The subcommissural organ (SCO) is a phylogenetically ancient and conserved structure. During ontogeny, it is one of the first brain structures to differentiate. In many species, including the human, it reaches its full development during embryonic life. The SCO is a glandular structure formed by ependymal and hypendymal cells highly specialized in the secretion of proteins. It is located at the entrance of the aqueduct of Sylvius. The ependymal cells secrete into the ventricle core-glycosylated proteins of high molecular mass. The bulk of this secretion is formed by glycoproteins that would derive from two different precursors of 540 and 320 kDa and that, upon release into the ventricle aggregate, form a threadlike structure known as Reissner's fiber (RF). By addition of newly released glycoproteins to its proximal end, RF grows caudally and extends along the aqueduct, fourth ventricle, and the whole length of the central canal of the spinal cord. RF material continuously arrives at the dilated caudal end of the central canal, known as the terminal ventricle or ampulla. When reaching the ampulla, the RF material undergoes chemical modifications, disaggregates, and then escapes through openings in the dorsal wall of the ampulla to finally reach local blood vessels. The SCO also appears to secrete a cerebrospinal fluid (CSF)-soluble material that is different from the RF material that circulates in the ventricular and subarachnoidal CSF. Cell processes of the ependymal and hypendymal cells, containing a secretory material, terminate at the subarachnoidal space and on the very special blood capillaries supplying the SCO. The SCO is sequestered within a double-barrier system, a blood-brain barrier, and a CSF-SCO barrier. The function of the SCO is unknown. Some evidence suggests that the SCO may participate in different processes such as the clearance of certain compounds from the CSF, the circulation of CSF, and morphogenetic mechanisms.

The subcommissural organ.

Publisher Summary This chapter highlights the cell biology of the subcommissural organ (SCO) and discusses its structure–function relationships to provide a basis for further experimental work. SCO is a complex of nonneuronal secretory cells covering and penetrating the posterior commissure. This complex protrudes toward the third ventricle, occupies the posterior portion of the diencephalic roof caudal to the pineal organ, and marks the entrance to the Sylvian aqueduct. SCO is formed by two populations of secretory cells, which, in many species, are arranged into two distinct layers: the ependyma and the hypendyma. The bulk of the secretory products of the SCO, apparently a complex containing different glycoproteins, are released into the ventricular cerebrospinal fluid (CSF). In the ventricle, the secretion is condensed into a thread-like structure, Reissner's fiber (RF) that terminates in the ampulla caudalis of the central canal. The blood vessels of the SCO represent a highly specialized vascular structure within the CNS. The SCO is sequestered within a double-barrier system of unknown functional significance, thereby indicating the unique character of this circumventricular organ.

Cell biology of the subcommissural organ.

Identification and partial characterization of the secretory glycoproteins of the bovine subcommissural organ-Reissner's fiber complex. Evidence for the existence of two precursor forms.

Within the vestibular system of virtually all vertebrate species, gravity and linear acceleration are detected via coupling of calcified masses to the cilia of mechanosensory hair cells. The mammalian ear contains thousands of minute biomineralized particles called otoconia, whereas the inner ear of teleost fish contains three large ear stones called otoliths that serve a similar function. Otoconia and otoliths are composed of calcium carbonate crystals condensed on a core protein lattice. Otoconin-90 (Oc90) is the major matrix protein of mammalian and avian otoconia, while otolith matrix protein (OMP) is the most abundant matrix protein found in the otoliths of teleost fish. We have identified a novel gene, otoc1, which encodes the zebrafish ortholog of Oc90. Expression of otoc1 was detected in the ear between 15 hpf and 72 hpf, and was restricted primarily to the macula and the developing epithelial pillars of the semicircular canals. Expression of otoc1 was also detected in epiphysis, optic stalk, midbrain, diencephalon, flexural organ, and spinal cord. During embryogenesis, expression of otoc1 mRNA preceded the appearance of omp-1 transcripts. Knockdown of otoc1 mRNA translation with antisense morpholinos produced a variety of aberrant otolith phenotypes. Our results suggest that Otoc1 may serve to nucleate calcium carbonate mineralization of aragonitic otoliths.

Otoc1: A Novel Otoconin-90 Ortholog Required For Otolith Mineralization In Zebrafish

The subcommissural organ and Reissner's fiber complex. An enigma in the central nervous system?

Antibodies were raised in rabbit against crude subcommissural organ (SCO) extract of 19 day old chick embryos. After absorption with crude brain extract, the IgG fraction was purified by ion exchange chromatography. The specificity of the antibodies was controled by immunostaining and by a competition test between lectins (Concanavalin A-Con A- and wheat germ agglutinin-WGA-) and antibodies (A74 IgG).

Glycoprotein synthesis in the subcommissural organ of the chick embryo. I. An ontogenetical study using specific antibodies.

In the chick embryo, A74 immunoaffinity chromatography allowed to purify specific glycoproteins relevant to the SCO ventricular secretory process. The eluted fractions of the subcommissural organ (SCO), the cerebral hemispheres (CH) and the medulla oblongata (MO) were compared using the Concanavalin A (Con A) and wheat germ agglutinin (WGA) staining procedures after western-blotting. Analysis of the optical density of the reactive bands allowed to estimate the relative concentration of the various glycopeptides in the eluted fractions. In the SCO-eluted fractions at least ten Con A-positive glycopeptides were identified, their apparent molecular weight ranging from 240 to 42 kD. Only three of these appeared to be WGA-positive (98, 88, and 52 kD). In the CH-eluted fractions only a 52 kD Con A-and WGA-positive glycopeptide was revealed, while in the MO-eluted fractions a 32 kD glycopeptide was also Con A- and WGA-positive. These results are discussed in regard to the known biosynthesis pathway of complex type glycoproteins.

Y. Croisille

Papers

Glycoprotein synthesis in the subcommissural organ of the chick embryo. I. An ontogenetical study using specific antibodies.

Glycoprotein synthesis in the subcommissural organ of the chick embryo. II. An immunochemical study.