amnionless

Amnionless is an adjective used in biology to describe the absence of the amnion, the innermost membranous sac that surrounds the developing embryo in mammals. In human embryology, amnion formation is a normal step early in gestation; an amnionless embryo would lack this membrane and is typically associated with severe developmental anomalies or pregnancy loss. The term appears mainly in embryology and comparative anatomy literature.

In genetics, Amnionless (gene symbol AMN) encodes the amnionless protein, a component of the intestinal receptor for the vitamin B12-intrinsic factor complex. Along with cubilin (CUBN), amnionless forms a receptor complex on enterocytes that mediates endocytosis of the B12-IF complex, enabling dietary cobalamin uptake.

Mutations in AMN cause Imerslund-Gräsbeck syndrome, an autosomal recessive disorder characterized by selective vitamin B12 malabsorption, leading to megaloblastic anemia and often proteinuria. Diagnosis relies on biochemical evidence of B12 deficiency and genetic testing; treatment is lifelong parenteral or high-dose B12 supplementation, which corrects the anemia and improves outcomes. The syndrome highlights the role of the AMN-CUBN complex in nutrient absorption rather than embryonic membrane formation.