1.9.2 Normal Airway Epithelium 
Together, the epithelium of the bronchi, submucosal glands, and bronchioles 
is comprised of 6 major cell types: ciliated cells, goblet cells, serous 
cells, undifferentiated columnar cells, basal cells, and Clara cells (Fig- 
ures 1.9.2-A, 1.9.2-B; see Breeze and Wheeldon, 1977; Basbaum and 
Finbeiner, 1989; Massaro, 1989 for general reviews). Although some investi- 
gators describe the goblet and serous cells in the submucosal glands sepa- 
rately from their counterparts in the epithelial cell layer on the air 
surface, as there is no evidence they are clearly different or play a 
different role in CF, they will be considered together. Also, airway epi- 
thelium includes a number of minor cell types (minor in number, not neces- 
sarily in relevance to airway function) such as "K cells" (neuroepithelial 
cells), brush cells, and mast cells; however, as there is no evidence these 
cells play a primary role in the pathogenesis of CF, they will be ignored 
for this review. 
Ciliated cells - these columnar cells are found throughout the airways. In 
the bronchial epithelium there are approximately 5 times more ciliated 
cells than any other cell type. Ciliated cells form the lining of the 
opening of the collecting ducts of the submucosal glands. In the 
bronchioles the ciliated cells also dominate in numbers, but less so than 
in the upper airways. The cytoplasm of ciliated cells is relatively sparse, 
but rough endoplasmic reticulum (RER) and Golgi apparatus are present. On 
their apical surface, each cell contains approximately 250 cilia, each 6 /ttm 
x 0.3 nm. The cilia beat in a coordinated fashion toward the pharynx at 
about 12-20 Hz. 
Goblet cells - also referred to as "mucus cells", these columnar cells are 
called "goblet" cells because they are shaped like wine glass in longitudi- 
nal section. Goblet cells are present in the bronchial epithelium and the 
submucosal glands. In normal bronchi, there are approximately 6.8 x 10 3 
goblet cells/mm 2 . Toward the peripheral airways, their numbers progressive- 
ly decrease, and they are absent in the bronchioles. Goblet cells are 
characterized by a cytoplasm dense in mucus granules and extensive RER and 
Golgi. The mucus granules coalesce toward the apical surface and are se- 
creted through pits or pores on the surface. When discharged from the 
surface, the granules often retain an intact limiting membrane, although 
they are also discharged as confluent clumps. 
Serous cells - these columnar cells are common in the submucosal glands, 
but are also present in the surface epithelium of the bronchi, although to 
a lesser extent than goblet cells . The cytoplasm contains a variable number 
of secretory granules. The serous cells have a small number of surface 
microvilli. The contents of the secretory granules are discharged as drop- 
lets that individually fuse with the surface. 
Undifferentiated columnar cells - these are columnar cells of similar 
dimensions to ciliated cells. The cytoplasm is relatively sparse and, as 
the name suggests, the cell has no special features. These cells are found 
in bronchi, glands, and bronchioles. In bronchi, these cells represent 
approximately 12% of the airway epithelial cells (Breeze and Wheeldon, 
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