Table 2 Classification of non-sinusoidal fenestrated blood capillary microvasculature

•Fenestrated endothelial cells •Continuous anionic basement membrane rich in sulphated proteoglycans[64, 86] •Anionic glycocalyx matrix of endothelial cell surfaces & clefts rich in sialyated glycoproteins & of fenestrated spaces rich in sulphated proteoglycans[166–170]

Diaphragmed fenestrae

•Diameters of fenestrae range between 60 and 80 nm

•Widths of closed membranous central diaphragms range between 10 and 30 nm

•Eight to twelve, 2 to 7 nm wide, outwardly radiating membranous septae from central diaphragm

•Arc widths of fenestrated open spaces between ~6 and ~12 nm [72–76, 82]

•Arc widths of open spaces devoid of membranous components (central diaphragm and septae) delineate the upper limits of pore size

•Diaphragms of diaphragmed fenestrae constitute the barriers to the transvascular flow of macromolecules

•Anionic glycocalyx matrix over fenestrated spaces charge barrier to the transvascular flow of anionic macromolecules

•Skin* [164, 165]

•Testis [171–173]

•Connective tissue [174, 175]

•Eye Choriocapillaris [82, 102–104, 176, 177]

•Exocrine Glands [105–108]

•Kidney Peritubular [72, 178]

•Endocrine Glands [73, 106, 179–185]

•Intestinal Mucosa [186–191]

•Peripheral Ganglia [158, 192–194]

•Nerve Epineurium [160]

•Circumventricular Organs [109, 110, 195–199]

•Choroid Plexus [109–113]

•Pre-Ovulatory Follicle [114, 115]

•Eye Ciliary Process [116–120]

Open fenestrae

•Open 'non-diaphragmed' fenestrae with avg. diameters of 65 nm devoid of the central diaphragm & septae [61, 72, 200]

•Narrow interspacing of glycocalyx matrix fibers is the barrier to the transcapillary flow of macromolecules larger than ~15 nm in diameter

•Kidney Glomerulus [60, 72, 101, 123–126, 201, 202]╬

╬Slit diaphragms at the level of podocyte foot processes restrict the filtration of plasma proteins larger than 6 nm in diameter (i.e. hemoglobin, albumin)