Limb buds appear at week 4 (the upper buds first, day ~26; the lower buds 1–2 days later) and complete their basic patterning by week 8. Each bud is a column of lateral-plate mesoderm (bone and connective tissue) ensheathed by ectoderm and infiltrated by somite-derived myoblasts (skeletal muscle). The bud's three orthogonal axes are each set up by a separate signaling center, and each center is central to dysmorphology because the same gene mutations show up across many limb syndromes.
| Axis | Direction | Signaling center | Key signal | Mutation phenotype |
|---|---|---|---|---|
| Proximodistal | Shoulder → fingertip | AER (apical ectodermal ridge, thickened distal ectoderm) | FGF8, FGF4 | Loss → truncated limb; experimental removal of AER stops outgrowth |
| Anteroposterior | Thumb → pinky | ZPA (zone of polarizing activity, posterior mesenchyme) | SHH (regulated by ZRS enhancer) | Ectopic anterior SHH → preaxial polydactyly |
| Dorsoventral | Back of hand → palm | Dorsal ectoderm | WNT7a (dorsal); engrailed-1 (ventral) | Mutation → ventralization (palms-on-both-sides) or vice versa |
The AER and ZPA maintain each other through a feedback loop: AER FGFs → ZPA SHH → maintains AER. Disrupting one collapses the other.
Hox genes are activated in nested rostral-to-caudal domains and determine "what kind of segment forms here." In limbs, Hox activation gradients specify which bones form at which levels (humerus vs. radius vs. carpals vs. fingers). Synpolydactyly results from HOXD13 mutations (poly-alanine expansion).
Hands and feet form initially as paddle-shaped plates. Programmed cell death in the interdigital mesenchyme during weeks 7–8 sculpts out the spaces between digits. Failure of apoptosis → cutaneous (soft-tissue) syndactyly. Failure of bone separation → bony syndactyly.
Polydactyly
- Preaxial polydactyly (extra thumb side): often involves SHH/ZRS regulatory mutations. Familial; can be isolated or part of Greig cephalopolysyndactyly (GLI3) or Pallister-Hall (GLI3).
- Postaxial polydactyly (extra ulnar side): often isolated, autosomal dominant; associated with Bardet-Biedl, McKusick-Kaufman, Pallister-Hall, trisomy 13.
- ZRS (Zone of polarizing activity Regulatory Sequence) is a long-range enhancer ~1 Mb upstream of SHH; it activates SHH only in the posterior limb. Point mutations in ZRS cause ectopic anterior SHH expression → preaxial polydactyly. ZRS is the textbook example of a regulatory-element mutation causing disease.
Syndactyly
- Cutaneous syndactyly: failure of interdigital apoptosis.
- Apert syndrome (FGFR2): severe symmetric bony syndactyly of hands and feet ("mitten hands"), craniosynostosis, midface hypoplasia.
- Synpolydactyly: HOXD13; central syndactyly with extra digit in the syndactylous web.
Ectrodactyly / split-hand-foot malformation
- "Lobster claw" deformity. Mutations in DLX5/6, TP63 (EEC syndrome: ectrodactyly + ectodermal dysplasia + cleft lip/palate), or chromosome 7q deletion.
Holt-Oram syndrome
- TBX5 mutations → upper-limb (radial ray) anomalies + cardiac septal defects (ASD, VSD).
- Embryologic logic: TBX5 patterns both the forelimb mesenchyme and the developing heart, so a single mutation produces "heart-hand syndrome."
- Phenotype: thumb anomalies (absent/hypoplastic/triphalangeal), radial hypoplasia, sometimes a limb amelia, ASD/VSD, conduction abnormalities.
Thrombocytopenia-absent radius (TAR) syndrome
- RBM8A: bilateral radial aplasia with thumbs preserved (distinguishes from Fanconi anemia, where thumbs are usually involved); congenital thrombocytopenia.
Skeletal dysplasias affecting limbs
- Achondroplasia (FGFR3): short proximal limbs (rhizomelic), trident hands, macrocephaly, narrow foramen magnum.
- Thanatophoric dysplasia (FGFR3): severe short limbs, narrow thorax, typically lethal.
- Mechanism: gain-of-function FGFR3 inhibits chondrocyte proliferation at the growth plate.
Achiropodia, Adams-Oliver, amniotic band sequence
- Amniotic band sequence: physical disruption of developing limbs by amniotic strands; produces irregular, asymmetric digit/limb amputations and constriction rings. Sporadic, non-genetic; important to recognize because counseling differs entirely from genetic limb defects.
| Pathway disrupted | Resulting phenotype |
|---|---|
| FGF8 / AER | Truncated limb (intercalary or terminal) |
| SHH / ZRS | Preaxial polydactyly, holoprosencephaly |
| WNT7a | Dorsoventral patterning loss |
| FGFR2 | Apert / Crouzon / Pfeiffer (craniosynostosis + syndactyly) |
| FGFR3 | Achondroplasia, thanatophoric dysplasia |
| GLI3 | Greig cephalopolysyndactyly, Pallister-Hall |
| HOXD13 | Synpolydactyly |
| TBX5 | Holt-Oram (radial + cardiac) |
| RBM8A | TAR syndrome |
| TP63 | EEC, AEC, limb-mammary |
"AER → outgrowth (FGF). ZPA → digit identity (SHH). WNT7a → dorsal." The three axes.
"Heart-hand": Holt-Oram (TBX5).
"Mitten hand": Apert (FGFR2).
"Thumbs preserved" in TAR distinguishes it from Fanconi anemia.
- Radial ray defect + congenital heart disease (ASD/VSD) → think Holt-Oram (TBX5). The "heart-hand" syndrome.
- Preaxial polydactyly with no syndromic features → consider ZRS regulatory mutation. Classic example of a long-range enhancer disease.
- Symmetric bony syndactyly + craniosynostosis + midface hypoplasia → Apert (FGFR2).
- Bilateral absent radii + congenital thrombocytopenia + thumbs present → TAR (RBM8A). Thumbs absent → think Fanconi anemia.
- Preserved thumbs in radial ray defect is the discriminator.
- Asymmetric, ragged digit amputations with constriction rings → amniotic band sequence, not genetic.
- Achondroplasia = rhizomelic (proximal) shortening, trident hands, normal-length trunk; FGFR3 G380R is the most common mutation.