A. The lower limb buds appear in week 4 (about 4 days after the upper limb bud) as small bulges oriented in a coronal plane.
B. The lower limb buds undergo a horizontal flexion in week 6 so that they are now oriented in a parasagittal plane.
C. The lower limbs rotate medially 90° during weeks 6—8 so that the knee points anteriorly, the extensor compartment lies anteriorly, and the flexor compartment lies posteriorly. This rotation causes the originally straight segmental pattern of innervation (dermatomes) to be modified slightly in the adult (Figure 20-2).
D. Note that the upper limbs rotate laterally 90°, whereas the lower limbs rotate medially 90°. This rotation sets up the following anatomic situations:
1. The flexor compartment of the upper limb is anterior, whereas the flexor compartment of the lower limb is posterior.
Week 5. Lateral plate mesoderm condenses (hatched). Week 6. Hyaline cartilage (light shading) model of future bones forms. Weeks 7-9. Primary ossification centers within the diaphysis appear such that bone (dark shading) forms (osteogenesis).
At birth. The diaphysis consists of bone (dark shading), whereas the epiphysis remains hyaline cartilage. This is important to note when interpreting radiographs of newborns. The radiograph of a newborn at the hip region (I = femur; 2 = ilium) shows the portions of the hyaline cartilage model that have been replaced by radiodense bone (white). Note that the epiphyseal end of the femur (white arrow) is still hyaline cartilage at birth and will therefore appear radiolucent (dark). The radiograph of a newborn at the ankle and foot shows the portions of the hyaline cartilage model that have been replaced by radiodense bone (white) in the tibia (1), fibula (2), calcaneus (3), talus (4), metatarsals (5), and phalanges (6). Note that the epiphyseal ends of the tibia and fibula are still cartilage and are, therefore, radiolucent (white arrows).
Childhood. During childhood, secondary ossification centers form in the epiphyseal ends of the bones. During childhood and adolescence, the growth in length of long bones occurs at the epiphyseal grcwth plate. Note the radiograph of a 6-year-old child at the hip region (1 = femur; 2 = ilium). Because secondary ossification centers are present within the epiphyseal ends, the head of the femur is now radiodense (white arrow), and the epiphyseal growth plate (arrowhead) where hyaline cartilage is present remains radiolucent (dark). This should not be confused with a bone fracture. Oil the radiograph of a 6-year-old child at the foot, the diaphyseal and epiphyseal portions of the metatarsals (5) and phalanges (6) as well as their epiphyseal growth plates (arrowheads) can be observed. The remaining tarsal bones have begun to ossify. 3 = calcaneus; 4 = talus; 7 = cuboid; 8 = lateral cuneiform; 9 = intermediate cuneiform; 10 = medial cuneiform; 11 = navicular. (From Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 230.)
Figure 20-2. Dermatome pattern in the adult lower limb and limb bud. The 90° medial rotation of the lower limb bud causes the originally straight segmental pattern of innervation in the embryo to be somewhat modified (i.e., "twisted in a spiral") such that the dermatome pattern in the adult is altered. However, an orderly dermatome pattern can still be recognized in the adult if the lower limb is positioned in a parasagittal plane with the big toe pointing superiorly (as shown). The dermatomes from LI can be counted distally down the superior border of the lower limb (arrow) to L5 and then back proximally up the inferior border of the lower limb (arrow) to S2. (From Cudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 234.)
2. The extensor compartment of the upper limb is posterior, whereas the extensor compartment of the lower limb is anterior.
3. Flexion at the wrist joint is analogous to plantar flexion at the ankle joint.
4. Extension at the wrist joint is analogous to dorsiflexion at the ankle joint.
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