Ontogeny of the conus papillaris of the lizard Gallotia galloti and cellular response following transection of the optic nerve: an immunohistochemical and ultrastructural study

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Variable functional recovery and minor cell loss in the ganglion cell layer of the lizard Gallotia galloti after optic nerve axotomy

The lizard Gallotia galloti shows spontaneous and slow axon regrowth through a permissive glial scar after optic nerve axotomy. Although much of the expression pattern of glial, neuronal and extracellular matrix markers have been analyzed by our group, an estimation of the cell loss in the ganglion cell layer (GCL) and the degree of visual function recovery remained unresolved. Thus, we performed a series of tests indicative of effective visual function (pupillary light reflex, accommodation, visually elicited behavior) in 18 lizards at 3, 6, 9 and 12 months post-axotomy which were then processed for immunohistochemistry for the neuronal markers SMI-31 (neurofilaments), Tuj1 (beta-III tubulin) and SV2 (synaptic vesicles) at the last timepoint. Separately, cell loss in the GCL was estimated by comparative quantitation of DAPI(+) nuclei in control and 12 months experimental lizards. Additionally, 15 lizards were processed for electron microscopy to monitor relevant ultrastructural changes in the GCL, optic nerve and optic tract throughout regeneration. Hypertrophy of RGCs was persistent, morphology of the regenerated nerves varied from narrow to neuroma-like features and larger regenerated axons underwent remyelination by 9 months. The estimated cell loss in the GCL was 27% and two-third of the animals recovered the pupillary light reflex which involves the pretectum. Strikingly, visually elicited behavior involving the tectum was only restored in two specimens, presumably due to the higher complexity of this pathway. These preliminary results indicate that limited functional regeneration occurs spontaneously in the severely injured visual system of the lacertid G. galloti.