The human body is an amazing thing, right down to the earliest processes of development. In this lesson, we’ll discuss the process of organogenesis in humans as we develop in the womb.

Human Development

At some point in our lives we have all wondered, ‘Where do babies come from?’ The basic answer is simple, but the biological processes that create a baby are actually quite complex. The fact that we start off as a few microscopic cells that transform into a baby is amazing.

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If we were to look at a fetus at only a few days old, we look almost identical to a frog, salamander, bird, fish, squirrel, the list goes on! Our development closely mirrors that of other organisms, and so our taxonomic classification is broken down to reflect that.

What Is Organogenesis?

Organogenesis is the process by which three germ layers turn into the internal organs of animals. The outer layer is called ectoderm; the middle layer, the mesoderm; and the inner layer, the endoderm. Human organogenesis starts off very similarly to other organisms. Eventually, though, it becomes more complex, making us humans the way we are rather than like other organisms.

Animals that lay eggs, called amniotes, have a similar organogenesis as placental mammals, those with a placenta like humans. There are a few differences, however.During organogenesis we see three different kinds of morphological changes, or changes in form.

There are folds, splits, and dense clustering of cells in the embryo. These will give rise to our organs. We see this occurring in humans somewhere around the third to eighth week of gestation. The neural tube, which eventually becomes the brain and spinal cord, is one of the first major organs to form inside of the embryo. They form out of the mesoderm by condensing into the tissues and then folding over. Both of these structures become the central nervous system of a human adult.

All beings with a notochord are chordates, part of the phylum Chordata.Strips of mesoderm, called somites, also form in this time span. They separate into blocks to become the vertebrae of the back. Organogenesis continues on as the germ layer gives rise to specific organs. The ectoderm gives us our skin and many of the other linings of the mouth, rectum, eyes, and glands.

The mesoderm typically gives way to many of the major systems we see, such as the nervous, skeletal, and muscular systems. The endoderm, like the ectoderm, is responsible for many other linings, such as the digestive tract, as well as many of the major organs themselves, such as the thyroid, liver, bladder, and more.

Lesson Summary

During human embryonic development we move through a number of different stages. One stage, organogenesis, occurs in humans anywhere between the third to eighth week of gestation. During this stage we see the three germ layers of the embryo form into the major organs of the body system.

In the chordates, those with a notochord, we see the neural tubes form first. Afterwards, we will see the major organs form from the endoderm; skin and many of the linings of organs form from the ectoderm; and the nervous, skeletal, and muscular systems form from the mesoderm.


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