Different tissues of the body

A tissue is a structure made up of cells that perform the same task for the whole organism. It is possible to differentiate four basic types of tissue based on their mode of development, structure, and function.

- An organ is made up of different types of tissue:

- The parenchyma: composed of the cells responsible for the proper function of the organ;

- The stroma or interstitial compartment (from the Latin interstitium, intermediate compartment): forms the framework of the organ; it contains connective tissue (CT), more rarely epithelial tissue and muscle cells, as well as vessels and nerves.

- The intercellular space is filled with an intercellular substance, or extracellular matrix means the exchange of substances between the blood and the cells, mechanical function (connective and support tissue).

Let’s find together different types of tissues in the human body.

Epithelial tissue

Superficial cell structure; covers the external and internal surfaces of the body, organic cavities and ducts, and channels (covering tissue).

There are various forms.

Basement membrane

Basement membrane (MB): (approximately one μm) separates the epithelium from the underlying connective tissue.

Importance in malignant tumors: if the MB is not yet ruptured, it means no communication with the blood or lymphatic vessels, no metastasis, and a very good prognosis.

Intercellular contacts

The epithelia often have a protective and barrier function, and they form particular intercellular contacts:

- desmosomes (macula adherens): mechanical junction formed by the thickening of membrane segments of two neighboring cells, associated with the presence of an intermembrane cement;

- zonula adherens: structurally similar, it unites epithelial cells;

- tight junctions (zonula occludens): close to free surfaces, they prevent the uncontrolled exchange of substances;

- Communicating junctions (nexus): small "connecting channels," which cross the intercellular space that control the exchange of substances.

Polarity of cells

The appearance of the epithelial cells is not the same on the apical side (near the surface) as on the basal side (near the MB). Explanation: these two sides have different functions:

- basal pole: mechanical connection with the underlying connective tissue;

- Apical pole: depends on the epithelial cell's function: for example, microvilli (brush border), reabsorption, or mobile eyelashes (kinetic).

Superficial (coating) epithelium

The superficial (covering) epithelia cover the internal and external faces of the organism:

- skin epithelium: protects against environmental influences and water loss;

- the lining epithelia of the body's interior: lining, among other things, the hollow organs and the excretory ducts of the glands:

- protect the tissues of the body located deeper from aggressive substances;

- often contain secretory cells as well as glands

- form mucous membranes (mucous or mucous coat, from the Latin mucus, mucus).

Types of epithelia

- They differ in the shape of their cells:

- squamous epithelium ;

- cubic epithelium (cells as wide as they are tall);

- Prismatic or cylindrical epithelium (taller than wide cells).

- The arrangement of cells differentiates them:

- simple (a single layer of cells): all cells are in contact with MB;

- pseudostratified: the cells are in contact with MB, but not all of them reach the epithelium;

- laminate: only the lower cellular layer is in contact with the MB

Functions

- Simple squamous epithelium (single layer of cells): smooth surface; occurs in the pulmonary alveoli, pleura, peritoneum, pericardium, heart chambers ( endocardium ) and blood vessels ( endothelium )

- Laminated prismatic epithelium: lines the respiratory tract, most often the presence of apical eyelashes.

- Stratified squamous epithelium: protects from mechanical, chemical, and thermal influences:

- keratinized: epidermis

- Non-keratinized: oral cavity, nasopharynx, esophagus, vagina.

- Simple prismatic epithelium: lines the digestive tract from the stomach to the rectum as well as the gallbladder. In the form of ciliated epithelium in the small bronchi as well as in part in the uterus and the oviducts.

Connective and support tissues

Connective tissue and support tissue → shape the body and allow it to be maintained. Connective tissue (CT): can be loose, dense, reticular, or adipose. 

- Cell composition: TC contains few cells that are very far from each other and incorporated into the extracellular matrix. One exception: the fatty tissue.

- Main TC cells (e.g., fibroblasts): produce the extracellular matrix.

- Free mobile cells: mainly belong to the mononuclear phagocyte system (monocytes and macrophages) defense 

- Extracellular matrix: gives the various connectives and support tissues their mechanical properties, their resistance, their shape, and their solidity; all the exchanges of substances pass through it.

- Fundamental substance (viscous solution formed mainly of water, proteins, and carbohydrate compounds).

- Fibers: each type of connective tissue is characterized by a mixture of one or more fibers and a specific fundamental substance.

