Capsules of duck muscle spindles.

Duck muscle spindles show a large outer capsule enclosing a number of intrafusal muscle fibres which are individually encircled by an inner capsule. The outer capsule consists of a multilayer of 4-6 flattened cells with elongated nuclei, and are usually lined by a basement membrane. They resemble and are linked with perineural cells of the nerve bundle that innervates the spindle. There is overlapping and interdigitation between branching processes of these cells. Some apposing surfaces develop junctional complexes in the form of hemi-desmosomes and desmosomes. The cytoplasmic content shows numerous pinocytotic vesicles, some polyribosomes, lamellated cisternae of rough endoplasmic reticulum and microfilaments. The inner capsule consists of cells showing, at the mid-equatorial region, a large ovoid nucleus and extensive branching of thin and long processes that overlap, forming numerous layers around each intrafusal muscle fibre. Bundles of collagen fibrils in a crescent-shaped configuration occur circumferentially between the multilayer of processes and the muscle fibre. Some linkages between the inner and the outer capsule cell processes provide a network that subdivides the large periaxial space into compartments. There is no basement material lining the inner capsule cells and the processes. Some cytoplasmic area occurs around the ovoid nucleus and the cytoplasm varies in density, showing some dilated short profiles of rough endoplasmic reticulum, a few pinocytotic vesicles and microfilaments.