Choroid Plexus

CHORIOIDEUS PLEXUS, vascular plexus, represents glandular formations [choroid glands (glandulae chorioideae, Mott)] located in the cavities of the brain ventricles. The topography of the choroid plexus in most mammals is approximately the same (Rusconi); it is customary to distinguish the anterior choroid plexus (corresponding to the lateral and third ventricles) and the posterior choroid plexus, located in the fourth ventricle. The choroid plexus of the third ventricle in humans and higher apes has the shape of a triangle, the apex of which reaches the columns of the fornix (see figure), and the base corresponds to the splenium.

Plexus chorioideus ventriculi lateralis and tela chorioidea ventriculi III: 1-truncus communis venae Galeni; 2-v. magna Galeni; 3-v. corporis striati; 4-v. plexus chorioidei; 5-v. septi pellucidi; 6-v. thalami optici; 7-v. cornu Ammonis; 8-v. calcaris avis; 9-v. cuneo-limb.; 10-v. centri semiovalis; 11-v. corporis quadrigem.

of the corpus callosum; it consists of dorsal and ventral leaflets which are connected to each other by means of subarachnoid tissue, while at the edges of the plexus the dorsal leaflet turns into the ventral one. This edge of the plexus passes into the choroid plexus of the lateral ventricle, which extends from the foramen of Monro to the end of the inferior horn. At the place of the collateral trigone, the choroid plexus forms lobular, villous reddish processes (glomus chorioideum). Both plexuses of the lateral ventricles run forward into the narrow space between both foramina of Monro, into which they bend and pass here into each other. In the choroid plexus of the lateral ventricle, the dorsal leaflet of the plexus is covered with vascular villi, and in the choroid plexus of the third ventricle, the ventral one. The lateral edge of the lateral choroid plexus is attached to the taenia chorioidea (see below), and the medial edge to the free lateral edge of the fornix. The epithelium of the plexus is continuous with the ependyma of the ventricles. The choroid plexus of the third ventricle is attached laterally to the stria medullaris thalami, and at its posterior end the line of its attachment passes to the habenula and to the surface of the pineal gland (Vicq d'Azyr's plexus). The layer of brain substance covering the terminal vein (vena terminalis) continues in the form of a thin "attached lamina" (lamina affixa) onto the neighboring part of the optic thalamos, fusing with the latter structures as early as the embryo; in the direction from front to back, the lamina affixa first becomes wider, and then narrows and is completely absent in the posterior horn. It passes directly into the epithelial lamina of the choroid plexus of the lateral ventricle (lamina chorioidea epithelialis), after the removal of which there remains a thin rim with an uneven, serrated edge, forming the "strap" of the choroid plexus (taenia chorioidea). In the formation of the choroid plexus, the main part is played by the pia mater, the process of which is tucked into the transverse fissure of the brain, where it constitutes the vascular roof of the third ventricle (tela chorioidea ventr. III). The latter represents a duplication of the pia mater, has a triangular shape (in man and higher apes) and ends blindly in front. This roof consists of 1) the upper lamina, adjacent to the lower surface of the corpus callosum and fornix, and 2) the lower lamina, covering most of the upper surface of the optic thalami. The choroid plexus of the fourth ventricle consists of the epithelial roof of the fourth ventricle, extending laterally to the taenia ventriculi IV, and forward to the velum medullare, and the lamina of the pia mater adjacent to the epithelium. The choroid plexus occupies here in width the entire line between both cerebellar flocculi, with the middle point of its attachment falling on the nodulus of the inferior vermis, and its apex located at the posterior end of the fourth ventricle on the calamus scriptorius. In the fourth ventricle, two paired lateral plexuses (pl. laterales) and a median unpaired one (pl. medialis) can be distinguished. The first choroid plexuses are directed sideways from the nodulus to the lateral recesses of the fourth ventricle; the median plexus consists of two strips adjacent to each other, running back to the nodulus. In contrast to the completely closed tela chorioidea of the third ventricle, the tela chorioidea of the fourth ventricle has three secondary breaks in the form of one unpaired opening - the median aperture of the fourth ventricle (foramen