Key Concepts:

Figure 1. Regeneration of the caudal fin in adult and larval zebrafish. Regeneration of the adult fin (a) the adult caudal fin is a bi-lobed appendage whose main structural elements are the ray bones (R). (b) Amputation through the fin results in a wound healing response during which fin epidermis migrates as a sheet over the injury. After wound closure, a thickened wound epidermis (We) forms at the site of injury. (c) With the formation of the wound epidermis, cells underneath begin to proliferate at the distal tips of the fin rays to form blastemas (Bl). These cells lack any clear defined differentiated morphology. (d) As the blastema cells continue to proliferate, they coalesce to form an outgrowth front. New tissues are generated at the proximal end of the blastema. (e) This outgrowth process continues until the fin has reached it original length. (f) Cross-section through a regenerating fin ray shows the accumulated cells of the blastema (Bl) surrounded by the pro-regenerative wound epidermis (We). Cells at the interface between the blastema and the tissues of the stump contribute as newly differentiated tissues: scleroblasts (s), fibroblasts (f), segmented lepidotrichia (l) and nerve (n). The source of the differentiating cells is currently unclear. Regeneration of the larval fin. (g) The caudal fin of the larva contains several tissues. The notochord (nc) and neural tube (nt) are surrounded somatic skeletal muscle (sm). These tissues are enclosed by an overlying epithelial layer that protrudes ventrally and dorsally along the body and posteriorly at the end of the tail to form an epithelial fin fold (ff). The skeletal structure is a scaffold of actinotrichia (a). Pigment cells (p). (h) Amputation of the fin fold results in wound closure. (i) Regeneration of the fin fold involves cell proliferation at the distal edge, and this results in regenerative outgrowth of the fin fold. As this process continues, pigment cells migrate distally. (j) This outgrowth process continues until the regenerate resembles a fin fold.

Figure 2. Neuron regeneration in the zebrafish brain and spinal cord. (a) The adult zebrafish brain (anterior pointing left) has precisely arranged regions that contain cells that continually proliferate and are considered to be stem cell niches (red). Around these regions, neurogenic zones (dark blue) form where cells from the stem cell niches migrate and differentiate into new neurons. Horizontal bars present the sections through the zebrafish brain shown in panels B, C and D. (b) One model for the continued growth of the adult brains is that stem cell niches surrounding the ventricle produce neuronal progenitors, and that these progenitors differentiate to form new neurons (new growth) along the periphery of the brain. The continued accumulation of the differentiated progeny of the progenitors along the periphery of the brain results in constant growth. (c) PCNA staining (blue) of cross-sections through the cerebellum of an adult transgenic nestin:GFP zebrafish brain, illustrating the structure of the adult neural stem cell niche in the cerebellum. The nestin gene is a marker for neuronal progenitors. Insert provides greater resolution of doubly labelled nestin:GFP-PCNA-positive cells. The plane of the cross-section through the brain is depicted by white diagram in lower left corner. Reproduced with permission from Kaslin et al. (2009). (d) Staining of cross-section through the ventral telencephalon (V) with PCNA identify a zone of proliferation along the ventral and lateral ventricle, and the retention of BrdU label after pulse-chase experiments indicate that after proliferation, these cells migrate into the ventricle to become neurons. The plane of the cross-section through the brain is depicted by white diagram in upper right corner. Reproduced with permission from Grandel et al. (2006). (e) The zebrafish spinal cord (Sc) runs from the brain (Br) to the base of the caudal fin. As for all vertebrates, transection of the spinal cord results in paralysis caudal to the transection site. (f) Histological cross-sections through the unlesioned spinal cord show different nerve types and a narrow ventricle; BrdU incorporation studies indicate that cells do not cycle. However, after injury to the spinal cord, cells along the ventricle do incorporate BrdU (green). Cell tracking of BrdU-labelled cells indicate that these cells migrate from the ventricular zone. Furthermore, immunohistochemistry with markers for neuronal differentiation (ChAT) and synapse formation (SV) indicate that these cells become functional neurons. (g) Over time, the new nerves extend over the transection site, and 27–47% of these are neuronal extensions from the brain, as demonstrated by the anterograde transport of vital dye (indicated green) past the transected site of spinal cord when injected into cell bodies in the brainstem. (h) Neuronal connections in the spinal cord also transport vital dye (green) retrograde into neurons in the brain; however, this occurs significantly less frequently. Only 0.3–1.2% of the neurons regenerate ascending axons into the brain stem. Drawings are after Grandel et al. (2006), Kaslin et al. (2008, 2009) modified and Reimer et al. (2008).

Figure 3. Formation and regeneration of the fish neural retina. The adult zebrafish eye maintains retinal stem cells in a circumferential germinal zone termed the ciliary marginal zone (CMZ). These cells continuously generate much of the neural cell types that comprise the retina: the cone photoreceptor cells and the rod photoreceptor cells of the outer nuclear layer (ONL), the neuronal cells (amacrine, bipolar and horizontal neurons) and the Müller glia of the inner nuclear layer (INL), the ganglion cells of the ganglion cell layer (GCL). Neuronal cells in the inner and outer nuclear layers are connected to each other through their extended axons in the inner plexiform layer (IPL). Injury to the retina (e.g. by intense light) kills neuronal cells of the retina. Zebrafish regenerate the central retina through the proliferation of Müller glia cells. Although some daughter cells remain Müller glia, others reproduce the lost neuronal cells to regenerate the retina. Drawing summarises work from several works including Braisted et al. (1994), Raymond et al. (2006), Bernardos et al. (2007) and Thummel et al. (2008).

Figure 4. Regeneration of the hair cells in the peripheral sensory system. (a) The lateral line in the zebrafish larva runs under the skin from the head to the tail. The lateral line consists of several neuromasts. (b) The lateral line of the adult zebrafish extends through to the lobes of the caudal fin. (c) Neuromasts also dot the lateral line of the adult caudal fin. (d) The neuromasts consists of hair cells that are supported by support cells and protected by mantle cells. The hair cells (sensory cells) are connected to lateral line cells by afferent nerves, which transmit the signals after the movement of the ‘hairs’ (kinocilia). (e) Loss of the hair cells eliminates the interaction with afferent nerve causing the nerves to contract. The support cells begin to proliferate (dark blue cells with green nuclei). (f) New hair cells are generated with some support cells differentiate into hair cells whereas others remain support cells. The afferent nerves selectively reconnect with the regenerated hair cells. Drawing summarises work from Dufourcq et al. (2006).

Figure 5. Heart regeneration. (a) The zebrafish heart contains two chambers: one atrium (A) and one ventricle (V). The bulbus arterioles (B) is the outflow tract of the heart. The ventricle consists of three tissue layers of the epicardium (epi) that lines the ventricular muscle (myo). The tissue lining the interior of the chamber is the endocardium (endo). The red arrows indicate the direction of blood flow. (b) Zebrafish can lose up to 20% of its ventricular muscle after amputation injury (dashed line). (c) Immediately after amputation, a durable clot forms to prevent further blood loss from the opened chamber of the ventricle. (d) Over the course of 1–2 months, new ventricular tissue regenerates from the residual ventricle. This process appears to involve the migration of endocardium, myocardium and epidcardium as myocardial cells proliferate. This regeneration process replaces the clot to form a thickened ventricular muscle wall where the amputation took place. (e) Over time, remodelling returns the ventricular wall to its original dimensions. The drawing summarises work from Poss et al. (2002b).