The parietal pleura structure is arranged in three layers, which are, in order from super cial to deep:

•\ Mesothelial layer, which is the outermost layer, formed by mesothelial cells.

•\ Super cial elastic layer.

•\ Connective tissue layer, thick, where the blood vessels are located. It constitutes the cleavage plane between the parietal pleura and the endothoracic fascia [7].

Parietal pleura thickness varies, reaching the maximum thickness at the level of the costal arches, and the minimum thickness at the level of the sternum and pericardium [8]. At the cervico-­ thoracic straight height, it thickens in the form of a septum or brous septum described by Bourgery.

capacity and signi cant metabolic activity, so they are responsible, to a certain extent, for pleural fuid composition and quantity. The cytoplasm is also rich in pinocytotic vesicles and mitochondria, which, in turn, are connected by a rich network of intercellular desmosomes.

Fine threads 150 nm in diameter emanate from the super cial glycocalyx between the microvilli, creating compartments between these threads and the last ones, trapping liquid intended to protect the pleural surface [8].

They have phospholipids similar to those of pulmonary surfactant, produced by mesothelial cells, which act by lubricating the pleural surfaces to facilitate movement during the respiratory cycle [9].

Mesothelial Cells Functions

Mesothelial cells functions are as follows:

Mesothelial Cells, Characteristics,

and Function

Cytological Characteristics

They are the cells that cover the pleural cavity inside, and they have the ability to stretch, varying in shape and size depending on the location, and can have a fattened, cubic, columnar, or cylindrical shape.

•\ Flattened: They are quiescent cells on the visceral surface or on the parietal surface on rigid structures such as the costal arches. They will be observed more fattened the greater the lung expansion, at the visceral pleura level [5].

•\ Cubic or columnar: They are cells associated with a fatty substructure.

From the ultrastructural point of view, mesothelial cells are characterized by being covered by an abundant and dense layer of microvilli 0.1 μm in length, which facilitates identi cation in mesothelioma microscopic study.

Mesothelial cells have abundant endoplasmic reticulum indicating a certain secretory

•\ Increase the effective surface favoring phagocytosis and transcytosis (liquid absorption).

•\ Trap hyaluronic acid, which acts as a lubricant to decrease friction between the lung surface and the chest wall.

Regarding its possible diagnostic utility, a small mesothelial cell number can be found in normal pleural fuid. They can also be found in pleural effusions in both transudate and exudate.

Pleural Space Defense Mechanism

Inside the pleural cavity, Kampmeier foci are located, which are macrophages, lymphocytes, histiocytes, mast cells, and undifferentiated mesenchymal cells aggregates that are found surrounding thick blood capillaries and lymphatic channels [10]. They are strategically located in the mediastinal pleura lower portion and are similar to the lymphoid tissue found in the tonsils. Among its functions, pleural space defense stands out through the following actions [11–16]: