Support Cells of Neuronal Sensilla - Cell list - Back to Contents
Socket and sheath cells are specialized interfacial epithelial cells that are associated with the ciliated endings of most sensory receptors (GliaFIG1-3). They are generally related by lineage to the sense cells that they protect (See Glia Table). Socket cells are more epithelial, join closely to hypodermis and secrete cuticle which lines the opening of several sensilla, whereas sheath cells are more glial, and form a protected environment for the cilium. The outermost portion of each sensillum is formed by the socket cell, which has a smaller cell body and tends to lie closer to the sensillum. The socket cell extends a short thick process to the tip of the sensillum where it forms a small, doughnut-like ring of tissue enclosing the distal tip of the cilium. The sheath cell forms a protective larger pocket around most of the cilium (GliaFIG2. The sections (A) to (D) are from anterior to posterior). The cilium often traverses the sheath cell cytoplasm in a narrow membraneous tube to enter this membrane-lined pocket, coming to lie within an extracellular environment which is sometimes exposed to the exterior of the animal via an opening in the socket cell (BodyFIG2). Each tube is sealed apically by a ring of adherens junction, tightly closing the sheath/cilium connection and blocking any flow of material between extracellular (seath pocket) and pseudocoelomic spaces (See Amphid image gallery, GliaFIG4). The distal portions of the socket and sheath cells form two rings of tissue surrounding the cilium, and linked to each other by an electron-dense ring of adherens junction. Less robust adherens junctions also connect the socket cell to the hypodermis.
Some sheath cells can be very large, particularly those for the amphids, which must each enclose 12 cilia (See Amphid image gallery). The amphid sheath cells contain distinctive parallel membrane lamellae near the pocket, as well as large vesicles which may transport material from the soma to the pocket, or from the lamellae to the pocket (See Amphid image gallery, GliaFIG4). The amphid pocket is filled by a granular electron dense material surrounding the cilia. The cell bodies of the amphid sheath cells are particularly large and also contain many membranous organelles. The sheath cells for smaller sensilla are less complex, but contain some of these features in less dramatic form, often including several small membrane lamellae and a few vesicles near the pocket. Distal portions of the amphid socket and sheath cells are strengthened locally by a network of intermediate filaments (see Perkins et al., 1986), this suggests that the reinforcement is needed to keep the amphid channel open, to resist forces such as muscle contraction or body turgor which would tend to close these sensilla.
CEP sheath cells in the head seem to serve a separate glial function in the region of their cell bodies. They extend distinctive thin lamellar processes which completely surround the outer edge of the nerve ring and ventral ganglion neuropil, separating it from adjacent hypodermis or sometimes from neuron cell bodies. See CEP image gallery (Also see Ware et al, 1975 Figs 24, 29, 33. Note that CEP's are called "LSM" and sheath extensions are labelled as "Gl from pocket cells" in these figures, and White et al, 1986 Fig.16).
It should be noted that URX and URY neurons are not associated with any sensilla though they reach the tip of the animal's head. URXL/R neurons have unciliated bulb-like endings between CEPshDL/R and AMshL/R. URYDL/R terminate in a thin sheet-like structure close to the ILshDL/R and OLQshDL/R, whereas, URYVL/R terminate close to ILshVL/R and OLQshVL/R. Similarly, FLP and BAG neurons do not have specific socket and sheath cells assigned to them. These ciliated neurons terminate close to the ILLsoL/R behind the cuticle and in bag and flap-shaped structures that wrap short projections from the ILL socket cells (Ward et al, 1975; Perkins et al., 1986). A schematic drawing of all head sensilla of the left side, excluding the anterior deirid, is seen below (Glia FIG3. Figure on the right (BodyFIG2) points to the positions of sensillar openings on the lips (Also see cross section of sensory receptors near the tip of the head).
In the male tail, there are many additional sensilla that feature sheath cells and socket cells as well (See Glia Table) (Sulston et al, 1980). In the sensory rays of the fan, a specialized structural cell is associated with each ray that serves combined sheath and socket cell function to enclose two ciliated neuron endings and form a narrow opening to the exterior (Sulston et al 1980, Chow et al, 1995). The very large cuticle-covered spicules require two sheath cells each to form a protective pocket over their sensilla and 4 socket cells each to provide the cuticular covering. The spicules slide in and out of a cuticle-lined cavities in the cloaca (the gubernaculum) that are formed by additional transitional epithelial cells (Sulston et al 1980). Complete cloacal structures will be described more in detail in a later chapter.
Local hypodermal cells or seam cells often take part in the formation of each papilla or sensory specialization. Thin extensions from the hypodermis may form a wrapping over the sense ending and become firmly attached by adherens junctions to the socket cell (if present). In early development, a hypodermal or seam stem cell such as T and its daughters may play the role of socket in forming a sensory opening prior to the birth of the true socket cell (Sulston et al 1980, Sulston and Horvitz, 1977).
List of support cells of the sensilla
(full sensillum will also include 1-12
sensory dendrites, and sometimes a hypodermal wrapping)
1. Amphid (left) (See Amphid image gallery)
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