LUNGFISH

Ventricles  The lateral ventricles are relatively large, vertically orientated and lenticular in shape. They communicate with the third ventricle towards their caudal end, and at this level the third ventricle is rhomboid in shape, with a large everted paraphysis and a midline choroid plexus which extends from the interventricular foramen to the level of the pineal gland and optic chiasm, where the ventricle is narrow and vertical. There is an infundibular recess which extends in a ventro-caudal direction to underlie the midbrain and overlie the adenohypophysis. The midbrain has a narrow midline ventricle and the optic lobes are fused so that there is no bilateral optic ventricle. The fourth ventricle is flat and transversely orientated at its rostral end, and extends dorsi-laterally into the lobes of the cerebellum. Caudal to the cerebellum, the fourth ventricle has lateral recesses which extend alongside the cerebellum for some distance, and the main body of the fourth ventricle forms a dorsal recess where the roof consists of choroid plexus. Further caudally there is a small midline dorsal recess and beyond this level the ventricle is a small rhomboid structure situated centrally within the medulla, extending about 1mm and is continuous with the small regular circular central canal of the spinal cord.

Hindbrain roof  The caudal end of the roof of the fourth ventricle, where it forms the narrow dorsal recess (the so-called posterior tela) is a thin single-celled ependymal layer which is intact, and the ventricular system in the lungfish is closed.

Meninges  The meningeal covering consists of an investing endomeninx which stains positive for collagen, is closely attached to the neuraxis and follows blood vessels into the glia for a short distance. Surrounding this is a loose layer of areolar meningeal tissue which is well separated from the endosteal dura, and there is a large subdural space, which may well be artefactual. The investing meningeal tissue is condensed over the dorsal aspect of the choroid plexuses of the third and fourth ventricles, where they are adjacent, with longitudinally running sinusoids and an endolymphatic system intervening. The loose areolar meningeal tissue is continuous with similar tissue in the inner ear, and also accompanies the nerve roots to their formanina. The extensive endolymphatic system is also continuous with the endolymphatic sacs derived from the inner ear. Just caudal to the cerebellum, where the roof of the fourth ventricle consists of ependyma only, there is a condensation of the areolar meningeal tissue and blood vessels and the fibrous investing meninx is not present. Around the ventral aspect of the midbrain the loose meninx extends towards the adenohypophysis and around the third ventricle it contains a collar of blood vessels running between the hypothalamus and the thin-walled infundibular recess. The investing meningeal layer is present all round the forebrain, and dorsally around the paraphysis and the ependymal roof of the third ventricle with blood vessels intervening between these structures and the cranial vault. The pia mater is thus well-defined but there is no specific arachnoid layer, and the only apparent space is subdural.

Ependyma The ependyma of the lateral ventricles is columnar and ciliated excepting over the medial wall where it is somewhat flattened cuboidal and non-ciliated. In the lateral walls of the third ventricle it is columnar and ciliated, but ventrally it is a thin flattened layer at the rostral end, and at the caudal end the infundibular recess has cuboidal epithelium ventrally, columnar ciliated epithelium laterally and a thin low cuboidal ciliated dorsal ependyma, adjacent to the saccus vasculosus. The roof of the third ventricle has flattened ependyma at the junction between the ventricle and the dorsal recess, and it is extremely thin where it extends dorsally to become the choroid plexus. At the caudal end of the third ventricle, behind the choroid plexus, the ependyma is a highly pigmented thick columnar epithelium with some dorsal vesicles where it forms the sub-commissural organ. The midbrain ependyma is columnar and ciliated, and throughout the floor and lateral walls of the fourth ventricle it is similar. Over the rhombic lip the ependyma is thin and flattened where it joins the choroid plexus folds, and at this point it is reinforced on the outside by a thin extension of the investing meningeal layer. In the medulla and spinal cord the ependyma is very tall columnar ciliated epithelium with basally placed nuclei and the crowded cells are somewhat irregular.

Choroid plexuses The choroid plexuses of the lateral ventricles are elaborate well-developed structures consisting of a central stroma of fibrous tissue and blood vessels covered by ciliated cuboidal epithelium with centrally placed nuclei. They are attached dorsally alongside the interventricular foramen and are separate from the third ventricle choroid plexus, which consists of a fairly simple fold from the centre of the recess of the third ventricle, and contains a stroma of blood vessels with pigmented cells, covered by a cuboidal ciliated epithelium and centrally placed nuclei. At the rostral end there is an elaborate dorsal extension of epithelium similar to choroid plexus although of somewhat lower cuboidal type, and accompanied by rather large blood vessels, and this is the paraphysis. The choroid plexus of the fourth ventricle is arranged in villi which are narrow and hang down from the fourth ventricle roof, and consist of a core of blood vessels without much fibrous tissue, and covered by cuboidal to columnar epithelium with centrally placed nuclei, well-stained cytoplasm and cilia. There are a few pigmented cells in the core of the villi and epiplexus cells present on the surface. The blood vessels and areolar tissue within the choroid plexuses are continuous with the loose areolar tissue surmounted by the large sinusoidal vessels dorsally in the subdural space.

CSF-vascular relationship The CSF of the ventricular system is separated by cuboidal epithelium from the blood vessels of the tela choroidea, and by similar cuboidal epithelium from the meningeal blood vessels around the infundibular recess. The ventral ependyma at the rostral end of the third ventricle is a thin, flattened layer, as it is also in the lateral walls of the recess of the third ventricle and the rhombic lip choroid plexus interval of the fourth ventricle, and in the midline of the fourth ventricle roof just caudal to the cerebellum; at all of these points a thin flattened ependyma separates the ventricular csf from the prominent meningeal blood vessels. The sinusoidal vessels running between the tela choroidea of the third and fourth ventricles and the endocranium are very prominent. Elsewhere the csf of the ventricular system is separated from the sub-ependymal vessels of the brain by columnar ciliated epithelium.

Illustrations - Lungfish

Conclusions