Early Embryonic Development part 4

Summary

This transcript delves into the process of gastrulation in embryonic development, describing the key stages and cellular movements involved. It explains how mesoderm cells, initially located on the outer surface, migrate inward through a process of ingress and migration, driven by changes in cellular adhesion forces. The formation of the archenteron, the precursor to the digestive tract, is detailed, along with the role of extracellular matrix changes and cell interactions in this process. Additionally, the involvement of filopodia in guiding mesoderm cells and the formation of structures like spicules is covered, ultimately leading to the creation of the embryo's body plan.

Takeaways

• 😀 Gastrulation follows blastulation and is crucial for positioning embryonic cells into three distinct layers: ectoderm, mesoderm, and endoderm.
• 😀 Ectoderm cells remain at the outermost layer, while mesoderm and endoderm cells must move inward during gastrulation.
• 😀 Mesoderm cells, including primary and secondary mesenchyme, migrate from the vegetal pole into the blastula cavity during the ingression process.
• 😀 Adhesion strength between mesoderm cells and surrounding structures changes during migration, allowing cells to detach and move inward.
• 😀 Filopodia, fine extensions from mesoderm cells, help them navigate through the extracellular matrix to reach their proper positions in the blastula.
• 😀 The mesoderm forms spicules, calcium carbonate structures, as it moves inward, which later support the larval body.
• 😀 Endodermal cells at the vegetal pole undergo invagination, folding inward to form the archenteron, a precursor to the digestive tract.
• 😀 Invagination is driven by changes in the extracellular matrix, which causes inner cell layers to expand while outer layers remain intact.
• 😀 The archenteron extends and narrows as lateral cells move toward the center, reducing the diameter of the blastopore and increasing its height.
• 😀 The roof of the archenteron is pulled toward the animal pole through filopodia formed by secondary mesenchyme cells, contributing to the formation of the mouth.
• 😀 The stomodeum forms when the archenteron roof fuses with the ectoderm, eventually developing into the mouth opening of the embryo.

Q & A

What is the primary purpose of gastrulation in embryonic development?

Gastrulation's primary purpose is to position the cells of the embryo in their correct locations, ensuring that ectoderm cells remain on the outer side, while mesoderm and endoderm cells migrate inward to form the proper structures.

How do mesoderm cells migrate during gastrulation?

Mesoderm cells, such as primary and secondary mesenchyme, migrate into the blastula cavity through a process known as ingress. This migration is driven by changes in cell adhesion properties, enabling them to detach from surrounding structures and move inward.

What changes occur in the adhesion forces of mesoderm cells during gastrulation?

During gastrulation, mesoderm cells experience a decrease in adhesion strength with surrounding cells, while their adhesion to the extracellular matrix increases. For example, adhesion to the hyaline layer decreases from 10^-5 to 10^-7, while adhesion to the matrix inside the blastula increases from 10^-7 to 10^-5.

What role does the hyaline layer play in mesoderm cell migration?

The hyaline layer plays a crucial role in the migration of mesoderm cells by providing a surface on which these cells can move. The adhesion changes between mesoderm cells and the hyaline layer facilitate their detachment and migration into the blastula cavity.

How do mesoderm cells stop migrating during gastrulation?

Mesoderm cells stop migrating when they reach the ventrolateral blastocoel, where they aggregate and form a circle. At this point, they stop moving and begin to function by forming calcium carbonate spicules, which contribute to the larval skeleton.

What is the role of filopodia in mesoderm cell migration?

Filopodia, particularly thin filopodia, help mesoderm cells recognize and bind to the ectoderm cells located at the ventrolateral blastocoel. These filopodia guide the mesoderm cells to their correct positions during migration.

How do mesoderm cells recognize their target location during migration?

Mesoderm cells use fine filopodia to recognize the ectodermal cells at the ventrolateral blastocoel. When these filopodia encounter the ectoderm cells, the mesoderm cells stop migrating and begin forming spicules, contributing to the skeleton.

What is the significance of the archenteron in gastrulation?

The archenteron, formed during invagination of the endoderm, is the precursor to the digestive tract. It forms as a result of the folding of the vegetal plate, and its development marks an important step in the formation of internal structures.

How does the extracellular matrix contribute to invagination during gastrulation?

The extracellular matrix, particularly the hyaline layer, undergoes changes that allow for invagination. When secretory vesicles containing proteoglycans such as chondroitin sulfate are released, they absorb water, causing the inner lamina to expand and initiate the invagination of the endoderm cells.

What is the final step of gastrulation, and how does it contribute to the embryo's development?

The final step of gastrulation is the pulling of the archenteron roof towards the animal pole of the embryo. This is facilitated by secondary mesoderm cells at the archenteron tip, which form filopodia that attach to the animal pole. This movement results in the fusion of the archenteron roof with the ectoderm, forming the stomodeum, which later becomes the mouth.