Musculoskeletal System Development – Key Vocabulary

Vocabulary flashcards summarizing essential terms and definitions from the Musculoskeletal System Development lecture, covering embryologic tissue origins, spinal and vertebral formation, limb development, muscle and innervation patterns, and associated congenital anomalies.

Gastrulation

Early embryonic process that forms the three germ layers—ectoderm, mesoderm, and endoderm—from the trilaminar disc.

Primitive streak

First visible sign of gastrulation appearing at the caudal end of the trilaminar disc where cells ingress.

Ectoderm

Outer germ layer that gives rise to neuroectoderm (brain, spinal cord) and surface ectoderm (epidermis).

Mesoderm

Middle germ layer that differentiates into paraxial (somites), intermediate, and lateral plate mesoderm.

Endoderm

Innermost germ layer that forms the epithelial lining of the gut and associated organs.

Somite

Segmented block of paraxial mesoderm that differentiates into dermomyotome and sclerotome.

Dermomyotome

Somite region that produces dermatome (dermis) and myotome (skeletal muscle).

Sclerotome

Somite portion that migrates around the neural tube and notochord to form vertebrae and ribs.

Resegmentation

Process where caudal half of one sclerotome fuses with cranial half of the next to permit spinal movement.

Neural crest

Ectoderm-derived cells that form sensory ganglia, including dorsal root ganglia (DRG).

Dorsal root ganglion (DRG)

Cluster of sensory neuron cell bodies derived from neural crest located outside the spinal cord.

Bipolar neuroblast

Immature neuron with two opposite processes formed as neuroblasts migrate into the mantle layer.

Multipolar neuroblast

Neuron precursor whose bipolar processes become a primitive axon and dendrite, maturing into a neuron.

Basal plate

Ventral region of spinal cord where motor neuroblasts generate axons that exit as ventral motor roots.

Ventral motor root

Bundle of motor axons emerging from the ventral horn to innervate skeletal muscle.

Spina bifida occulta

Failure of vertebral arch formation covered by skin; mild form of spina bifida.

Meningocele

Spina bifida variant where meninges and CSF protrude through vertebral defect forming a cyst.

Meningomyelocele

Severe spina bifida with meninges and neural tissue herniating dorsally in a cystic sac.

Myeloschisis

Most severe neural tube defect with open, unfused neural plate exposed at the surface.

Hemivertebra

Congenital absence of one half of a vertebra leading to congenital scoliosis.

Klippel-Feil syndrome

Failure of cervical sclerotome resegmentation causing fused cervical vertebrae and short neck.

Intramembranous ossification

Direct conversion of mesenchyme into bone (e.g., flat skull bones, part of clavicle).

Endochondral ossification

Bone formation via a cartilage model that later ossifies (e.g., long bones, vertebrae).

Apical ectodermal ridge (AER)

Specialized ectodermal ridge at limb tip that maintains underlying mesenchyme in a proliferative, undifferentiated state.

Fibroblast growth factor 10 (FGF10)

Signal from parietal lateral plate mesoderm initiating limb outgrowth in a proximal-to-distal direction.

Stylopod

Proximal limb segment forming the humerus or femur.

Zeugopod

Middle limb segment forming radius/ulna or tibia/fibula.

Autopod

Distal limb portion forming carpals/tarsals and digits.

Digital rays

Condensations of cartilage in hand/foot plates that prefigure individual digits.

Apoptosis (limb webbing)

Programmed cell death in the AER between digital rays creating separate fingers or toes.

Hox genes (limb positioning)

Homeotic genes that determine limb bud placement along the craniocaudal axis.

Hox code of the limb

Specific combination of Hox gene expression (groups 9–13) that patterns stylopod, zeugopod, and autopod.

Primaxial domain

Region derived from somites forming back, posterior neck, and intercostal muscles.

Abaxial domain

Region where myoblasts migrate into parietal lateral plate mesoderm to form limb and body-wall muscles.

Myoblast

Mesenchymal cell committed to muscle lineage that elongates then fuses to form myotubes.

Myotube

Multinucleated, early muscle fiber formed by fusion of myoblasts; matures after innervation.

Undifferentiated zone

Proliferative mesenchyme beneath the AER that elongates the limb bud.

Costal cartilage

Cartilage bars formed by abaxial sclerotome cells that crossed into the parietal lateral plate mesoderm.

Sternum origin

Sternum develops entirely from parietal lateral plate mesoderm, not from sclerotome.

Gastroschisis

Ventral body-wall defect where abdominal viscera protrude through an open lateral abdominal wall and are bathed in amniotic fluid.

Omphalocele

Herniation of abdominal viscera into the proximal umbilical cord covered by peritoneum.

Pectus excavatum

Sunken sternum due to abnormal ventral body-wall development.

Pectus carinatum

Protruding sternum caused by overgrowth of costal cartilages.

Amelia

Complete absence of a limb, typically from very early loss of FGF signaling.

Meromelia (phocomelia)

Partial absence or truncation of limbs resulting from later disruption of FGF signaling.

Brachydactyly

Congenitally short digits.

Syndactyly

Fusion of digits due to failed apoptosis in interdigital tissue.

Polydactyly

Presence of supernumerary (extra) digits.

Ectrodactyly (cleft foot)

Split hand/foot with absence of central digits and cleft between metacarpals 2 and 4.

Amniotic band sequence

Constriction of limbs or digits by fibrous amniotic bands causing ischemia and deformities.

Limb rotation

Embryonic torsion: upper limb rotates laterally, lower limb rotates medially, establishing adult orientation.

Dermatome

Strip of skin innervated by a single spinal nerve; limb rotation produces radial dermatome patterns.