Technical considerations of the microscopic evaluation of intrapulmonary vessels.

The methods of microscopic evaluation and measurement of the intrapulmo­ nary blood vessels as reported in the literature are reviewed. Difficulties in these procedures and their practical application are considered. The intrapulmonary vasculature has been studied in the past two decades and now forms an important part of our body of knowledge of cardiopulmonary pathophys­ iology. This review will cover some aspects of the methods of microscopic evaluation of the intrapulmonary vessels as used by different workers and will consider some of the difficulties and practicalities of proce­ dures. S ta in in g T echn ique The staining of the microscopic sections for examination of the blood vessels must demonstrate elastic tissue and include a nuclear stain and a counterstain to distin­ guish collagen and muscle tissue. One of the more popular elastic stains is the Verhoeff method using an iodine-ferric chloride-hematoxylin mixture, which also stains the nuclei; it is commonly counterstained with the Van Gieson picrofuchsin stain to distinguish collagen and muscle. This method provides a sharp contrast with black elastic fibers, but the stain seems to lack sensitivity for the fine elastic fibers in the smallest blood vessels. O’Neal et al14 found that overdifferentiation of the Verhoeff stain was a hazard, especially in the lungs of infants. The histotechnologist com­ monly relies on the large vessels to judge the degree of differentiation of the stain and may use a section of aorta as a stain­ ing control. It is difficult for the technol­ ogist to examine the small vessels in an uncovered slide taken from a water solu­ tion, which would be necessary for micro­ scopic control of the staining procedure. Civin and Edwards, using the Verhoeff elastic stain, recorded that the age was variable for the development of the elastic lamina of the pulmonary arteriole, but that the elastic lamina had appeared by the 20th postnatal year.2 On the other hand, O’Neal et al14 used the aldehyde fuchsin stain for elastic tissue and could easily identify the elastic laminae of arterioles in older fetuses. The differences in these ob­ servations may be related to the elastic staining technique. The Weigert resorcin-fuchsin elastic stain is a sensitive agent for elastic tissue and stains deeply to give a sharp contrast. This stain is an iron resorcin lake with basic