Glutaraldehyde fixation of central nervous tissue: an electron microscopic evaluation in the isolated rabbit retina

Optimizing optical coherence tomography and histopathology correlation in retinal imaging

OBJECTIVE

To develop a methodology to correlate optical coherence tomography (OCT) images and histopathological sections from the same eye. Part 1: To determine the best fixative for optimal OCT and histopathological analysis in post-mortem eyes. Part 2: A protocol is proposed to correlate histopathological features and OCT scans from the same post-mortem eyes.

DESIGN

Experimental study.

PARTICIPANTS

Part 1: Twenty-three rabbit eyes and 14 post-mortem human eyes. Part 2: Nineteen post-mortem human eyes.

METHODS

Part 1: Six different fixatives were tested, and specimens were evaluated on 4 criteria: globe shape, structure opacification, retinal detachment, and nuclear details. Part 2: Based on the findings from Part 1, fixed human eyes were imaged using OCT. Orientation-controlled histopathological processing was performed to obtain serial tissue sections from paraffin embedded tissue, which were matched to corresponding OCT images.

RESULTS

Part 1: Of the 6 fixatives, 2% glutaraldehyde and Davidson's solution met the proposed criteria in rabbit eyes. Of these, glutaraldehyde showed similar results in human eyes and was selected for Part 2. Part 2: Using anatomical landmarks, cross-sectional histopathological sections in the same orientation as the OCT images were correlated to their corresponding OCT images. Retinal lesions such as a macular hole, an epiretinal membrane, and the presence of drusen were easily correlated, proving the reliability of our methodology. Moreover, the photoreceptor's inner/outer junction was correlated to a hyperreflective band on OCT.

CONCLUSIONS

A standardized protocol was developed to correlate OCT images and histopathological findings by generating serial cross-sections of the retina, which can be used to better understand otherwise ambiguous OCT findings.

A quantitative morphological study of osmotically induced swelling and shrinkage in nervous tissue

The rabbit retina was used to study, in vitro, the responses of central nervous tissue to changes in extracellular osmolarity. After isolation, retinas were incubated in either hypertonic or hypotonic medium containing 80 milliosmols more or 80 milliosmols less sodium chloride than the isotonic control medium. After fixation and embedding, comparable areas of each retina were sectioned and studied with the phase and electron microscopes. The diameters of receptor cell inner segments, synapses, nuclei, and mitochondria were measured on micrographs; mean nuclear areas and volumes were calculated. Cutouts from micrographs also provided areas and volumes of the receptor cell nucleus and its 'surround' of axons, dendrites, glial processes, and extracellular space. In general, hypertonic incubation produced decreases in the linear dimensions, areas, and volumes of the receptor cell, its nucleus, and its mitochondria that were consistent with their behaviour as osmometers. After hypotonic incubation, the increases in the diameters of inner segments, synapses, and mitochondria were in the predicted range. The increases for the nuclei themselves, and the nuclei and their 'surround' were less than expected. This may have been due to the failure of the preparative techniques to maintain the swollen state of these larger structures.

Shrinkage and distortion of the rabbit corneal endothelial cell mosaic caused by a high osmolality glutaraldehyde-formaldehyde fixative compared to glutaraldehyde

The purpose of this study was to quantitatively compare cell dimensions and cell layer organization of the corneal endothelium after chemical fixation. Rabbit corneas (9-10 weeks of age) were prepared immediately postmortem for transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using either a widely used high osmolality fixative (glutaraldehyde-formaldehyde, after Karnovsky; 1% formaldehyde, 2.5% glutaraldehyde, 0.1 M cacodylate, pH 7.6, 850 mOsm/kg) or a glutaraldehyde fixative (2% glutaraldehyde in 80 mM cacodylate, pH 7.4, 330 mOsm/kg). With the glutaraldehyde-formaldehyde fixative, TEM revealed gross shrinkage (up to 40%) and distortion of the cytoplasm and organelles, while the regions of the cell-cell junctions were not attenuated but included dilated extracellular space. With the glutaraldehyde fixative, TEM also revealed shrinkage but the cytoplasm was less compact than with the high osmolality fixative. The overall cell shrinkage and relative accentuation of the cell-cell borders was confirmed by SEM, which also revealed that the 11 to 20% area shrinkage was also related to the number of cell sides.

