BRONCHIAL SUBMUCOSAL
GLANDS
The submucosal glands of the human airways are of the branched tubuloacinar
type: tubulo refers to the main part of the secretory tubule and acinar
to the blind end of such a tubule.
Three-dimensional reconstruction of
the gland reveals its various zones:
1. The origin is referred to as the ciliated duct and
is lined by bronchial epithelium with its mixed population of cells. With the
naked eye, the origin of the gland is seen as a hole of pinpoint size in the
surface epithelium of the bronchus.
2. The second part of the duct expands to form the collecting
duct and is lined by a columnar epithelium in which the cells are
eosinophilic after staining with hematoxylin and eosin. Ultrastructural
examination shows these cells to be packed with mitochondria, resembling the
cells of the striated duct of the salivary gland (except that they lack the
folds of membrane responsible for the appearance of striation). The collecting
duct may be up to 0.25 mm in diameter and 1 mm long. It passes obliquely from
the airway lumen, so the usual macroscopic section does not include the full
length of the duct. It is usually seen as a rather large “acinus” composed of
cells without secretory granules.
3. About 13 tubules rise from each collecting duct.
These may branch several times and are closely intertwined with each other. The
secretory cells lining these tubules are of two types: mucous and serous.
Mucous cells line the central or proximal part of a tubule; serous cells line
the distal part. Outpouchings or short-sided tubules may arise from the sides
of the mucous tubules, and these are lined by serous cells. The peripheral
portion of a tubule usually branches several times, and each of the final blind
endings is lined with serous cells.
The gland tissue is internal to a
basement membrane. In addition to the cell types described above, the following
are found: (1) myoepithelial cells; (2) “clear” cells; and (3) nerve fibers,
including motor fibers. Outside the basement membrane, there are rich vascular
and lymphatic networks and the nerve plexus.
In histologic cross-sections, the
submucosal gland is seen as a compact structure. In a main bronchus of an
adult, the gland is about 0.2 mm in diameter or less than one-third the
thickness of the airway wall (measured from the luminal surface to the
cartilage layer). This ratio is similar in both children and adults and is
consistent throughout airways at various levels of branching. The ratio of
gland size to wall thickness (sometimes
referred to as the Reid index) is a useful way of assessing
abnormalities in gland size because gland hypertrophy is a hallmark of a number
of inflammatory diseases of the large airways.
In humans, the secretory tubules of
the mucous and serous cells contain mainly an acid glycoprotein, either sialic
acid or its sulfate ester.
The concentration of bronchial
submucosal openings in the trachea is on the order of one gland opening per mm2.
The glands become sparser towards the periphery of the lung, their decrease in
number and concentration being parallel to the diminution in the amount of
cartilage in the airway.
