Notes for Musculoskeletal Development
1. Development of the Skeletal System: Overview
a. Skeletal system develops from:
i. Mesoderm
1. Paraxial, lateral plate
ii. Neural Crest
b. How?
i. Intraembryonic mesoderm next to notochord and neural tube
thicken to form 2 longitudinal columns called Paraxial
mesoderm
ii. Paraxial mesoderm further divides into somites (occipital to
sacral) and somitomeres in the head
iii.
iv. 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])
v. Somatic mesoderm of the lateral mesoderm (dorsal portion)
forms the pelvic and pectoral girdles and long bones of ribs
vi. The Neural crest forms the bones of the face and skull
2. Skull development
a. Develops from cells from the paraxial mesoderm and the neural crest
i. Neural crest forms the frontal portion of the face and skull
ii. Paraxial mesoderm forms the parietal and occipital portion of
the skull
b. Neurocranium and viscerocranium have membranous and
cartilaginous components
Notes for Musculoskeletal Development
3. Neurocranium
a. Develops from the neural crest (frontal skull bone) and occipital
sclerotome (parietal and occipital portions of the skull)
b.
c. Divided into 2 portions
i. 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
a. Frontal (2)
b. Parietal (2)
c. 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
a. Frontal suture (between both frontal skull
bones)
b. Sagittal suture (between both parietal skull
bones)
c. Lamboid suture (between parietal bones and
occipital bone)
d. Coronal suture (between frontal and parietal
bones)
e. Squamous suture (between parietal and
squamous temporal bones)
Notes for Musculoskeletal Development
5. Fontanelles are fibrous areas where sutures meet
a. Anterior fontanelle
i. Where both frontal and both parietal
bones meet (Coronal and Frontal sutures)
b. Posterior fontanelle
i. Where parietal bones and occipital bone
meet (lamboid and sagittal sutures)
c. Sphenoid fontanelle
i. Where frontal, parietal and squamous
temporal bones meet (coronal and
squamous sutures)
d. Mastoid fontanelle
i. Where parietal, occipital, and squamous
temporal bones meet (lamboid and
squamous sutures)
ii. 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])
4. Viscerocranium
a. Consists of bones of the face
b. Formed mainly from cartilages of the 1st 2 pairs of pharyngeal arches
c. Divided into 2 portions
i. Membranous viscerocranium
1. The dorsal portion (maxillary process) undergoes
intramembranous ossification
2. The ventral portion (mandibular process) contains
Meckel’s cartilage (forms mandible)
ii. Cartilagenous viscerocranium
1. Meckel’s cartilage
2. Reichert’s cartilage
5. Skull development Clinical Correlates
a. Cranioschisis
i. Failure of cranial vault to form
ii. Cranial neuropore fails to close
iii. Brain is exposed to amniotic fluid and degenerates
(anencephaly)
b. Craniosynostosis
i. Premature closure of skull bone sutures results in abnormal
head shape
Notes for Musculoskeletal Development
6. Limbs and girdle development
a. Limb buds appear toward the end of week 4 and are well
differentiated by week 8
i. Upper limb buds develop prior to lower limb buds
ii. 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
a. 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
iii. During week 6, the distal end of the limb buds become
flattened to form hand and foot plates (paddle shaped)
iv. 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
b. Rotation of the limbs
i. 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
7. Clinical Correlations for Limb Development
a. Limb malformations are relatively rare, hereditary, and assoc. with
other birth defects
i. Teratogen-induced defects can also occur (thalidomide)
b. Amelia
i. Absence of limb
c. Meromelia
i. Absence of a part of the limb
d. Phocomelia
i. Hands and feet are attached to abbreviated arms and legs
ii. Hands at shoulders like a seal’s flipper
e. Polydactyly
i. Extra digits on hands or feet
f. Syndactyly
i. Webbed fingers or toes (most common)
Notes for Musculoskeletal Development
8. Vertebral, rib, and sternal development
a. Mesodermal cells from sclerotome migrate to condense:
i. Around the notochord (Centrum, aka vertebral body)
ii. Around the neural tube (vertebral arches)
1. Pedicles, laminae, spinous and articular and transverse
processes
iii. In the body wall (costal processes or ribs)
9. Clinical Correlations for Vertebral, rib and Sternal development
a. Accessory lumbar ribs
i. Most common
b. Accessory Cervical ribs
i. Attached to C7
ii. Quiz question: during fx may put pressure on brachial plexus
and subclavian artery
c. Spina bifida occulta
i. Failure of vertebral arches to form or fuse
d. Scoliosis
i. Failure of vertebral body to form
e. Spondylolisthesis
i. Pedicles fail to fuse with vertebral body
ii. Results in Lordosis (vertebral body moves anteriorly relative
to vertebra below)
f. Sternal Cleft
i. Incomplete fusion of the sternal bars
g. Pectus excavatum (funnel Chest)
i. Most common
10. Muscular System Development: Overview
a. Development
i. Cardiac and Smooth muscle is derived from Splanchnic
Mesoderm
ii. Skeletal Muscle is derived from myotome of somites
b. Myotome organization – Completed by week 5
i. Ventrolateral portion
1. Forms the hypomere
a. Innervated by ventral rami
b. Hypomere forms hypaxial mm (intercostal,
abdominal, limb)
ii. Dorsomedial portion
1. Forms the epimere
a. Innervated by dorsal rami
b. Epimere forms extensor mm of the vertebral
column (erector spinae)
c. Upper limb mm development
i. During week 5, mesoderm from C4-T1 somites migrates into
limb buds and forms anterior (flexor) and posterior (extensor)
condensations
Notes for Musculoskeletal Development
d. Lower limb mm development
i. During week 5, mesoderm from L1-S2 somites migrate into
limb buds and forms anterior (extensor) and posterior (flexor)
condensations