Topography and ultrastructure of sensory nerve endings in the joint capsules of the Kowari (Dasyuroides byrnei), an Australian marsupial

Summary

The present investigation is concerned with the topography and ultrastructure of sensory nerve endings in the joint capsules of the Kowari (Dasyuroides byrnei), an Australian marsupial. Material for light and electron microscopy was obtained from shoulder, elbow and knee joint capsules.

On the basis of differences in the organization of the connective tissue belonging to the fibrous layer, 3 variants of capsule structure have been distinguished: a rigid, a flaccid and an intermediate type. Whilst the rigid type is characterized by dense connective tissue in the clearly demarcated fibrous layer, the flaccid type shows loose, irregularly arranged connective tissue in the fibrous layer which merges into the synovial layer of the joint capsule. the morphology of the intermediate type corresponds to an intermediate stage between the former two types.

In the fibrous layer of the joint capsules three different types of sensory nerve endings were observed: free nerve endings, Ruffini corpuscles and lamellated corpuscles. The free nerve endings are supplied by myelinated afferent axons (1–2 μm in diameter); the terminal thickenings of which are incompletely surrounded by a terminal Schwann cell. Ruffini corpuscles are present in three different varieties: 1. small corpuscles without a perineural capsule predominantly within the flaccid part of the capsule; 2. slightly larger corpuscles with an incomplete perineural capsule and 3. large corpuscles resembling Golgi tendon organs which predominantly occur in the rigid parts of the capsule. The afferent myelinated axons measure 2–4 μm in diameter. The lamellated corpuscles show two variants: 1. small corpuscles with a 2 to 4-layered perineural capsule in the rigid parts of the joint capsules and 2. large corpuscles with two longitudinal clefts of the inner core in the flaccid parts. Both types are supplied by myelinated axons of 3–5 μm in diameter. Thus, in the fibrous layer of the rigid type of joint capsules large Ruffini and small lamellated corpuscles predominate, whereas the fibrous layer of the flaccid type coincides with small Ruffini and large lamellated corpuscles. The present data, therefore, corroborate the concept that the morphology of mechanoreceptors depends upon the texture of the surrounding connective tissue.