Embryo - Development of Skeletal System Flashcards
Vertebrae form as _______ structures derived from adjacent _______.
intersegmental; sclerotomes
There are originally ___ cervical somites, but fusion results in __ cervical nerves
8 cervical somites; 7 cervical nerves
Cervical spinal nerves exit ______ (above/below) vertebra of same number, while other spinal nerves exit ______ (above/below).
Cervical nerves exit above
Other nerves exit below
_______ form neural arches surrounding the developing spinal cord
Sclerotomes
Congenital Scoliosis
Abnormal lateral curvature of the spine, resulting from disruption of normal vertebral development
Types of Congenital Scoliosis
A - failure of formation of vertebrae
B - failure of segmentation of vertebrae
C - mixed
Where do ribs arise from?
Zones of condensed mesenchyme lateral to the body of the vertebra
Develop from costal processes
The sternum develops from:
Cartilaginous sternal bars in ventral body wall
In what direction do the sternal bars fuse?
Cranial-caudal direction
Defects of the Sternum
Sternal Cleft
Pectus excavatum
Pectus Carinatum
Pectus Excavatum
Assumed to be caused by overgrowth of the costal cartilage. Costal cartilage overgrowth restricts the expansion of the ribs and pushes the sternum inward.
Pectus Carinatum
Overgrowth of cartilage causing sternum to protrude. May occur as solitary congenital abnormality or in association with other genetic disorders or syndromes. Can be evident at birth or in adolescent males during growth spurt (most common)
Sternal Foramen
Ossification anomaly found in 4-10% of population.
Can be mistaken for bullet wound or sternal disease by CT scan.
Common acupuncture point
Directly over heat
When does outgrowth of the body wall begin?
Late in week 4 of development, FL>HL
Positioning of the limbs along the craniocaudal axis is regulated by the ____ genes expressed along the axis
HOX genes
Once positioning along the craniocaudal axis is determined, limb growth must be regulated along the:
proximodistal, anteroposterior, dorsoventral axes
Limb outgrowth in the forelimb is initiated by:
TBX5 and FGF10
Limb outgrowth in the hindlimb is initiated by:
TBX4 and FGF10
Meromelia
Part of limb is missing
Amelia
All of limb is missing
Phocomelia
Feet and hands arise close to the trunk
Mesomelia
Shortened forearm or leg elements
Radial Club Hand
Congenital absence or hypoplasia of the radial structures of the forearm and hand
Hypoplastic or absent muscular structures and radial nerve
Treatment is best if early passive stretching exercises and corrective casting is started
Once limb outgrowth is initiated, BMPs, expressed in ventral ectoderm, induce formation of the _____ _______ ______
Apical Ectodermal Ridge (AER)
Apical Ectodermal Ridge (AER)
Thickened ectoderm on apex of limb bud
Directs limb bud organization along proximo-distal axis
Maintains dorsal/ventral axes
After AER is established, it expresses _____ and _____, which maintain the progress zone.
FGF4 and FGF8
Zone of Polarizing Activity (ZPA)
Mesodermal cells located at base of limb
What does the ZPA produce?
Produces Retionic Acid which initiates expression of SHH
The ZPA directs:
Organization of limb bud and pattering of digits Determines anterior (preaxial/thumb), posterior (postaxial)
Misexpression of retinoic acid and SHH results in:
A mirror image duplication of the limb structures
Polydactyly
Extra digits
Commonly medial or lateral, rather than central
Dominant trait
In what direction do limbs grow and develop?
Proximo-distally
Progress Zone (PZ)
Where mitosis and limb lengthening occurs
Zone of Cell Division (Progress Zone)
Region of actively dividing cells
Zone of Differentiation
Region of cell specialization
_____ genes regulate type/shape of bones
HOX
AER secretes FGFs that influence the closest cells (progress zone) to develop into _____ structures. Cells no longer within range of AER remain _____ in nature.
Distal
Proximal
Thalidomide
Disrupts signal from AER down to the progress zone
Cells in progress zone may divide a little or not at all
(Hansons disease)
Syndactyly
Most common limb abnormality
Webbed fingers or toes
Failure of programmed apoptosis in digital ray
Simple dominant or simple recessive inheritance
Where is syndactyly most frequent?
Between 3rd & 4th fingers and 2nd & 3rd toes
Brachydactyly
Uncommon
Reduction in the length of phalanges
Inherited as dominant trait
Short stature
Cleft hand or foot
Uncommon
Lobster-claw
Absence of one or more central digits or between digits 2 & 4
Cleft hand/foot is divided into 2 parts that oppose each other
Remaining digits are partly or completely fused
Contribution of Lateral Plate Mesoderm to developing limb bud:
Skeleton and Vasculature
Contribution of Hypaxial Mesoderm to developing limb bud:
Muscles
Contribution of Neural Crest to developing limb bud:
Schwann cells
Contribution of Neural Tube to developing limb bud:
Motor/Sensory axons (dermatome map)
How do upper limbs rotate?
90 degrees laterally
How do lower limbs rotate?
90 degrees medially
Malformations
Overgrowth
Undergrowth
Focal Defects
Associated w/ other general/syndromic skeletal abnormalities
Sirenomelia
“Mermaid Syndrome”
Different Classifications
Separation surgery possible in some cases
Achondroplasia
Most prevalent form of dwarfism
Mutation on FGF-R3
Pathogenic changes at epiphyseal plate; zones of proliferation & hypertrophy are narrow and disorganized
Autosomal dominant
Osteogenesis Imperfecta/Brittle Bone Disease
Bone fragility that predisposes to fractures and deformities related to connective tissue abnormalities (Collagen deformation)
Blue Sclera
Not always evident at birth
Talipes equinovarus (Club Foot)
Most common type of club foot
The sole of the foot is tuned medially and the foot is inverted.
1/1000 births, two times more frequent in males
Flexible Club Foot
Results from abnormal positioning or restricted movement of the lower limbs in utero.
Structurally normal feet
Usually correct themselves spontaneously
Rigid Club Foot
Abnormal development of the ankle and foot joints during the 6th and 7th week
Bony deformities, particularly the talus