of Magendie) and two lateral ones located in the lateral recesses - the lateral apertures of the fourth ventricle (foramen of Luschka); the indicated openings serve to regulate the pressure of the cerebrospinal fluid (Rauber). It should be noted that Rusconi's studies indicate a great variability of the vascular plexus of the fourth ventricle in animals. According to the histological structure [see separate table (vol. XXXIII, pp. 551-552), fig. 6], the so-called portio villosa, the connective tissue of which is a product of the pia mater, and portio non villosa, in the structure of which the arachnoid and stratum trabeculare also take equal part, are distinguished in the choroid plexus according to Vialli and Imamura. The latter department consists of 4 layers located from outside to inside: 1) epithelium, 2) connective tissue rich in vessels and originating from the pia, 3) connective tissue originating from the arachnoid, and 4) stratum trabeculare; between the third and fourth layers no sharp difference is observed - both of them are stained yellow by van Gieson's method. The relationship between portio villosa and portio non villosa changes not only in various classes of vertebrates, but even in their various species. The epithelium of the choroid plexus is for the most part single-layered according to the opinion of the majority of authors, doubling and tripling only in a pathological state. It should be kept in mind, however, that Zalka does not agree with this universally recognized statement and considers the multilayered epithelium of the choroid plexus to be normal. Another controversial issue is the difference between epithelium and ependyma. Some authors (Luschka, Saxer, Hart) deny this difference altogether, others (Benda, Bittorf, Vonwiller, Askanazy, Zand) recognize this kind of distinction contrary to Coupin's studies on fish, who believes that the cells lining the cavities of the ventricles and the cells of the villi of the choroid plexus are equivalent to each other. Epithelial cells generally have a prismatic shape with two surfaces: a) free and b) basal. In its size, the epithelium of the choroid plexus belongs to the most volumetric and varies greatly in various classes of animals in its width and height, changing also depending on age. The epithelium of the choroid plexus has villi on its surface (especially well noticeable in embryos and newborns and described in vertebrates by Coupin); these villi may be absent in individual cases in certain places (e.g., the paraphysis in amphibians). The nucleus of the epithelial cells is relatively large, vesicular, with a clearly expressed membrane, with a low chromatin content and with one or more nucleoli; with a retracted shape of the cells, the nucleus has an oval configuration. In young subjects, epithelial cells have a rounded shape, in elderly ones - polygonal. Mitoses have been noted very rarely and were found only in fish (Cyprinus carpio) and reptiles (Lacerta muralis) in the growth period or during hibernation. Active phenomena of secretion in the nucleus, as well as in the cell plasma, were described by Galeotti and Engel. Granular and rod-shaped inclusions were established in the cytoplasm. Chondriomes (in the form of short rods or longer threads) are not a constant finding and change in their appearance in the series of vertebrates. In mammals, the form of chondrioconts (rods or grains) predominates, while in fish, amphibians, and reptiles, both chondrioconts and mitochondria are encountered equally often. The Golgi apparatus, described in the vascular epithelium by Biondi and Kopsch, was observed in birds by Comini; intercellular canaliculi, analogous to Holmgren's trophospongium, can also be referred here. "Spheres" in the form of diplosomes or dots were described by Schmid. Fat in the choroidal epithelium is in the form of grains, drops or "granular drops", and sometimes in the form of "globoplastic" half-moon formations. Their presence, as well as that of lipoids, varies in different animal species. The amount of lipoids increases sharply according to Morosi's data during pregnancy. Glycogen in the vascular epithelium was noted by Vialli in fish, amphibians, and a newborn wild cat. Pigment in the choroid plexus is found in the form of melanin or hemosiderin: the former in lower animals, the second in higher ones; besides, other pigments are observed in the epithelium of the choroid plexus, which due to their easy solubility in alcohol or xylene are very difficult to distinguish. Epithelial cells rest on a connective tissue base (stroma), which