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Chordata Germ Layers

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

Members of the phylum Chordata have three main germ cell layers, each with its own unique role in development. In this lesson, we'll discuss the structure, function, and derivation of the germ cell layers of chordates.

What Is a Germ Layer?

In spite of the name, germ layers have nothing to do with germs or getting sick. A germ layer is a layer of cells in a developing embryo. Each germ layer interacts with the adjacent layer as the embryo grows, and each germ layer contributes to the formation of all of the fully-developed organism's tissues and organs.

To best understand how germ layers contribute to the growth and development of a chordate, or an animal possessing a specialized cartilage rod called a notochord, we will first need to take a look at the basics of embryology in the phylum Chordata.

Introduction to Embryology in Chordata

The phylum Chordata encompasses all living and extinct chordates. We, as humans, are chordates. In our embryological development, we possess a notochord--the flexible cartilage rod that becomes part of our intervertebral cartilage disks as adults. At some point during embryological development, humans, as well as other chordate embryos, also have:

  • Gill slits
  • A hollow nerve tube at our dorsal side, or along our back
  • A throat groove known as the endostyle
  • A post-anal tail

Humans are vertebrates (animals with a backbone), and all other animals with backbones are also chordates--birds, reptiles, other mammals, and fish. There are some invertebrate chordates as well, such as the aquatic animals known as sea squirts and lancelets. All of us are united within the phylum Chordata by the ways in which our germ layer cells differentiate into specific tissues and organs.

The blastula forming a gastrula, with the three main germ layers and the archenteron space
gastrulation

These germ layers arise early in embryological development in chordates. Shortly after sperm and egg meet to become a fertilized cell, or zygote, that fertilized cell begins to divide. First, the resulting nearly-solid ball of cells is called a morula. Eventually, the morula cells migrate to the edges of the sphere, and a hollow center forms. The ball of cells is now called a blastula, which then folds inward to form a gastrula. Think of this in-folding as if you were pressing your fist into a deflated balloon: The balloon would still be semi-inflated, but it would form a u-shaped cavity that is open to the air.

It is in this gastrula stage that chordate germ layers begin to develop. The three germ layers in the chordate gastrula are called the endoderm, mesoderm, and ectoderm. In our following description of the structure and function of these germ layers, we will move from the innermost germ layer--the endoderm--and proceed outward to the outermost germ layer--the ectoderm.

Endoderm Structure and Function

An easy way to remember where the endoderm forms is that the prefix 'endo-' means 'internal' or 'within.' The endoderm germ layer forms on the inside of the in-folded gastrula.

These cells, which are at first very thin and flat squamous cells, can become more columnar, or column-like in shape, as they develop into fully-formed tissues.

The endoderm germ layer forms several organs and tissues in chordates, including the thymus gland; thyroid gland; area around the pharynx, or throat; parts of the trachea, also known as the windpipe; liver and pancreas; stomach and intestines; urinary bladder; and lungs.

Endoderm differentiation
endoderm

As you can see, most of these organs involve thin layers of gas or nutrient exchange, such as oxygen and carbon dioxide exchange in the lungs, or nutrient absorption in the intestines.

Mesoderm Structure and Function

Mesoderm germ layer cells (from the prefix 'meso-' or 'middle') form in the middle of the two other chordate germ layers.

The mesoderm is made up of a variety of sub-types, each of which is signaled to differentiate into organs and tissues by cell-to-cell chemical interactions with the endoderm and ectoderm.

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