The epithelial-mesenchymal tropic unit is a structure recently described in the upper airway of the lung. [It is diagrammed and described in http://www.inhalation.net/Fibroblast_Sheath.htm  .] The epithelial-mesenchymal tropic unit consists of layers of mesenchymal and epithelial cells separated by the basement membrane zone (BMZ). (See Figure 1 below) The basement membrane or basal lamina divides epithelium from mesenchyme or extracellular matrix in all tissues. The basement membrane consists of 3 distinct layers as examined with EM. Collagen accumulation in the lamina reticularis layer of the BMZ in large airways of humans is related to asthma and supepithelial fibrosis. The collagen accumulation and associated remodeling causes thickening thought to protect against asthma-induced airway narrowing and air trapping. It is not known how this remodeling affects the functions of the basement membrane. The study presented here addresses the effects of remodeling in infant monkeys.

Functions of the epithelial-mesenchymal tropic unit are currently known to include:

1) Attachment of the epithelium to the extracellular matrix.

2) Binding of growth factors, ions, hormones.

Fibroblast growth factor-2 (FGF-2) is stored in the BMZ of all lung epithelium and endothelium and smooth muscle cells. It is stored with the heparin sulfate proteoglycan, perlecan.

3) Communication between cells.

4) A physical barrier between the airway lumen and subepithelial region of the lung.

 

This study questioned whether remodeling of the BMZ occurs in infant rhesus monkeys sensitized to HDMA. Structural changes were detected in the BMZ of young monkeys by measuring collagens III, IV, and I. Distribution of perlecan and FGF-2 revealed functional changes in the BMZ.

The development of the BMZ occurs during the first 6 months after birth in rhesus monkeys. FGF-2, the topic of this study is produced by the basal cells during development of the BMZ. It is the main growth factor stored in the BMZ of monkeys as well as rats. The purpose of FGF-2 is thought to be to enable the lung to respond rapidly to environmental changes. FGF-2 is bound to perlecan when stored resulting in perlecan acting as a regulator of FGF-2 function. There is significant remodeling of the BMZ in adult rhesus monkeys sensitized with house dust mite allergen (HDMA) and later exposed to HDMA.

Twelve 30 d old rhesus monkeys (Macaca mulatta) from the California Regional Primate Center were used in this experiment. Six of these were sensitized to HDMA by subcutaneous injection and i.p. injection of Bordetella pertussis cells. They were exposed to an aerosol of HDMA. The other 6 were sham exposed to filtered air. More details of this monkey model of sensitization can be found in Schelegle, 2001 (see reference below).

Tracheal rings were used for morphology and immunohistological studies after sacrifice. Five um paraffin sections were stained with H and E. Collagen was visualized with an antibody kit as was perlecan.   Morphometry was used to measure the width of the BMZ.

Tracheal ring diameter was similar in both HDMA-treated and control groups as was the height of the columnar epithelium. Morphology of the epithelium was also similar in both groups. The BMZ appeared wider and more goblet cells appeared to be present in the epithelium in the HDMA group. Leukocyte trafficking was observed in focal areas of the BMZ in the HDMA group. Areas with leukocyte trafficking had thinner BMZ and weaker staining for both perlecan and collagen.

There was a significant increase in collagen in the BMZ of HDMA treated animals compared to controls. There was also significantly more perlecan and FGF-2 in the BMZ of HDMA treated animals than in controls.

When BAL is collected from asthmatic humans, baseline levels of FGF-2 are higher than those in controls. Similarly there is rapid release of FGF-2 from the BMZ after oxygen-induced pulmonary injury in mice and rats. Thus FGF-2 seems to be important for repair of the lung, specifically the basement membrane in mammals. Increased FGF-2 in primate basal cells is related to the growth of the basement membrane zone in developing airways. This suggests that the BMZ was being remodeled in the HDMA treated group. The finding of increased BMZ width in HDMA treated animals allows potential storage of increased volumes of FGF-2. This leads to the conclusion that increased amounts of FGF-2 exist in the tracheal BMZ of infant rhesus monkeys sensitized and exposed to HDMA compared to control monkeys. Thus there should be a large pool of FGF-2 available in response to stress on the lung.

