Early Embryonic Development of Amphibian part 1

Summary

This transcript explains the early embryonic development of amphibians, particularly focusing on Xenopus (a species of frog). It covers stages from fertilization to the formation of the morula and blastula, including details about cleavage patterns and cell divisions. The text delves into molecular mechanisms, like the role of cadherins, and the influence of factors like the rotational movement of the cortical region during fertilization. It also explains how the embryo transitions from the blastula stage to gastrulation, highlighting key processes and the regulation of gene expression that prepare cells for movement and differentiation in later stages of development.

Takeaways

• 😀 The class Amphibia includes vertebrates like salamanders and frogs, such as Xenopus, whose eggs have a mesolecithal type.
• 😀 Mesolecithal eggs have more yolk (yellow yolk) compared to isolecithal eggs, and undergo holoblastic cleavage during development.
• 😀 Xenopus embryos undergo radial holoblastic cleavage, where division happens throughout the embryo from the animal to vegetal pole, similar to echinoderms and amphioxus.
• 😀 The first division in Xenopus embryos occurs meridionally, with cleavage at the animal pole moving toward the vegetal pole.
• 😀 The cleavage speed is faster at the animal pole, about 1 mm per minute, but significantly slower at the vegetal pole (0.02 to 0.03 mm per minute), due to higher yolk content in the vegetal region.
• 😀 Xenopus embryos undergo successive divisions, including a third equatorial cleavage that results in smaller cells at the animal pole (micromeres) and larger cells at the vegetal pole (macromeres).
• 😀 By the fourth cleavage, the embryo has 16 cells, and further cleavages double the number of cells, progressing from 16 to 32, 64, and eventually forming a morula (64 cells).
• 😀 At 128 cells, Xenopus embryos form a blastula with a central cavity (blastosol), which is located near the animal pole, unlike other species where it is centrally located.
• 😀 A significant feature of amphibian embryos is the presence of the gray crescent, a region formed due to cortex rotation at the animal pole after fertilization, which influences future development.
• 😀 The gray crescent plays a crucial role in the formation of the blastopore and dorsal structures in the embryo, with cleavage planes affecting its symmetry and the final embryo organization.

Q & A

What is the difference between mesolecithal and isolecithal eggs?

Mesolecithal eggs, like those of amphibians such as Xenopus and salamanders, contain more yolk, especially in the vegetal pole, compared to isolecithal eggs, which have evenly distributed yolk. This difference influences the pattern of cleavage during early embryonic development.

How does cleavage occur in Xenopus embryos?

In Xenopus embryos, the first cleavage is meridional, dividing the embryo into two equal parts. Subsequent cleavages follow a similar meridional pattern, with the third cleavage occurring equatorially, resulting in unequal-sized cells due to the location of the cleavage plane.

What are micromeres and macromeres in Xenopus embryonic development?

Micromeres are the smaller cells formed at the animal pole during cleavage, while macromeres are the larger cells formed at the vegetal pole. These cells differ in size and contribute to the development of different structures within the embryo.

Why does cleavage speed differ between the animal and vegetal poles in amphibian embryos?

Cleavage is slower at the vegetal pole because the yolk content is higher in this region. The presence of more yolk reduces the ability of the cells to undergo rapid cleavage, resulting in slower development at the vegetal pole compared to the animal pole.

What is the significance of the gray crescent or 'Sabit Kelabu' in amphibian embryos?

The gray crescent is a critical region formed due to the rotation of the cortical cytoplasm in the animal pole after fertilization. It plays an important role in organizing the dorsal structures of the embryo, including the formation of the blastopore during gastrulation.

What is the blastula stage in Xenopus embryos, and how is it identified?

The blastula stage in Xenopus is characterized by the formation of a fluid-filled cavity called the blastocoel, which develops as the embryo divides. At the 128-cell stage, the embryo is called a blastula, and its cells are referred to as blastomeres.

How does the presence of the blastocoel affect cell fate in Xenopus embryos?

The blastocoel helps maintain cellular separation, preventing cells at the roof of the cavity from interacting with those at the bottom. This segregation is crucial for ensuring that cells destined to become the ectoderm or mesoderm retain their developmental fate.

What role does the Epicadherin molecule play in Xenopus blastula?

Epicadherin, a molecule found in the blastula, facilitates the adhesion between blastomeres, maintaining the structure of the blastula. It also plays a role in allowing cells to undergo morphogenetic movements during gastrulation.

What happens when Epicadherin function is disrupted in Xenopus embryos?

Disrupting Epicadherin function using antisense molecules causes the blastomeres to lose their cohesion, leading to the collapse of the blastocoel and a failure to maintain the normal structure of the blastula. This prevents the embryo from progressing properly through development.

How does the gastrulation process begin in Xenopus embryos?

Gastrulation in Xenopus embryos begins when cells in the blastula become motile and start migrating. This process is regulated by specific transcription factors that activate different genes, allowing cells to change position and form the three germ layers: ectoderm, mesoderm, and endoderm.