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Limb Development

Notes

Limb Development

Overview

  • During week 4, the limb buds (which are the precursors of the limbs) form.
  • During week 5, skeletal and muscular precursors form.
  • During week 6, hyaline cartilage forms and digital rays form in the hand.
  • During week 7, endochondral ossification begins and digital rays form in the foot.
  • During week 8, the great toe and thumb rotate in opposite directions.

Anatomical Components

Weeks 4 through 8.

  • Week 4
    • First, draw an embryo.
    • Next, draw an upper limb bud.
    • Then, draw a lower limb bud. (The upper limb forms slightly before the lower limb.)
    • Show that each limb bud comprises a core of loose mesenchyme covered in ectoderm, with an apical ectodermal ridge (AER) at the tip.
    • The AER controls the growth and development of the limb; without it, the limb does not develop.
  • Week 5
  • During week 5, show that ectodermal ridge and apical ectodermal ridge have extend distally, and that the distal end has begun to flatten.
  • Draw a pair of mesenchymal models of bones within the mesenchymal core which form from lateral plate mesoderm.
  • Show muscle anterior and posterior masses, derived from paraxial mesoderm (somites).
  • Week 6
  • During week 6, show that the mesenchymal models begin to transition to hyaline cartilage (via chondrification).
  • Show the surrounding muscle masses for reference.
  • Then, show that cartilaginous digital rays (fingers) and carpals begin to form within the hand, itself.
  • Week 7
  • Show that at the beginning of week 7, the thumb and big toe are oriented in the same direction.
    • Include the digital rays within the hand and foot.
    • Remember: the lower limb lags slightly behind that of the upper limb.
  • Next, show that endochondral ossification begins.
  • Week 8
    • Show that during week 8, the thumb and big toe are oriented in opposite directions because the upper limb has rotated 90° laterally, and the lower limb has rotated 90° medially. (The rotation actually initiates during week 7 and completes only after bipedal walking begins).
  • Extensors vs Flexors
  • We bundle muscle groups of the limbs as flexors or extensors.
  • To understand this terminology, draw the upper extremity with the arm flexed (like a waiter holding a tray), so that the elbow points posteriorly.
  • Show that the extensors lie posteriorly and the flexors lie anteriorly.
  • Then, draw the lower extremity flexed at the knee (as though its poised to kick a soccer ball), so that the knee points anteriorly.
  • Show that the extensors lie anteriorly and the flexors lie posteriorly.
  • Clinical Correlations
    Now, let's address some clinically important consequences of failure of appropriate limb development.
  • Amelia is a congenital absence of a limb; it often results from the absence of an AER.
  • Meromelia is a partial absence of a limb (subvarieties exist).
  • Syndactyly is fusion (webbing) of digits.
  • Polydactyly is the presence of extra digits.
  • Brachydactyly is a shortening of the bones of the digits (subvarieties exist).

Intertextual discrepancies exist regarding the precise timing and regulation of embryological development; the timeline here is an approximation for simplicity.