EMBRYONIC DEVELOPMENT OF AMPHIBIANS (EMBRYOGENESIS) - BIOLOGY - 2025

The embryonic development of amphibians, also known as embryogenesis, refers to the early stage of formation and development of the embryo. This period ranges from the formation of the zygote - a cell formed by the union of male and female gametes - until birth.

Amphibians are characterized by drastic physiological transformations during their development. This process is known as metamorphosis.

Amphibians are classified as multicellular organisms and belong to the class amphibia, which means "both means" in Greek.

These vertebrates are classified as multicellular organisms and belong to the amphibia class, which means "both means" in Greek, because they live between water and land.

Among the amphibians, toads, frogs and salamanders stand out.

5 stages of embryonic development of amphibians

During the mating season, the female can only mate once, while the male can mate multiple times.

1- Fertilization

It refers to the union of the two parental gametes, the ovum and the sperm, to form a zygote. After the fertilization of the sperm to the egg, the zygote begins the process of cell division to become an embryo.

In amphibians, fertilization can occur externally or internally. In external fertilization, the male releases the sperm into the water while the female expels the ovum. The eggs must be fertilized in water because they do not have a shell.

During the mating season, the female can only mate once, while the male can mate multiple times.

2- Segmentation

Segmentation refers to the mitotic divisions that the egg undergoes to create small, nucleated cells.

In amphibians, two southern divisions occur and segmentation is later hampered by the distribution of yolk, defined as the nutrients that feed the egg.

The yolk is found in greater quantities in the plant pole than the animal; therefore, when the first equatorial division occurs at the animal pole, it slowly extends to the plant pole.

Segmentation in amphibians affects the entire egg and creates two sizes of blastomeres (each cell that arises as a consequence of the division of the ovum that has been fertilized). Therefore, amphibians present a total and uneven segmentation.

3- Blastulation

Segmentation is preceded by the development of blastomeres. Blastomeres are undifferentiated cells that coalesce to form a cavity in the center of the morula, or early-stage embryo. This cavity is called a blastocele.

The blastula forms two cell layers that prevent complete invagination during gastrulation, a stage that occurs after blastulation.

In the case of amphibians, embryos that have between 16 and 64 blastomeres are considered morula.

4- Gastrulation

Gastrulation fulfills several functions in amphibians. It begins by migrating the embryo to the places destined to form the endodermal organs, allows the formation of the ectoderm around the embryo and correctly positions the mesodermal cells.

In amphibians, not all species perform gastrulation in the same way, but different gastrulation processes lead to the same functions.

Amphibians have a gastrulation by epibolia, where the cells of the animal pole multiply until they cover the cells of the vegetative pole.

5- Neurulation

Primary neurulation begins with morphogenetic changes in the ectoderm. During neurulation, the neural tube develops, later becoming the central nervous system. Somites and notochord also develop.

The embryo is now called a neurula and resembles a tadpole. In the neurula the main characteristics of a vertebrate embryo are identified.

Organ formation, or organogenesis, begins with neurulation and ends with full development of the tadpole before it enters the water.

References

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