Biomedical Engineering Reference
In-Depth Information
4
Plastic Embedding of Ocular Sections
Soft plastics as methyl methacrylate (MMA) and the preferred
glycol methacrylate (GMA) may be used for embedding ocular
tissues. Soft ocular tissues containing hard implants or bone require
the use of a hard plastic, such as polymethylmethacrylate (PMMA)
[
1
,
106
]. In addition, methacrylate-based material such as intraoc-
ular lenses may dissolve or become deformed in MMA resin, thus
the monomer of PMMA or GMA-based resins may be used as
substitutes in these cases [
1
].
For GMA, the ocular tissue is fixed by systemic perfusion with
glutaraldehyde, and then globes are trimmed along a medial plane
as for paraffin embedding, and then processed following a schedule
with monomer infiltration. This involves agitation to facilitate infil-
tration and eventual embedding in a mold for polymerization with
an oven. The blocks are removed from the molds and the GMA is
trimmed from the edges (Fig.
1f
). Trimmed blocks are mounted on
a chuck of a microtome and sections are collected, placed on a water
bath, and adhered to glass slides for staining. Embedding in GMA
results in a reduction in shrinkage artifacts and provides better
preservation of the cellular detail. Limitations of using GMA
include a limit on the size of the tissue embedded requiring
trimming of intact globes in smaller tissues and the generation of
heat by the GMA blocks during processing that may result in tissue
artifacts such as vacuolation of the retinal nerve fiber layer. Also,
special equipment is needed and staining of GMA sections may be
more challenging.
Processing ocular tissues that contain hard medical devices
requires initial fixation followed by dehydration to prepare for
plastic infiltration and embedding in a hard plastic, such as
PMMA [
1
]. The ocular tissue is initially trimmed to decrease the
size of the tissue and to have the implanted hard medical device
near the sectioning surface. This is followed by infiltration, place-
ment in embedding molds, and polymerization. The polymerized
block is removed from the embedding mold and the side of the
block that is not of interest is mounted on a glass slide using a
mounting media. The block mounted on a glass slide undergoes a
grinding process to obtain the desired location within the block
using coarse to fine grit of sandpaper through a microgrinding
system. Following grinding, a parallel glass slide is affixed onto
the block and the block is cut with a specialized saw. The sections
obtained from the block by the saw are about 200
m thick. These
μ
are then ground to about 45-60
m thick using the microgrinding
system. Tissue sections are stained with stains used for paraffin-
embedded sections, generally after etching the slide face.
μ
