Three types of fibre, dark (type 1), light (type 2) and intermediate, were distinguished in the caval sphincter muscle of the diaphragm in the harp seal (Pagophilus groenlandicus) using histochemical and electron-microscopic techniques. The dark fibre contained large peripheral aggregations of mitochondria, numerous lipid droplets and dense aggregates of glycogen granules. The same features were observed in the dark fibre of the diaphragm muscle too. In the light of the low oxidative enzyme activity and high lipase activity observed in the diaphragm and other skeletal muscles of the harp seal in previous studies, it is postulated that these mitochondrial aggregations and lipid droplets represent an adaptation for the generation of heat through non-shivering thermogenesis comparable to that in the brown adipose tissue. Such ability to generate and maintain heat by uncoupling the oxidative phosphorylation process should enable the caval sphincter muscle to function efficiently in regulating the cardiac return of blood from the inferior vena cava during a dive when the body temperature as a whole drops. The lack of the regional differences in the fibre composition of the harp seal diaphragm, as were reported in the rat diaphragm, is attributed to the seal’s large body size, lower metabolic rate and diving habit.
<p class="abstract">Thyroglossal duct remnants and branchial arch anomalies are the most common congenital neck masses. These anomalies typically present in childhood or early adulthood as cysts, sinuses or cartilaginous remnants, but may rarely present in late adulthood. Although both thyroglossal duct remnants and branchial cysts may be encountered individually, these anomalies are rarely encountered together in the same individual. We report the third such case of co-existing branchial cyst and thyroglossal cyst occurring in the same individual with review of embryological development of thyroglossal duct cysts and second branchial arch anomalies.</p>
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