Introduction to Cellular dynamics

The late 1800s saw a shift in the way scientists viewed the cell. They came to understand the cell as the basic unit of anatomy and physiology, which prompted embryologists to focus their field on the cell as well. However, unlike the adult organism, embryonic cells did not stay put and retained their shape, leading to the conclusion that they were dynamic and in constant motion.

Types of Cells in the Embryo

In the embryonic stage, there are two main types of cells, epithelial cells and mesenchymal cells. Epithelial cells are tightly connected to one another, forming sheets or tubes, while mesenchymal cells are loosely connected and can act as individual units. These cells bring about morphogenesis through variations in cellular processes such as cell divisions, shape changes, migration, growth, death, and changes in membrane composition or secreted products.

Cell Divisions

The number and orientation of cell divisions are critical in determining the final shape and form of the developing embryo. For instance, the faces of two dog breeds will have different shapes based on the number and orientation of cell divisions, as seen in breeds like German Shepherds and Poodles. Similarly, the skeleton-forming cells of different dog breeds will undergo varying numbers of cell divisions, leading to different leg lengths.

Cell Shape Changes

Cell shape changes play a crucial role in embryonic development. The changing shape of epithelial cells often leads to the formation of tubes from sheets, such as the neural tube. A transformation from epithelial to mesenchymal cells is necessary for individual cells to migrate away from the epithelial sheet, as seen in muscle cell formation. This epithelial-to-mesenchymal change is also seen in cancer, where cancer cells migrate and spread from the primary tumor.

Cell Migration

For cells to reach their final destinations, they must migrate. This is seen in the germ cells, which migrate into the developing gonad, and the primordial heart cells, which meet in the middle of the vertebrate neck and migrate to the left part of the chest.

Cell Growth

Cells can also change in size during embryonic development. The most notable changes occur in the germ cells, where the sperm eliminates cytoplasm and becomes smaller, while the developing egg conserves and adds cytoplasm, becoming comparatively larger. Other cells undergo asymmetric cell division, leading to one big cell and one small cell, each with a different fate.

Cell Death

Cell death is an integral part of life, and in the embryonic stage, certain cells die at specific times and places. For example, the cells of the webbing between our toes and fingers and the cells of our tails die before birth. The orifices of our mouth, anus, and reproductive glands all form through apoptosis, the programmed death of specific cells.

Changes in Membrane Composition and Secreted Products

The cell membrane and secreted cell products influence the behavior of neighboring cells. For instance, extracellular matrices secreted by one set of cells can allow the migration of neighboring cells, while those secreted by other cell types can prohibit migration. In this way, paths and guiderails are established for migrating cells.

Conclusion

Embryonic cells are dynamic and in constant motion, and the final shape and form of the developing embryo are the result of cellular processes such as cell divisions, shape changes, migration, growth, death, and changes in membrane composition or secreted products. Through these processes, cells establish paths and guiderails, ensuring that they reach their final destinations and contribute to the formation of the fully developed organism