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Morula: Definition & Concept

Instructor: Derrick Arrington

Derrick has taught biology and chemistry at both the high school and college level. He has a master's degree in science education.

Shortly after fertilization takes place, a lot of cellular activities begin to occur. In this lesson we will investigate the formation of a morula as well as what happens to the morula.

Background Information

New life begins with the process of fertilization. Fertilization occurs when a male gamete unites with a female gamete. Male gametes are known as sperm and female gametes are known as eggs.

Once a sperm has penetrated an egg, the egg undergoes a chemical change that prevents other sperm from entering it. The sperm inside the egg loses its tail and swells to form a nucleus. The egg also forms a nucleus. The two nuclei then move toward each other and join. The process of fertilization is complete when the 23 chromosomes from the sperm have united with the 23 chromosomes from the egg, giving the new individual a full set of chromosomes containing genetic information.

Definition of a Morula

At this point, the fertilized cell is referred to as a zygote. A few hours after fertilization, the zygote begins a process of rapid internal cellular division. First it divides into two cells, then into four cells, then eight cells and so on, doubling the number with each division. This process of cleavage, or cell division of a zygote, is known as segmentation. Segmentation transforms the zygote into a cluster of cells known as the morula.

The cell division process that forms the morula
A diagram of morula formation.

Function of the Morula

A few days after fertilization, cells on the outside of the morula become tightly bound together and form desmosomes and gap junctions. A desmosome is a structure made of protein that attaches two cells together. A gap junction is similar to a desmosome, but provides a channel for cells to exchange molecules and ions.

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