Hilum (anatomy)

In anatomy, a hilum is a depression or fissure on the surface of an organ where its vessels, nerves and ducts enter or leave.

The hilum is therefore the physical gateway between an organ and the rest of the body, and its contents define the organ's vascular and neural connections.

Well-known examples include the hilum of the lung, the hilum of the kidney, the splenic hilum and the hilum of the liver, each with a characteristic arrangement of structures.

Definition and etymology

• Hilum is Latin for a small thing or a trifle, and classical writers used hilus for the same idea, which is why both hilum and hilus appear in older anatomical texts.
• The plural is hila in modern usage, and older texts sometimes use hili when following the hilus form.
• A hilum is a localized depression rather than the whole surface of the organ, and it is usually marked by a change in the covering membrane.
• The hilar region contains the main vessels and often the first-order branches of the duct system, which makes it a surgical bottleneck.
• A hilum differs from a pedicle because the pedicle is the bundle of structures themselves, while the hilum is the surface region where that bundle attaches.
• The term is also used in pathology for the point where vessels enter a lesion, as in the hilum of a lymph node or the hilum of an ovarian follicle.

Examples

• The hilum of the kidney is a vertical slit on the medial border through which the renal artery, renal vein, renal pelvis, lymphatics and autonomic nerves pass.
• The renal hilum is surrounded by perirenal fat and is the site where the ureter emerges from the renal pelvis.
• The splenic hilum is on the medial surface of the spleen and transmits the splenic artery, splenic vein, lymphatics and sympathetic nerves.
• The hilum of the lung is a triangular depression on the mediastinal surface where the root of the lung enters, containing the main bronchus, pulmonary artery, pulmonary veins and bronchial vessels.
• The right and left pulmonary hila differ in their arrangement, and the right hilum lies anterior to the oesophagus while the left lies below the aortic arch.
• The hilum of the liver, or porta hepatis, transmits the portal vein, proper hepatic artery, common bile duct, lymphatics and hepatic nerves.
• The ovarian hilum is the point where the ovarian artery, vein and nerve enter the ovary, and it contains specialised hilar cells.
• The hilum of a lymph node is the indented region where efferent lymphatic vessels leave and where blood vessels enter.
• The hilum of the adrenal gland is a small area on the medial border where the adrenal vein exits, usually the right adrenal vein into the inferior vena cava.

Structure

• Hilar structures are packed in a predictable order, and the order differs between organs and between the right and left sides.
• The hilar region is usually covered by a reflection of the visceral peritoneum or pleura, which continues onto the organ surface.
• The connective tissue of a hilum is continuous with the organ capsule and with the adventitia of its vessels, forming a conduit for lymphatics.
• Hilar fat surrounds the vessels and allows them to move slightly, but it also obscures their arrangement on imaging.
• The hilar vessels branch soon after entering the organ, so the hilum is a transition zone between named vessels and segmental branches.

Function

• The hilum funnels vessels, nerves and ducts into the organ and protects them from compression by surrounding structures.
• Because all major vessels pass through a narrow region, the hilum acts as a point of vascular control during surgery and as a site for selective clamping.
• The hilar region also concentrates lymphatics, which drain the organ and provide a route for tumour spread.
• In the lung, the hilum transmits both the airway and the pulmonary vessels, so it couples ventilation and perfusion at a single anatomical gateway.

Clinical significance

• Hilar lymphadenopathy is a common imaging finding in sarcoidosis, tuberculosis, lymphoma and metastatic lung cancer.
• Hilar cholangiocarcinoma, or Klatskin tumour, arises at the confluence of the hepatic ducts within the porta hepatis and often involves the portal vein.
• Renal hilar tumours can invade the renal vein and inferior vena cava, and their resection requires vascular control at the hilum.
• Splenic hilar varices form in portal hypertension and are a risk during splenectomy or splenic biopsy.
• Video-assisted thoracic surgery often begins with hilar dissection, and the hilar lymph nodes are sampled for staging.
• Hilar cell hyperplasia in the ovary can produce androgen excess, although it is far less common than stromal hyperthecosis.
• During liver transplantation the porta hepatis is dissected to isolate the portal vein, hepatic artery and bile duct for anastomosis.

Development

• The hilum forms as the organ's vascular pedicle condenses and the surrounding mesenchyme differentiates into capsule and hilar connective tissue.
• The position of the hilum is determined by the direction in which the organ grows and by the point at which its vessels enter the embryonic primordium.
• During lung development the hilum marks the entry of the bronchial bud, and the pulmonary arteries follow the airway branches into the same region.
• The renal hilum rotates from a fetal ventral position to its adult medial position as the kidney ascends from the pelvis.
• Hilar cells of the ovary are derived from the ovarian stroma and are thought to be related to Leydig cells.

Comparative anatomy

• A hilum is recognisable in the lungs, kidneys, spleen and liver of all mammals, although the relative position of the structures varies with posture and organ shape.
• Birds have hilar regions in their lungs and kidneys, but the avian lung is rigid and its hilar arrangement differs from the mammalian pattern.
• Fish kidneys open their vessels into a hilar region that is less compact than in mammals, and amphibian kidneys show a similar loose arrangement.
• The splenic hilum is most prominent in species with a large spleen, such as carnivores, and less distinct in species with a small spleen.

History

• The Latin hilum was used by classical authors for a small opening or a trifle, and early anatomists applied it to the entry point of vessels.
• The distinction between hilum and porta was gradually fixed in the nineteenth century, with porta hepatis retained for the liver.
• Radiologists in the twentieth century adopted hilar as an adjective for the region around the lung root, and hilar adenopathy became a standard reporting term.
• Terminologia Anatomica lists hilum renale, hilum pulmonis, hilum splenicum and porta hepatis as preferred terms.

Research and controversies

• The definition of hilar cholangiocarcinoma varies between staging systems, which makes comparison of surgical series difficult.
• Whether hilar lymph node sampling should be routine in all lung cancer surgery is debated, since the procedure increases operating time but improves staging.
• Three-dimensional reconstruction has shown that the hilar vessels branch more variably than classical descriptions suggest.
• Some authors use hilum and pedicle interchangeably, but most surgical anatomists reserve pedicle for the bundle and hilum for the surface depression.