Embryology

Question 1

What is the development of the musculoskeletal system in the human embryo?

Answer 1

End of 4th week: begins
Day 24= upper limb buds
1-2d after = lower limb buds

Through the activation of mesenchymal cells in the somatic lateral mesoderm (homeobox (HOX) genes regulate patterning eg determine which skeletal element is form from which gene expression)

Limb buds elongate by the proliferation of mesenchyme

• upper limb buds develop opposite the caudal cervical segments
• lower limbs form opposite the lumbar and sacral segments

At the apex of each bus the ectoderm thickens to form an apical ectodermal ridge (AER). Has an inductive influence on the limb mesenchyme. Cells adjacent to it consist of undifferentiated, rapidly proliferating cells. Proximal cells differentiate into blood vessels and cartilage bone models.

Distal ends flatten into hand plates and foot plates. By end of 6th week tissue in the hand plates have formed digital rays, and by end of the 7th week the same in feet. Apoptosis is responsible for tissue breakdown between digits.

Question 2

How can abnormalities in development lead to limb deformities?

Answer 2

Birth defects of the limbs are usually associated with other defects eg hereditary or teratogenic

Can have: total absence (amelia), partial absence (meromelia), some long bone absence (phocomelia), all parts present but small (micromelia)

The critical period of limb development is 24-36 days after fertilisation

Examples include polydactyly (extra digits), syndactyly (cutaneous or osseous), congenital clubfoot aka talipes equinovarus (possible causes: multifactorial, abnormal positioning or restrictive movement in utero), developmental dysplasia of the hip (1: abnormal development of the acetabulum, common after breech births, 2: generalised joint laxity which is multifactorial, associated with congenital dislocation of hip)

Question 3

What is limb rotation in embryology?

Answer 3

The upper and lower limbs rotate.

• upper: 90* laterally so extensors are on the lateral and posterior surfaces (thumb lateral)
• lower: 90* medically so that the extensors are on the anterior surface (big toe is lateral)

Question 4

How do the dermatomal patterns develop?

Answer 4

During the 5th week, peripheral nerves grow outwards from the developing limb plexus into the mesenchyme of the limbs eg brachial plexus, C4-T2, lumbar plexus L2-S3