The Juxtaligamental Cells of Echinoderms and Their Role in the Mechano-Effector Function of Connective Tissue

The term 'juxtaligamental cell' refers to neurosecretory-like cells associated with connective tissue structures of echinoderms. Many of these structures have the capacity to change their mechanical properties drastically, either reversibly or irreversibly, within a timescale of seconds. Since such mutability is under the control of the nervous system and since the juxtaligamental cells (JLCs) are key elements interposed between the motor nervous system and connective tissue, it is hypothesized that these cells are the effectors directly responsible for the mechanical adaptability of the connective tissue. This chapter firstly reviews available information on morphological aspects of JLCs - their anatomical distribution, histological organization, ultrastructure, and relations with the nervous system. There follow considerations of what is known about the physiology, functioning, and presumptive roles of JLCs, and what can be inferred about evolutionary aspects of JLCs, including their homology, the evolution of juxtaligamental 346systems within the Echinodermata and the origins of their effector function. The final section highlights gaps in current knowledge.