Limb development and teratogens
Limb and musculoskeletal development, teratogens and twinning: FCPS embryology
Somites, limb buds, limb rotation and ossification, teratogen periods with classic drug and defect pairs, and how twins form, for FCPS Part 1. Explains syndactyly, accessory nipples, twin chorionicity and twin–twin transfusion from first principles.
Somites and the axial skeleton
- Paraxial mesoderm segments into 42–44 pairs of somites from day 20 (about 34–35 pairs at day 30). Each somite divides into:
| Part | Forms |
|---|---|
| Sclerotome | Vertebrae and ribs, surrounding the notochord and neural tube |
| Myotome | Skeletal muscle of trunk and limbs |
| Dermatome | Dermis of the back |
- Each myotome splits into a dorsal epimere and ventral hypomere. The epimere forms the epaxial muscles (deep back muscles, erector spinae), supplied by dorsal rami. The hypomere forms hypaxial muscles (intercostals, abdominal wall, limb muscles), supplied by ventral rami. Destroying epimere cells leaves erector spinae absent but limb muscles intact.
- Each vertebral body forms from the caudal half of one sclerotome fused with the cranial half of the next (resegmentation), so spinal nerves pass between vertebrae and myotomes bridge joints, allowing movement.
- Klippel–Feil syndrome: fused cervical vertebrae; short neck, low hairline, limited movement.
- Hemivertebra causes congenital scoliosis.
Limb development
- Limb buds appear in week 4–5 (upper before lower) as lateral plate mesoderm covered by ectoderm.
| Signalling centre | Signal | Controls | Fault |
|---|---|---|---|
| Apical ectodermal ridge (AER) | FGF | Proximal to distal outgrowth | Removal gives truncated limb |
| Zone of polarising activity (ZPA) | Sonic hedgehog | Anterior–posterior (thumb to little finger) | Extra ZPA gives mirror duplication, polydactyly |
| Dorsal ectoderm | WNT7a | Dorsal–ventral | Ventralised limb |
- Digits separate when cells in the webs die by apoptosis (weeks 6–8). Failure gives syndactyly (webbed digits).
- Limbs rotate in week 7–8: the upper limb laterally by 90° (thumb lateral, elbow posterior, extensors behind); the lower limb medially by about 90° (big toe medial, knee anterior, extensors in front). This explains the spiral dermatomes of the leg.
- Ossification: limb bones, vertebrae and the skull base by endochondral ossification (cartilage model first); skull vault, facial bones and most of the clavicle by intramembranous ossification. The clavicle is the first bone to ossify (week 5–6).
- Mammary glands: epidermal thickenings along the milk lines from the axilla to the groin on each side. Normally only the pectoral part persists; remnants give accessory nipples (polythelia) or accessory breast tissue anywhere on this line, axillary tissue may swell in pregnancy and lactation.
Common limb and bone anomalies
| Anomaly | Basis |
|---|---|
| Amelia, meromelia, phocomelia | Absent or short limbs; thalidomide (days 24–36) |
| Achondroplasia | FGFR3 gain of function; short limbs, normal trunk, autosomal dominant, often new mutation |
| Osteogenesis imperfecta | Type I collagen; fractures, blue sclera |
| Developmental dysplasia of the hip | Shallow acetabulum; girls, breech, first-born; Barlow and Ortolani tests |
| Talipes equinovarus | Club foot; oligohydramnios, neuromuscular disease |
| Amniotic bands | Constriction rings and amputations |
Critical periods
| Period | Effect of a teratogen |
|---|---|
| Weeks 1–2 (before the disc forms) | All or nothing: the embryo dies or recovers fully |
| Weeks 3–8 (embryonic, organogenesis) | Major structural malformations; each organ has its own peak window |
| Weeks 9–38 (fetal) | Growth restriction, functional and minor defects (brain, genitalia, teeth) |
Teratogens
| Agent | Characteristic defect |
|---|---|
| Thalidomide | Phocomelia |
| Valproate | Neural tube defects (highest risk antiepileptic), facial dysmorphism, developmental delay; avoid in women of childbearing potential |
| Phenytoin | Fetal hydantoin syndrome: cleft lip and palate, nail and distal phalanx hypoplasia |
| Carbamazepine | Neural tube defects |
| Isotretinoin and vitamin A excess | Craniofacial (cleft palate, small ears), cardiac and CNS defects |
| Warfarin | Nasal hypoplasia, stippled epiphyses (weeks 6–12); fetal bleeding later |
| ACE inhibitors and ARBs | Fetal renal failure, oligohydramnios, skull hypoplasia (2nd and 3rd trimesters) |
| Lithium | Ebstein anomaly |
| Tetracyclines | Tooth discoloration, enamel hypoplasia |
| Aminoglycosides | Deafness (VIII nerve) |
| Methotrexate | Miscarriage, skeletal and craniofacial defects |
| Alcohol | Fetal alcohol syndrome: smooth philtrum, thin upper lip, short palpebral fissures, microcephaly, learning difficulty; the commonest preventable cause of intellectual disability |
| Smoking | Growth restriction, prematurity, placental abruption |
| Diethylstilbestrol | Vaginal clear cell adenocarcinoma, uterine anomalies in daughters |
| Maternal diabetes | Caudal regression, transposition, neural tube defects, macrosomia, neonatal hypoglycaemia |
| Radiation | Microcephaly, intellectual disability |
| Congenital rubella | Cataract, deafness, PDA |
- Caudal regression (absent sacrum, small legs, neurogenic bladder) is the classic marker of poorly controlled maternal diabetes; HbA1c before conception best predicts the risk.
