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We analysed the astroglia response that is concurrent with spontaneous axonal regrowth after optic nerve (ON) transection in the lizard Gallotia galloti. At different post-lesional time points (0.5, 1, 3, 6, 9 and 12 months) we used conventional electron microscopy and specific markers for astrocytes [glial fibrillary acidic protein (GFAP), vimentin (Vim), sex-determining region Y-box-9 (Sox9), paired box-2 (Pax2)¸ cluster differentiation-44 (CD44)] and for proliferating cells (PCNA). The experimental retina showed a limited glial response since the increase of gliofilaments was not significant when compared with controls, and proliferating cells were undetectable. Conversely, PCNA(+) cells populated the regenerating ON, optic tract (OTr) and ventricular wall of both the hypothalamus and optic tectum (OT). Subpopulations of these PCNA(+) cells were identified as GFAP(+) and Vim(+) reactive astrocytes and radial glia. Reactive astrocytes up-regulated Vim at 1 month post-lesion, and both Vim and GFAP at 12 months post-lesion in the ON-OTr, indicating long-term astrogliosis. They also expressed Pax2, Sox9 and CD44 in the ON, and Sox9 in the OTr. Concomitantly, persistent tissue cavities and disorganised regrowing fibre bundles reaching the OT were observed. Our ultrastructural data confirm abundant gliofilaments in reactive astrocytes joined by desmosomes. Remarkably, they also accumulated myelin debris and lipid droplets until late stages, indicating their participation in myelin removal. These data suggest that persistent mammalian-like astrogliosis in the adult lizard ON contributes to a permissive structural scaffold for long-term axonal regeneration and provides a useful model to study the molecular mechanisms involved in these beneficial neuron-glia interactions., (© 2013 Anatomical Society.)

We studied the myelination of the visual pathway during the ontogeny of the lizard Gallotia galloti using immunohistochemical methods to stain the myelin basic protein (MBP) and proteolipid protein (PLP/DM20), and electron microscopy. The staining pattern for the PLP/DM20 and MBP overlapped during the lizard ontogeny and was first observed at E39 in cell bodies and fibers located in the temporal optic nerve, optic chiasm, middle optic tract, and in the stratum album centrale of the optic tectum (OT). The expression of these proteins extended to the nerve fiber layer (NFL) of the temporal retina and to the outer strata of the OT at E40. From hatching onwards, the labeling became stronger and extended to the entire visual pathway. Our ultrastructural data in postnatal and adult animals revealed the presence of both myelinated and unmyelinated retinal ganglion cell axons in all visual areas, with a tendency for the larger axons to show the thicker myelin sheaths. Moreover, two kinds of oligodendrocytes were described: peculiar oligodendrocytes displaying loose myelin sheaths were only observed in the NFL, whereas typical medium electron-dense oligodendrocytes displaying compact myelin sheaths were observed in the rest of the visual areas. The weakest expression of the PLP/DM20 in the NFL of the retina appears to be linked to the loose appearance of its myelin sheaths. We conclude that typical and peculiar oligodendrocytes are involved in an uneven myelination process, which follows a temporo-nasal and rostro-caudal gradient in the retina and ON, and a ventro-dorsal gradient in the OT.

Cilia are a common characteristic of the embryonic neurons of the thalamus and striated centers of Gallotia galloti from E.35. The ciliogenesis process occurs around the stages E.33 and E.34. The ciliary morphology shows no substantial variations between the different stages and the ciliary structure in the embryonic period is 8 + 1. On the other hand, the adult cilia has a 9 + 2 arrangement of microtubules. We do not rule out the possibility of other axonemes.

We have studied the development of the mesencephalon of Gallotia galloti and we have observed two classes of cellular death: 'nuclear' or 'Type I' and 'cytoplasmatic' or 'Type II'. The first appears in stages around E-34 and the second one is observed from stages around hatching to the adult lizard. The degenerative cells have been observed in the profundus, torus semicircularis, the 5th pair of the trigeminal nerve nuclei as well as the 4th and 5th layers of the optic tectum where this last nucleus mentioned is situated.

In the striatum, thalamus and cerebellum of a Lacertid reptile, we have found three types of cellular death during embryonic development, both at the light and electron microscopic level. The first affects the undifferentiated neuro-epithelial cells and is commonest during the early stages (E. 32-E. 36). The second corresponds to the type of 'nuclear' death described in the bibliography and reaches a maximum in the middle embryonic period (E. 37-E. 39); nevertheless important variations were observed in different zones. The third is the same as the 'cytoplasmic' death type and appears in the perinatal stages. Phagocytosis involved in the elimination of dead cells is of two types. One is associated with early death and is carried out by undifferentiated neuro-epithelial cells. The other is carried out by microglial cells which appear around Stage 37. Much cellular debris was observed in the intermediate zone and this was associated with the second type of phagocytosis. In both cases lipid production was associated with the degenerative process. Comparison of the temporal cellular death pattern with synaptogenesis, gliogenesis and maturation of neuronal processes is consistent with the view that the various types of cellular death found by us had different causes.