is nothing more than processes of the pia and arachnoid (Robertson, Findley, Zand). The connective tissue of the choroid plexus consists mainly, on the one hand, of reticular cells and fibers, and on the other, of collagen fibers. The shape, quantity, and distribution of both these components vary greatly. Collagen tissue can be considered as the skeleton of choroidal formations. Elastic fibers are contained not only in the pars non villosa of the choroid plexus and in large vessels, but also in the villi and in small vascular branches. Vialli distinguishes cells associated with connective tissue from those cells that are related to collagen fibers, and singles out as main categories: a) reticulo-endothelial elements, b) fibroblasts, c) mast cells, d) melano-chromatophores, and e) large vacuolated cells (choroidal tissue). It should be kept in mind that according to Fieschi's data, there are very few connective tissue elements in the choroid plexus: the bulk of the choroid plexus is represented by epithelium and vessels. - Regarding the nerve elements of the choroid plexus, Shapiro, using silver treatment methods, was able to prove the presence of myelinated and unmyelinated fibers, partly passing into peculiar terminal endings. With the exception of the nerve fibers of the smooth musculature of vessels, the nerve endings of the choroid plexus depart from nerve trunks originating in the dorsolateral department of the medulla oblongata. In the medial parts of the choroid plexus, the nerve endings are branches of those fibers which, starting in the medulla oblongata, reach here through the taenia ventr. IV.

These fibers, according to Clark, differ in their appearance from the fibers of the choroid plexus of the lateral recess. With the exception of vasomotor branches, the nerve endings of the choroid plexus are sensory in nature. Clark points out further that the nerve endings show a substantial difference in newborn and adult animals. The strong development of the choroid plexus in embryos was noted by older authors (His, Ziegler), and Luschka long ago, based on the presence of glucose in the embryonic cells of the choroid plexus, concluded that it has a possible significance for the nutrition and development of the embryonic brain ("cerebral placenta"). Ontogenetically, the choroid plexus develops relatively very early: in a two-month embryo, this formation can be seen powerfully developed and occupying the greater part of the ventricle, and sagittal sections particularly clearly demonstrate the close connection of the choroid plexus with the pia mater. Physiology of the choroid plexus. The choroid plexus should be considered primarily as an organ that secretes cerebrospinal fluid (see). The secretory activity is served mainly by the epithelium of the choroid plexus, and then by the ependymal lining of the ventricles. According to Vialli's study, not all parts of the choroid plexus secrete simultaneously. The indicated author draws attention to the fact that the villous part of the choroid plexus produces more fluid than the non-villous part; the cells of the choroid plexus that are at rest are less high than the secreting cells. In addition, Vialli was able to establish that the relative amount of substances secreted by the choroid plexus varies to a strong degree in individual vertebrate classes. Further differences are noted in the purely chemical structure of the epithelial cells of embryos and adults: in the former, according to Vialli, glycogen, for example, predominates over other substances, the amount of which, however, rapidly drops after birth; thus, in embryos, the cerebrospinal fluid has a different composition than in newborns. Sand distinguishes essentially two crucial moments in the activity of the choroid plexus: 1) the retention of substances foreign to the cerebrospinal fluid through the mediation of the blood-brain barrier, and 2) the actual formation of the cerebrospinal fluid. Regarding the first moment, it should be borne in mind that the blood-brain barrier is a special control and regulatory mechanism between the blood and the cerebrospinal fluid (L. Stern), consisting of the choroid plexuses, the endothelium of the pia mater, the walls of the capillaries, and the ependyma of the ventricles ("reticulo-histiocytic system") and functioning as a semipermeable membrane, electively passing various substances circulating in the blood into the cerebrospinal fluid depending on their physicochemical properties: crystalloids easier than colloids, cations harder than anions, etc. Among the elements