Studies on the choroid plexus

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Development of the choroid plexus and blood-CSF barrier

Well-known as one of the main sources of cerebrospinal fluid (CSF), the choroid plexuses have been, and still remain, a relatively understudied tissue in neuroscience. The choroid plexus and CSF (along with the blood-brain barrier proper) are recognized to provide a robust protective effort for the brain: a physical barrier to impede entrance of toxic metabolites to the brain; a “biochemical” barrier that facilitates removal of moieties that circumvent this physical barrier; and buoyant physical protection by CSF itself. In addition, the choroid plexus-CSF system has been shown to be integral for normal brain development, central nervous system (CNS) homeostasis, and repair after disease and trauma. It has been suggested to provide a stem-cell like repository for neuronal and astrocyte glial cell progenitors. By far, the most widely recognized choroid plexus role is as the site of the blood-CSF barrier, controller of the internal CNS microenvironment. Mechanisms involved combine structural diffusion restraint from tight junctions between plexus epithelial cells (physical barrier) and specific exchange mechanisms across the interface (enzymatic barrier). The current hypothesis states that early in development this interface is functional and more specific than in the adult, with differences historically termed as “immaturity” actually correctly reflecting developmental specialization. The advanced knowledge of the choroid plexus-CSF system proves itself imperative to understand a range of neurological diseases, from those caused by plexus or CSF drainage dysfunction (e.g., hydrocephalus) to more complicated late-stage diseases (e.g., Alzheimer's) and failure of CNS regeneration. This review will focus on choroid plexus development, outlining how early specializations may be exploited clinically.

In vivo analysis of choroid plexus morphogenesis in zebrafish

Background

The choroid plexus (ChP), a component of the blood-brain barrier (BBB), produces the cerebrospinal fluid (CSF) and as a result plays a role in (i) protecting and nurturing the brain as well as (ii) in coordinating neuronal migration during neurodevelopment. Until now ChP development was not analyzed in living vertebrates due to technical problems.

Methodology/Principal Findings

We have analyzed the formation of the fourth ventricle ChP of zebrafish in the GFP-tagged enhancer trap transgenic line SqET33-E20 (Gateways) by a combination of in vivo imaging, histology and mutant analysis. This process includes the formation of the tela choroidea (TC), the recruitment of cells from rhombic lips and, finally, the coalescence of TC resulting in formation of ChP. In Notch-deficient mib mutants the first phase of this process is affected with premature GFP expression, deficient cell recruitment into TC and abnormal patterning of ChP. In Hedgehog-deficient smu mutants the second phase of the ChP morphogenesis lacks cell recruitment and TC cells undergo apoptosis.

Conclusions/Significance

This study is the first to demonstrate the formation of ChP in vivo revealing a role of Notch and Hedgehog signalling pathways during different developmental phases of this process.

Cell proliferation and differentiation from ependymal, subependymal and choroid plexus cells in response to stroke in rats

We tested the hypothesis that populations of ependymal, subependymal and choroid plexus cells proliferate and differentiate after stroke in adult rats. Rats were subjected to 2 h of middle cerebral artery occlusion (n=70) and euthanized at 1, 2, 4, 7, 14, 21 and 28 days (10 per time point). Hematoxylin and eosin staining and immunostaining were performed using antibodies against bromodeoxyuridine, neuronal nuclear antigen and glial fibrillary acidic protein after stroke. In normal nonischemic rats (n=10), single layers of ependymal and choroid plexus cells do not react with bromodeoxyuridine, neuronal nuclear antigen or glial fibrillary acidic protein. Individual subependymal cells express glial fibrillary acidic protein and bromodeoxyuridine, but not neuronal nuclear antigen. After stroke, increased bromodeoxyuridine reactivity was present in multiple layers of ependymal cells and nodules of subependymal cells and in scattered choroid plexus cells from 2 to 28 days and peaked at 7 days. Bromodeoxyuridine immunoreactivity colocalized with neural phenotypes of neuronal nuclear antigen (approximately 0.1-3.5%) and glial fibrillary acidic protein (approximately 8.6%) immunoreactivity in cells in the ventricular zone and the subventricular zone, as well as in the choroid plexus of the ischemia affected hemisphere. Our data suggest that ependymal, subependymal and choroid plexus cells are potential neural precursor cells in the adult mammalian brain.

