A light microscopy section of part of a murine lung

B=Bronchiole
TB=Terminal bronchiole
RB=Respiratory bronchiole
AD=Alveolar duct
A=Alveolus

The Aspergilli are inhaled in the form of spores(conidia) and to a lesser extent of hyphal fragments released from colonised substrates such as grain and decaying plant remains. Histologically, all mammalian lungs have a gross similarity. From the trachea downwards generations of airways of progressively diminishing size lead eventually to the alveoli in which the primary lung function of gaseous exchange takes place. In the larger bronchi, which are supported by cartilage, submucosal glands secrete mucus augmented by lysozyme and IgA. Further mucus is added by goblet cells which are a component of the columnar layer of ciliated cells lining the bronchial lumen. The upwardly directed beating of the cilia propels the mucus towards the larynx from which it is swallowed or expectorated. The mechanism is generally referred to as the mucociliary escalator. Submucosal glands are absent from the remaining lower levels of the airways and only a few goblet cells occur in the ciliated epithelia of bronchioles. The integrity of the lumina of bronchioles downwards is assured by the occurrence of smooth muscle as far as the respiratory bronchioles. At the level of the respiratory bronchioles, alveoli first make their appearance and become prolific at the alveolar ducts.

A transmission electron micrograph of murine lung parenchyma

A=Alveolus
C=Capillary
L=Capillary lumen
PNI=TypeI Pneumocyte
PNII=TypeII Pneumocyte
RBC=Red blood corpuscle

Most of the wall of an alveolus is composed of blood capillary and the flattened plate-like processes of Type I Pneumocytes, the organelles of which are confined to a perinuclear area. The processes are regarded as controlling the movements of fluids. This type of cell does not regenerate following injury and therefore would allow flooding of adjoining alveolar spaces. Type II Pneumocytes are much fewer in number. They are rounded, possess microvilli and also, significantly, osmophilic laminated bodies which store the surfactant which they manufacture. The main functions of surfactant are to provide a coherent monolayer lowering surface tension in the lung, to equalise pressures and to keep the alveoli open by preventing collapse during expiration. The cells are capable of regeneration following injury. Surfactant is a mix of lipids and proteins. Two proteins SP-A and SP-D are capable of binding to the surface carbohydrates of viral, bacterial and fungal pathogens causing agglutination followed by killing due to phagocytosis by macrophages and neutrophils. The role of such proteins in blocking allergenic sites to the binding of IgE is also probable. It is to the respiratory airways that inhaled conidia and hyphal fragments are introduced. The degree of their penetration within the system is dependent primarily on size, aerodynamic shape and surface texture. It is generally assumed that particles greater than 10 microns are unlikely to penetrate beyond the nasopharynx and are cleared by sneezing or coughing. Those in the region of 5 and less than 10 microns will enter the mucus blanket to be transported upwards to the larynx and be expectorated or swallowed. Particles of about 2 to 4 microns will enter the alveolar system. Many conidia of Aspergillus species would fall into this category. Initially, cellular defence against infection is mainly associated with macrophages which occur within alveoli and probably represent a resident population. These are actively phagocytic cells which engulf foreign particulate materials and pathogens. They can then migrate towards the mucociliary escalator or alternatively to the blood capillaries or the lymphatics and eventually eliminate their phagocytozed contents.

Any comments to Dr.W.Blyth wblyth01@genie.co.uk

Walter Blyth worked on Medical Mycology as a chosen research activity in the Department of Botany at Edinburgh University where he set up an Experimental Mycoses Unit. He retired from lecturing early in the 1980s.
During the last few years he converted slides and prints for computer use, amongst them a Medical Mycology collection.
Dr Blyth published many of these images originally on his website, but they are now contained in the relevant species section of th image library here. The material for this came from work done together with members of Staff of the Respiratory Unit of the then Northern General Hospital in Edinburgh and in the Experimental Mycoses Unit.

This material and images are reproduced here with kind permission from Dr Blyth.