Human subcommissural organ, with particular emphasis on its secretory activity during the fetal life

The subcommissural organ (SCO) is a conserved brain gland present throughou t the vertebrate phylum. During ontogeny, it is the first secretory structu re of the brain to differentiate. In the human, the SCO can be morphologica lly distinguished in 7- to 8-week-old embryos. The SCO of 3- to 5-month-old fetuses is an active, secretory structure of the brain. However, already i n 9-month-old fetuses, the regressive development of the SCO-parenchyma is evident. In 1-year-old infants, the height of the secretory ependymal cells is distinctly reduced and they are grouped in the form of islets that alte rnate with cuboid non-secretory ependyma. The regression of the SCO continu es during childhood, so that; at the ninth year of life the specific secret ory parenchyma is confined to a few islets of secretory ependymal cells. Th e human fetal SCO shares the distinct ultrastructural features characterizi ng the SCO of all other species, namely, a well-developed rough endoplasmic reticulum, with many of its cisternae being dilated and filled with a fila mentous material, several Golgi complexes, and secretory granules of variab le size, shape, and electron density. The human fetal SCO does not immunore act with any of the numerous polyclonal and monoclonal antibodies raised ag ainst RF-glycoproteins of animal origin. This and the absence of RF in the human led to the conclusion that the human SCO does not secrete RF-glycopro teins. Taking into account the ultrastructural, lectin-histochemical, and i mmunocytochemical findings, it can be concluded that the human SCO, and mos t likely the SCO of the anthropoid apes, secrete glycoprotein(s) with a pro tein backbone of unknown nature, and with a carbohydrate chain similar or i dentical to that of RF-glycoproteins secreted by the SCO of all other speci es. These, as yet unidentified, glycoprotein(s) do not aggregate but become soluble in the CSF. Evidence is presented that these CSF-soluble proteins secreted by the human SCO correspond to (1) a 45-kDa compound similar or id entical to transthyretin and, (2) a protein of about 500 kDa. Microsc. Res. Tech. 52: 573-590, 2001. (C) 2001 Wiley-Liss. Inc.