Lumen (anatomy)

In biology, a lumen is the space enclosed by a tubular structure, and the word is used for structures as large as the gut and as small as a microtubule.

The Latin lumen originally meant an aperture or opening, and the term entered English through anatomy before being adopted by cell biology.

The defining feature of a lumen is not its size but its topology, because a lumen is a space bounded by cells, membranes or extracellular matrix and connected to a transport or signalling pathway.

General definition

• A lumen is the interior space of a tube, vessel, duct, gland or organelle, and its boundary is an epithelium, an endothelium or a membrane.
• The plural lumina is preferred in anatomy, while lumens is common in general biology and in physics.
• A lumen is not empty, because it contains fluid, air, mucus, blood, bile, urine or secreted products depending on the organ.
• The surface area of a lumen can be increased by folds, villi, microvilli or cristae, which is a recurring solution to the problem of exchange across a narrow space.
• Luminal is the adjective for the inside of a lumen, and abluminal or basolateral describes the opposite surface of the same cell.
• The lumen is often the first compartment to be affected by disease, because it is exposed to ingested, inhaled or secreted noxious agents.

Lumina in the body

• The lumen of an artery, vein or capillary is the central space through which blood flows, and its diameter is a major determinant of resistance.
• The lumen of the gastrointestinal tract runs from the mouth to the anus and is continuous with the external environment, which is why it is not a sterile space.^[1]
• The bronchial tree contains a continuous lumen that branches from the trachea to the alveoli, and its surface is lined by ciliated epithelium and mucus.
• Renal tubules and collecting ducts have a lumen through which filtrate becomes urine, and their luminal membrane carries transporters that modify the filtrate.
• The female genital tract has a single vaginal lumen that splits into two uterine horns in some mammals, and in humans the uterine cavity is a single flattened lumen.
• The blastocoel is the fluid-filled lumen of the blastocyst, and its formation is one of the earliest lumen-forming events in development.
• Lymphatic vessels, bile ducts, pancreatic ducts and salivary ducts all have lumina that are lined by specialised epithelia.
• The lumen of a gland acinus is the small central space into which secretory cells release their products before the acinus joins a duct.

Cell biology

• In cell biology a lumen is a membrane-bounded space inside an organelle, and its composition is kept distinct from the cytosol by the surrounding membrane.
• The thylakoid lumen is the aqueous space inside a thylakoid, and the light-driven pumping of protons into it powers ATP synthesis.
• The endoplasmic reticulum lumen is the site where newly synthesized proteins are folded, glycosylated and assembled before onward transport.
• The Golgi lumen is divided into cisternae with different enzymatic contents, so a protein is progressively modified as it passes from the cis to the trans face.
• Lysosomes maintain an acidic lumen, usually around pH 4.5 to 5.0, because their acid hydrolases require low pH for activity.
• The mitochondrial lumen, usually called the matrix, contains the enzymes of the citric acid cycle and the machinery for oxidative phosphorylation.
• Microtubules have a narrow lumen that is largely solvent-filled, although cryo-electron tomography has identified luminal particles including tubulin-binding cofactors.
• Nuclear pores connect the nucleoplasm to the cytoplasm, and the nuclear envelope itself encloses a perinuclear lumen continuous with the endoplasmic reticulum.

Formation

• Lumen formation begins when cells create a sealed boundary and then transport ions across it, so that water follows osmotically.
• In closed lumina, ion pumps create an osmotic gradient inside a cluster of cells, and water flows in to inflate the space.
• The blastocoel forms when extracellular fluid is pumped into the embryo and microlumens coalesce into one central cavity.
• Epithelial lumina form by the opening of a central cavity within a solid epithelial cord, a process that requires coordinated apical membrane trafficking.
• Cell death, cell shape change and cell rearrangements can all contribute to lumen opening, and different organs use different combinations.
• The physics of lumen growth is described by tissue hydraulic models that treat the lumen as a pressurized fluid compartment within a viscoelastic tissue.

Transluminal procedures

• Transluminal procedures are operations performed through the lumen of a hollow organ, often without a skin incision.
• Natural orifice transluminal endoscopic surgery passes an endoscope through the mouth, vagina, bladder or colon to reach the abdominal cavity.
• Percutaneous transluminal angioplasty widens a narrowed blood vessel by inflating a balloon within its lumen.
• Percutaneous transluminal commissurotomy opens a fused cardiac valve commissure, most often in mitral stenosis, by a catheter passed through the lumen.
• Transluminal stenting and thrombectomy rely on the same principle of reaching a lesion from inside the vessel rather than from outside.
• The main advantage of transluminal access is the absence of a surgical wound, but the main risk is injury to the wall of the lumen during passage.

Clinical significance

• Luminal stenosis narrows a hollow organ and can obstruct flow, as in coronary artery stenosis, ureteric stricture or intestinal stricture.
• Luminal thrombosis occludes a vessel from within, and the size and location of the thrombus determine the ischaemic territory.
• Luminal obstruction of the bile duct causes jaundice, whereas luminal obstruction of the ureter causes hydronephrosis.
• Luminal pH is clinically important in the stomach and duodenum, and it is measured by pH monitoring in reflux disease.
• Luminal bacteria in the gut outnumber human cells, and their composition is studied by stool metagenomics rather than by culture alone.
• Luminal surface markers are used by pathologists to determine whether a tumour is of epithelial origin, since luminal keratins face the cavity.
• Endoluminal imaging with optical coherence tomography or intravascular ultrasound resolves the lumen wall and is used to guide stenting.

Comparative anatomy

• Luminal organisation is ancient, and even sponges have water-filled channels that function as a primitive lumen system.
• Vertebrate guts show a conserved lumen lined by epithelium, but the folding pattern ranges from smooth in fish to villous in mammals.
• The avian lung has a rigid parabronchial lumen rather than the compliant alveolar lumen of mammals, reflecting a different airflow design.
• Insect tracheae and vertebrate blood vessels are not homologous, but both use a tubular lumen to distribute gases or blood.
• Plant xylem vessels have a lumen that is dead at maturity and conducts water without a living lining, unlike animal vessels.

History

• Lumen was used in classical Latin for an opening or a window, and anatomical writers applied it to the cavity of a vessel or duct.
• Microscopists in the nineteenth century described the lumen of glands and tubules, but the molecular study of lumen formation began only in the late twentieth century.
• The discovery of membrane trafficking in the 1970s and 1980s explained how organelles maintain distinct luminal environments.
• Transluminal surgery developed from balloon angioplasty in the late 1970s and expanded into endoscopic and hybrid procedures in the 2000s.

Research and controversies

• How cells coordinate apical membrane growth with lumen expansion is still an open problem, and several competing models exist.
• Whether lumina form mainly by vesicle fusion, by cell death or by membrane separation depends on the organ, and no single mechanism explains all cases.
• The role of luminal pressure in organ size control is debated, because measurements in vivo are difficult and often indirect.
• Transluminal procedures raise infection and closure concerns that are still being studied in clinical trials.
• Single-cell and organoid methods now allow lumen formation to be watched in real time, which may resolve long-standing disagreements about mechanism.