of the protective barrier, a special role is played by those cells which Cushing designates as meningocytes, Oberling as meningoblasts, Mechnikov as macrophages, Maksimov as polyblasts, Ranvier as clasmatocytes, Sand as histiocytes, Weed as arachnoid cells, etc., considering them in the brain as representatives of the "reticuloendothelial system" (Aschoff). To these cells, Sand attributes the property of protecting nervous tissue from the action of foreign substances soluble in the blood. The question of the participation of the choroid plexus in absorption must be decided in the positive sense, according to Italian authors (Vialli, Gianelli, Chiancone), especially if based on the experiments of Fieschi and Klestadt. The named authors see the proof of the absorptive activity of the choroid plexus, among other things, in the dimorphism of the epithelial cells of various parts of the choroid plexus, viewing this dimorphism as confirmation of the dual function of the choroid plexus: secretory and absorptive. The doctrine of the protective and filtrative significance of the choroid plexus found supporters in the persons of various scientists (Milian, Schmorl, Mestrezat, etc.), as well as in the experiments of Goldman, Schläpfer, and others. These experiments could establish that after intravenous injection of dyes, all organs are stained, while the brain and cerebrospinal fluid remain unstained. From this, the role of the choroid plexus as a barrier (see above) or filter between the blood and the cerebrospinal fluid (or nervous tissue) becomes obvious. These data were verified by Monakow, Gautier, and L. Stern on the basis of a large embryological, comparative anatomical, experimental, and pathological material and led to the conclusion that the choroid plexus is one of the ingredients of that functional complex of the ecto-mesodermal barrier, which serves not only as a filter for neurotoxic substances, but simultaneously possesses its own hormonal function and, besides, is permeable to other hormones; disputes here concern whether this hormonal activity is associated with the entire surface of the epithelium of the choroid plexus or with certain sites of it. Summarizing all of the above, one can be convinced that the function of the choroid plexus is basically 1) secretory, 2) absorptive, and 3) hormonal; the question of the resorptive significance of the choroid plexus cannot at present be considered finally resolved in a negative or positive sense. All these data are convincing as to the correctness of the view according to which the choroid plexus is regarded as glandular formations (choroid glands). Pathological anatomy of the choroid plexus. Based on the data of Monakow and his school, the choroid plexus is generally speaking extremely labile and sensitive to all kinds of harms; besides, its structure changes physiologically with age. These age-related changes, described in detail by Tsalka and Auersperg, consist in the appearance of brown pigment and vacuoles, in the flattening of the epithelium, in the development of diffuse or focal sclerosis of the connective tissue, and in the appearance of cysts [according to Brack, cysts are never lined with epi-endothelium (the so-called "Luschka's hydatids")], in the formation of psammoma bodies, hyaline droplets, as well as in calcification (Beals proved the presence of lime in the choroid plexus in 28% of X-ray images). The presence of the so-called corpora arenacea is always noted in the choroid plexus of elderly subjects; there are especially many of these bodies near the aforementioned hydatids. Corpora arenacea have an affinity for many dyes (hematoxylin, eosin, Sudan, fuchsin, Nile blue). Regarding the genesis of the described bodies, the most diverse opinions exist in the literature. Age-related changes in the vessels can in turn lead to secondary changes in the choroid plexus. If one compares the choroid plexus of a newborn and an elderly subject, it will turn out that in the first case, there is a delicate connective tissue stroma and numerous villi in the choroid plexus, while in the second case, exactly the opposite relationships are ascertained. Pathological changes in the choroid plexus have been the subject of many works and have been described both in acute and chronic diseases (meningitis, syphilis, alcoholism, arteriosclerosis, general infections, lethargic encephalitis, malaria, etc.). Monakow, Kitabayashi, and Sierra published changes in schizophrenia which they were inclined to view as specific for this disease; however, Hoch and Tsalka believe that the