Local regeneration in the retina of the goldfish.

We have studied regeneration of the retina in the goldfish as a model of regenerative neurogenesis in the central nervous system. Using a transscleral surgical approach, we excised small patches of retina that were replaced over several weeks by regeneration. Lesioned retinas from three groups of animals were studied to characterize, respectively, the qualitative changes of the retina and surrounding tissues during regeneration, the concomitant cellular proliferation, and the quantitative relationship between regenerated and intact retina. The qualitative and quantitative analyses were done on retinas prepared using standard methods for light microscopy. The planimetric density of regenerated and intact retinal neurons was computed in a group of animals in which the normal planimetric density ranged from high to low. Cell proliferation was investigated by making intraocular injections of 5-bromo-2'-deoxyuridine (BUdr) at various survival times to label proliferating cells and processing retinal sections for BUdr immunocytochemistry. The qualitative analysis showed that the surgery created a gap in the existing retina that was replaced with new retina over the subsequent weeks. The BUdr-labeling experiments demonstrated that the excised retina was replaced by regeneration of new neurons. Neuroepithial-like cells clustered on the wound margin and migrated centripetally, appositionally adding new retina to the old. The quantitative analysis showed that the planimetric density of the regenerated neurons approximated that of the intact ones.     

Stimulating regeneration in the damaged spinal cord.

Great progress has been made in recent years in experimental strategies for spinal cord repair. In this review we describe two of these strategies, namely the use of neurotrophic factors to promote functional regeneration across the dorsal root entry zone (DREZ), and the use of synthetic fibronectin conduits to support directed axonal growth. The junction between the peripheral nervous system (PNS) and central nervous system (CNS) is marked by a specialized region, the DREZ, where sensory axons enter the spinal cord from the dorsal roots. After injury to dorsal roots, axons will regenerate as far as the DREZ but no further. However, recent studies have shown that this barrier can be overcome and function restored. In animals treated with neurotrophic factors, regenerating axons cross the DREZ and establish functional connections with dorsal horn cells. For example, intrathecal delivery of neurotrophin 3 (NT3) supports ingrowth of A fibres into the dorsal horn. This ingrowth is revealed using a transganglionic anatomical tracer (cholera toxin subunit B) and analysis at light and electron microscopic level. In addition to promoting axonal growth, spinal cord repair is likely to require strategies for supporting long-distance regeneration. Synthetic fibronectin conduits may be useful for this purpose. Experimental studies indicate that fibronectin mats implanted into the spinal cord will integrate with the host tissue and support extensive and directional axonal growth. Growth of both PNS and CNS axons is supported by the fibronectin, and axons become myelinated by Schwann cells. Ongoing studies are aimed at developing composite conduits and promoting axonal growth from the fibronectin back into the spinal cord.     

Limb regeneration: re-entering the cell cycle.

Understanding the cellular plasticity that enables urodeles to regenerate many tissues is important for determining why mammals repair those same tissues with scar. The answer may lie partly in a recently discovered differential responsiveness of urodele cells to factors present in serum at the wound site.     

Fibrinolysis and liver regeneration.

The plasmin and plasminogen activator proteases of the plasma fibrinolytic system were investigated as potential blood-borne mediators of the proliferative activation of hepatocytes by partial hepatectomy. Partial (68%) liver resection, as well as proliferatively activating the remaining hepatocytes, rapidly (by 30 minutes) doubled the level (or activity) of circulating plasminogen activator but later (2 hours) greatly depressed this level. This later depression of the activity of circulating plasminogen activator lasted for eight to ten hours before returning to the normal level two to four hours before the hepatocytes in the liver remnant began to synthesize DNA. This sequence of changes in the fibrinolytic potential was not abolished by prior thyroparathyroidectomy which is known to inhibit the initiation of hepatocyte DNA synthesis and to prevent the secretion of the calcium homeostatic hormones, another early systemic consequence of partial liver resection. Since the early rise in plasminogen activator activity did not cause the appearance of active (free) circulating plasmin, and since the injection of large doses of the fibrinolytic and protease inhibitors, EACA and Trasylol®, during this early, post-operative period of hyperfibrinolytic potential did not prevent hepatocytes from initiating DNA synthesis, it is unlikely that either plasmin or its activator protease are blood-borne initiators of hepatocyte proliferative development.     

Cytokines in neural regeneration.

Growth factors with already established multiple effects on non-neural cells continue to be of considerable interest to researchers with regard to the nervous system, where regulation of cell maintenance and plasticity in relation to lesion and regeneration is part of their functional repertoire. Fibroblast growth factors, interleukins, and type beta transforming growth factors are prominent representatives of such proteins. Ciliary neurotrophic factor is another multifunctional neurokine. The proposed role of this molecule as a 'lesion factor', however, is still not firmly settled.     

Macrophages and nerve regeneration.

Macrophages are not only phagocytic cells but also secrete a plethora of growth factors that are potentially important for regeneration. This review will examine the emerging evidence of a likely contribution by macrophages to axonal regeneration.     

Neurotrophic factors in regeneration.

Nerve growth factor, fibroblast growth factor, and ciliary neurotrophic factor can protect selected populations of neurons from some of the degenerative changes that otherwise follow axonal injury or other insults. The function of diffusible neurotrophic factors in axonal regeneration is still unclear, however. Knowledge of the nerve growth factor congeners, brain-derived neurotrophic factor and neurotrophin-3, is advancing rapidly as is the identification of neurotrophin receptors, several of which are membrane-bound tyrosine kinases.