Lymphatic capillaries

Vasa lymphocapillaria

Also known as: initial lymphatics, lymph capillaries, lacteals

Blind-ended, highly permeable vessels that begin the lymphatic system in the interstitium; their overlapping endothelial flaps act as one-way valves letting fluid, protein, cells and microbes in.

Overview

Lymphatic capillaries (initial lymphatics) form networks interwoven with blood capillary beds in almost every vascularised tissue. They begin blindly, are slightly wider and more irregular than blood capillaries, and consist of a single layer of thin, oak-leaf-shaped endothelial cells with discontinuous "button" junctions. The edges of adjacent cells overlap loosely, forming primary (endothelial) valves. There is little or no basement membrane and no pericytes or smooth muscle.

Anchoring filaments (fibrillin-containing microfibrils) tether the endothelial cells to surrounding collagen. When interstitial fluid accumulates and tissue pressure rises, the filaments pull the cell edges apart, opening the flaps; when pressure inside the capillary exceeds that outside, the flaps are pushed shut, so fluid cannot leak back. Their porosity means they absorb not only fluid but also proteins, chylomicrons, cells (dendritic cells, lymphocytes) and unfortunately bacteria and cancer cells.

In the small intestine each villus contains a central lymphatic capillary, the lacteal, which absorbs chylomicrons. Lymphatic capillaries are absent from avascular tissues (epidermis, cartilage, cornea), bone marrow and the brain and spinal-cord parenchyma — although true lymphatic vessels have now been identified in the dura mater, draining CSF-derived fluid to the deep cervical nodes.

Location

Throughout the connective tissues of the body alongside blood capillary beds; absent from epidermis, cartilage, cornea, bone marrow and CNS parenchyma.

Parts & subdivisions

Overlapping endothelial cells (primary valves)

Oak-leaf-shaped cells with button-like junctions whose overlapping edges open inwards like swinging doors.

Anchoring filaments

Microfibrils linking the abluminal endothelial surface to the extracellular matrix; they keep the vessel open when tissues swell.

Lacteals

Central lymphatic capillaries of intestinal villi that absorb chylomicrons, forming chyle.

Precollectors

Transitional vessels linking capillaries to collecting vessels, acquiring the first secondary valves and sparse smooth muscle.

Anatomy

Histology

Simple squamous endothelium (markers: LYVE-1, PROX1, VEGFR-3, podoplanin/D2-40 — used in pathology to identify lymphovascular invasion by tumours), discontinuous junctions, absent or incomplete basal lamina, no mural cells.

Development

Arise from PROX1-positive endothelial cells that bud from the cardinal veins under VEGF-C/VEGFR-3 signalling (weeks 5–6).

Function

Absorb excess interstitial fluid and plasma proteins.

Absorb dietary fat (lacteals).

Provide the entry route for dendritic cells, lymphocytes and antigen travelling to lymph nodes.

Physiology

Filling depends on the pressure gradient between interstitium and lumen; the intrinsic and extrinsic pumps of downstream collecting vessels lower intraluminal pressure and aid filling. Dendritic cells enter via chemokine (CCL21) gradients produced by lymphatic endothelium.

Clinical significance

Lymphovascular invasion and metastasis

Because lymphatic capillaries have no continuous basement membrane, carcinoma cells enter them easily — the main route of spread for carcinomas (sarcomas prefer blood vessels). Lymphovascular invasion seen on histology (D2-40 staining) is an adverse prognostic factor.

Test yourself

Interstitial fluid enters lymphatic capillaries mainly because…

Their endothelium is fenestrated and actively pumps fluid inwards
Overlapping endothelial cell edges act as one-way flaps, held open by anchoring filaments when tissue pressure rises
Their thick basement membrane filters protein from the interstitium
Smooth muscle in their walls generates negative pressure

Answer: Overlapping endothelial cell edges act as one-way flaps, held open by anchoring filaments when tissue pressure rises

Lymphatic capillaries have loosely overlapping "oak-leaf" endothelial cells with button-like junctions and an incomplete basement membrane. Anchoring filaments pull the vessel open when interstitial pressure rises, and the overlapping edges behave as primary valves, letting fluid, protein and cells in but not out.

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