Presence and structure of innervated myotendinous cylinders in sheep extraocular muscle.

Innervated myotendinous cylinders (IMCs) were for the first time described in a sheep extraocular muscle (EOMs). They were found at the distal myotendinous junction of a medial rectus and were investigated by light and transmission electron microscopy. The IMCs are enveloped by a multi-layered capsule of fibrocytes and each contains the terminal portion of one multiply-innervated muscle fibre and its corresponding tendon. The tendinous compartment of the IMC is entered by a single nerve fibre which, inside, spreads into several terminal branches. Numerous terminal branches were found among the collagen fibrils but few on the muscle fibre tips. Nerve terminals contain mitochondria and are full of clear vesicles. Within the nerve terminals, vesicles are often concentrated in an area where the axolemma exhibits dense patches. Innervated myotendinous cylinders of sheep EOMs exhibit the same ultrastructural features as those earlier described as palisade endings or myotendinous cylinders in cat, monkey and man.