In the human body there are many types of cells, but all of them are organized in only four basic types of tissue :
- Epithelial tissue
- Connective tissue
- Muscle tissue
- Nervous tissue
Each type of tissue has specific functions and all participate in the health and maintenance of the body as a whole. The combination of one or more types of tissue forms the organs, the body work unit. Understanding how tissues work is fundamental to understanding how organs work.
The four types of fabrics
Epithelial tissue, or epithelium, can be described as leaves or layers of tissue lining surfaces in the human body . These surfaces can be found in cavities in contact with the exterior and in internal cavities and conduits. The epithelial tissue also forms the secretory surface in glands.
In general, the epithelial tissue is formed by closely joined cells and always shows an apical face , which is the one that remains on the surface exposed to the outside of the body or to the light of an internal cavity, and an opposite face called the basal face . The basal layer is usually anchored to a layer of underlying connective tissue.
Among its many functions, we find tissue with marked protective character , for example the skin, absorption , for example the intestine, filtration , as in the kidneys, or secretion , mainly in the glands.
It is common that the epithelial tissue is innervated, but on the contrary, it does not present blood vessels ; to obtain nutrients and oxygen depends on the irrigation of the underlying connective tissue.
Types of epithelial tissue
The epithelial tissue can be classified according to the predominant form of its cells and the number of cell layers. Depending on the cellular form, the epithelial tissue can be :
- scaly : is formed by layers of flattened cells, often in the form of scales.
- cuboidal : formed by layers of cells approximately equal of high that of wide, what of a more or less square or rounded form.
- columnar : the cells are visibly higher than wide.
According to the number of cell layers:
- simple : consist of a single layer of cells.
- stratified : it has two or more layers of cells.
- pseudostratified : there is a single cell layer but they overlap in some areas where it may seem to be stratified.
The categories of each group can be combined, with simple squamous epithelia, stratified squamous, etc.
Connective tissue is the most abundant and widely distributed type of tissue in the human body. Its function is related to the functions of support and support to organs and tissues, both mechanical and metabolic. For example, it envelops and protects organs, stores nutrients, forms tendons and ligaments, and appears in many organs to increase their mechanical resistance, for example in the deep layers of the skin.
The connective tissue can be as different as bone and blood . The first is hard and rigid tissue, the second is liquid but is also considered a type of connective tissue.
What characterizes the connective tissue is the presence of three components : fundamental substance, protein fibers and cells. The fundamental substance is a matrix in which cells and fibers are suspended; it is usually described as a hydrated substance with a gelatinous appearance, with extremes in the degree of hydration, such as those mentioned in bone tissue, whose matrix is ??calcified, and blood with a liquid colloidal matrix.
Types of connective tissue
The connective tissue is classified into three large groups:
- Loose connective tissue: areolar connective tissue, adipose tissue, connective reticular tissue, etc.
- Dense connective tissue: regular dense connective tissue, the fibers are arranged in one direction, as in tendons and ligaments, or irregular dense connective tissue, with the fibers arranged in various directions, for example in the skin.
- Specialized connective tissue : blood, bone tissue, cartilage, etc.
Muscle tissue is characterized by its contractile capacity . Among other functions, it is responsible for body movement, mechanical digestion or the movement of blood.
The contraction occurs when the muscle tissue receives an electrical stimulation from the nervous system, or in the case of the heart, from its own autonomous system: the atrioventricular node and the sinoatrial node .
Muscle tissue of the heart, or myocardium , is one of three types of muscle tissue . The other two are skeletal muscle , striated or voluntary, and smooth muscle tissue . Skeletal muscle is the tissue of the skeletal muscles and its contraction is controlled voluntarily. Smooth muscle is found in the inner walls of blood vessels, the intestine, urinary system and other internal organs, and is usually controlled inadvertently.
The characteristic feature of nerve tissue is the transmission of electrical impulses that carry information to and from the central nervous system and the rest of the body. In its histological organization it is possible to differentiate clearly between the brain, with the white substance and the gray matter , the spinal cord and the peripheral nervous fibers.
The neuron is the cell responsible for the electrical transmission and form the nervous tissue next to the glial and microglial cells .
Tissue membranes represent the simplest tissue combination . Tissue membranes are layers that cover the surfaces of body cavities, envelop organs or coat the interior of the articular cavities.
The tissue membranes may be formed by connective tissue alone or in combination with epithelial tissue:
- Connective tissue membranes : they are covering organs, such as the kidney or heart (pericardium), and encapsulates them. The synovial membrane is also formed by connective tissue and is responsible for secreting the synovial fluid that lubricates the joints .
- Membranes of epithelial tissue : these membranes are formed by a layer of epithelial tissue anchored to a layer of connective tissue, for example mucous membranes and serous membranes . The mucous membranes cover cavities and ducts that connect with the outside, for example the digestive, respiratory, excretory and reproductive tracts. The serous membranes are a kind of bags that cover internal cavities that do not open to the outside and that usually contain more than one organ, for example the abdominal mesentery. The skin is also a type of epithelial membrane, sometimes called cutaneous membrane, it is also formed by layers of epithelial tissue on connective tissue .
In the human body there are more than 200 types of cells that are organized to form tissues that can be of four basic types:
- Epithelial tissue : covers organs and body cavities, both external and internal, and controls the passage of substances between surfaces.
- Connective tissue : forms various parts of the body that give mechanical and metabolic support, from the transport and storage of nutrients, to the protection and damping of organs.
- Muscle tissue: it is contractile tissue that allows the movement of the body.
- Nervous tissue : is responsible for the transmission of information.
The simplest combination of tissues are the tissue membranes, formed by connective tissue or by a combination of epithelial tissue and connective tissue.