Twinning
- Dizygotic (fraternal), about two-thirds: two ova fertilised by two sperm; each implants separately, so always dichorionic diamniotic (placentas may fuse but circulations do not). Runs in families, rises with maternal age, ovulation induction and IVF. No more alike than ordinary siblings.
- Monozygotic (identical): one zygote splits; genetically identical, so a kidney from an identical twin is an isograft that is not rejected without immunosuppression.
| Time of split | Chorion and amnion | Frequency among MZ twins |
|---|---|---|
| Days 0–3 (two-cell to morula) | Dichorionic diamniotic | About 30% |
| Days 4–8 (inner cell mass of blastocyst) | Monochorionic diamniotic | About 70%, the commonest |
| Days 8–13 (bilaminar disc) | Monochorionic monoamniotic | About 1%; cord entanglement |
| After day 13 | Conjoined | Rare |
- A shared placenta (monochorionic) means monozygotic twins; two placentas can be either type.
- Twin–twin transfusion syndrome: only in monochorionic twins, from arteriovenous anastomoses in the shared placenta. The donor becomes anaemic, growth-restricted with oligohydramnios (stuck twin); the recipient becomes polycythaemic and overloaded with polyhydramnios and heart failure. Treated with laser ablation of the connecting vessels.
Remember Weeks 3–8 are the danger weeks; before that it is all or nothing.
Exam trap Twin chorionicity depends on when a monozygotic embryo splits: days 4–8 give monochorionic diamniotic, the commonest type.
Exam trap In twin–twin transfusion the donor has oligohydramnios and the recipient has polyhydramnios.
How FCPS asks about this
- Webbed fingers: failure of apoptosis of interdigital tissue.
- Extra nipple in the groin or axilla: persistent milk line tissue (axilla to groin).
- Absent sacrum and neurogenic bladder: caudal regression with maternal diabetes.
- Drug–defect pairs: thalidomide phocomelia; lithium Ebstein anomaly; warfarin nasal hypoplasia; valproate neural tube defect.
- Shared placenta with two amniotic sacs: monozygotic twins splitting at days 4–8.
- Two ova fertilised by two sperm: dizygotic twins, always dichorionic diamniotic.
- Twins with linked placental vessels and unequal amniotic fluid: twin–twin transfusion syndrome.
Test yourself on this topic
5 of the 34 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
The bones of the face, including the maxilla, mandible and zygomatic bones, develop mainly from which embryonic tissue?
Answer: Cranial neural crest mesenchyme
Cranial neural crest cells migrate into the frontonasal prominence and pharyngeal arches and form most of the facial skeleton, including the nasal bones, maxilla, zygomatic bone, mandible and the ossicles, as well as the cartilage and connective tissue of the face. Paraxial mesoderm contributes to the occipital and posterior skull base and parts of the vault. This is why disorders of neural crest, such as Treacher Collins syndrome, cause facial bone hypoplasia.
Facial skeleton
- From cranial neural crest
- Maxilla, mandible, zygoma
- Also ossicles and nasal bones
- Crest defects: facial hypoplasia
Remember Face bones are crest bones.
Exam trap Neural crest forms bone in the head only; trunk crest does not form skeleton.
Why the other options are wrong
- Paraxial mesoderm of the head Contributes to the posterior skull base and vault, not most of the face.
- Lateral plate mesoderm Forms limb bones and the sternum.
- Pharyngeal endoderm Lines the pharynx and forms glands, not bone.
- Occipital somites Form the occipital bone and tongue muscles.
Synovial joints such as the knee form between the cartilage models of two adjacent bones. Which process creates the joint cavity?
