Notes for Musculoskeletal Development



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  1. Development of the Skeletal System: Overview

    1. Skeletal system develops from:

      1. Mesoderm

        1. Paraxial, lateral plate

      2. Neural Crest

    2. How?

      1. Intraembryonic mesoderm next to notochord and neural tube thicken to form 2 longitudinal columns called Paraxial mesoderm

      2. Paraxial mesoderm further divides into somites (occipital to sacral) and somitomeres in the head



      3. Each somite consists of a sclerotome (mesenchymal cells that surround the neural tube and notochord to become the future vertebral column and ribs) and a dermamyotome (gives rise to dermis of skin [dermatome] and skeletal muscles [ventrolateral and dorsomedial myotome])

      4. Somatic mesoderm of the lateral mesoderm (dorsal portion) forms the pelvic and pectoral girdles and long bones of ribs

      5. The Neural crest forms the bones of the face and skull

  2. Skull development

    1. Develops from cells from the paraxial mesoderm and the neural crest

      1. Neural crest forms the frontal portion of the face and skull

      2. Paraxial mesoderm forms the parietal and occipital portion of the skull

    2. Neurocranium and viscerocranium have membranous and cartilaginous components




  1. Neurocranium

    1. Develops from the neural crest (frontal skull bone) and occipital sclerotome (parietal and occipital portions of the skull)



    2. Divided into 2 portions

      1. Membranous portion

        1. Made of flat bones surrounding the brain (forms calvaria)

        2. Derived from neural crest and paraxial mesoderm (somites)

        3. Flat bones form 5 ossification centers

          1. Frontal (2)

          2. Parietal (2)

          3. Occipital (1)

        4. Gaps between the developing bones of the skull (sutures [lines] and fontanelles [gaps/corners]) allow flat bones to “mold” during childbirth and expand with brain growth

          1. Frontal suture (between both frontal skull bones)

          2. Sagittal suture (between both parietal skull bones)

          3. Lamboid suture (between parietal bones and occipital bone)

          4. Coronal suture (between frontal and parietal bones)

          5. Squamous suture (between parietal and squamous temporal bones)




        1. Fontanelles are fibrous areas where sutures meet

          1. Anterior fontanelle

            1. Where both frontal and both parietal bones meet (Coronal and Frontal sutures)

          2. Posterior fontanelle

            1. Where parietal bones and occipital bone meet (lamboid and sagittal sutures)

          3. Sphenoid fontanelle

            1. Where frontal, parietal and squamous temporal bones meet (coronal and squamous sutures)

          4. Mastoid fontanelle

            1. Where parietal, occipital, and squamous temporal bones meet (lamboid and squamous sutures)

      1. Cartilaginous portion

        1. Chondrocranium – forms the bones at the base of the skull

        2. Made up of cartilages anterior to the rostral border of the notochord (neural crest) and cartilages posterior to the rostral border of the notochord (occipital sclerotome [paraxial mesoderm])

  1. Viscerocranium

    1. Consists of bones of the face

    2. Formed mainly from cartilages of the 1st 2 pairs of pharyngeal arches

    3. Divided into 2 portions

      1. Membranous viscerocranium

        1. The dorsal portion (maxillary process) undergoes intramembranous ossification

        2. The ventral portion (mandibular process) contains Meckel’s cartilage (forms mandible)

      2. Cartilagenous viscerocranium

        1. Meckel’s cartilage

        2. Reichert’s cartilage

  2. Skull development Clinical Correlates

    1. Cranioschisis

      1. Failure of cranial vault to form

      2. Cranial neuropore fails to close

      3. Brain is exposed to amniotic fluid and degenerates (anencephaly)

    2. Craniosynostosis

      1. Premature closure of skull bone sutures results in abnormal head shape




  1. Limbs and girdle development

    1. Limb buds appear toward the end of week 4 and are well differentiated by week 8

      1. Upper limb buds develop prior to lower limb buds

      2. Limb bud tissue arises from 3 sources

        1. Somatic mesoderm (bones), dermomyotome of somites (skeletal muscle), and ectoderm (skin)

        2. Each limb bud has an apical ectodermal ridge (AER) of thickened ectoderm at its distal end

          1. The AER exerts an inductive influence on the mesenchyme in the limb bud that promotes growth and development of the limb

        3. Development proceeds proximal to distal as cells farthest from the AER begin to differentiate into cartilage and muscle

      3. During week 6, the distal end of the limb buds become flattened to form hand and foot plates (paddle shaped)

      4. Phalanges form due to cell death in the AER dividing the plates into 5 parts

        1. Further digital growth depends on the influence of the remaining ridge ectoderm and condensation of the mesenchyme to form digital rays and subsequent tissue death

    2. Rotation of the limbs

      1. In week 7, the limbs rotate in opposite directions

        1. Upper limb rotates 90 degrees LATERALLY (think lateral rotation from praying pose)

        2. Lower limb rotates 90 degrees MEDIALLY

        3. Thumb is then lateral and great toe is medial

  2. Clinical Correlations for Limb Development

    1. Limb malformations are relatively rare, hereditary, and assoc. with other birth defects

      1. Teratogen-induced defects can also occur (thalidomide)

    2. Amelia

      1. Absence of limb

    3. Meromelia

      1. Absence of a part of the limb

    4. Phocomelia

      1. Hands and feet are attached to abbreviated arms and legs

      2. Hands at shoulders like a seal’s flipper

    5. Polydactyly

      1. Extra digits on hands or feet

    6. Syndactyly

      1. Webbed fingers or toes (most common)




  1. Vertebral, rib, and sternal development

    1. Mesodermal cells from sclerotome migrate to condense:

      1. Around the notochord (Centrum, aka vertebral body)

      2. Around the neural tube (vertebral arches)

        1. Pedicles, laminae, spinous and articular and transverse processes

      3. In the body wall (costal processes or ribs)

  2. Clinical Correlations for Vertebral, rib and Sternal development

    1. Accessory lumbar ribs

      1. Most common

    2. Accessory Cervical ribs

      1. Attached to C7

      2. Quiz question: during fx may put pressure on brachial plexus and subclavian artery

    3. Spina bifida occulta

      1. Failure of vertebral arches to form or fuse

    4. Scoliosis

      1. Failure of vertebral body to form

    5. Spondylolisthesis

      1. Pedicles fail to fuse with vertebral body

      2. Results in Lordosis (vertebral body moves anteriorly relative to vertebra below)

    6. Sternal Cleft

      1. Incomplete fusion of the sternal bars

    7. Pectus excavatum (funnel Chest)

      1. Most common

  3. Muscular System Development: Overview

    1. Development

      1. Cardiac and Smooth muscle is derived from Splanchnic Mesoderm

      2. Skeletal Muscle is derived from myotome of somites

    2. Myotome organization – Completed by week 5

      1. Ventrolateral portion

        1. Forms the hypomere

          1. Innervated by ventral rami

          2. Hypomere forms hypaxial mm (intercostal, abdominal, limb)

      2. Dorsomedial portion

        1. Forms the epimere

          1. Innervated by dorsal rami

          2. Epimere forms extensor mm of the vertebral column (erector spinae)

    3. Upper limb mm development

      1. During week 5, mesoderm from C4-T1 somites migrates into limb buds and forms anterior (flexor) and posterior (extensor) condensations

    4. Lower limb mm development

      1. During week 5, mesoderm from L1-S2 somites migrate into limb buds and forms anterior (extensor) and posterior (flexor) condensations


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