Cytokinesis is the final stage of cell division, following the division of the nucleus, where the cytoplasm of a single eukaryotic cell divides to form two distinct daughter cells. This process ensures that each new cell receives its own complete set of organelles and cytoplasmic components. The general purpose of cytokinesis is to complete cell reproduction, allowing for growth, tissue repair, and the propagation of organisms.
Cytokinesis in Animal Cells
In animal cells, cytokinesis initiates with the formation of a cleavage furrow, which is a visible indentation that appears on the cell’s surface. This furrow forms around the equator of the cell. The formation of this furrow is driven by a contractile ring, a dynamic structure located just inside the plasma membrane.
The contractile ring is primarily composed of two proteins: filamentous actin and motor protein myosin II. These proteins work together, with myosin II pulling on the actin filaments, causing the ring to constrict. This constriction action effectively “pinches” or “strangles” the cell in the middle. The cleavage furrow deepens progressively, moving inward from the cell’s periphery towards its center. This inward pinching separates the original cell into two independent daughter cells, each enclosed by its own plasma membrane.
Cytokinesis in Plant Cells
Plant cells, unlike animal cells, cannot form a cleavage furrow due to the presence of a rigid outer cell wall. Instead, cytokinesis in plant cells involves the construction of a new cell wall between the two forming daughter nuclei. This process begins with the formation of a cell plate, which emerges in the center of the cell and grows outwards towards the existing cell walls.
The cell plate is assembled from vesicles that originate primarily from the Golgi apparatus. These vesicles, carrying cell wall components, are transported to the equatorial plane of the cell, guided by a specialized microtubule structure called the phragmoplast. The phragmoplast forms between the separating nuclei and acts as a scaffold, directing the vesicles to fuse and coalesce. As more vesicles fuse, the cell plate expands centrifugally, meaning it grows from the center to the periphery, until it connects with the original parent cell wall. The membranes of these fused vesicles then form the new plasma membranes for the daughter cells, while their contents contribute to the new cell wall material.
Fundamental Divergence: Why the Differences Exist
The fundamental reason for the distinct mechanisms of cytokinesis in animal and plant cells lies in their structural differences, specifically the presence or absence of a rigid cell wall. Animal cells lack a cell wall, possessing only a flexible plasma membrane as their outermost boundary. This flexibility allows the animal cell to be physically constricted and pinched in two by the contractile ring, forming the cleavage furrow. The plasma membrane can readily deform and fuse to complete the separation.
Conversely, plant cells are encased by a rigid cell wall that provides structural support and protection. This strong, non-flexible barrier prevents the cell from undergoing the inward pinching characteristic of a cleavage furrow. Therefore, plant cells have evolved an alternative strategy: building a new wall from the inside out to divide the cellular contents. The cell plate mechanism, guided by the phragmoplast and reliant on Golgi-derived vesicles, is an adaptation to this structural constraint, allowing for the precise and orderly deposition of new cell wall material to bisect the cell.
Implications for Cellular Structure and Function
The distinct modes of cytokinesis have profound implications for the overall structure and function of multicellular animal and plant organisms. In animals, the flexible nature of the cleavage furrow mechanism allows for the formation of diverse tissues with varying shapes and arrangements. This flexibility is important for processes like embryonic development, wound healing, and the movement of organs, where cells need to migrate, rearrange, and form complex structures. The ability of animal cells to round up and then pinch apart contributes to the dynamic and often fluid organization of animal bodies.
In contrast, the cell plate formation in plants, which results in the creation of a new, rigid cell wall, contributes to the fixed and often highly organized structure of plant tissues. The new cell wall effectively glues the daughter cells together, forming a continuous and stable tissue architecture. This mechanism facilitates the characteristic upright growth patterns of plants and provides the mechanical strength necessary for their structure, such as the trunks of trees and the stems of smaller plants. The precise placement of new cell walls dictates the overall morphology and rigidity of the plant body, supporting its stationary lifestyle.