The interpretation of trans‐synovial fluxes requires a knowledge of the number of capillaries present around the joint cavity, and their depths below the surface of the joint lining. Measurements of capillary numbers, depths and dimensions were therefore carried out by light microscopy on synovial tissue sections from five rabbit knees. In addition the surface area of synovial tissue lining the joint was estimated, and the thickness of the synovium measured. Synovial capillary density was found to be uneven, with respect to both anatomical location in the joint cavity and depth below the synovial surface. The large area of synovium over areolar or adipose tissue was highly vascular (67000–83000 capillaries.cm−2 section; mean intercapillary distance 35–55, µm; 228–329 cm2 endothelium.cm−3 synovium). The small area of synovium over tendons was of very low vascularity (2000 capillaries.cm−2 section). From these data and a previous estimate of net filtration capacity, mean synovial endothelial conductivity was estimated as ∼ 16 x 10−7 cm.s−1.mmHg−1. The depth distribution of the capillaries was extremely skewed, with a sharp peak at 6–11 µm depth. The rapid decay in capillary density at depths 〉 6–11 µm was partly due to an uneven density of capillaries within the synovium itself; and partly due to a lower density in subsynovial tissues. It was concluded that most synovium has a rich capillary bed which is very superficially located and that the depth of periarticular tissue which is effective in exchange processes with the joint cavity is less than 25 µm.

SUMMARY1. A synovial cavity is separated from plasma by synovial intima in series with capillary endothelium. Because 20 % of the intimal surface is bare interstitium, the system is a convenient model for the study of passive transport through serial endothelial and interstitial layers. Here hydraulic flow across the composite barrier was investigated in forty-seven knees of isolated, blood-perfused rabbit hindquarters, at intra-articular pressures between 4 and 30 cmH2O.2. In order to measure barrier conductance at constant intra-articular pressure, pressure on the opposite side of the barrier was varied, i.e. capillary blood pressure (Pc). Capillary pressure was changed by alteration of vascular perfusion pressures, and the resulting changes in rate of absorption of Krebs solution from the synovial cavity (Qs) were recorded. Trans-synovial absorption was a negative linear function of PC at each joint pressure, in verification of the applicability of Starling's hypothesis to this system. 3. The hydraulic conductance ofthe blood-joint barrier was calculated as dQs/dPc.Conductance was independent of intra-articular pressure below 9 cmH2O and was 0 12 + 0.015 ,al min' mmHg-' (mean + S.E. of mean).4. Barrier conductance increased as a curvilinear function of intra-articular pressure above 9-4 cmH2O (yield pressure). At 30 cmH2O conductance averaged 0-60 + 0-06 #1 min' mmHg-', a 5-fold increase.

SUMMARY1. The pressure-volume relationship in the synovial cavity of the rabbit knee was investigated by the infusion of a non-absorbable oil into the cavity.2. The pressure (P)-volume (V) curve was sigmoid. Its slope dP/dV (elastance) was steep at subatmospheric, physiological pressures, least steep at 5 cm H20, and steepened progressively at higher, pathological pressures.3. Elastance dP/dV was a good approximation to linear functions of P below atmospheric pressure and above 5 cm H20. Each part of the pressure-volume relationship could therefore be described by an exponential expression.4. Flexion of the joint increased pressure at a given volume. This was mainly due to steepening of the pressure-volume curve, and also partly due to a shift towards the pressure axis.5. Pressure-volume curves determined by the infusion of normal saline or Krebs solution were grossly distorted, because these fluids were absorbed across the permeable synovium.6. The physiological significance ofthe synovial pressure-volume curve is discussed, and mechanical explanations of the curve are suggested.

SUMMARYThe ultrastructure of the barrier to water and solute exchange between blood and a synovial cavity was studied morphometrically in five rabbit knees. The synovial surface consisted of 80 % synoviocytes and 200% interstitium in direct contact with synovial fluid. The surface intercellular gaps were wide (0 1-55 ,um). A rich network of capillaries lay just beneath the surface, the commonest (modal) depth of the capillary perimeter being 5 ,sm at its nearest point. Of these capillaries, 91 % were fenestrated in adipose synovium, 44% in areolar synovium and 210% in posterior synovium; the remainder were continuous capillaries. The number of fenestrae per fenestrated capillary was low (-05,um-1, circumference: -6 ,m-2). The fenestrae (width 52 nm; membrane thickness 3-4 nm) occupied -1 % of the fenestrated endothelial surfacearea. Nuclear segments and intercellular junctions were roughly twice as frequent in continuous capillaries as in fenestrated capillaries of the same circumference, indicating that continuous endothelial cells are of smaller surface area than fenestrated cells. The basement membranes of continuous and fenestrated synovial capillaries were relatively thick (98 and 110 nm respectively), perhaps supporting the synovial endothelium during joint motion. In both fenestrated and continuous capillaries the nuclei were preferentially oriented away from the synovial surface (92 and 670% respectively), which presumably facilitates exchange with the surface.

The rat granular retrosplenial cortex (GRS) is a simplified cortex, with distinct stratification and, in the uppermost layers, distinct modularity. Thalamic and cortical inputs are segregated by layers and in layer 1 colocalize, respectively, with apical dendritic bundles originating from neurons in layers 2 or 5. To further investigate this organization, we turned to reelin-deficient reeler mouse and Shaking rat Kawasaki. We found that the disrupted lamination, evident in Nissl stains in these rodents, is in fact a patch-matrix mosaic of segregated afferents and dendrites. Patches consist of thalamocortical connections, visualized by vesicular glutamate transporter 2 (VGluT2) or AChE. The surrounding matrix consists of corticocortical terminations, visualized by VGluT1 or zinc. Dendrites concentrate in the matrix or patches, depending on whether they are OCAM positive (matrix) or negative (patches). In wild-type rodents and, presumably, mutants, OCAM(+) structures originate from layer 5 neurons. By double labeling for dendrites (filled by Lucifer yellow in fixed slice) and OCAM immunofluorescence, we ascertained 2 populations in reeler: dendritic branches either preferred (putative layer 5 neurons) or avoided (putative supragranular neurons) the OCAM(+) matrix. We conclude that input-target relationships are largely preserved in the mutant GRS and that dendrite-dendrite interactions involving OCAM influence the formation of the mosaic configuration.