The human body has many levels of structural organization. The simplest level is the chemical level, which includes small building blocks like atoms.
Cells are the smallest functional units of life.
The simplest living creatures are single-celled creatures, but in complex life forms, such as humans, cells also exist at the tissue level.
In biology, tissue is a level of cellular organization between cells and a complete organ. A tissue is a collection of similar cells and their extracellular matrix of the same origin that together carry out a specific function. Organs are formed by the functional grouping of multiple tissues.
The word in English is derived from the French tissu, which means something that is woven, from the verb tisser, “to weave.” The study of human and animal tissues is known as histology or, in relation to disease, histopathology. For plants, the discipline is called plant anatomy.
The classic tools for studying tissues are the paraffin block in which the tissue is embedded and then sectioned, histological staining and the light microscope.
In recent decades, advances in electron microscopy, immunofluorescence, and the use of frozen tissue sections have improved the details that can be observed in tissues.
With these tools, the classic appearances of tissues can be examined in health and disease, allowing considerable refinement of medical diagnosis and prognosis. Each type of tissue has a characteristic role in the body:
- The epithelium covers the surface of the body and the lines of the body cavities.
- The muscle provides movement.
- Connective tissue supports and protects the body’s organs.
- Nervous tissue provides a means of rapid internal communication by transmitting electrical impulses.
The term was introduced in anatomy by Marie François Xavier Bichat in 1801. He argued that the functions of the body would be better understood by taking tissues as a unit of study, and not organs. Bichat distinguished 21 types of elemental tissues for the human body, a number later reduced by other authors.
Types of fabrics
Each organ is made up of two or more tissues, groups of similar cells that work together to perform a specific task. Humans and other large multicellular animals are made up of four basic tissue types: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
Epithelial tissue consists of tight cell sheets that cover surfaces, including the outer part of the body, and line body cavities. For example, the outer layer of your skin is epithelial tissue, and so is the lining of your small intestine.
Epithelial cells are polarized, which means they have an upper side and a lower side. The upper, apical side of an epithelial cell faces the interior of a cavity or the exterior of a structure and is generally exposed to fluid or air.
The lower, basal side faces the underlying cells. For example, the apical sides of intestinal cells have finger-like structures that increase their surface area to absorb nutrients.
Epithelial cells are tightly packed, and this allows them to act as barriers to the movement of potentially harmful fluids and microbes. Cells are often joined by specialized junctions that hold them together to reduce leakage.
Connective tissue consists of cells suspended in an extracellular matrix. In most cases, the matrix is made up of protein fibers such as collagen and fibrin in a solid, liquid, or gelatinous substance. Connective tissue supports and, as the name suggests, connects other tissues.
Loose connective tissue is the most common type of connective tissue. It is found throughout the body, and supports organs and blood vessels, and links epithelial tissues with the muscles underneath.
Dense or fibrous connective tissue is found in tendons and ligaments, which connect muscles to bones and bones to each other, respectively.
Specialized forms of connective tissue include adipose tissue, body fat, cartilage, and blood, in which the extracellular matrix is a fluid called plasma.
Muscle tissue is essential for keeping the body upright, allowing it to move and even pump blood and push food through the digestive tract.
Muscle cells, often called muscle fibers, contain the proteins actin and myosin, which allow them to contract. There are three main types of muscles: skeletal muscle, heart muscle, and smooth muscle.
Skeletal muscle, which is also called striated stripe muscle, is what we call muscle in everyday life. Skeletal muscle is attached to the bones by tendons and allows you to consciously control your movements. For example, the quads in the legs or the biceps in the arms are skeletal muscles.
The heart muscle is found only in the walls of the heart. Like skeletal muscle, heart muscle is striated or lined. But it’s not under voluntary control, so luckily, you don’t need to think about getting your heart pumping.
Individual fibers are connected by structures called intercalated discs, which allow them to contract in sync.
Smooth muscle is found in the walls of blood vessels, as well as the walls of the digestive tract, uterus, urinary bladder, and various other internal structures.
Smooth muscle is not streaked or striated, it is involuntary, and it is not under conscious control. That means you don’t have to think about moving food through your digestive tract!
The nervous tissue is involved in the detection of stimuli, external or internal signals, and the processing and transmission of information. It is made up of two main types of cells: neurons or nerve cells and glia.
Neurons are the basic functional unit of the nervous system. They generate electrical signals called conducted nerve impulses or action potentials that allow neurons to transmit information very quickly over long distances. The glia acts primarily to support neuronal function.
An organ is a structure that is made up of at least two or more types of tissues and performs a specific set of functions for the body. The liver, stomach , brain, and blood are all different organs and perform different functions.
Each organ is a specialized functional center responsible for a specific function of the body.
At the organ level, complex functions become possible due to the specialized activities of various tissues. Most organs contain more than one type of tissue.
For example, the stomach consists of smooth muscle tissue to agitate movement while innervated, but it is also provided by blood, which is a connective tissue. The next level is the level of the organ system.
Many organs that work together to achieve a common goal create an organ system. For example, the heart and blood vessels of the cardiovascular system circulate blood and carry oxygen and nutrients to all cells in the body.