Ultrastructural characterization of the accessory lobes of Lachi in the lumbosacral spinal cord of the pigeon with special reference to intrinsic mechanoreceptors

Abstract

The lumbosacral spinal cord of birds is unique among vertebrates in that segmentally organized accessory lobes protrude from the ventrolateral spinal cord into the vertebral canal. Recently, it has been suggested that these lobes may be part of an extralabyrinthine sense organ of equilibrium. For a better understanding of such a function, a complete analysis of the structural basis was performed by means of electron microscopy. The lobes consist of multipolar neurons, myelinated and unmyelinated axons, glia-derived glycogen cells, glial cells, and capillaries. The dorsal part of the lobe is covered by a loose mesh of pia mater. Ventrolaterally, an arachnoidal trabecle is in close contact with the lobe. Extracellular lacunae extend from the periphery deep into each lobe. The lacunae are separated from the subarachnoidal space by a loose mesh of processes of the glycogen cells with its basal lamina. The lacunae are filled by a network of processes of glycogen cells, glial cell, dendrites, and small axons. Both neuronal somata and dendrites are contacted by numerous axon terminals that form rather uniform synapses. Finger-like processes emerge from both the somata and the dendrites. The dendrites branch deeply into the extracellular lacunae and form lateral ramifications, which consist of narrow stalks with serially arranged bulbous portions, from which finger-like processes emerge. Finger-like processes are well-known elements in mechanotransduction. Glycogen cells and lacunae may contribute to transmission of hydrostatic pressure changes during movements of the body.

Chick spinal accessory lobes contain functional neurons expressing voltagegated sodium channels generating action potentials

“…The acutely dissociated chick AL cells with the clear cytoplasm sometimes had short processes, and the cells with the rich cytosolic structures sometimes had some dendrites or axons with many branches. Such a morphology is consistent respectively with the reported properties of the glycogen cells and neurons in pigeon ALs (17). It has been proposed that AL neurons function as sensory neurons of equilibrium (14).…”

“…Considering that AL consists of glycogen cells and neurons (12,16,17), the dissociated cells with the clear cytosol and no voltage-gated ionic currents may be glycogen cells derived from astroglial cells. In contrast, the other type of cells with the rich cytosolic structures showed voltage-gated currents, indicating that they express functional VGSCs and voltage-gated K + channels.…”

“…The majority of cells in AL are glycogen-rich glial cells (glycogen cells). Somata of the paragriseal neurons are scattered in the pool of the glycogen cells and AL neurons have been reported to show some morphological properties of mechanoreceptive neurons (16,17). In birds, the vertebrae are fused at the lumbosacral region.…”

The structure and development of avian lumbosacral specializations of the vertebral canal and the spinal cord with special reference to a possible function as a sense organ of equilibrium

“…This idea was corroborated by the lesion experiments which showed that disturbances of the system led to a severe impairment of walking on the ground whereas flight was normal (Necker et al 2000). The neurons in the accessory lobes are thought to represent the sensory elements and it has been shown that they are mechanosensitive and equipped with finger-like processes typical of mechanoreceptors (Necker 2002;Rosenberg and Necker 2002).…”

“…A Arachnoidea, P pia, SAS subarachnoidal space. Scale bar 500 lm Gennaro 1982; Schroeder and Murray 1987;Rosenberg and Necker 2002). However, little attention has been given to the structure of the vertebral canal and to the surrounding of the accessory lobes.…”