Outer brain barriers in rat and human development.

Christian B Brøchner,Kjeld Møllgård,Camilla B Holst

doi:10.3389/fnins.2015.00075

Christian B Brøchner, Kjeld Møllgård + Show 1 more

Open Access

https://doi.org/10.3389/fnins.2015.00075

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Abstract

Complex barriers at the brain's surface, particularly in development, are poorly defined. In the adult, arachnoid blood-cerebrospinal fluid (CSF) barrier separates the fenestrated dural vessels from the CSF by means of a cell layer joined by tight junctions. Outer CSF-brain barrier provides diffusion restriction between brain and subarachnoid CSF through an initial radial glial end feet layer covered with a pial surface layer. To further characterize these interfaces we examined embryonic rat brains from E10 to P0 and forebrains from human embryos and fetuses (6–21st weeks post-conception) and adults using immunohistochemistry and confocal microscopy. Antibodies against claudin-11, BLBP, collagen 1, SSEA-4, MAP2, YKL-40, and its receptor IL-13Rα2 and EAAT1 were used to describe morphological characteristics and functional aspects of the outer brain barriers. Claudin-11 was a reliable marker of the arachnoid blood-CSF barrier. Collagen 1 delineated the subarachnoid space and stained pial surface layer. BLBP defined radial glial end feet layer and SSEA-4 and YKL-40 were present in both leptomeningeal cells and end feet layer, which transformed into glial limitans. IL-13Rα2 and EAAT1 were present in the end feet layer illustrating transporter/receptor presence in the outer CSF-brain barrier. MAP2 immunostaining in adult brain outlined the lower border of glia limitans; remnants of end feet were YKL-40 positive in some areas. We propose that outer brain barriers are composed of at least 3 interfaces: blood-CSF barrier across arachnoid barrier cell layer, blood-CSF barrier across pial microvessels, and outer CSF-brain barrier comprising glial end feet layer/pial surface layer.

Highlights

IntroductionFor decades the meninges have been considered to be no more than a protective shield, but increasing evidence suggests that the complex outer barriers of the brain function as morphogenic signaling centers (Siegenthaler and Pleasure, 2011; Decimo et al, 2012), dynamic transport systems (Yasuda et al, 2013), a stem cell niche (Decimo et al, 2012) and regulators of immune cell entry to the CNS (Stolp et al, 2013)
We aimed to characterize the interfaces at the surface of the developing brain revealing a claudin-11 positive arachnoid blood-cerebrospinal fluid (CSF) barrier, collagen 1 positive subarachnoid space and pial surface layer, stage-specific embryonic antigen-4 (SSEA-4)/YKL-40 positive leptomeningeal cells and SSEA-4/YKL-40/IL-13Rα2/brain specific lipidbinding protein (BLBP) positive radial glial end feet
Research regarding the brain barrier system has evolved considerably over the last decades and the focus is on both physical, transport, and metabolic barrier properties

Summary

Introduction

For decades the meninges have been considered to be no more than a protective shield, but increasing evidence suggests that the complex outer barriers of the brain function as morphogenic signaling centers (Siegenthaler and Pleasure, 2011; Decimo et al, 2012), dynamic transport systems (Yasuda et al, 2013), a stem cell niche (Decimo et al, 2012) and regulators of immune cell entry to the CNS (Stolp et al, 2013). Ultrastructural electron microscopical (EM) and freeze fracture studies described the arachnoid as a multi-layered epithelium with tight junctions between cells of the outer continuous 1–3 cellular layers—the arachnoid barrier cell layer—that forms an effective seal covering the inner dural surface (Nabeshima et al, 1975; Rascher and Wolburg, 1997). The layers of developing human meninges were investigated and defined by O’Rahilly and Muller (1986) starting with the primary meninx comprising neural crest-derived cells (Decimo et al, 2012) that initially surround the neural tube and at around 6th week post-conception (wpc) are present around most parts of the brain. Later 6 layers develop consisting of subcutaneous tissue, the skeletogenous layer (the dense mesenchyme between the subcutaneous layer and at first the primary meninx), dura mater, the dural limiting layer (probably contributing to both dura and arachnoid), leptomeninx (arachnoid and pia), and cerebral wall (O’Rahilly and Muller, 1986)

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