Blood–CSF barrier function in the rat embryo

Abstract

Blood-cerebrospinal fluid (CSF) barrier function and expansion of the ventricular system were investigated in embryonic rats (E12-18). Permeability markers (sucrose and inulin) were injected intraperitoneally and concentrations measured in plasma and CSF at two sites (lateral and 4th ventricles) after 1 h. Total protein concentrations were also measured. CSF/plasma concentration ratios for endogenous protein were stable at approximately 20% at E14-18 and subsequently declined. In contrast, ratios for sucrose (100%) and inulin (40%) were highest at the earliest ages studied (E13-14) and then decreased substantially. Between E13 and E16 the volume of the lateral ventricles increased over three-fold. Decreasing CSF/plasma concentration ratios for small, passively diffusing molecules during embryonic development may not reflect changes in permeability. Instead, increasing volume of distribution appears to be important in this decline. The intracellular presence of a small marker (3000 Da biotin-dextranamine) in plexus epithelial cells following intraperitoneal injection indicates a transcellular route of transfer. Ultrastructural evidence confirmed that choroid plexus tight junctions are impermeable to small molecules at least as early as E15, indicating the blood-CSF barrier is morphologically and functionally mature early in embryonic development. Comparison of two albumins (human and bovine) showed that transfer of human albumin (surrogate for endogenous protein) was 4-5 times greater than bovine, indicating selective blood-to-CSF transfer. The number of plexus epithelial cells immunopositive for endogenous plasma protein increased in parallel with increases in total protein content of the expanding ventricular system. Results suggest that different transcellular mechanisms for protein and small molecule transfer are operating across the embryonic blood-CSF interface.

Low levels of Na, K-ATPase and carbonic anhydrase II during choroid plexus development suggest limited involvement in early CSF secretion

“…The presence of AQP1 at E15 strongly suggests CSF formation at this age. This is further supported by the observed expansion of the lateral ventricles immediately after the choroid plexuses first appear [12]. This occurs at a time in development when the pores in roof of the fourth ventricle have not yet formed [15] and the water added to the ventricular space at this time cannot escape and thus contributes to the pressure required for ventricular expansion and proper brain development [7].…”

“…We have previously suggested that the high CSF protein concentration and specific protein transfer present early in choroid plexus development may be involved in driving CSF formation and ventricular expansion [12,14]. The aim of the present study was to investigate the ontogeny of NKA and CAII in the lateral ventricular choroid plexus in the rat in order to determine if these enzymes also play a role in CSF secretion during early brain development.…”

“…During this period the cerebral ventricles more than double in size [12] and AQP1 water channels are already present in an adult-like pattern [13] implying secretion of fluid. The low levels of NKA and CAII during this period suggest that alternative mechanisms are likely to be involved.…”

The transcription factor Otx2 regulates choroid plexus development and function

“…Samples of CSF were collected from the fourth ventricle using microcapillary samplers (E12-14, typically 0.5-2 μl) (see Johansson et al, 2006). All CSF samples were microscopically examined for traces of blood over a white background and samples that showed any sign of blood contamination were discarded.…”

“…The ChPs are formed between embryonic day (E) 11 and 14 in the mouse, with the fourth ventricular (hindbrain) ChP differentiating first followed by the two lateral ventricular and later the third ventricular ChP (Dziegielewska et al, 2001). The ChPs acquire barrier, secretory and transport capacities shortly after formation (Møllgård et al, 1976;Ek et al, 2003;Johansson et al, 2005;Johansson et al, 2006;Liddelow et al, 2009;Ek et al, 2010). This early functionality and their localization inside the cerebral ventricles, together with their appearance during the period of neurogenesis, make them uniquely suited for influencing CSF composition and thereby regulating development of the neural stem cells that are in contact with the ventricle along the entire neuraxis (e.g.…”