Chapter 69. Development of The Limbs - Review of Medical Embryology Book

THE LIMB BUD PRIMORDIA appear at the end of week 4 as small elevations of the ventrolateral body wall, but most development occurs in week 6. Early developmental stages for both the upper and lower limbs are identical, except that the arm buds precede those of the leg buds by several days
THE BUDS are formed by a series of reciprocal inductions of mesoderm and ectoderm

The lateral mesoderm of the somatopleure induces a transitory longitudinal thickening of the surface ectoderm, the wolffian crest, a fold which can be seen in front of the somite column. The middle portion of the column disappears rapidly, leaving only 2 nodes at the extremities of the crest which are at the level of the future osseous girdles

The arm buds develop opposite the caudal cervical segments; the leg buds develop opposite the lumbar and upper sacral segments

The ectodermal nodule or apical ectoderm ridge, located on the proximal side of the column, induces the mesenchyme to grow and develop the limbs, in successive waves. There is no apparent contribution from the myotome regions of the somite

Each bud is thus initially a mass of mesenchyme, of somatic mesodermal origin, covered by ectoderm

Innervation of the limbs is an early phenomenon. The upper limb is supplied from the last 6 cervical and first 2 thoracic metameres (brachial plexus); the lower limb from the last 4 lumbar and first 3 sacral metameres (lumbosacral plexus)

The first primordium of the upper limb appears about the 24th day and that of the lower limb at about day 2 The essential basic constituents of the limbs are distinguishable at day 3
The distal ends of the limb buds flatten into paddle-shaped hand or foot plates, and the respective digits form at the margins of these plates
The limb acquires its distal segment in week Shortly after this, a groove divides the proximal segment, and the limb now consists of its 3 definitive segments. Development of the upper limb is more advanced than that of the lower
Chondroblasts appear in the precartilaginous matrix which fragments to form the various skeletal parts. Between them, the first joint structures make their appearance toward week
As the bones form and limbs elongate, myoblasts aggregate and form the large muscle masses in each limb

The muscle masses separate into dorsal (extensor) and ventral (flexor) components

Early in week 7, the limbs move ventrally, and the developing arms and legs rotate to different degrees and in opposite directions

Initially, the flexor surface of the limbs is ventral and the extensor surface is dorsal, with the preaxial and postaxial borders being cranial and caudal, respectively
With rotation, the upper limbs rotate laterally through 90 degrees on their long axes, the elbows come to face posteriorly, and the extensor muscles come to lie on the outer or dorsal aspect of the arm
With rotation, the lower limbs rotate medially through 90 degrees on their long axes, the knees face forward or ventrolaterally, and the extensor muscles come to lie on the ventral aspect of the legs