Cytokinesis is the final stage of cell division, following nuclear division (mitosis or meiosis). This intricate biological event ensures the partitioning of the cytoplasm and all cellular components into two distinct daughter cells. Its purpose is to guarantee that each newly formed cell receives a complete set of organelles and resources to function independently. This division is necessary for the growth, development, and integrity of multicellular organisms.
The Unique Nature of Plant Cell Division
Cytokinesis in plant cells proceeds differently from that observed in animal cells. This distinction arises from the presence of a rigid cell wall surrounding plant cells. Unlike animal cells, which can form a cleavage furrow that pinches inward to divide the cell, the inflexible cell wall of plant cells prevents such an indentation. Instead, plant cells construct a new cell wall and plasma membrane from the center outwards to separate the daughter cells. This method of division, known as cell plate formation, creates a new partition that becomes the shared boundary between the two new cells. The process of division in plants is centrifugal, expanding from the cell’s interior towards its periphery.
Orchestrating the New Cell Wall: The Phragmoplast
The initial steps of plant cytokinesis involve the formation of a structure called the phragmoplast. This structure emerges during the late stages of mitosis, in the equatorial plane where the new cell wall will form. The phragmoplast is a network composed of microtubules and actin filaments. It appears as a barrel-shaped array, serving as a scaffold for the assembly of the new cell partition.
The phragmoplast plays an organizing role, guiding vesicles derived from the Golgi apparatus to the cell’s center. Microtubules within the phragmoplast provide structural support and act as tracks, facilitating the transport of these vesicles. Actin filaments also contribute to the guidance and expansion of this structure. As the cell plate begins to form and expand, the phragmoplast itself expands outwards, directing the building materials to the growing edge.
Building the Cell Plate: From Vesicles to New Wall
The construction of the new cell partition, the cell plate, is driven by the fusion of vesicles originating from the Golgi apparatus. These vesicles carry components for the new cell wall, including pectin, hemicellulose, and precursors for cellulose, as well as membrane materials. Guided by the phragmoplast, these vesicles travel to the cell’s midline and begin to fuse, forming a disc-like structure.
This fusion process expands centrifugally, moving outwards from the cell’s center. Initially, the cell plate forms as a network of tubules and vesicles that gradually consolidates into a continuous sheet. As it grows, the cell plate eventually fuses with the existing parental cell walls and plasma membranes, completely dividing the original cell into two. The membranes of the fusing vesicles become the new plasma membranes for the two daughter cells.
During this formation, pectin and hemicellulose are deposited as major matrix polysaccharides, forming the initial framework, with cellulose added later for strength. Simultaneously, microscopic channels called plasmodesmata are established. These connections form when strands of the endoplasmic reticulum become trapped within the developing cell plate during vesicle fusion. These trapped strands become desmotubules, creating direct cytoplasmic bridges that allow communication and transport between the daughter cells.