The ultrastructure of the barriers between blood and lymph. Where do we stand to-day?

Issue title: Microcirculation, Interstitium, Lymph, Pathophysiology and Disease. Proceedings of the International Symposium, Villa La Principessa, Lucca, Italy, June 19–20, 1981

Guest editors: Siegfried Witte

Article type: Research Article

Authors: Hammersen, F. | Hammersen, E.

Affiliations: Department of Anatomy, Technical University München, F.R. Germany

Abstract: (I) The blood capillary endothelial cell (EC): Three different structures play a more or less decisive rôle in capillary permeability characteristics: (1) a delicate endothelial layer of glycoproteic nature, (2) intracytoplasmic vesicles and/or channels of different stereological configurations with a max. inner diameter of 50 nm, and (3) interendothelial clefts 15-20 nm wide of variable length and course, fitted with attachment decives of variant fine structure. (II) The interstitial space (ISS): To evaluate the barrier functions of this compartment with the limited arsenal of morphological techniques is extremely difficult, and therefore the results are controversial. Using electron-dense tracers of variant molecular seizes, the very first component of the ISS directly beyond the endothelial cells, i.e. their basal lamina, does not seem to function as a general, ubiquitous sieve for molecules up to a seize of 5 nm. Caution, however, is emphasized against data obtained and conclusions drawn from model experiments performed with a very special, yet frequently employed representative of endothelial basal laminae, i.e. the glomerular basement membrane. The adjoining interstitial space proper (ISS) consists of three major components: (1) a fluid phase, (2) a phase of dissolved macromolecules, and (3) the matrix which is composed of long chain molecules of fibrous proteins (e.g. collagen) and glucosaminoglycans (mucopolysaccharides). While the first two of these constituents are more or less beyond the reach of any structural analysis, the third component can be studied more closely with both histochemical techniques and electron microscopy to elucidate its chemical and structural composition. Not this, however is the objective of the present contribution, but the barrier functions of the ISS, which depend to a variant extent on all its components. Off these it is the matrix again which allows to demonstrate at an ultramicroscopic level at least one of its permeability characteristics, namely the phenomenon of ‘exclusion’. Another proposition held over more recent years emphasized the regular occurence of nonendothelialized, preformed ‘tissue channels’ which should serve as ‘prelymphatics’ of variable length and diameter in almost every tissue and organ. The methods, between, however, by which the existence of such ‘channels’ has been proven are open for severe criticism due to the many artifacts involved in the techniques employed. At this stage it seems, therefore, to be premature to correlate already these structurally defined ‘channels’ with those postulated to exist on the grounds of permeability data and calculations. (III) The endothelium of the initial (terminal) lymphatics: As compared to the blood capillary endothelial cells, lymphatic endothelium appears to be more uniform with less topical spezializations. It differs from blood capillary ECs in several ways: (1) It is extremely thin over large areas except its nucleated portion. (2) Micropinocytic vesicles are less numerous and most of them tend to form lysosomal vacuoles instead of ferrying materials across the endothelial barrier. (3) The interendothelial clefts vary in width and they are often loosely arranged with no or less elaborate attachment decives. Open gaps of up to several microns width are regularly encountered. (4) The basement membrane is discontinous and ‘anchoring filaments’ insert in a densely staining substance of the abluminal endothelial plasmalemma. They extend into the surrounding connective tissue and they exert tension on the lymphatic wall thus stabilizing the vessel, keeping its lumen open and under certain conditions they may even pull adjoining ECs apart.

Keywords: Endothelial cell, ultrastructure, interstitial space, initial lymphatic, vesicular transport

DOI: 10.3233/CH-1982-25-605

Journal: Clinical Hemorheology and Microcirculation, vol. 2, no. 5-6, pp. 425-440, 1982