It should be noted that remodeling of the BMZ also involves increased deposition of collagens I, III, and V causing thickening of the region. Tenacin and fibronectin are also found where there is BMZ thickening. Although other growth factors as well are involved in airway remodeling, only FGF-2 is stored in the BMZ.

Focal areas of BMZ observed with leukocyte trafficking allowing influx of immune cells and eosinophils into the airway lumen were also noted in the HDMA group. The leukocytes are known to enter through pores or by degradation of the BMZ by proteases. This protease degradation of the BMZ is also seen in human asthmatics. The authors conclude that proteases released by leukocytes degrade collagen, perlecan and FGF-2 in the BMZ of HDMA treated animals. Most allergens in the air are potent proteinases leading to another conclusion that repeated exposures to HDMA cause focal damage to the BMZ and stimulate immune responses.

As FGF-2 decreased in areas of leukocyte trafficking in the BMZ, there is increased expression of FGF-2 in and around basal cells of the tracheal epithelium. The receptor for FGF-2 in the basal cells is FGFR-1 (fibroblast growth factor receptor-1). It is only found in the basal cells where is can initiate signaling pathways. The authors conclude that FGF-2 associated with basal cells is synthesized and used in a paracrine or autocrine manner by these cells.

This research indicates an important relationship between basal cells, the basement membrane zone, and FGF-2. The same relationship is seen in skin and prostate. Because of their central position in the trophic unit recently described in the lung and diagrammed below, basal cells are able to interact with the remaining columnar epithelium of the lung, neurons, migrating immune cells, eosinophils, basement membrane and the mesenchymal cells below the BMZ. A variety of molecules associated with the above processes have been identified as expressed by basal cells.

It seems that FGF-2 signaling via basal cells is integral to essential processes in the developing airway of the rhesus monkey. These include growth, differentiation, and the response to leukocyte trafficking. The result of these processes is remodeling of the lung.

Editorial comment: This paper may not seem, at first glance, relevant to the field of inhalation toxicology. However it is a significant paper, which helps to explain the anatomical changes that occur during development of allergic responses. These anatomical changes lead to "remodeling" of the lung. In this case, an increase in the thickness of the basement membrane is demonstrated. It is not clear whether the inner diameter of the trachea also changes in the allergen challenged animals. However, it is possible that changes in the diameters and branching patterns of airways may be altered in allergen treated animals. This could result in an altered deposition pattern of inhaled particles and gases. Structural changes in human lungs are known to occur in certain occupational diseases. (see http://www.inhalation.net/Occupational_respiratory_disease_htm.  )

It is also fascinating to find that the basal cell seems to have a pivotal role in regulation of the functions of the entire epithelial-mesenchymal unit when only a few years ago the function of the basal cell was unknown. It was originally thought to be the stem cell of the airway epithelium, a theory which later came into hot dispute.

Figure 1: Cross section of trachea illustrating the epithelial-mesenchymal tropic unit. It consists of 3 parts: 1) The BMZ (basement membrane zone) shown in light pink. It is between the epithelium on the inside of the trachea and the attenuated fibroblast sheath (magenta color) on the outside. The fibroblasts are on the outside of the trachea. 2) The pseudo-ciliated columnar epithelium is as labeled. 3) The mesenchymal cells, which are the fibroblast layer, are on the outside of the trachea. The fibroblast nuclei are only seen in the left side of the illustration. Note that the fibroblasts cover the outside of the trachea and are thought to continue down throughout the entire pulmonary tree.

Surrounding the fibroblast sheath is interstitial space containing more fibroblasts as well as blood vessels, migratory cells and nerves. The entire trachea is surrounded by cartilage and muscle.

Modified from: http://www.inhalation.net/Fibroblast_Sheath.htm.  (Illustration by Susan G. Shami, ScD) The diagram was originally drawn to illustrate the attenuated fibroblast sheath and now serves to illustrate the concept of the epithelial-mesenchymal tropic unit.

Reference for model: Schelegle ES et.al. American Journal of Pathology 158:33-341 (2001).

By: Susan G. Shami, ScD, Editor
    http://www.susanshami.com/index.html
    20  Feb 03