what-when-how
In Depth Tutorials and Information
Vitreous
The vitreous is composed of 98% water and 2% struc-
tural proteins, extracellular matrix components and other
compounds. The major structural protein of the vitreous
is collagen, with type II collagen comprising 75% and
type IX collagen accounting for 15%, with minor contri-
butions from type XVIII collagen, a progenitor of endo-
statin, an inhibitor of angiogenesis. The major function
of the vitreous is to maintain transparency within the
eye to help optimize light transmission and focus onto
the retina. In early life, the vitreous is normally closely
adherent to the retina at the vitreous base near the pars
plana, optic nerve head, along retinal blood vessels and
the macula and fovea, the location in the retina reserved
for inest central vision. Over time, the vitreous may
separate from the retinal surface, resulting in a posterior
vitreous separation or posterior vitreous detachment
that may result in floater symptoms due to shadows cast
from condensed vitreous components onto the retina.
With close adherence between the vitreous and retina
during vitreous separation, the vitreous may cause both
traction on the retina and a retinal tear that may develop
into a retinal detachment, a problem that may occur in
patients with and without OI.
loose collagenous zone, the middle layer of elastic fibers,
the outer loose collagenous zone and the basement mem-
brane of the endothelium of the choriocapillaris. The
blood circulation for the inner retina is supplied by the
central retinal artery and vein while the outer retina (from
the outer third of the inner nuclear layer and beyond) and
RPE are supplied by the choriocapillaris and choroid.
Beyond the choroid is the sclera of the eye. As light inter-
acts with the photoreceptors at the outer retina, vitamin
A-derived molecules bound to apoproteins called opsins
undergo a conformational change that leads to graded
membrane potential changes in the photoreceptors. These
photoreceptor membrane responses are modified by hori-
zontal and bipolar cells synapsed to the photoreceptors
in the inner nuclear layer. The bipolar cells also undergo
a graded membrane polarization change and these cells
synapse with ganglion cells. The ganglion cells have
axons that course within the innermost nerve fiber layer
of the retina and converge to form the optic nerve at the
posterior pole of the eye. The ganglion cells summate the
response of bipolar and amacrine cells and develop action
potentials that are conducted through the optic nerve to
the dorsolateral geniculate nucleus in the brain. The cen-
ter of the retina is called the macula and is responsible
for the sharp, central visual acuity used for reading and
identifying faces. Loss of vision may occur from diseases
affecting the retina including age-related macular degen-
eration, retinal and choroidal neovascularization, retinal
vascular occlusions and retinal detachment.
Retina, Retinal Pigment Epithelium, Bruch
Membrane and Choroid
The retina is the specialized neurosensory tissue of
the eye that is intimately linked to the vitreous and the
outer wall structure provided by the retinal pigment epi-
thelium, choroid and sclera. The retina, developmentally
an outpouching extension of the central nervous sys-
tem, is composed of multiple layers histologically from
inner to outer retina: the internal limiting membrane, the
nerve fiber layer, the ganglion cell layer, the inner plexi-
form layer, the inner nuclear layer, the outer plexiform
layer, the outer nuclear layer, the external limiting mem-
brane, and the rod and cone inner and outer segments.
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The rods and cones are photoreceptors with nuclei in the
outer nuclear layer and inner and outer segments extend-
ing more peripherally. These photoreceptors form close
connections with the underlying retinal pigment epithe-
lium (RPE). The RPE absorbs stray light, recycles pho-
totransducing molecules from the photoreceptors (by
phagocytosis of outer segments), removes fluid from the
subretinal space, metabolizes retinal and polyunsaturated
fatty acids and forms the outer blood-retinal barrier. The
retinal pigment epithelium is supported by the Bruch
membrane.
The Bruch membrane is composed of collagen types
I and IV and other molecules including laminin, fibro-
nectin and elastin. The Bruch membrane histologically
includes the basement membrane of the RPE, the inner
Optic Nerve and Visual System
As the optic nerve exits the eye, it leaves the bony
orbit of the skull through the optic canal of the lesser
wing of the sphenoid bone with partial crossing of nerve
fibers at the optic chiasm. Optic tracts from the chiasm
synapse with the dorsolateral geniculate nucleus of the
brain. Nerve pathways from the dorsolateral geniculate
nucleus of the brain eventually traverse along optic radi-
ations that end at the occipital cortex of the brain, where
visual centers interpret the nerve signals into the percep-
tion of images seen.
OI AND THE EYE: AN OVERVIEW OF EYE
FINDINGS RELATED TO OI
This section provides an overview of eye findings -
both very rare and more common - in OI reported in
histology studies, case reports, case series and small com-
parative studies of OI eyes with normal eyes. The actual
prevalence of each of these conditions within the OI pop-
ulation and among the different OI types is unknown.
These conditions may also be found in people without
OI. Many of the findings are likely very rare among OI
