![]() A secondary ossification center will appear in each epiphysis (expanded end) of these bones at a later time, usually after birth. By the twelfth week, a primary ossification center will have appeared in the diaphysis (shaft) region of the long bones, initiating the process that converts the cartilage model into bone. This process begins as the mesenchyme within the limb bud differentiates into hyaline cartilage to form cartilage models for future bones. On what days of embryonic development do these events occur: (a) first appearance of the upper limb bud (limb ridge) (b) the flattening of the distal limb to form the handplate or footplate and (c) the beginning of limb rotation? Ossification of Appendicular BonesĪll of the girdle and limb bones, except for the clavicle, develop by the process of endochondral ossification. Watch this animation (no sound) to follow the development and growth of the upper and lower limb buds. In contrast, the lower limb undergoes a 90-degree medial rotation, thus bringing the big toe to the medial side of the foot. During the seventh week of development, the upper limb rotates laterally by 90 degrees, so that the palm of the hand faces anteriorly and the thumb points laterally. The early outgrowth of the upper and lower limb buds initially has the limbs positioned so that the regions that will become the palm of the hand or the bottom of the foot are facing medially toward the body, with the future thumb or big toe both oriented toward the head. Also during the sixth week of development, mesenchyme within the limb buds begins to differentiate into hyaline cartilage that will form models of the future limb bones. Within the paddle, areas of tissue undergo cell death, producing separations between the growing fingers and toes. Shortly after this, a second constriction on the limb bud appears at the future site of the elbow or knee. The wrist or ankle areas then appear as a constriction that develops at the base of the paddle. This region will become the hand or foot. ![]() ![]() In this way, the limb develops along a proximal-to-distal axis.ĭuring the sixth week of development, the distal ends of the upper and lower limb buds expand and flatten into a paddle shape. As the limb bud elongates, cells located farther from the apical ectodermal ridge slow their rates of cell division and begin to differentiate. This ridge stimulates the underlying mesenchyme to rapidly proliferate, producing the outgrowth of the developing limb. The ectoderm at the end of the limb bud thickens to form a narrow crest called the apical ectodermal ridge. Initially, the limb buds consist of a core of mesenchyme covered by a layer of ectoderm. Figure 8.5.1 – Embryo at Seven Weeks: Limb buds are visible in an embryo at the end of the seventh week of development (embryo derived from an ectopic pregnancy). The upper limb bud appears near the end of the fourth week of development, with the lower limb bud appearing shortly after ( Figure 8.5.1). Limb GrowthĮach upper and lower limb initially develops as a small bulge called a limb bud, which appears on the lateral side of the early embryo. Thereafter, the development of the upper and lower limbs follows similar patterns, with the lower limbs lagging behind the upper limbs by a few days. Development of the limbs begins near the end of the fourth embryonic week, with the upper limbs appearing first. Mesenchyme gives rise to the bones of the upper and lower limbs, as well as to the pectoral and pelvic girdles. Discuss the appearance of primary and secondary ossification centersĮmbryologically, the appendicular skeleton arises from mesenchyme, a type of embryonic tissue that can differentiate into many types of tissues, including bone or muscle tissue.Describe the growth and development of the embryonic limb buds.By the end of this section, you will be able to:ĭescribe the embryonic formation and growth of the limb bones
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