What is Notochord? - Definition, Formation & Function

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  • 0:03 What is a Notochord?
  • 1:17 Formation
  • 2:10 Role
  • 2:50 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
Humans are animals but so are sponges, flies, jellyfish, and crabs. Yet, humans are nothing like any of these other animals. What makes us different? One of the key differences is our backbone, or in more primitive species, our notochord.

What is a Notochord?

Most schoolchildren know what a spine is and could even point it out on a diagram. Even if it was a diagram of a non-human animal like a dog, the spine is pretty easy to figure out. But even adults don't often know what a notochord is.

A notochord is a primitive beginning to the backbone. It appears in embryos as a small flexible rod made from cells from the mesoderm, which is one of the three layers of cells of embryos. Notochords are only found in the phylum chordata, a group of animals that includes humans. In fact, it is the defining feature of that particular phylum. Notochords, and furthermore backbones, are what separate us from the more primitive living creatures that we encounter.

The notochord is oriented head to tail and is found between the digestive tube and the nerve cord. Since it is composed of stiffer tissue, it allows for skeletal support of the organism. In certain chordates, like the lamprey and the sturgeon, the notochord remains there for life. In vertebrates, such as humans, a more complex backbone appears with only portions of the notochord remaining. As our backbone fully forms, the discs in our back is where the pieces of notochord are found as a sort of gelatinous material.

Formation

All chordates start out as one cell, a zygote. Eventually we divide into multiple cells in a single layer called a blastula. Then in a process called gastriculation, the blastula develops into 3 embryonic layers, which are collectively called the embryonic germ layers. These are the ectoderm, or outer layer, the mesoderm, or the middle layer, and the endoderm, or inner layer.

The product of gastriculation: a gastrula or 3-layered embryo.
Gastrulation

The one we are most focused on here is the mesoderm. It is this layer that gives rise to the notochord, skeletal system, muscular system, and a whole bunch of other systems in the body.

The notochord forms from mesoderm cells as they begin to condense and become more rigid. As we progress further along through development, the notochord will elongate and help to stretch the embryo out lengthwise. In humans, more mesodermal cells will begin to wrap around the notochord, forming the vertebrae that we all have in our backs.

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