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Look in the mirror. What do you see? Skin? Hair? Eyes? All of these structures develop from the same embryonic tissue called the ectoderm. Keep watching to find out what other structures are derived from the ectoderm.

Germ Layers

During development, most animals form a gastrula. A gastrula is an embryo that has undergone gastrulation, or the process that produces different germ layers. Germ layers are the initial tissue layers of an embryo, and animals can have up to three of them.

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  1. Ectoderm: the outermost layer of cells
  2. Mesoderm: the middle tissue layer
  3. Endoderm: the innermost germ layer

A few groups of animals, such as cnidarians, like jellyfish, corals, and sea anemones, and ctenophores, such as comb jellies, only have two germ layers, which makes them diploblastic.

Most other animals, including you, have a triploblastic gastrula. The mesoderm layer is found only in triploblastic animals, but all animals with tissues have at least an endoderm and ectoderm layer.Each germ layer will develop into different adult tissues and structures. For example, in humans the endoderm forms the lining of internal structures like the stomach and intestines. The mesoderm develops into most of your organ systems, like the skeletal and muscular systems. The ectoderm develops into most external structures, like the skin, mouth, eye, and nervous system.

Skin & Mouth

When you look at yourself in the mirror, most of the structures you see are derived from your embryonic ectoderm layer.

This includes the outermost layer of your skin, called the epidermis. It also includes derivatives of your epidermis, such as:

  • Hair
  • Nails
  • Sweat glands
  • Oil glands
  • Mammary glands

If you open up your mouth and look inside at the walls of your cheeks, you are looking at a derivative of ectoderm. The white outer layer of your teeth, or enamel, also develops from the ectoderm layer.

Eyes

The cornea is the clear, outermost covering located in front of the colored part of your eye.

The lens is located behind the colored part of your eye. Even further back is the retina, a region that has light receptors. These three structures of your eye (the cornea, lens, and retina) are all derived from the ectoderm.

Nervous System

Most of your nervous system also develops from the ectoderm layer. Your nervous system includes:

  • Brain
  • Spinal cord
  • Neurons, or the communication cells that are packaged in nerves
  • Nerves, which connect the parts of the body to the brain and spinal cord
  • Neuroglial cells, which support cells that nourish and repair neurons

Lesson Summary

Animals that undergo gastrulation will form two or three germ layers.

Diploblastic animals have ectoderm and endoderm layers. Triploblastic animals have ectoderm, endoderm, and mesoderm layers. These embryonic tissue layers will develop into the different tissues and structures found in an adult animal.The endoderm is the innermost germ layer that develops into internal structures.

The mesoderm is the middle germ layer that forms most organ systems. The ectoderm is the outer layer of an embryo that develops into the epidermis, derivatives of the epidermis, parts of the mouth and eye (like the cornea, lens, and retina), and most of the nervous system.

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