Minimal structural changes, not detected by qualitative
assessment require quantitative methods for recognition. This
work is concerned with the development of such a quantitative
technique, and its application to minimal change situations, namely
the visual cortices of rats with retinal dystrophy (Campbell strain)
and of rats which had received a low grade retinotoxic insult.
As well as the anatomy and normal development, the
literature survey included those factors which can influence the
normal development and function of the visual system, and hence
the quantitative data. The quantitation review demonstrated the
diversity of approach of the various authors working in this field,
and revealed how little quantitative data exists on the rat cerebrum.
No reports could be found on visual system quantitation of either of
the situations investigated in this work.
In the section on the problems of quantitation, the high
technical demands and inherent errors are discussed, and the methods
and correction factors to overcome these problems are detailed.
Strict comparability of sections was attained by devising a method
which ensured similar anatomical location in every case, despite
great variation in brain size.
A technique was devised to measure the variable shrinkage of
processing, a problem ignored by many authors. By means of this
Reduction Factor, the exact volume of tissue in a section is known
and quantitative data can be calculated in a standard comparable
From a total of 1,500 sections, after screening by
ophthalmoscopy, post mortem examination, section thickness
control and histological scrutiny, some 950 sections were selected
for quantitation. The 950 sections, from 190 individuals provided
the large sample, necessary because of individual variation.
The quantitative results are presented in graphical and
statistical forms, subdivided by age, sex and strain. These results
show the method revealed differences between the dystrophic
(Campbell) and control (P.V.G.) visual cortices. The Campbell
cortex has a smaller mean cell size, which is produced by two
factors, namely an increase in small cell density and a decrease
in the density of the largest cells. Ultrastructural and special
light microscopic investigation identified these cells as microglia
and neurones respectively.
These results, which have never previously been reported in
the rat compare well with those produced in other species with
visual deprivation and are consistent with the results of biochemical
investigation of the Campbell visual cortex.
The retinotoxic drugs, two diaminophenoxy compounds and
iodoacetate which are retinotoxic in other species at the dose
schedule used, failed to cause any changes in the P.V.G. rat by
ophthalmoscopic, histological or quantitative investigation. It
would seem the rat eye is relatively resistant to such drugs, and
it is suggested that species other than the rat should be used to