During fin regeneration, a blastema, a group of de-differentiated cells, forms underneath the wound epidermis. As regeneration proceeds, cells leave the proximal blastema and enter the differentiation zone. Adjacent to the differentiation zone, a subset of cells in the basal epidermal layer (BEL) express sonic hedgehog a (shha). Cells that come in contact with BEL differentiate into osteoblasts and joint cells, enabling the formation of bone segments at the end of each fin ray. Generally, fin regeneration occurs similarly in males and females. However, breeding tubercles (BT), keratinized epidermal structures on the male pectoral fin, result in regenerative differences when compared to females. In this thesis, three aspects of zebrafish fin regeneration were studied: 1) Cell lineage tracing of shha-expressing cells in the caudal fin regenerate; 2) The differentiation of joint cells and osteoblasts in the caudal fin regenerate; 3) Regeneration of pectoral fin BTs. Studies on caudal fin regenerates suggest osteoblasts and joint cells originate from a common cell lineage, but are committed to different cell fates. Joint cells follow a genetic pathway in which evx1 occurs downstream or parallel to hoxa13a and upstream of pthrp1. In the absence of Evx1, presumptive joint cells are committed to an osteoblast cell fate. Furthermore, joint cells do not regenerate following laser cell ablation, suggesting joint cell differentiation occurs only at specific intervals during osteoblast regeneration. Collectively, these results suggest a mechanism for joint cell differentiation during caudal fin regeneration. Studies on pectoral fins indicate androgens induce and estrogens inhibit BT formation. BT regeneration in males and androgen-treated females follows the initiation of revascularization, but occurs concomitantly with a novel second wave of angiogenesis. The inhibition of angiogenesis in androgen-treated females prevents BT formation. Altogether, these results suggest the growth and regeneration of BTs requires a v hormonal stimulus and the presence of an additional blood vessel network naturally found in males. In conclusion, these studies have increased the overall knowledge of key aspects of zebrafish fin regeneration. A gain in understanding zebrafish regeneration provides a basis in which treatments can be developed to induce regeneration in species with limited regenerative capabilities.

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Polypterus is an extant fish that is used as a model to understand the fin-to-limb evolutionary transition. Polypterus exhibits muscle phenotypes relevant to this transition. In particular, plastic changes in bone and muscle in Polypterus have been shown in response to spending time in a terrestrial environment. Muscle fiber changes are usually associated with changes in the performance demand placed on those muscles. We hypothesize that muscle fibers are recruited differently between aquatic and terrestrial environments to explain the change in fiber type. How pectoral fin muscle activity changes between swimming and walking is mostly unknown. Hence, this study utilizes electromyography (EMG) and high-speed videography to understand how the muscle activity pattern and function of all four pectoral fin muscle groups change during swimming and walking in aquatically raised fish. In this experiment, aquatically raised fish were placed in water and on land to observe changes in fin muscle function between behaviours. This study aims to understand how the instantaneous changes in the behaviour of the fish, particularly in the pectoral fin, could explain the muscle plasticity found in previous research. This study showed that fish adduct their pectoral fins much faster with increased muscle effort during walking compared to swimming. The adductor muscle also had the biggest change in function, activating for the majority of the fin-stroke cycle and therefore undergoing eccentric contraction. The increase in muscle effort seen in this study is consistent with the muscle fiber transition seen in fish that spend long periods on land, and the dramatic change of EMG magnitudes found in the adductor muscle may explain muscle damage previously found following acute walking.