Search

The ending of Reissner's fiber (RF) and structural features associated with gaps or fissures in the rostral part of the filum were investigated using light, scanning, and transmission microscopic techniques in young and mature possums of both sexes. To the best of the author's knowledge the report contains results of the first successful application of the SEM for a study of RF in the spinal cord. Some new observations suggest that while the bulk of RF is formed by the subcommissural organ and moved caudally, additional secretory products may be added by ependymal cells in the sacral and possibly other regions of the spinal cord. Evidence is provided in support of the view that RF may pass through gaps in the ependymal lining in the rostral part of the dorsal wall of the filum terminale and caudal end of the sacral cord to reach the periependymal loose tissue and possibly the subarachnoid space. The region of the gap shows the surface of the ependyma facing the lumen of the filum to be covered with microvilli and cilia, and to be in direct continuity with the external surface of the ependyma covered with basement membrane with glial processes and collagen fibers in close proximity. The present results confirm and extend observations reported by Wislocki et al. ('56).

The subcommissural organ (SCO) of the Bradypus tridactylus was studied from the histological and ultrastructural point of view. The methods of GOMORI (alum-chromic-hematoxylin-phloxine), AZAN (M. Heidenhain), KLUVER and BARRERA (for nerve cells and myelinated fibers) and silver impregnation according to PALMGREN and CAJAL-DE CASTRO, were used. The material prepared for ultrastructural analize was submitted to the sequence of gluteraldehyde, osmium tetroxide, uranyl acetate and lead citrate. In function of the secretory phenomenon, a systematic study is made of different hystological and ultrastructural characteristics of the SCO and hypendymal rosettes, which were considered in the B. tridactylus as a morphofunctional unit. Fine vascular and nerve networks were observed, by the basal parts of the compact tubule alveolar system (hypendymal rosettes) which composes part of the SCO. Nerve fibers were seen between its cells reaching even up to the lumen, and they may end there in bulbar dilatations which seem to show secretory characteristics at the ultrastructural examination.

Since morphological data do not allow to draw conclusions concerning the phyletic evolution of the subcommissural organ (SCO), a "metabolic" criterion is considered. Histoenzymology permits a comparison of metabolic activity in the subcommissural organ and that found within the ependyma. During the course of phyletic evolution, one notes various changes. Indeed, in lower vertebrates, the SCO presents a greater degree of enzyme activity than the ependyma while in birds the ependyma demonstrates a more intense activity than the SCO. The results are also compared with those previously reported in reptiles and mammals. In all vertebrates, glucid catabolism seems predominent in this areas.

Reissner's fibre was investigated in brains and cervical spinal cords of mice aged 28 and 31 months using scanning electron microscopy. At both 28 and 31 months Reissner's fibre was present. It arose from a fusing together of numerous smaller fibres in the subcommissural organ and was observed in the aqueduct, fourth ventricle and central canal of the medulla and spinal cord. For most of its length it was a ridged structure similar to a stick of celery but in places small processes could be seen arising from its surface. Cilia, supraependymal cells and other debris appeared to be adherent to the fibre and this may support the view that Reissner's fibre has a detoxifying function which is still present in old age.

By means of various techniques and on the basis of a large sampling of the brush-tailed possum, Trichosurus vulpecula, a detailed description is given of the morphology of the neural elements associated with the subcommissural organ (SCO) and the leptomeninges on the dorsal surface of the midbrain. It is shown that the SCO of the possum is richly innervated, the nerves reaching the cerebral aqueduct by passing through the organ; there is, however, no evidence of a neural link between the SCO and the pineal gland. No typical axo-glandular synapses were found; however, many axo-glandular synaptoid junctions between axons and secretory cells of the SCO were demonstrable within the parenchyma of the SCO and on the aqueductal surface of the organ. The possible functional implications of the findings are: (1) The secretory activity of the SCO may be modified by nerves associated with it; this is in accord with some recently published experimental studies. (2) The nerves that innervate the SCO may also be responsible for ciliary activity on the aqueductal surface of the SCO, thus directing the apical secretory product of the SCO and debris flowing caudally in the cerebrospinal fluid into the poorly ciliated median groove where Reissner's fibre is found.

The subcommissural organ (SCO) exhibits anatomical characteristics of an endocrine organ: The secretion is released either into the blood (hypendymal capillaries) or the CSF of the 3rd ventricle; excretory ducts are absent; the active secretory activity of the ependymal cells can be regulated by humorally transmitted messages or by neural input. The rate of production of the Reissner's fibre (RF) by the SCO is rather fast, and the secretory material is stored in the ampulla caudalis (AC) and must be continuously discharged accordingly. Structures jointly involved in depletion of the AC and the decomposition and removal of the massa caudalis (MC) are collectively called the terminal organ (TO). The TO of the SCO-complex is formed by an assemblage of different structures in the caudal segment of the spinal cord (neurogenic part) and in the tissues (non-neurogenic part) which encompass this part of the cord. The different parts of the TO are characterized, even at the cellular level, by specializations which support the discharge as well as the dissolution of the material of the MC. The RF may be a detoxicator for the CSF, but also a carrier of hormonally active substances. In this case the TO is a site of release of hormones. The function of the entire complex is still under discussion, particularly its role in endocrine integration.

The light microscopic analysis of serial sections of the subcommissural organ (SCO) of the rainbow trout (Salmo gairdneri) shows that the form of the groove-like (in cross section) organ varies over its total length. Its rostral origin is a tunnel-like structure anterior to the orifice of the hollow pineal stalk. The SCO forms the dorsal wall of the brain. Caudally the SCO is increasingly displaced from the surface of the brain by the fibers of the posterior commissure; the organ ends in a tabular area beyond the latter. The orifice of the pineal stalk is surrounded by the ependyma of the SCO that invaginates like a funnel and joins with the ependyma of the pineal stalk after a considerable narrowing. The rudimentary parapineal organ is located on the left side of the brain and is connected with the left habenular ganglion through the parapineal tract. It contacts the third ventricle with a short channel within the ependyma of the SCO. The histological organization of the ependymal and hypendymal cells of the SCO is typical of teleosts. Secretory material is located basally and apically in relation to the nucleus, but there is no indication of a basal secretory release. Basal ependymal processes terminate with broadened endings at the membrana limitans externa. The apical product is discharged into the third ventricle, where it aggregates into the thread-like structure of Reissner's fibre. The SCO cells have no direct contact with cerebral or meningeal blood vessels.