The endothelium has to be the most unique organ in the human body and perhaps in all higher animals. There are two reasons why. First of all, it flies in the face of the definition of an organ. Secondly, its functions far exceed its simple structure.
The biological definition of an organ is a structure of the body composed of a group of tissues that performs a specific function or group of functions. According to the traditional definition it consists of main tissue that performs the primary function and other tissues that either provide structural support or that perform secondary functions.
In higher animals an organ system is a group of organs that work together to perform a specific body function or task. Organs that make up one organ system oftentimes share functions with one or more other systems. The kidney is one prime example. On one hand it is the main organ of the urinary system. On the other hand it works with the lungs of the respiratory system to maintain a balanced pH in the body. In addition, it also works with the bone marrow of the hematologic system to produce red blood cells by its production of erythropoietin.
The endothelium is the joined group of cells that lines all blood vessels and lymphatic vessels of the body. It is one cell thick with no stacking of the cells. Therefore, it is very thin. The layer that lines the blood vessels – arteries, capillaries and veins – is the vascular endothelium. The layer that lines lymphatic vessels is the lymphatic endothelium. The focus of this discussion is the vascular layer, about which most is known. More times than not, use of just the term endothelium, usually refers to the layer that lines blood vessels.
Endothelial cells that form the endothelium are a type of epithelial cell in that they are the covering of body structures. They are squamous cells. That means they are flat and have the appearance of fish scales. Their overlapping junctions help to seal the vessels and prevent blood from leaking from the vessels into surrounding tissue.
The point of view regarding the endothelium has changed in recent years. The former concept of it was one of it being just one of the tissue layers of blood vessels with a simple barrier function. But extensive research in the last 25+ years has revealed that it has a host of functions and adaptive capabilities. Because of these attributes scholars in the field now recognize it as an organ of the body. It refutes the requirement that an organ has to have more than one type of cell because the endothelium is comprised of only the squamous cells that form it.
Endothelial cells have different functions depending on their context and needs at the time. The context is the particular organ they pass through and the tissue of that organ which surrounds them. Their ability to adapt to their surroundings gives them the versatility to perform specific functions based on their context and its need at the time. Thus, their functions are both location and time specific.
All cells in the body except for mature red blood cells have the same full set of genetic material on their chromosomes within their nuclei. The part of the genetic code expressed though, determines what a cell looks like and what it is capable of doing however. Those attributes in turn determine an organ’s identity, except in the case of the endothelium.
The endothelium is a group of differentiated cells with varying functions. But they are identical in size, shape and other physical aspects. Their functions differ by virtue of adaptations they make based on the needs of their surrounding tissues. They do so by two-way signaling. In essence, they are parts of the same structure, but function like diverse organs in different parts of the body by means of shared communication between themselves and other nearby cells.
The endothelium receives and sends signals to neighboring tissues. Signals received might cause it to release molecules that mediate certain functions. Signals they send might cause the activation of surrounding cells that act as mediators of functions directly or by their release of substances. In some cases, the signals received by the endothelium might result in the expressing of a part(s) of its genome which was, up until that time inactive, but which is necessary for performing a needed function(s).
The one keyword that summarizes the functions of the vascular endothelium is homeostasis. Despite some functions that are in opposition to one another, in the healthy state they serve to maintain a balance throughout the body where blood flows.
Because of the endothelium’s ability to produce numerous vasoactive substances, some medical scholars have likened it to an endocrine organ, even though it is not a part of the endocrine system. Vasoactive refers to the regulation of vascular tone – the degree of constriction relative to full dilation of blood vessels. The type of vasoactive substance(s) released depends on what is necessary for maintaining a state of balance in a given part of the body at a given time.
One well recognized vasodilator the endothelium produces is nitric oxide. In addition to helping maintain a normal blood pressure, it has some other functions. One is involvement in the achieving of penile erection.
In addition to vasoactive molecules the vascular endothelium also produces substances that play a role in metabolism of cells. One such substance is vascular endothelial growth factor (VEGF). It is a signaling protein that stimulates the formation of new blood vessels.
Research in the last 25-30 years has been very revealing. It has shown that the various substances produced by the endothelium:
- promote structure and integrity of blood vessels
- stimulate new blood vessel formation
- affect growth and metabolism in surrounding tissues
- participate in the repair and regeneration of organs
- inhibit unhealthy blood clotting in vessels
- prevent hemorrhage from blood vessels
- participate in innate and cellular immunity
- promote inflammation
- inhibit inflammation
- prevent atherosclerosis
These are but some of the functions of the endothelium. There are probably also others not yet discovered. The endothelium is an organ within many organs, unique in its structure and functions. All in all, it qualifies as the most unique organ of the body.