Cirri of the stalked crinoid Metacrinus rotundus: neural elements and the effect of cholinergic agonists on mechanical properties.

Sea lilies are enigmatic animals due to their scarcity and their biology is comparatively neglected. Cirri, arranged in whorls of five along the sea lily stalk, anchor and support the animal. They consist of ossicles interconnected by collagenous ligaments and by a central canal. Cirri have a well-developed nervous system but lack muscular cells. A light and electron microscopic study was performed to clarify the morphology of the nervous system of the cirri. Two cellular networks were found, one of neuron-like cells and one of cells filled with bullet-shaped organelles. Both networks ramify throughout the cirral ossicles up to the interossicle ligaments. Mechanical tests were performed to analyse the influence of cholinergic agonists on the mechanical properties of these ligaments. In the tests, the cirral ligaments softened after the application of acetylcholine, muscarinic agonists and nicotinic agonists. The reaction time to muscarinic agonists was much slower than to acetylcholine and nicotinic agonists. At low concentrations, muscarinic agonists caused active development of force. No reaction to stimuli was observed in anaesthetized cirri. The data clearly establish the existence of catch connective tissue which can change its mechanical properties under nervous control mediated via nerves with cholinergic receptors. The possible sources of the observed force production are discussed and it is concluded that active contraction of collagenous ligaments causes movement of cirri.

Peptides controlling stifness of connective tissue in sea cucumbers.

We present the first evidence of a system of four bioactive peptides that affect the stiffness of sea cucumber dermis. The body wall dermis of sea cucumbers consists of catch connective tissue that is characterized by quick and drastic stiffness changes under nervous control. The peptides were isolated from the body wall, their amino acid sequences determined, and identical peptides synthesized. Two peptides, which we named holokinins, are homologous with bradykinin. We tested the effect of the peptides on the mechanical properties of sea cucumber dermis. Both of the holokinins softened the dermis, and a pentapeptide that we designated as NGIWYamide stiffened it. Both effects were reversibly suppressed by anesthesia with menthol. We called the fourth peptide stichopin; it had no direct effect on the stiffness of the dermis but suppressed action of the neurotransmitter acetylcholine reversibly. The results suggest that the peptides are neuropeptides and are part of a sophisticated system of neurotransmitters and neuromodulators that controls the connective tissue stiffness of sea cucumber dermis.

Contractile connective tissue in crinoids.

Active movements in animals are usually attributed to cellular protein engines, e.g., the actin-myosin system of muscle cells. Here we report the first evidence of an extracellular contractile connective tissue, which we have found in sea lilies and feather stars (Echinodermata, Crinoida). These marine animals have arm muscles that are antagonized, not by other muscles, but by ligaments consisting of extracellular fibrils interspersed with neuron-like cell processes. Contractile cells are lacking, yet these arm ligaments actively contracted upon stimulation. The ligaments stayed in a contracted condition even after the stimulus had stopped. The stresses generated were lower than those of typical skeletal muscles. Additional data from crinoid cirri, which lack muscles entirely, corroborate the hypothesis that the connective tissue of the ligaments is contractile.

To be Stiff or to be Soft-the Dilemma of the Echinoid Tooth Ligament. I. Morphology.

The morphology of the tooth ligament of the echinoid Eucidaris tribuloides was investigated. The tooth ligament connects the middle part of the rod-shaped tooth to the jaw. It consists of collagen and microfibrils that are synthesized by fibroblasts in the aboral growth zone of the tooth. Histochemical staining showed that the tooth ligament contains highly sulfated acid proteoglycans that seem to connect the collagen fibrils. We did not find any cell bodies in the ligament, but we observed a small number of cell processes. Dense vesicle cells and supporting cells in the jaw near the ligament send these processes among the collagen fibrils. Dense vesicle cells have connections to a newly described jaw nerve. Supporting cells possess a unique podocyte-like filtration structure that probably serves for selective pinocytosis. In the oral region, where the working part of the tooth protrudes from the jaw, collagen fibrils swell and disappear. Sometimes phagocytosis of collagen fibrils could be observed. Autoradiographic data indicate that the collagen of the tooth ligament is not subject to quick turnover. The tooth ligament resembles a catch connective tissue that can change its mechanical properties under nervous control.

To be Stiff or to be Soft-the Dilemma of the Echinoid Tooth Ligament. II. Mechanical Properties.

The teeth of sea urchins are connected to jaws by means of ligaments. Their sliding along the jaw during continuous growth requires a pliant ligament, whereas scraping on rocks for feeding requires a stiff ligament for firm support. We investigated the mechanical properties of the tooth ligament of Diadema setosum to clarify how sea urchins solve this dilemma. In creep tests a load of 30 g caused a shift of the tooth that continued until the tooth was pulled out of the jaw. The creep curve had three phases: an initial phase of high creep rate, a long phase of constant creep rate, and a final phase of accelerating creep rate. The ligaments had a shear viscosity of about 550 MPa {middot} s. Viscosity increased reversibly after stimulation with seawater containing a high concentration of potassium ions or acetylcholine. Frozen and rethawed ligaments did not show an increase of viscosity after stimulation. The data indicate that sea urchins can change the stiffness of their tooth ligaments through nervous control. We suggest that the tooth ligament is a catch connective tissue.