The mechanisms by which different genotypes of Giardia duodenalis result in different symptoms remain unresolved. In particular, we lack detailed knowledge on which transport mechanisms (transcellular or paracellular) are affected by different Giardia isolates. Using horse radish peroxidase (HRP) and creatinine as transcellular and paracellular probes, respectively, we developed a robust assay that can be used with an Ussing chamber to investigate epithelial transport, as well as short-circuit current as an indicator of net ion transport. We investigated 2 Giardia isolates, both Assemblage A, one a lab-adapted strain and the other a field isolate. Results indicate that products from sonicated Giardia trophozoites increase both transcellular and paracellular transport. A non-significant increase in transepithelial electrical resistance (TEER) and short-circuit current were also noted. The paracellular transport was increased significantly more in the field isolate than in the lab-adapted strain. Our results indicate that while both transcellular and paracellular transport mechanisms may be increased following exposure of cells to Giardia trophozoite sonicate, perhaps by inducing non-specific increases in cellular traffic, it is important that in vitro studies of Giardia pathophysiology are conducted with different Giardia isolates, not just lab-attenuated strains.

Using a combination of retrograde horseradish peroxidase (HRP) labelling, silver staining, and electron microscopy, we have assessed the relationship between retinal ganglion cell soma size and axon diameter in the adult ferret (Mustela putorius furo). Retinal ganglion cells were labelled following injections of HRP into the lateral geniculate nucleus (LGN), superior colliculus (SC), or LGN+SC. The soma size distributions following LGN, SC, or LGN+SC injections were all unimodal showing considerable overlap between different cell classes. This was confirmed for alpha cells, identified on the basis of dendritic filling or from neurofibrillar-stained retinae. Analysis of the soma size and axon diameters of a population of heavily labelled retinal ganglion cells showed a significant correlation between the two. However, the overall distribution of intraretinal axon diameter was bimodal with an extended tail. Analysis of the ganglion cell distributions in the adult ferret indicates that beta cells comprise about 50.5–55%, gamma 42.5–47%, and alpha 2.5% of the ganglion cell population. This implies that the proportion of gamma, beta, alpha cells in both cat and ferret retina is highly conserved despite differences in visual specialization in the two species.

Physiological studies of intracellular messengers frequently employ intracellular injections of large molecules that either monitor or modulate the metabolism of the messenger cascade. Injected molecules have unknown mobility in the cytosol and unknown accessibility to various cytosolic compartments, including those postulated to be traversed by intracellular messenger molecules. In order to determine whether injected molecules have access to the confined spaces through which messenger molecules must diffuse, we injected 5-nm colloidal gold or horseradish peroxidase, or both, into Limulus ventral photoreceptors. Injections were made by applying pressure pulses to the back of an intracellular micropipette that also monitored membrane voltage. The tissue was fixed at varying times after injection and processed for electron microscopy by conventional techniques. Cells fixed 1–3 min after injection contained HRP reaction product only in the cell body. HRP reaction product was found at varying distances down axons in direct relation to the interval between injection and fixation. Colloidal gold particles were found throughout the cell body but not in axons of tissue fixed 1–3 min after injection. Both HRP reaction product and 5-nm colloidal gold particles were observed within the microvillar projections of internal and external rhabdomere, as well as within the extracisternal spaces of endoplasmic reticulum. We conclude that large molecules injected from an intracellular micropipette into an arbitrary locus of ventral photoreceptor cells have access to all of the presumed sites of the phototransduction cascade.

The cellular connectivity of the lacertilian parietal eye is not well understood. Because the intercellular connections establish the foundation for information processing, we have investigated cellular connectivity of one cell type in this simple vertebrate retina. We also developed an in vitro preparation to study the anatomy of the parietal eye visual system. Horseradish peroxidase transport in the in vitro preparation revealed a class of displaced ganglion cells occupying positions among the photoreceptors, in a location where the presence of interneurons had been suggested. Three-dimensional reconstruction at the electron-microscopic level showed that the morphology and synaptic input of these displaced ganglion cells is different from that of the previously known ganglion cells. The displaced ganglion cells receive an average of about 13 ribbon synapses from photoreceptors. The ribbon input is equally distributed between the soma and dendritic arbor. Junctional membrane measurement and ethnolic phosphotungstic acid-staining provided evidence for the existence of non-ribbon synaptic contacts (synaptoid junctions). Displaced ganglion cells make about 20 synaptoid junctions, 65% of which are on the dendritic arbor. The morphology of the displaced ganglion cell is such that a significant measure of synaptic input to the dendritic arbor will be transmitted to the soma.

The peripheral retina of the sea lamprey develops in a 5-year-long process in which only certain neurons differentiate each year. The growth of cell layers, the differentiation of the neurons, and the morphology of their dendrites and axons were studied with normal, HRP, and Golgi preparations. Ganglion cells are differentiated in 3-year-old larvae, amacrine and horizontal cells in 4-year-old larvae, photoreceptor cells in stage I transformers, and bipolar cells in stage III transformers. Each new development is expressed as a radial gradient of differentiation. As a result of this protracted and stepped process, lamprey retinal neurons, particularly ganglion cells, differentiate in the absence of other cells to which they will ultimately be connected and may express their individual genetic programs more fully than in other vertebrate retinas. This could account for the unusual relationship of the ganglion cell, inner plexiform, and optic nerve layers and for the very high ratio of displaced to orthotopic ganglion cells.