The Morphology and Transport of Mucus in Mammalian Airways.
Date
1975
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Abstract
The mucociliary clearance mechanisms in mammalian pulmonary airways have been
re-examined. In this investigation Wistar rats, housed both under specific pathogen
free (SPF) and normal animal house conditions, were examined. An intact airway
. system from the trachea down to the level of the terminal bronchioles was used.
The airway preparation was rapidly isolated and examined under carefully controlled
in vitro conditions. Specimens remained viable for at least 10 h. Mucociliary activity
was observed through the intact bronchial wall with the aid of incident light. This
function could be examined at all levels of the pulmonary tree in the same specimen.
In contrast to the previously described presence of a continuous mucous bl~ket, the
morphology of mucus in the airways of the rat has been shown to be discontinuous.
Mucus is present as discrete particles of varying size. Under the light microscope
these particles appeared to fall into three categories: droplets less than 4 μm in
diameter; flakes 10-70 μm in diameter; and plaques which are conglomerations of
droplets and flakes. Scanning electron microscopy reveals that droplets (i.e. single
particles) may be as small as 0,5 μm and that composite particles made up of
numerous aggregated droplets may be as small as 5 μm in diameter. Plaques are
conglomerations of these particles. In the normal intrapulmonary airways only
smaller particles are seen and are transported over the individual metachronal fields.
In the larger extralobar airways these particles move together to be transported in
well defined streams which may be up to 500 μm wide. These streams may follow
a winding course up the trachea and more than one may be in operation at a time.
Under conditions of hypersecretion such as occurs with chronic respiratory disease in
rats (CRD) the number of particles increase peripherally and plaques may be found in
small airways. The transport of mucus is however still intermittent and it never becomes
confluent. Acute bronchitis results in wide-spread abnormalities of ciliary activity
and mucus transport, which leads to total disorganisation of pulmonary clearance.
"Chronic bronchitis" associated with CRD results in more organised abnormalities of
mucociliary activity. Cilia may become inactive, reverse the .direction of their effective
stroke, beat retrogradely, and exhibit abnormal beat patterns which result in impaired
mucus clearance. Squamous metaplastic areas further impede mucus transport. In
general mucus transport rates were found to be faster in rats with "chronic bronchitis"
than SPF rats, provided that the extent of the damage to the mucous membrane was
(
not too great in the "bronchitic" animal. This finding was confirmed by the
examination of airway preparations approximately 19 h after the exposure to a
charcoal aerosol. While significant amounts of charcoal were retained at the bifurcations
of bronchi in SPF rats, most of the charcoal was cleared in "bronchitic rats'.'. The
only areas where particles were seen were on bronchitic patches or on whirlpools.
The findings of this study indicated that mucus was present in a discontinuous form,
and that in both SPF and non-SPF animals no evidence for a mucous blanket was
found.
Description
A· Thesis submitted to the Faculty· of Medicine,University of the Witwatersrand, Johannesburg
for the degree of Doctor of Philosophy in Medicine Johannesburg.