[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Skeleton, muscles, limbs and skin](https://durgical.com/en/embryology/#a-musculoskeletal) › Limb defects: amelia, meromelia, polydactyly and syndactyly [Embryology](https://durgical.com/en/embryology/) · [Skeleton, muscles, limbs and skin](https://durgical.com/en/embryology/#a-musculoskeletal) # Limb defects: amelia, meromelia, polydactyly and syndactyly - Malformation A limb can be missing completely or in part, have extra digits or joined digits; nearly all of it is decided between weeks 4 and 8. Updated 2026-11-01 **See it in 3D:** [Humerus](https://durgical.com/en/anatomy/humerus/) · [Radius](https://durgical.com/en/anatomy/radius/) · [Ulna](https://durgical.com/en/anatomy/ulna/) · [Femur](https://durgical.com/en/anatomy/femur/) · [1st metacarpal](https://durgical.com/en/anatomy/1st-metacarpal/) · [5th metacarpal](https://durgical.com/en/anatomy/5th-metacarpal/) · [Proximal phalanx, little finger](https://durgical.com/en/anatomy/proximal-phalanx-of-the-little-finger/) ## Key points - Amelia = whole limb missing; meromelia = part missing (phocomelia). - Thalidomide in weeks 4 to 6 → phocomelia. - Syndactyly = failed interdigital apoptosis. - Radial defect: look for heart (Holt–Oram), blood (Fanconi, TAR) and VACTERL. - Amniotic bands: asymmetric amputations with rings. ## What it is Limb defects arise in weeks 4 to 8, while the buds grow and the digits form. A limb can be completely absent (amelia) or partly absent (meromelia, such as phocomelia), have extra digits (polydactyly) or fused digits (syndactyly). Many are isolated, but a radial or thumb defect calls for a search for heart, kidney or blood anomalies. ## Embryological cause - Amelia and meromelia: failure of the apical ectodermal ridge or of bud mesoderm in weeks 4 to 6; thalidomide is the classic example. - Phocomelia: the long segments are missing and the hand or foot sits close to the trunk. - Polydactyly: excess anteroposterior signalling (SHH from the ZPA or its regulator); postaxial (little-finger side) is the most common. - Syndactyly: failure of interdigital apoptosis in weeks 6 to 8; it can join skin only or bone as well. - Cleft hand or foot: the central ray (third metacarpal and its phalanges) is missing. - Amniotic bands: constriction rings and amputations from amnion adhesions; not a genetic error. ## Markers - SHH - TBX5 - HOXD13 - FGF8 - Thalidomide ## How it presents - Thalidomide causes phocomelia mainly with exposure between days 24 and 36 after fertilisation. - Isolated postaxial polydactyly: common and inherited, especially in people of African descent; also in trisomy 13. - Most frequent syndactyly: between the second and third toes; seen in craniosynostosis syndromes (Apert). - Radial or thumb defect: think of Holt–Oram (TBX5), Fanconi anaemia, TAR (thrombocytopenia–absent radius) and VACTERL. - Some reduction defects come with heart defects, omphalocele and gastroschisis. ## Diagnosis - Second-trimester ultrasound: count the long bones and digits of all four limbs. - X-ray at birth to see which bones are missing or fused. - Echocardiogram, renal ultrasound and blood count if there is a radial defect. - Karyotype or genetic testing if there are other anomalies. ## Memory aid If the thumb is missing, check the heart and the blood: the radius rarely travels alone. ## Test yourself Think of the answer, then tap the question to see it. ## Which mechanism fails in syndactyly? Apoptosis of the mesenchyme between the digits, which normally separates them between weeks 6 and 8. ## What is phocomelia, and which drug is it linked to? Absence of the long segments, with hands or feet close to the trunk; it is linked to thalidomide in weeks 4 to 6. ## Which tests does a newborn with an absent thumb or radius need? Echocardiogram, renal ultrasound and blood count, looking for Holt–Oram, Fanconi anaemia, TAR syndrome or VACTERL. ## Which type of polydactyly is the most common? Postaxial, with an extra digit on the little-finger side; often isolated and inherited. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/limb-defects-amelia-meromelia-polydactyly-and-syndactyly/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Humerus](https://durgical.com/en/anatomy/humerus/) - [Radius](https://durgical.com/en/anatomy/radius/) - [Ulna](https://durgical.com/en/anatomy/ulna/) - [Femur](https://durgical.com/en/anatomy/femur/) - [1st metacarpal](https://durgical.com/en/anatomy/1st-metacarpal/) - [5th metacarpal](https://durgical.com/en/anatomy/5th-metacarpal/) - [Proximal phalanx, little finger](https://durgical.com/en/anatomy/proximal-phalanx-of-the-little-finger/) ## Related - [Limb development: the apical ectodermal ridge, ZPA and limb rotation](https://durgical.com/en/embryology/limb-development-the-apical-ectodermal-ridge-zpa-and-limb-rotation/) - [Thalidomide and limb reduction defects](https://durgical.com/en/embryology/thalidomide-and-limb-reduction-defects/) - [Fetal membranes: amnion, chorion, yolk sac and allantois](https://durgical.com/en/embryology/fetal-membranes-amnion-chorion-yolk-sac-and-allantois/) - [Malformation, disruption, deformation, sequence, syndrome, association](https://durgical.com/en/embryology/malformation-disruption-deformation-sequence-syndrome-association/) - [Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acid](https://durgical.com/en/embryology/molecular-regulation-shh-wnt-fgf-bmp-hox-genes-and-retinoic-acid/) - [Poland sequence and prune belly syndrome](https://durgical.com/en/embryology/poland-sequence-and-prune-belly-syndrome/) - [Craniosynostosis](https://durgical.com/en/embryology/craniosynostosis/) [← All of embryology](https://durgical.com/en/embryology/) Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given. Sources: [Limb Reduction Defects | Birth Defects | CDC](https://www.cdc.gov/birth-defects/about/limb-reduction-defects.html) (Centers for Disease Control and Prevention, 2026); [Polydactyly: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003176.htm) (MedlinePlus, US National Library of Medicine, 2026); [Webbing of the fingers or toes: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003289.htm) (MedlinePlus, US National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 12: Limbs (Wolters Kluwer, 2015). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/limb-defects-amelia-meromelia-polydactyly-and-syndactyly/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/limb-defects-amelia-meromelia-polydactyly-and-syndactyly/