Tissue Level: What is it? History of the Concept, Types of Tissues and Structure of Organs

The human body has many levels of structural organization. The most superficial level is the chemical level, which includes small building blocks like atoms.

Cells are the minor functional units of life.

The simplest living creatures are single-celled creatures, but cells also exist at the tissue level in complex life forms, such as humans.

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 exact origin that carry out a specific function. The functional grouping of multiple tissues forms organs.

The word in English is derived from the French tissue, which means something woven, from the verb tisser, “to weave.” The study of human and animal tissues is known as histology or, about the 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 with histological staining, and the light microscope.

In recent decades, advances in electron microscopy, immunofluorescence, and frozen tissue sections have improved the details 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 distinct 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 offers a means of rapid internal communication by transmitting electrical impulses.

Concept history

The term was introduced in anatomy by Marie François Xavier Bichat in 1801. He argued that the body’s functions 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 comprises 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

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 upper and lower sides. The upper, apical side of an epithelial cell faces the interior of a cavity or the structure’s exterior 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, allowing 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

Connective tissue consists of cells suspended in an extracellular matrix. In most cases, the matrix comprises 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, 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 each other.

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

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, 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 control your movements consciously. 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, the 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 intercalated discs, which allow them to contract in sync.

Smooth muscle is found in the walls of blood vessels and 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 not under conscious control. That means you don’t have to think about moving food through your digestive tract!

Nervous tissue

The nervous tissue is involved in detecting stimuli, external or internal signals, and the processing and transmission of information. It comprises 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 over long distances quickly. The glia acts primarily to support neuronal function.


An organ is a structure made up of at least two or more types of tissues and performs a specific set of functions for the body. The liver, stomachbrain, and blood are all different organs and perform other functions.

Each organ is a specialized functional center responsible for a specific body function.

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, 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.