MSK embryology Flashcards
When does the development of limbs begin?
Near end of 4th week
What part of the germ layer does the development of limbs originate from?
Mesenchymal cells activated in somatic lateral mesoderm
What genes regulate patterning of limb development?
Homeobox genes. Determine which skeletal element is formed
What homeobox genes code for expression in upper limb?
HOXd 9-13. Eg remove HOXa 11 and HOXd 11 and radius and ulna don’t form
What proliferates causing the limb buds to elongate?
Mesenchyme
What do upper limb buds develop opposite?
Caudal cervical segments
What do lower limb buds develop opposite?
Lumbar and sacral segments
What happens at the apex of each limb bud?
Ectoderm thickens to form apical ectodermal ridge (AER)
Within the AER, what does the mesenchyme consist of?
Undifferentiated, rapidly proliferating cells
Proximal to this undifferentiated zone what occurs?
Mesenchymal cells differentiating into blood vessels and cartilage bone models
When do the distal ends of limb buds flatten and condense to form digital rays?
End of 6th week (UL) and 7th week (LL)
What causes the tissue breakdown in the interdigital regions?
Apoptosis mediated by bone morphogenetic proteins
How do cartilaginous precursors of limb bones develop?
By chondrification within mesoderm in 6th week
When do primary centres of ossification appear?
Weeks 8-12
What is the only limb bone that has membranous ossification with the rest having endochondrial?
Clavicle
How do the upper and lower limbs rotate?
- Upper limbs: 90 degrees laterally so extensors are on lateral and posterior surfaces (thumb is lateral)
- Lower limbs: 90 degrees medially so extensors are on anterior surface (big toe is medial)
How do dermatomal patterns of limbs form?
Peripheral nerves grow from developing limb plexuses in 5th week
What can cause birth defects to arise?
Hereditary or teratogens
What is total/partial absence of limbs called?
- Total absence: amelia
- Partial absence: meromelia
What is long bone absence known as?
Phocomelia
What is small limbs known as?
Micromelia
Is lobster claw foot autosomal dominant of recessive?
Autosomal dominant
When is the critical period of limb development?
24-36 days after fertilisation
What is polydactyly?
Presence of more than 5 digits on hands/feet. Either inherited or teratogen induced
What are the two types of syndactyly (common defect in hand/foot)?
Cutaneous syndactyly - simple webbing between digits. Common
Osseous syndactyly - fusion of bones. When notches between digital rays fail to develop
What defects can occur due to maternal ingestion of thalidomide?
- Meromelia
- Intestinal atresia
- Cardiac abnormalities
Acts by disrupting cell adhesion and angiogenesis
What happens in club foot/talipes equinovarus (common birth defect)
Sole of foot turned medially and foot inverted
Causes of club foot?
1) Multifactorial (genetic and env.)
2) Abnormal positioning or restricted movement of fetus lower limb (disputed)
Discuss developmental dysplasia of hip?
Common after breech (buttock) deliveries, breech posture during late part of pregnancy (Causes abnormal development of acetabulum)
Joint laxity can be multifactorial and is associated with congenital hip dislocation
