**1.1 Bronchial structure**

The bronchial wall is microscopically composed of the following five sections: mucosa, muscle, submucosa, cartilage, and peribronchial connective tissue (adventitia) (**Figure 1**) [3].

The epithelial and lamina propria layers constitute the bronchial mucosa layer, which has the characteristics of the respiratory mucosa. The initial part of the bronchi exhibits a similar structure to that of the trachea, which is a pathway responsible for the transmission of air taken from the external environment into the lungs. The structure of the bronchial wall changes histologically at the point where it enters the lungs and transforms into intrapulmonary bronchi. In the beginning, the bronchial mucosa comprises a layer of respiratory epithelium with the same cellular composition as the trachea. The height of the cells of this ciliated layer, also known as the pseudostratified columnar epithelium, decreases in proportion to the diameter of the bronchus.

#### **Figure 1.**

*Light microscopic view of the bronchial wall, rat lung (H-E). Black star: bronchial lumen, black arrow: respiratory epithelium layer, white arrow: lamina propria layer, red arrow: smooth muscle layer, yellow arrow: submucosa layer, white star: distinctive lung tissue (LT) showing the many empty spaces of pulmonary alveoli.*

## *Bronchus-Associated Lymphoid Tissue (BALT) Histology and Its Role in Various Pathologies DOI: http://dx.doi.org/10.5772/intechopen.99366*

The prominent cell types in the epithelium are ciliated cells, goblet cells, basal cells, brush cells, and neuroendocrine cells. The epithelial layer is separated from other mucosal layers by a basement membrane [4].

The basement membrane is prominent in the primary bronchi; however, it rapidly decreases in thickness and disappears as a separate structure in the secondary bronchi. The lamina propria layer is similar to the trachea, but it decreases in proportion to the diameter of the bronchi. The lamina propria layer, which appears as a typical loose connective tissue with abundant elastic and collagen threads, is rich in cellular structures. In addition to the cell types such as plasma cells, mast cells, eosinophils, and fibroblasts, it comprises a large number of lymphocyte cells. The lymphocytes in this layer gather in the form of infiltrates at some places and lymph follicles at some [3].

The muscularis layer, which comprises multiple rows of circular smooth muscle cells, is a continuous layer of smooth muscles in the large bronchi. However, in the small bronchi, it is weakly and loosely organized because it may appear discontinuous due to its spiral route. This layer is responsible for determining the appropriate airway diameter for airflow regulation. In the large bronchi, the loose connective tissue submucosa layer is evident, whereas in the small bronchi, it is only observed as a narrow patch. In addition to the venous plexus and lymph follicles, bronchial glands known as GI. bronchioles are quite common in this layer. These glands, similar to salivary gland tissue, comprise a mixture of serous and mucinous cells and decrease in quantity as the diameter of the bronchi decreases (**Figure 2**) [3, 5].

The cartilage layer is observed as a whole in the trachea, whereas it is irregularly present at the beginning of the bronchi in the form of hyaline cartilage. As the diameter of the bronchus decreases, the fragmented cartilage layer becomes smaller and appears as elastic cartilage. On the other hand, the peribronchial connective tissue (adventitia) layer is dense that limits the bronchi from the alveoli and is rich in nerve and elastic fibers in addition to large blood and lymph vessels [3].

#### **Figure 2.**

*A higher power light microscopic view of the bronchial wall, rat lung (H-E). Black arrow: respiratory epithelium layer, white arrow: lamina propria layer, red arrow: smooth muscle layer.*