changes in the choroid plexus described by the indicated authors are by no means characteristic, but are ordinary age-related changes in the choroid plexus. Of particular interest is the study of the pathology of the choroid plexus in hydrocephalus. Claisse and Levi described an idiopathic form of hydrocephalus with an increase in the size of the choroid plexus (macroplexia), whereas Kitabayashi and Gassin saw, on the contrary, a decrease in the choroid plexus. Weber, Burr, and others noted infiltration in the choroid plexus in hydrocephalus, however, Saito finds nothing specific in this, since he saw a number of cases of hydrocephalus without the presence of infiltrates. Monakow attributes the changes in the choroid plexus in hydrocephalus, as is known, to his group IV, i.e., basically to atrophic-sclerotic changes in the connective tissue; it must be borne in mind that these changes in the choroid plexus are nevertheless refuted by the works of Tsalka. Sand believes that in the idiopathic form of hydrocephalus, one should speak more of the insufficiency of the function of the choroid plexus than of an increase in its activity. Auersperg's observation runs counter to this viewpoint of Sand, since this author in a 56-year-old patient with congenital hydrocephalus was able to establish the indisputable participation of the choroid plexus in the increased production of cerebrospinal fluid. In head injuries, significant changes can be ascertained in the choroid plexus and in the ependyma, with the exception only of those cases where death occurred very quickly. These changes in the choroid plexus reduce to the following points: 1) the cells of the choroid plexus enlarge 2–3 times compared to the norm, 2) the border line with the ventricle (for the ependyma) becomes irregular, 3) vacuoles enlarge in the cells (sometimes in the form of one large one, sometimes in the form of several small ones), 4) the nucleus swells, deforms, shrinks, 5) large vacuoles are visible in the stroma of the villi, 6) iron-containing fibrin can be ascertained in the stroma, 7) connective tissue elements are also pathologically altered; besides, an increased amount of pigment in the choroid plexus is observed in such cases. Circulatory disorders sometimes cause hyperemia, sometimes hemorrhages. Wüllenweber described an aneurysm of the choroid plexus, noted in the literature only 6 times before him. Amyloidosis and hyalinosis are noted in the vessels of the choroid plexus.

It should be kept in mind that general arteriosclerosis is not always necessarily accompanied by sclerosis of the choroid plexus. Regarding the latter condition of the choroid plexus, it should be noted that Tannenberg distinguishes three groups of choroid plexus sclerosis, and Tsalka five, depending on the transitions from more severe degrees of involvement to milder forms with an unclearly outlined histopathological picture; in such cases it is sometimes very difficult to establish the boundary between incipient sclerosis and the normal structure of the choroid plexus (Tsalka). Tumors of the choroid plexus are of primary and secondary (metastatic) origin. Literature data show that among primary tumors, papilloma should be placed in first place; the latter is most frequently found in the choroid plexus of the fourth ventricle, less frequently in the third, and very rarely in the lateral ventricles (Kaufmann). According to data by Boudet and Clunet, up to 1910 only 36 cases of neoplasms of this kind had been described (of which, in the opinion of Davis and Cushing, only 13 are indisputable). Vonwiller distinguishes two groups of choroid plexus papillomas: 1) in the first group, the epithelium of the tumor develops from glandular epithelium, and 2) in the second group, the epithelium has an embryonic appearance. If the described neoplasm has an infiltrating character, Hart regards it as a cancer; the malignant form of papillomas is very rare. Besides papillomas, among primary tumors the following have been observed: xanthoma, vascular cholesteatoma, perithelioma, adenoma, neuroepithelioma, meningioma. Chernyshev, Kopylov, and Terian described a choroid plexus tumor weighing 154 g in the right lateral ventricle of a 10-year-old boy, diagnosed during life by radiography; histopathological examination of this tumor revealed a calcified meningioma (Bailey and Cushing) or psammoma. Among metastatic tumors in the choroid plexus, sarcomas and carcinomas have been observed most frequently. Here it should be pointed out that primary carcinoma of the choroid plexus is extremely rare (Esser).