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This lesson on the early stages of mammalian development covers the processes following fertilization, such as cleavage, the morula formation and blastula formation. Watch to learn what the inner cell mass, trophoblast cells and blastocyst are, as well as their functions within embryonic development.

Early Mammalian Development

The earliest stages of mammalian development following fertilization are very difficult to study. First of all, the gestational period of most mammals is long, which slows down the pace of research dramatically. In addition, most mammals produce only a few offspring at a time, and many, like humans, only produce one or two at a time, so there are fewer embryos to study.Almost all mammals develop inside the mother, which makes observation and manipulation difficult. In addition, mammalian zygotes are much smaller than other vertebrate zygotes. For instance, Xenopus laevis is a frog species that is often used to study development and the volume of one of its eggs is one thousand times larger than a human zygote.

Illustration of the cleavage process
Illustration of the Cleavage Process

Now let’s say that you want to study very early human development. You already can’t see the single zygote or embryo because it’s inside a person, and it’s too small to study with ultrasound or other non-invasive imaging techniques. On top of that you have many more ethical limitations when studying humans. You can’t, for example, go out and breed people to produce embryos for scientific study. For these reasons, most developmental biology is done in non-mammalian species. Mice are most commonly used when studying mammalian developmental biology because they have one of the shortest mammalian gestational periods, they are easy to house and breed in large numbers, and they can have more than ten pups in a single litter.

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Cleavage and Formation of the Morula

Soon after fertilization, the mammalian zygote begins the process of cleavage, which is the process by which the zygote rapidly divides without growing to become multicellular. Cleavage uses the process of mitosis to replicate the genome and then divide the cells in half; however, unlike normal mitosis, there is no growth phase between divisions during cleavage.The first division occurs about a day after fertilization, and subsequent divisions occur every 12 to 24 hours after that. The zygote divides several times to form a mass of cells called a morula, which is an embryonic stage consisting of a solid, compact mass of 16 or more cells. The morula is the first embryonic stage where mammalian cells can be categorized as being either internal or external.

The cells continue to divide, and when the mammalian morula reaches the 64 cell stage, the internal and external cells become separate lineages.

Illustration of a blastula
Illustration of a Blastula

The internal cells are called the inner cell mass or ICM for short. The ICM will eventually become the embryo itself and its surrounding membranes. The external cells are called trophoblast cells.

The trophoblast cells will play a key role in the process of implantation in the uterine wall and will eventually become the chorion, which is the embryonic portion of the placenta.

Blastula Formation

Soon after the cells of the mammalian morula differentiate, the trophoblast cells secrete fluid into the center of the morula to create a fluid-filled space inside, called the blastocoel. Now that the embryo has taken the form of a hollow ball of cells, it is called a blastula. The trophoblast cells remain in a single outer layer surrounding the blastocoel, and the cells of the ICM form a mass of cells on the inner surface on one side of the blastula.

All vertebrates form a blastula during their development; however, in non-mammalian species, there is no inner cell mass. Instead, the blastula consists of just one cell layer. Unlike mammals, other vertebrates do not attach to the uterine wall and do not form a placenta, so there is no need for the differentiation of trophoblast cells and the ICM.

Illustration of mammalian and non-mammalian blastula
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