Answer: Cavitation of the interzone mesenchyme
Where two bones will meet, the mesenchyme between their cartilage models stays as a dense interzone. Its outer layers form the articular cartilage and capsule, and its central part breaks down by cell death to form a cavity, which becomes the joint cavity lined by synovium. Fetal movement is needed to maintain the cavity; reduced movement can lead to joint contractures (arthrogryposis).
Synovial joint formation
- Interzone between cartilage models
- Outer layers: capsule, articular cartilage
- Centre cavitates: joint cavity
- Needs fetal movement
Remember Joint cavity = hollowed interzone.
Exam trap Fibrous and cartilaginous joints form when the interzone does not cavitate.
Why the other options are wrong
- Invasion of the bone by blood vessels This forms the primary ossification centre, not a joint.
- Fusion of two epiphyseal centres Fusion would eliminate a joint, not create one.
- Apoptosis of the apical ectodermal ridge AER regression ends limb outgrowth; it does not form joints.
- Ossification of the perichondrium Forms the periosteal bone collar of the shaft.
A baby is born with webbed fingers (syndactyly). Failure of which process during development explains this?
Answer: Apoptosis of interdigital tissue
Apoptosis shapes the embryo: the tissue between the digits is removed by programmed cell death in weeks 6–8, separating fingers and toes. Failure leaves syndactyly. Other physiological apoptosis: removal of the Müllerian or Wolffian ducts, deletion of self-reactive lymphocytes in the thymus, shedding of the endometrium, involution of the lactating breast after weaning, and turnover of crypt cells.
Physiological apoptosis
- Interdigital webs (syndactyly if it fails)
- Duct regression, thymic selection
- Endometrial shedding, breast after weaning
Remember Apoptosis carves the fingers.
Why the other options are wrong
- Necrosis of digital rays Necrosis is not part of normal development.
- Hyperplasia of the apical ectodermal ridge The AER directs limb outgrowth; not the cause of webbing.
- Metaplasia of mesenchyme Not involved.
- Neural crest migration Not involved.
In week 4 the age of an embryo is estimated by counting its somites. About how many pairs are present at day 30?
Answer: 34 to 35 pairs
Somites appear on day 20 at the cranial end and form at about three pairs a day in a cranial to caudal sequence. By day 25 there are about 17 to 20 pairs, and by day 30 about 34 to 35 pairs. Eventually 42 to 44 pairs form, though several caudal ones disappear. Because they are easily counted, somites are used to stage the embryo in this period.
Counting somites
- First pair on day 20
- About 3 pairs a day
- Day 30: 34–35 pairs
- Total formed: 42–44 pairs
Remember Day 30, about 35 pairs.
Exam trap The final count is higher than the adult number of vertebrae because caudal somites regress.
Why the other options are wrong
- 4 to 6 pairs The number on about day 21.
- 12 to 14 pairs The number on about day 23 to 24.
- 20 to 22 pairs The number on about day 26.
- 42 to 44 pairs The final total, reached around the end of week 5.
A newborn has a midline defect of the upper sternum through which the heart can be seen pulsating under the skin. What is the developmental cause?
Answer: Failed fusion of the sternal bars
The sternum develops from a pair of vertical mesenchymal sternal bars in the ventral body wall, derived from somatic lateral plate mesoderm. They chondrify and fuse in the midline from cranial to caudal, and the ribs join them laterally. If fusion fails, a sternal cleft results; an upper cleft lets the heart pulsate under thin skin, and a complete defect with absent overlying tissue gives ectopia cordis.
Sternal cleft
- Sternum from paired sternal bars
- From lateral plate mesoderm
- Fuse cranial to caudal
- Failure: sternal cleft
Remember A split sternum is two bars that never met.
Exam trap The sternum is not from the sclerotome, unlike the ribs and vertebrae.
Why the other options are wrong
- Failed fusion of the costal elements The costal elements form ribs; they do not fuse in the midline.
- Failed segmentation of the sternebrae Segmentation defects alter sternebrae, not a midline cleft.
- Persistence of the septum transversum Gives diaphragm defects, not sternal clefts.
- Failed closure of the neural tube Gives spinal defects, not anterior chest wall defects.
Where this sits in the CPSP syllabus
- Paper 1: General and systemic embryology, Musculoskeletal and limb development; teratogens and twinning
- Surgery & Allied, Paper 2: Embryology for surgery, Musculoskeletal and limb development; teratogens and twinning
- Diagnostic Radiology, Paper 2: Embryology, Development of the limbs and skeleton; common congenital anomalies
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