Structure of Microlymphatics

Compared with the high density of the microvasculature, the microlymphatic network is sparse. Individual microlym-phatics originate in the tissue and form bifurcating trees and in some organs, meshworks. Their detailed network morphology depends on the tissue and organ involved.

The lymphatics can be divided into two general classes, the initial lymphatics and the contractile lymphatics (Figure 1). Preceding the initial lymphatics there are also prelymphatic channels. We will describe the three microstructures in the sequence through which tissue fluid is transported.

Prelymphatic Channels

In mesentery, extracellular tissue fibers are connected to the terminal endings of the initial lymphatics. The prelym-phatic channels are formed by tissue fibers oriented in the direction towards the lymphatic endothelial cells. In the brain, the delineation of tracers injected into the cere-brospinal space identifies prelymphatic channels. Such tracers tend to follow the perivascular space of arterioles and eventually drain into the initial lymphatics.

The Initial Lymphatics

These consist of a continuous but highly attenuated endothelial lining, and they have no smooth muscle media. In organs such as the skin and intestine, the initial lymphatics form a highly regular hexagonal branching pattern. Initial lymphatics are frequently associated with structures that have smooth muscle (arterioles, respiratory bronchioles, and so on). They are located in the adventitia of arterioles in close proximity to the smooth muscle cells, nerve fibers, adipocytes, and what is a continuous line of mast cells (Figure 2). In contrast, other parts of the tissue parenchyma may be relatively depleted of initial lymphatics. For example, the capillary space in skeletal muscle has no initial lymphatics. Instead, initial lymphatics form a meshwork that is tightly paired with the arcade arterioles and larger venules in this organ [1].

In contrast to the cylindrical geometry of most blood vessels, the lumen cross section of the initial lymphatics has an irregular shape in many tissues; the initial lymphatics almost never have circular cross sections. The lumen may, in fact, be completely collapsed. Yet in normal tissue, the initial lymphatics are readily identified based on morphology and specific location in the tissue. In tumor tissues, initial lymphatics may be more difficult to detect on histological

Figure 1 Schematic of initial lymphatics without smooth muscle and contractile lymphatics with smooth muscle in the mesentery microcirculation. The vessel dimensions are not drawn to scale. The initial lymphatics tend to be associated with paired arcade arterioles and venules.

Figure 2 Light microscopic cross section of initial lymphatic in rat skeletal muscle. The vessel has a single endothelial lining, is positioned in vicinity of adipose cells (A), a terminal arteriole (TA), embedded in collagen fibers(C), and positioned between two arcade arterioles (AA) with vascular smooth muscle media (VSM). (see color insert)

Figure 2 Light microscopic cross section of initial lymphatic in rat skeletal muscle. The vessel has a single endothelial lining, is positioned in vicinity of adipose cells (A), a terminal arteriole (TA), embedded in collagen fibers(C), and positioned between two arcade arterioles (AA) with vascular smooth muscle media (VSM). (see color insert)

sections and therefore require immunological labels (such as an antibody against LYVE-1, a lymphatic specific receptor for hyaluronan [2], or the lymphatic marker podoplanin) for identification as lymphatic endothelium.

The endothelium in initial lymphatics has a discontinuous basement membrane with openings in the vicinity of the interendothelial junctions. In contrast to capillary endothelium, there are no pericytes on lymphatic endothe-lium. Short interstitial fibers (anchoring filaments) connect the abluminal side of the lymphatic endothelium to the adjacent connective tissue. In the organs investigated to date, no adrenergic innervation of the initial lymphatics has been encountered. In the interlobular connective tissue of the rat liver, neuropeptide Y- and substance P-containing nerve fibers around initial lymphatics have been observed.

Initial lymphatics have the ability to collect and transport interstitial fluid, proteins, lipid particles, colloids, and cells, including metastatic cells. In tissues that are resting, peristaltic motion sufficient to expand or compress the lumen cross section as a fluid pump mechanism has yet to be observed in initial lymphatics. To date, no human lymphatic system has been described that is without initial lymphatics. Compared with contractile lymphatics, the initial lymphatics are considerably more numerous and make up the major portion of the lymphatic network.

The Contractile Lymphatics

In contrast to the initial lymphatics, the contractile lymphatics have a smooth muscle media and exhibit regular peristaltic motion with periodic compression and expansion of the lymphatic lumen. Contractile lymphatics frequently have circular cross sections so that a circumferential mechanical stress can be generated by the smooth muscle. The contractile lymphatics undergo spontaneous peristaltic contractions, a phenomenon not seen in the initial lymphatics. They are rarely associated with vascular smooth muscle and frequently form vessels that pass through the tissue in the form of isolated channels unpaired with the arterioles or large veins. Contractile lymphatics are positioned downstream of (i.e., proximal to) the initial lymphatics and serve to drain fluid from the initial lymphatics into the lymph nodes and the central thoracic ducts. Contractile lymphatics have pacemaker cells, and smooth muscle contraction travels in the form of peristaltic waves along the contractile lymphatic ducts.

The lymphatic smooth muscle cells exhibit many of the characteristics of arteriolar smooth muscle, with smooth muscle myosin heavy chain isoforms as well as cardiac alpha-actin, vascular alpha-actin, enteric gamma-actin, and skeletal alpha-actin.

The contractile lymphatics exhibit a myogenic response accompanied by generation of membrane action potentials and calcium influx into the endothelial cytoplasm. They also show nitric oxide-dependent vasodilation. The flow-dependent dilation can be mimicked by nitric oxide. The smooth muscle in contractile lymphatics has among many other receptors, stimulatory a-adrenergic and b-inhibitory adrenoreceptors [3], histamine receptors, and ATP-sensitive K+ channels. The contractile lymphatics are also responsive to a variety of inflammatory mediators, such as oxygen free radicals and substance P. Contractile lymphatics may have a noradrenergic innervation [4] with unmyelinated nerve fibers that reach into the subendothelial connective environment in close association with endothelial cells. These nerve fibers react with calcitonin gene-related pep-tide or substance P.

In mammalian organs, like the intestine or skeletal muscle, all lymphatics inside the tissue parenchyma are initial lymphatics. Contractile lymphatics originate only at strate gic transition points where the lymphatics exit the tissue parenchyma [5].

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