Histochemistry and immunocytochemistry of the developing ependyma and choroid plexus

The adult human ependyma expresses no intermediate filament proteins or secretory proteins; the fetal ependyma shows strong immunocytochemical (ICC) expression of vimentin, glial fibrillary acidic protein (GFAP), cytokeratins (CKs) of high molecular weight, glycoproteins, and S-100beta protein. Each has a precise and specific spatial distribution within the developing ependyma and a predictable time of appearance and regression in each region of the ventricular system. Several are coexpressed, but some appear earlier or persist longer than others. Secretory proteins of ependymal cells are important in several developmental processes such as the guidance of axonal growth cones. GFAP is not expressed in the floor plate ependyma at any stage of development, unlike vimentin and CK. The choroid plexus epithelium is a specialized ependyma, with an ICC profile that differs from the surface ependyma: vimentin, CK, and S-100beta protein continue to be expressed throughout fetal and adult life, but GFAP is not expressed. Certain cerebral malformations are associated with specific ICC abnormalities: ependymal S-100beta protein continues to be immunoreactive in disorders of neuroblast migration; ependymal vimentin is focally upregulated in Chiari malformations and congenital aqueductal stenosis. Other mammalian and nonmammalian species have characteristic profiles of ependymal immunoreactivity to the same proteins expressed in humans but exhibit interspecific differences.

An experimental and ultrastructural study on the development of the avian choroid plexus

The choroid plexus consists of the choroidal epithelium, a derivative of the neural tube, and the choroidal stroma, which originates from the embryonic head mesenchyme. This study deals with epithelio-mesenchymal interactions of these two components leading to the formation of the organ. Grafting experiments of the prospective components have been performed using the quail-chicken marker technique. Prospective choroidal epithelium of quail embryos, forced to interact with mesenchyme of the body wall of chicken embryos, gives rise to a choroid plexus showing normal morphogenesis and differentiation. The choroidal epithelium induces the differentiation of organ-typical fenestrated capillaries, which are highly permeable to intravenously injected horseradish peroxidase. The choroidal epithelium of the grafts constitutes a blood-cerebrospinal fluid barrier. On top of the choroidal epithelium, there are epiplexus cells displaying a typical ultrastructure. The experimental results show that these cells do not originate from the transplanted neural epithelium. Prospective choroidal stroma of chicken embryos does not exert a choroid plexus-inducing influence upon a quail embryo's neural epithelium isolated from parts of the brain that normally do not develop a choroid plexus. The experiments show that the choroidal epithelial cells are determined at least three days before the first organ anlage is detectable.

Proliferation of epithelial cells in the adult primate choroid plexus

An adult rhesus monkey was injected intraperitoneally with [H3] thymidine (2.3 microCi/gram body weight) and perfused 90 minutes later with a mixture of aldehydes. One and a half micrometer plastic sections were then cut and dipped into liquid emulsion for radioautography. Labeled cells were observed in the choroid plexus of the anterior lateral ventricle; cell identification was evaluated using electron micrographs taken from serial thin sections of re-embedded. radioautographic 1.5-micron sections. The ultrastructure and location of both mitotic figures and labeled cells confirmed the presence of undifferentiated basal choroid plexus epithelial cells in the adult primate central nervous system.

The choroid plexus of the mature and aging rat: the choroidal epithelium

The choroid plexus of mature and old rats has been examined by both scanning and transmission electron microscopy. It has been shown that the macrophages lying upon the ventricular surface of the choroid plexus have a close association with burr-like protrusions that extend from the apical surfaces of the choroidal epithelial cells. These protrusions have a dark cytoplasm filled with vesicles and tubules, and projecting from them are thin, shrunken microvilli. It is suggested that these protrusions are phagocytosed by the macrophages and that they are the source of some of the inclusions which become increasingly common within the cytoplasm of macrophages in older rats. The lateral surfaces of the choroidal epithelial cells have also been examined in the scanning electron microscope after exposure of the surfaces by dissection. In such preparations it is apparent that the elaborate interdigitations between adjacent cells are effected by irregular and vertically arranged folds confined to the basal portions of the lateral cell surfaces. Lastly, it has been shown that at the junction between the choroid plexus and the ependyma in the lateral ventricle, there are two modes of transition between the choroidal and ependymal epithelia. In one, typical choroidal and ependymal epithelial cells lie next to each other to produce a distinct and continuous bondary. In the other mode the boundary is also continuous, but there are modified ependymal cells present. These modified cells have short, relatively sparsely distributed microvilli and not more than one or two cilia.