How Cells Divide, Reproduce, and Specialize by Alyssa Simon & Amy Romano

Author:Alyssa Simon & Amy Romano
Language: eng
Format: epub
ISBN: 9781622758036
Publisher: Britannica Educational Publishing
Published: 2015-02-06T16:00:00+00:00

This diagram illustrates the difference between the development of embryos that will become identical twins (left) and fraternal twins (right). BSIP/Science Source

The cells that line the inside of the embryo are called the endoderm. Those cells will eventually become the animal’s digestive tract, as well as other internal organs. The cells layered on the outside of the embryo will become the organism’s ectoderm, including the skin, nervous system, and parts of the eye and inner ear. The third layer, located between the endoderm and ectoderm, is the mesoderm. “Meso” in mesoderm is defined as “middle” and it is in this layer that the heart, muscles, bones, blood, and sex organs will develop.

ORGAN FORMATION

With the three layers in place as a foundation, the embryo can begin building actual organs. This process is called organogenesis. As the embryonic cells divide, the daughter cells assume forms and functions different from the mother cell. In some cases, the two daughter cells will form altogether different tissues or organs.

Some cells are programmed to function for a short period, then self-destruct. This process is called programmed cell death, or apoptosis. Human embryos—like all embryos with a backbone or vertebrae, start out with tails and webbed hands and feet. Six weeks into development, those cells are sent “death signals,” which cause the cells to die. This leaves the embryo without a tail and with recognizable and distinct fingers and toes.

The yolk of the fertilized egg fuels this process for most animals. Mammal growth is more complex and requires more energy than the yolk can provide. Early in its development, the embryo plants itself in the mother’s uterine wall. There, the embryo forms a direct connection with the mother’s blood supply, where it can receive essential nutrients.

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