Ultrastructural Study of Normal and Degenerating Nerve Fibers in the Protocerebral Tract of the Crab <i>Ucides cordatus</i>

Abstract

Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Identity of the cells recruited to a lesion in the central nervous system of a decapod crustacean

“…The events that occur 24 h, as described here, and 7 and 40 days after axotomy as reported in detail by Corrêa et al (2005) are very clear: 24 h and 7 days after the injury, the axons did not show visible signs of degeneration; however, many hemocytes were seen around and a few inside the PCT, while 40 days after the injury many axonal degenerating events could be observed, and most of the hemocytes were seen within the tract near the glial cells. In this system, the most evident role for glial cells was that of phagocytosis of axon debris (Corrêa et al 2005).…”

“…In addition, most of the cells that migrated to the lesion or that differentiated near the lesion were labeled with the isolectin IB4. These observations suggest that the IB4 + cells, identified in the lesion site of PCT, may be similar to the microglia/macrophages that are recruited when there is an injury in the vertebrate CNS [it is notable that the neurodegenerative events in vertebrates start after 30-48 h (Martinez and Ribeiro 1998), therefore much earlier than in the species U. cordatus, which starts at 28 days (Corrêa et al 2005)]. Interestingly, both microglia from vertebrate CNS (Cuadros and Navascués 1998;Chan et al 2007) and hemocytes from the PCT have a hematopoietic origin.…”

“…In this system, the most evident role for glial cells was that of phagocytosis of axon debris (Corrêa et al 2005). However, since we have now shown that many granular hemocytes (that were not found in normal conditions, and were therefore possibly activated) were attracted from the hemolymph to the eyestalk after it was ablated, we can suggest that hemocytes are not only phagocytizing neural debris together with glia, but also that they may be concerned with structural replacement of damaged glial cells, as reported by Smith et al (1987).…”

“…1 of Corrêa et al (2005), in order to cause degeneration of the distal stump of the PCTs. After 24 h, the remaining part of the PCTs was dissected in 4% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M phosphate-buffered crustacean saline (crust-PBS), pH 7.4, prepared according to Lockwood (1961).…”

“…In the decapod crustacean central nervous system, nerve-fiber degeneration has only been described in the crayfish Procambarus clarkii (Tanner et al 1995;Blundon et al 1990), and in the crab Ucides cordatus by our research group (Corrêa et al 2005). By qualitative and quantitative ultrastructural analyses, we described the features and time course of the protocerebral tract (PCT) events following extirpation of the optic stalk.…”