A cura di Rossella Iuliano
THE LYMPHATIC SYSTEM
INTRODUCTION
The lymphatic system has always been
known, since Hippocrates, as the “white
blood” of organisms. It is a part of the
immune system used to remove fluids and
proteins from tissues and pour them back into
the bloodstream. The parts identified as the
ones it is composed of are : interstitium,
lymphatic vessels, lymphatic organs and
migrating cells[1]. It is involved in a lot of
diseases, such as cancer, lymphedema, asthma
and various inflammatory diseases. This is the
reason why everyone works in the sanitary
sector needs to pay attention on structure and
function of lymphatic vessels, whit all of
them branches directed to the ductus
thoracicus and ductus lymphaticus dexter as
far as large veins.
Hodgkin’s Linphoma
STRUCTURE AND FUNCTION
Studies on bioptic and on necrotic human
material had disclosed very difficulties into
the differentiation of blood capillaries and
lymphatic vessels, especially considering that
also the second ones have a kind of valves
[2]. Nevertheless in 1990 the
lymphangiogenesis and the endothelium of
lymphatic capillaries were analyzed and the
vascular endothelial growth factor receptor-3
was discovered, among others specific
molecule functional for this system, with its
ligand VEGF-C [3]. Baruk et al. described the
effect of growth factors on lymphatic vessels
in mice affected by chronic respiratory tract
infection. The inhibition of VEGFR-3 did not
let lymphatic vessels grow up but it was
different for blood vessels, while VEGF-C
and VEGF-D were able to cause a contrary
effect. Another important result was obtained
by Banjeri et al. [4] when they visualized the
endothelium of limphatic vessels thanks to
LYVE-1 antibodies, i.e. markers for a protein
on the surface of this tissue. However both
blood and lymphatic vessels are essential for
the microcirculation in the loose connective
tissue [5], composed of the two types of
vessels and of its own interstitium. The last
one contains matrix, with fibers (collagen,
elastic and reticular fibers) and amorphous
substance (glycosaminoglycans,
proteoglycans and glycoproteins). Hyaluronan
is the glycosaminoglycan), which acts as a
base of support for immune cells and is
degraded in regional lymph nodes or in the
liver after being let in bloodstream[6].
Leukocytes, instead, go away from blood and
recognize antigens to defend the body from
all the possible external attach by an outside
being. But the filtration in blood capillaries
and venules must be controlled because it can
be dangerous, originating an edema if the
lymphatic drainage is insufficient [7]. Edema
is avoided because of filtration, readsorbition
into blood capillaries, and clearance into
lymphatic capillaries. They take plasma
proteins when the edematous fluid has more
than 10 g/L of proteins but, in this case, also
macrophages can act and realize proteolysis.
Lymphatic vessels also move fluid against
hydrostatic gradient with them contractions,
regulated by catecholamines, which stimulate
adrenergic and cholinergic nerves in
lymphatic vessels walls. Moreover this
system transports blood cells, as it was seen
in limbs and sheeps [8], and a lymphocytes
physiological recirculation from postnodal
lymph to the bloodstream in “endothelial
venules” was described, too [9]. After the
lymphocytes migrate from the bloodstream
into the lymph [10]. The cycle is repeated for
12-24 hours, and if no antigens are found, the
lymphocytes die [11]. In case of inflammatory
affection, T lymphocytes enter into lymphatic
vessels thanks to chemokine receptor CCR7
[12]; the same recirculation from peripheral
tissues exists also during the ontogenetic
development [13], just to assure a suited
migration to the bloodstream to make an
immune response. For these reason it is very
important afferent lymphatic vessels’rule,
because they transport cells to the regional
lymph nodes, also the dendritic cells, which
present antigens to T lymphocytes before
entering prenodal lymph. The afferent lymph
contained apoptotic cells and there are cell
debris in the lymphatic capillaries of the
synovial membrane [14]. But the drainage
function recognized to this system also in
synovial membrane has been described as a
consequence of specialized inter-endothelial
junctions, too [15 ]. The externals extension
of the endothelial cells are, in fact, anchored
to the connective tissue but the internal ones
can enter the lymphatic capillaries and create
a communication among them and the
interstitium. When the pressure in the
connective tissue exceeds, the internal
extension cover up the external one. Because
of this function, but also for them
morphology, inter-endothelial junctions are
called primary valves.
Fig1. Lymph node
CONCLUSION
Lymphatic system is the guardian of tissues’
fitness, because it guarantees the immune
response by transporting lymphocytes,
stimulating antibodies production and
evoking the activation of lymphonodes.
Moreover it is involved into the migration of
neoplastic cells because they pass into its
vessels but are degraded, in the best of
chances, before causing a metastasys.
Fig 2. Infiltration of a lymph node
capsule by neoplastic cells.
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