Fundamental substance

- Viscous, homogeneous mass-produced by the cells of the connective tissue. Reservoir for extracellular fluids, exchange of substances between cells and blood.

- Composed of intercellular fluids, glycoproteins, and proteoglycans (giant molecules having a high content of polysaccharides and a low content of proteins; proteoglycans fix the water in tissues

- The fundamental substance is therefore viscous to solid). The supporting tissues (cartilage, bone): mainly a mechanical function.

Fibers

- Collagen fibers: present everywhere and, in particular, in the tendons and ligaments of the joints. High tensile strength

- Holding function. Low elasticity.

- Reticular fibers (reticulin fibers): these are made up of a collagen subtype. Their tensile strength is lower than that of collagen fibers; they, therefore, have certain elasticity (limited). They form a branched network that adapts to different forms. They are present mainly in organs with reticular CT (bone marrow,

- Tonsils, lymph nodes, spleen), and participate in their support. They represent an important component of the basement membrane.

Connective tissue

- Loose connective tissue: poor in fibers, loose arrangement of collagen fibers, contains little elastic and reticular fibers. Fills, in the form of the stroma, the spaces separating the organs. It also fills the spaces separating the different parts of the organs; it maintains the shape of organs and body; it surrounds vessels and nerves; it serves as a water reserve and a sliding layer. It contains a large number of inflammatory and immune cell → defense and regeneration processes.

- Dense connective tissue: They are rich in fiber.

- Non-oriented dense fibrous connective tissue: the collagen fibers form a dense tissue resembling felt, for example, the sclera of the eye, the dura mater; the capsules are surrounding the organs.

- Regular oriented fibrous connective tissue, with parallel fibers: in the tendons.

- Fibroblasts: main cells of loose or dense connective tissue; they are fusiform and provided with extensions. The fibrocytes are fibroblasts at rest.

- Reticular (or crosslinked) connective tissue resembles undifferentiated embryonic connective tissue. The crosslinked, star-shaped cells form a three-dimensional network. They are pressed against the fine crosslinked fibers and mostly seen in the bone marrow and lymphatic organs.

Fat tissue

Adipose tissue: a particular form of reticular connective tissue. 

MEDICAL CONCEPT

Fat

Body energy reserve: storage of lipid droplets composed of neutral fats (triglycerides) and water, in adipocytes. The lipid droplets increase in size and round-off if the body consumes less energy than it absorbs.

The fibers of the connective tissue gather adipocytes into small lobules. Fat tissue is made up of many fatty lobules. It is supplied by a vascular network originating from the capillaries.

- Reserve fats: storage of excess energy mobilization in the event of an energy deficit.

- Supporting fat: padding of certain mechanically stressed body regions (such as the arch of the foot, buttocks) and the thermally insulating layer. Many organs are held in place by support fat (perirenal fat, orbital fat). During organic degeneration, the adipose tissue can fill the spaces left empty. For example, the adipose involution of the bone marrow, which is to say the replacement by adipocytes of the bone marrow, which is no longer used for hematopoiesis)? In the case of starvation, the adipose support tissue is not attacked until all the reserve adipose tissue has been exhausted.

Cartilages

Cartilage is one of the supporting tissues. They are particularly resistant to compression; they resist mechanical stresses, in particular to shear forces.

- Chondrocytes (cartilage cells): are grouped in small clusters (cubicles).

- Perichondrium (cartilage membrane): maintains cartilage. Three types of cartilage can be differentiated according to their proportion in fibers and in fundamental cartilaginous substance.

Hyaline cartilage

as resistant to pressure as it is elastic, covers the articular surfaces, forms the cartilage of the ribs, larynx, rings of the trachea, and parts of the nasal septum (nasal septum). 

Elastic cartilage:

high elasticity because mainly composed of an elastic fibrillar network, typically yellow in color. It can be seen at the level of the epiglottis and the pinna.

Fibrous cartilage:

very rich in collagen fibers organized in a dense structure that is very resistant to mechanical stresses. It is observed at the level of the intervertebral discs of the spine, menisci (crescent-shaped cartilages of the knee joint), and the pubic symphysis (which connects the bones of the pubis).

Bone tissue

The most differentiated support tissue in humans, resistant to pressure, flexion and torsion (rotation around the major axis) due to the properties of its extracellular matrix, the bone matrix: a large amount of calcium salts is stored between the tensile strength collagen fibers.

Author: Vicki Lezama