Describe the process of gastrulation in the sea urchin – what molecular and cellular changes are needed to drive the movements that one observes?
Gastrulation is a critical stage of embryonic development in which a blastula is transformed into a gastrula, establishing the three germ layers (ectoderm, mesoderm, and endoderm). In the case of sea urchins, gastrulation is a complex process involving a series of molecular and cellular changes that drive the movements observed.
The initial step in sea urchin gastrulation is the formation of a small intion called the blastopore. This intion occurs at the vegetal pole of the blastula and leads to the formation of the archenteron, which will develop into the digestive tube of the embryo. The process of gastrulation in sea urchins involves the following stages:
1. Epithelial-to-mesenchymal transition (EMT): The first molecular change involves the induction of EMT in the vegetal cells of the blastula. EMT causes these cells to lose their fixed positions, undergo changes in cell shape, and acquire migratory properties. This change is driven by the activation of specific regulatory genes.
2. Ingression: Once EMT is initiated, some of the vegetal cells detach from the blastula surface and migrate inward towards the blastopore. This process is known as ingression. These ingressing cells, now called primary mesenchyme cells (PMCs), undergo extensive rearrangements and shape changes as they move towards the blastopore.
3. Filopodia extension and adhesion: During ingression, the PMCs extend filopodia, thin cellular extensions that explore the surrounding environment. Filopodia help sense and respond to directional cues, allowing the cells to migrate in a coordinated manner. Adhesive interactions between migrating cells and the extracellular matrix also contribute to their movement.
4. Migration and skeleton formation: As the PMCs migrate towards the blastopore, they secrete a calcium carbonate matrix, forming a spicule that will eventually become a part of the embryonic skeleton. The spicule serves as an anchor point for the PMCs and helps regulate their migration.
5. Convergent extension and archenteron formation: As the PMCs accumulate at the vegetal pole, they undergo a process called convergent extension, where the cells intercalate and become narrower in one dimension while elongating in another. This process shapes the embryo and drives the elongation of the archenteron.
Overall, the molecular changes associated with gastrulation in sea urchins involve the activation of specific genes and signaling pathways that regulate cell adhesion, migration, and differentiation. Understanding the coordination between these molecular and cellular processes is crucial for comprehending the complex movements observed during sea urchin gastrulation.