Postnatal development of glycinergic neurons in the rabbit retina

Certain neurons in the adult rabbit retina possess a high-affinity uptake mechanism for glycine and release it in response to elevated K+ concentrations in the medium. Although the evidence is not yet complete, these properties, together with pharmacological studies, suggest that the glycine-accumulating neurons may be a subpopulation of amacrine cells and may use glycine as a neurotransmitter. In the present study, we have used the uptake and K+-stimulated release of glycine as physiological probes to follow the emergence and maturation of putative glycinergic neurons during postnatal development of the rabbit retina. We show that certain neurons in the newborn retina already possess a specific high-affinity mechanism for glycine uptake. The positions and density of these cells in the developing retina suggest that they will become glycine-accumulating neurons of the adult retina. Thus, similar to our earlier study on the development of GABA-ergic neurons in this retina, the commitment by certain retinal neurons to be glycinergic, if indeed these cells use glycine as the transmitter, is made prenatally. These putative glycinergic neurons are, however, probably immature at birth, because they do not release the accumulated glycine in response to high K+ concentrations in the medium. In fact, there is practically no K+-stimulated release of preloaded glycine from the retina until about 7 days after birth, after which the release increases drastically to about 65% of the adult level on day 10 and 80% on day 12. Assuming that this release originates synaptically, our finding suggests that the putative glycinergic neurons may be functionally mature by 10-12 days after birth. Additionally, our results show that during development of the rabbit retina, the mechanism for high-affinity glycine uptake emerges and matures much earlier than the mechanism for K+-stimulated glycine release.

Postnatal development of GABA-ergic neurons in the rabbit retina

Uptake, synthesis, storage, and release of gamma-aminobutyric acid (GABA) are some of the characteristic properties of GABA-ergic neurons. In the present study, we have used these properties as physiological probes to follow the emergence and maturation of GABA-ergic neurons during postnatal development of the rabbit retina. There is autoradiographic, immunocytochemical, and pharmacological evidence that some amacrine cells and certain neurons in the ganglion cell layer probably use GABA as the neurotransmitter. These neurons take up GABA, contain the GABA-synthesizing enzyme L-glutamic acid decarboxylase (GAD, EC 4.1.1.15), and release the accumulated GABA by a CA++-dependent mechanism when depolorized with high extracellular K+ concentration. In this study, we show that certain neurons in the newborn retina already possess a specific mechanism for GABA uptake. The positions and numbers of these cells in the developing retina suggest that they will become GABA-ergic neurons in the adult retina. These putative GABA-ergic neurons are, however, probably immature at birth because newborn retinas contain only low levels of GABA and GAD. Additionally, there is relatively little K+-stimulated, Ca++-dependent release of (3H)-GABA from the newborn retinas. GABA concentrations and GAD activities in developing retinas increase steadily postnatally, reaching about 80% of the adult levels by day 9. The activities of the GABA-degrading enzyme, GABA-glutamate transaminase (GABA-T, EC 2.6.1.19), follow a similar pattern of maturation during retinal development. K+ stimulated GABA release, however, remains low until about day 6, and then increases dramatically from 20% to 85% of the adult level over the next 3 days. Taken together, our results indicate that in the rabbit retina, the commitment by certain neurons to use GABA as the transmitter is made prenatally. These neurons are immature at birth but are biochemically, physiologically, and probably functionally mature by about 9 days after birth.

Development of outer segments and synapses in the rabbit retina

The peripheral retina of rabbits aged 0 to 60 days was studied by electron microscopy. Ribbon and conventional synaptogenesis was studied with serial sections, and the density of synapses of the inner plexiform layer was measured on large (1,500 micrometer 2) montages. Photoreceptor and bipolar ribbon synapses seem to develop similarly in that processes of the prospective dyad or triad contact the presynaptic ribbon-containing terminal one at a time. No statistically significant difference in the lengths of ribbon lamellae was found at 11 and 30 days. Conventional synapses appear to result from the aggregation of synaptic vesicles on one side of junctions that first existed as symmetrical membrane densities without vesicles. The length of the synaptic membrane specialization constant between 0 and 30 days. The density of inner plexiform layer conventional synapses remains at a low and roughly constant level from 0 to 9 days, after which there is an abrupt increase to a plateau at about 20 days. After nine days the density of ribbon synapses also increases, with an initially steep time course similar to that of conventional synapses. All subcategories of synapse studied (amacrine-to-amacrine, amacrine-to-bipolar, serial, and reciprocal) participate in the general increase between 9 and 20 days. Functional circuits of the inner plexiform layer thus seem to be assembled primarily during the second and third weeks of life.