelom.

Why does it matter?

Embryological development is highly ‘conserved’ in biology. That is to say, closely related organisms, though they may appear very different as adults (e.g. sharks, snakes, and people) develop very similarly in the early embryonic stages. This can help us better understand deeper relationships among organisms.

(For example, did you know that vertebrates and echinoderms [sea urchins, sea stars, etc.] have incredibly similar embryological developments? Turns out that our closest non-vertebrate relatives are the echinoderms!)

Why did I read this?

I was preparing a lecture on the origins of the mesoderm, the coelom, and the notochord for my vertebrate paleontology class. This paper helped me better understand what is actually happening in vertebrates.

It was only later that I noticed that this paper was written in 1890. Yup. Over 100 years old. Good thing embryos haven’t changed much since then!