Abstract
Risso’s dolphins (Grampus griseus) are known for their heavily scarred skin, which is usually caused from teeth raking between dolphins, as well as circular markings from cookie-cutter sharks (Isistius brasiliensis). The scars are usually gray to white, making them more noticeable on Risso’s dolphins.1 But there is something more than a simple white scar on Risso’s dolphins; it has a unique and highly repetitive pattern. Healed white scars, deep wounds previously created by cookie-cutter sharks, are mainly divided into four parts from the center to the outermost layer: (1) The white scar in the center,(2) the migrating black lines,(3) the gray scar on the periphery,(4) the black scar on the outermost periphery. Furthermore, in mature healed white scars, the rete and dermal ridges, collagen bundles, and adipose tissue were restored under histologic examination. These structures weren’t found in the healed deep wounds in humans and pigs.2 The result exhibits a strong ability to restore normal skin structure in Risso’s dolphins, which is similar to the previous study in Fraser’s dolphins.3
For the first part, we quantified the number and distribution of melanocytes in different parts of normal skin and white scars. Our result showed that in normal skin of Risso’s dolphins, the darker the skin was, the higher numbers of melanocytes were found. This finding is inconsistent with the human skin studies, since the number of melanocytes remain approximately equal despite different skin tones in human.4 Generally, the result of melanocytes counting in normal skin of Risso’s dolphin corresponds to the finding in Fraser’s dolphin.5 As for white scars, there is no positive stained melanocyte in the center of white scar in Risso’s dolphin.
For the second part, we performed IHC staining of growth factors such as stem cell factor (SCF) and endothelin-1 (ET-1). SCF and ET-1 both play essential roles in stimulating proliferation, differentiation, and migration in melanocytes.6-8 Our result showed that in the black, dark-gray, light gray, and white normal skin, SCF and ET-1 signals are positive in almost every layer of epidermis. Interestingly, SCF and ET-1 signal are significantly reduced in the center of healed white scar, especially at the bottom of the rete ridges. It is suggested that the keratinocytes of the rete ridges fail to produce SCF and ET-1, leading to the disappearance of melanocytes. These findings will provide a better understanding of the mechanism of repigmentation in Risso’s dolphins.
Acknowledgements
We would like to thank the Marine Biology and Cetacean Research Center in National Cheng Kung University and Taiwan Cetacean Society for assistance with sample collection. All animal procedures were conducted with the approval of the Ocean Conservation Administration, Taiwan (Permit #1090002352).
*Presenting author
+Student presenter
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