Blastula

Blastula

The blastula is an early stage of embryonic development in animals. It follows the cleavage stage and precedes the gastrulation stage. The blastula is characterized by its hollow, spherical structure composed of a single layer of cells, known as blastomeres, surrounding a fluid-filled cavity called the blastocoel.

Formation

The formation of the blastula begins after the fertilized egg undergoes a series of rapid mitotic divisions, a process known as cleavage. During cleavage, the zygote divides without increasing in size, resulting in a cluster of smaller cells. These cells, or blastomeres, arrange themselves into a spherical layer around the blastocoel, forming the blastula.

Structure

The structure of the blastula can vary among different species, but it generally consists of:

• Blastomeres: The individual cells that make up the outer layer of the blastula.
• Blastocoel: The central fluid-filled cavity within the blastula.

In some species, the blastula may have additional structures, such as the trophoblast in mammals, which contributes to the formation of the placenta.

Types of Blastula

Different types of blastulae can be observed in various animal groups, including:

• Coeloblastula: A blastula with a large blastocoel, typical in many invertebrates.
• Stereoblastula: A blastula with a reduced or absent blastocoel, found in some mollusks and annelids.
• Discoblastula: A blastula with a disc-shaped blastocoel, seen in birds and reptiles.
• Periblastula: A blastula with a peripheral blastocoel, common in insects.

Function

The blastula stage is crucial for setting up the basic body plan of the organism. It provides a framework for the next stage of development, gastrulation, where the single-layered blastula reorganizes into a multi-layered structure known as the gastrula. During gastrulation, the primary germ layers—ectoderm, mesoderm, and endoderm—are formed, which will give rise to all the tissues and organs of the organism.

Clinical Relevance

Understanding the blastula stage is important in the field of reproductive medicine and developmental biology. Abnormalities in blastula formation can lead to developmental disorders or failure of the embryo to implant in the uterus. Techniques such as in vitro fertilization (IVF) often involve the culture of embryos to the blastula stage before implantation.

See Also

• Embryogenesis
• Zygote
• Morula
• Gastrulation

References

• Gilbert, S. F. (2010). Developmental Biology. 9th edition. Sinauer Associates.
• Wolpert, L. (2007). Principles of Development. 3rd edition. Oxford University Press.