What Are Tissues?

Plants, for the most part, do not rely heavily on rapid movement to survive, as animals do. They tend to have fairly rigid structures that change slowly. Plant cells contain a cell wall, a rigid carbohydrate structure that provides overall support and protection for the cell. It is found in some, but not all, cells. Adjacent plant cells are in direct contact by their cell walls. Plant cells can control the rigidity of the extracellular matrix by secreting varying amounts of substances, giving rise to tough, woody stems or soft, pliable leaves and petals as required. Plants can then alter the shape of a tissue either by growing more cells in a particular location or by adjusting the water content within the vacuoles of the cells of a certain tissue. The water pressure within the cell is known as turgor pressure.

When a plant cell newly forms, its cell wall is thin and flexible. This allows the cell to grow to the proper size. Once the cell has reached the proper size, it thickens and toughens the cell wall with cellulose microfibrils. A cellulose microfibril is a bundle of polysaccharide chains that contributes to plant cell wall structure. These fibers are actually synthesized outside of the cell membrane by enzymes embedded in the cell membrane itself. Cellulose microfibrils interweave with other polysaccharides and structural proteins, forming a dense, tough fiber network. This makes the cell wall resistant to bending, stretching, and breaking and also adds strength to the tissue. Because of this resistance, the plant cell must carefully align the network to allow for growth. The network of cellulose microfibrils orients in the direction of growth, allowing the plant cell to grow as needed without rupturing. The orientation of the cellulose microfibrils is guided by the orientation of the cytoskeleton within the cell. The cytoskeleton, a network of microtubules and other protein structures within the cell, forms tracks for enzymes that secrete cellulose to follow. Thus, the shape of the microfibrils outside the cell mirrors the shape of the cytoskeleton inside the cell.

Plants can also add to rigidity by retaining a network of dead cells held together by cellulose microfibrils. Large spaces between the cells are packed with a network of cellulose microfibrils, yet the cells are no longer living. The dead cells do not require access to vascular tissue, which provides nutrients and waste removal for living cells. Many of the dead cells are desiccated, making them hard and dry. The spaces within the dead cells are packed with lignin, an organic molecule that makes the cell very hard and difficult to deform. This adds to the rigidity of the tissue. This type of tissue, which is dense and tough, makes up a large portion of the trunks of trees, as well as other woody tissues in other plants.

To move water and nutrients through their tissues, plants employ small channels that connect cells together. A plasmodesma (plural, plasmodesmata) is a small channel connecting the cytoplasm of adjacent plant cells through which molecules pass. Plasmodesmata are lined with cell membrane, making the cell membrane actually continuous between cells.

Unlike plants, animals tend to move within their environments, responding more quickly to changes and stimuli. Thus, their tissues must be pliable enough for them to react and move.

There is a major distinction at the cellular level between connective tissues and the muscle, nervous, and epithelial types of animal tissues. Connective tissues are distinctive by having a ubiquitous extracellular matrix that carries the mechanical load. Other types of tissues have comparatively little extracellular matrix, and the cells themselves carry much of the mechanical load and are especially adapted for the transmission of electrical signals.

Collagen is the protein that makes up the bulk of the extracellular matrix in animal tissues. It is a protein that is unique to animals. A collagen fiber, a rope-like structure found in some tissues, is composed of a bundle of collagen fibrils, which are themselves composed of three single collagen chains twisted together. This protein structure is tremendously strong yet also highly flexible, similar to rope. In other tissues, the triple-helical chains of collagen peptides spread out in a matrix, similar to fiberglass. This sheetlike matrix provides a tough, flexible material that can deform without being damaged. A fibroblast is a cell that makes the extracellular matrix, including collagen. Fibroblasts also synthesize other proteins and saccharides for export and assembly in the extracellular matrix. To ensure the molecules end up where they are needed rather than accumulating just outside the fibroblast, most of the proteins and saccharides are secreted as precursor molecules. They are then modified and finally assembled at their destination, in the proper place in the extracellular matrix. Fibroblasts have also been observed moving along collagen fibers, orienting them in particular directions. It is thought that fibroblasts similarly create large-scale order in connective tissues, such as by organizing tendons. Fibroblasts are known to play an important role in the healing of wounds.

Fibronectin is an extracellular protein that attaches cell membrane proteins to collagen. This connection between animal cells and fibronectin is mediated by an integrin, a receptor protein embedded in the cell membrane. Integrin is attached to actin filaments inside the cell, allowing for linkage between the extracellular matrix and the cytoskeleton. By using integrin proteins to bind with fibronectin molecules on opposite sides of the cell simultaneously, cells are able to pull themselves along the extracellular matrix.

Collagen provides elasticity in animal tissues, but it does not resist compression. That role is filled by glycosaminoglycans. A glycosaminoglycan (GAG) is a polysaccharide chain of repeating disaccharides, usually linked to a core protein, that forms part of the extracellular matrix. These units often form a chain together, resulting in an enormous aggregate with a bottlebrush shape and a very high molecular weight.
Dense connective tissues, such as bones and tendons, have a larger concentration of collagen, while jellylike connective tissues, such as the tissues of the eye, have more GAGs. GAGs are hydrophilic and form gels in aqueous solutions, which draw in more water from their surroundings, adding osmotic pressure to the gel. The pressure gives the structure greater strength, which allows it to bear heavier loads. These aggregates, for example, make up much of the cartilage that lines the knee joint.