A decrease in the rate of cephalic shape change late in ontogeny has been documented for several species of trilobites, possibly associated with the cessation of segment release into the thorax. Qualitative descriptions of the ontogeny of Cryptolithus tesselatus Green, 1832, suggest that shape change in the cephalon was strongly influenced by the progressive accommodation of large funnel-shaped perforations ("fringepits") over several molts. The number and arrangement of fringe-pits was established early in ontogeny, however, before thoracic segment release was completed. Due to the unusual and highly convex shape of the cephalon, we use three-dimensional (3D) geometric morphometrics to quantify shape change in this species and determine if there is a rate shift, and at what point in development this shift occurred. Three-dimensional morphometrics was made possible by extracting fixed and semi-landmarks from surface reconstructions of C. tesselatus rendered from CT scans of silicified specimens. Results show that the cephalon continued to change shape into adulthood, but that a threshold model with a rate shift associated with the cessation of new fringe-pits is best supported. 2D landmarks taken from the dorsal view fail to capture the dramatic change in convexity of the cephalon during development, but model comparison results are consistent with those based on the 3D landmark dataset, allowing comparison of this aspect of ontogenetic change with other species. Based on these comparisons, it appears that 1) trajectories are often better characterized by threshold models than simple linear regression models; 2) the timing of shifts may not be phylogenetically conserved.

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Alteration of neurofilament (NF) proteins is considered a critical component and a causative factor for a number of neuropathologies, especially certain neurotoxicities. Correlative observations have supported this hypothesis; the current study tests this relationship by exposure of neurotoxicants to crayfish, a species lacking NFs. Morphological and immunological tests verified the absence of NFs in crayfish peripheral nerve axons. Tail injections of acrylamide (ACR), 2,5-hexanedione (2,5-HD), or 3,4-dimethyl-2,5-HD (3,4-DMHD) produced ataxia and paralysis. Morphological expression of axonal degeneration in a spatial and temporal pattern of progression comparable to mammalian species possessing NFs was observed. With gamma-diketones, time to onset was slower than observed in mammals but relative potency between neurotoxic analogues was maintained. Non-neurotoxic analogues failed to produce any functional signs of neurotoxicity. These data are consistent with the conclusion that NF accumulations are not cause-effect related to axonal degeneration in these models of neurotoxicity and raise questions as to the relationship between accumulation of NF proteins and axonal degeneration in other neuropathological conditions.

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Quantitative enzyme histochemical methods have been used to determine the effect of ablation of synergists on the oxidative metabolism of the alpha-motoneurons and muscle fibers of the rat soleus. Sixty days postablation, the NADH-tetrazolium reductase (NADH-TR) activity of soleus motoneurons decreased 12.5% from 0.327 +/- 0.005 (mean +/- SE; optical density units) to 0.286 +/- 0.007. In the muscle fibers, the alpha-glycerophosphate dehydrogenase activity (glycolytic enzyme) decreased from 0.114 +/- 0.010 to 0.074 +/- 0.009, a change of 35.1%, and the NADH-TR activity decreased 21.2% from 0.348 +/- 0.018 to 0.274 +/- 0.017. In both the motoneurons and the muscle fibers, the decrease was nonspecific for all cells, although a greater effect on the cells with higher enzyme activity was observed. The decreased NADH-TR activity represents a shift in the oxidative profile of the motoneurons and muscle fibers, indicating a decreased ability to use oxidative metabolism for periods of short-term high-energy demands. Furthermore, the parallel decrease in muscle fibers and motoneurons with high NADH-TR activity (fast-twitch oxidative-glycolytic fibers and presumably also motoneurons) demonstrates the tight correlation of the NADH-TR activity between these parts of the motor unit in both control and synergist-ablated muscles.