Some morphological aspects of the cremaster muscle in brown hare and domestic rabbit

The cremaster muscle is formed by muscle fibres that are descended from the caudal border of m. obliquus intemus abdominis in males. The extent and functioning of the cremaster muscle differs among mammals. In rodents and lagomorphs testes migrate between scrotum and abdominal cavity and therefore the cremaster is extensive and highly active. There were hypotheses that the functioning of cremaster muscle is correlated with the seasonal testicular regression in brown hare. We previously established that position of the testes in hares is strongly correlated to the ambient temperature and not to the reproductive activity. Our present aim was to establish fibre type profile of the cremaster muscle in comparison with its origin, the internal oblique abdominal muscle, in both brown hare and laboratory rabbits. Muscle samples from 6 hares (3 in quiescent and 3 in reproductively active period) and 6 rabbits were collected and subjected to enzyme histochemistry (fibre typing according to mATPase, SDH and ct-GPDH activity and glycogen content) and immunohistochernistry directed to different myosin heavy chain isoforms. The cremaster muscle completely surrounded the testicle and epididymis in both species. While enzyme- and immunohistology revealed m. obliquus intemus abdominis as a predominantly fast muscle, central bundles of cremaster muscle contained mainly slow fibre types. However, the peripheral parts of cremaster were predominantly composed of fast IIA fibre type indicating functionally important intra-muscular variability. In rabbits, which were housed in constant ambient temperature, the central "slow" bundle was substantially smaller. Furthermore m. cremaster had high glycogen content and high glycolitic capacity, including the presence of uncommon SOG fibres. No prominent differences in fibre types between breeding and non-breeding seasons were found.