Tissue (biology)

In biology, a tissue is an assembly of similar cells and their extracellular matrix. These cells share the same embryonic origin and work together to perform a specific function. Tissues occupy a unique organizational level between individual cells and complete organs. An organ is formed when multiple different tissues group together for a shared purpose. The English word "tissue" comes from the French word "tisser," which means "to weave."

The study of these structures is called histology. When scientists study tissues in relation to disease, the field is called histopathology.

To see these tiny structures, scientists use several classical tools. They often embed tissue in a paraffin block to create a stable sample. This sample is then cut into very thin sections. Researchers use histological stains to color the parts and an optical microscope to see them. Modern science has added even more detail through electron microscopy and immunofluorescence. Scientists can also use frozen tissue-sections to observe cells. These advanced tools allow doctors to examine tissues in health and disease. This helps them improve medical diagnosis and prognosis.

Plant anatomy is organized into three broad tissue systems. The epidermis is the outer surface of leaves and young plant bodies. Ground tissue is less differentiated and performs many different jobs. It manufactures nutrients through photosynthesis and stores reserve nutrients. Finally, vascular tissue acts as the plant's internal transport system. This system is made of two primary components: the xylem and the phloem.

These tissues move fluids and nutrients throughout the plant body.

Plant tissues are also categorized by their ability to grow. Meristematic tissue consists of cells that are actively dividing. These cells lead to an increase in the length and thickness of the plant. This tissue is found in specific regions like the tips of stems and roots. Meristematic cells are often spherical, polyhedral, or rectangular in shape. They have thin, elastic cell walls made of cellulose. These cells have a dense cytoplasm and a prominent nucleus. Because they focus on multiplying, they have very few vacuoles.

There are three main types of meristematic tissue: apical, lateral, and intercalary.

Apical meristem is found at the growing tips of stems and roots. It is responsible for primary growth, which is the linear increase in length. Lateral meristem causes secondary growth, which increases the diameter or girth of the organ. This usually occurs beneath the bark as cork cambium or in vascular bundles. Intercalary meristem is located between permanent tissues, often at the base of nodes or leaves. It helps increase the size of internodes and aids in branch formation. As meristematic cells grow and mature, they undergo cellular differentiation. This means they take on a permanent shape, size, and specific function.

Permanent tissues are groups of living or dead cells that have lost the ability to divide. They are placed at fixed positions within the plant. Simple permanent tissues consist of only one type of cell. Parenchyma is a common type that provides support and stores food. Some parenchyma, called chlorenchyma, contains chlorophyll for photosynthesis. In aquatic plants, a type called aerenchyma creates air cavities to help the plant float. Collenchyma provides mechanical support and elasticity to young stems. It has thickened corners made of pectin and cellulose. Sclerenchyma consists of thick-walled, dead cells that provide rigid support. These cells often contain lignin, a substance that makes them hard and impermeable to water.

Complex permanent tissues consist of more than one cell type working as a unit. These are often called conducting or vascular tissues because they transport materials. Xylem is the chief conducting tissue for water and inorganic solutes. It is organized in a tube-like fashion and includes vessels and tracheids. Vessels are long tubes made of cells that are open at each end. Tracheids are also present, but they are tapered and lack end openings. Instead, they use pit pairs to allow water to pass between cells.

Phloem is the other part of this plumbing system. It carries dissolved food substances like organic solutes throughout the plant. Phloem is composed of sieve-tube members and companion cells. Sieve tubes have small pores called sieve plates that allow cytoplasm to connect cells.

While sieve-tube members lack nuclei at maturity, companion cells help manage the conduction of food.

Animal biology follows a similar organizational pattern. Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial. Just like in plants, these tissues work together to form organs. For example, a lung is made of many different tissue types working in unison.

By studying these tissues, scientists can understand the complex systems that keep animals alive.