Retinal blood vessels consist of endothelial tubules surrounded by a single layer of
smooth muscle cells or pericytes, collectively called the mural cells. Coverage of the
vessel with mural cells is suggested to signal their stability, and requires the actions
of several locally secreted growth factors including: platelet-derived growth factor,
which promotes mural cell migration and proliferation; transforming growth factor-/)1,
that induces the differentiation of precursors cells into pericytes and smooth muscle
cells; and the angiopoietins, for maintaining the interactions between endothelial
cells and pericytes.
Stability of blood vessels is believed to be compromised in retinopathy of
prematurity (ROP), a potentially binding eye disease characterised by the invasion of
the vitreous by proliferating retinal blood vessels. In the past the use of high oxygen
levels for the treatment of extremely premature infants was associated with the
incidence of this condition. However, it was recently suggested that rather than
absolute oxygen levels, the variability of the transcutaneous (blood) oxygen levels
within clinically 'safe' limits during the first two weeks of life may be more
important in ROP. Thus, a clinically relevant animal model of ROP was developed
(Edinburgh model). The first 14 days of the transcutaneous oxygen profile of an
infant who developed severe ROP was selected and converted into percentage
inspired oxygen levels to produce the equivalent arterial oxygen in a rat. The aims of
this thesis were to investigate mural cell coverage and the expression of the factors
suggested to participate in vessel stabilisation during normal retinal development and
in the Edinburgh model.
Rat pups were raised in room air or exposed to the oxygen regime of the Edinburgh
model from birth to postnatal days 2, 7 and 14. The pups were then killed, the eyes
enucleated and analysed for mural cell coverage and expression of platelet-derived
growth factor-ß₁, transforming growth factor-/!i and angiopoietin-2 using
immunohistochemical, Western blotting, electron microscopy and in situ
In comparison to room air raised pups, the retinal vasculature of the pups raised in
the variable oxygen regime had reduced mural cell coverage; smooth muscle cell
coverage was only slightly reduced, however there was a marked and significant
reduction in pericyte coverage. The protein expression of both platelet-derived
growth factor-ß₁ and transforming growth factor-ß were also reduced.
Unfortunately, the analysis of angiopoietin-2 mRNA expression was inconclusive
due to insensitive detection methods.
In conclusion, the reduction in mural cell coverage and in the expression of platelet derived growth factor and transforming growth factor-ß₁ suggest that blood vessel
stability is compromised in the Edinburgh model of retinopathy of prematurity.