Limb Petterns EX Limb Patterns Exercises body that the lectures cover in the reg

Question

Limb Petterns EX Limb Patterns Exercises body that the lectures cover in the regional The limbs comprise the second of the three main parts of the vertebrate phylogenetic time and du structural patterns of the limbs that you encountered anatomy. As with the trunk, limb anatomy is governed by structural patterns eme and during embryonic development. The exercises in this chapter will help you reinforce t in the video presentation. Understanding these patterns can help you organize the details of limb anatomy Limb Development 1. Describe the stages associated with the development of the limb bones. What embryonic tissues do the limb bones arise from 2. Theillustrations below depict cross sections of an series of stages of limb development. The upper llustrations represent embryonic stages and the lower illustration depicts a postnatal section through the upper limb and thorax. Color-code the all the body wall musles green and the limb muscles rd. Clearly and concisely describe the important steps that are occurring in these developmental sequences

Explanation / Answer

  I.        Limb primordia

                     A.        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

                     B.        THE BUDS are formed by a series of reciprocal inductions of mesoderm and ectoderm

1.        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

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

                                     2.        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

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

                                     3.        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)

    II.        Limb development

                                     1.        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

                                     2.        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

                                     3.        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

                                     4.        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

                                     5.        As the bones form and limbs elongate, myoblasts aggregate and form the large muscle masses in each limb

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

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

                                                     a.        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

                                                     b.        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

                                                     c.        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

Limb bones arise from the mesoderm.

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