Fig. 5.

Histologic and magnetic resonance images from a different piglet than that shown in Figures 3 and 4 illustrate other aspects of the common response to ischemia at eight weeks. This response was predominant in six of eight hips followed to eight weeks. A: Photomicrograph of another entire femoral head is shown. Fibrous tissue ingrowth is seen at the lower part of the secondary ossification center laterally above the physis. The cartilage surface is much thicker than normal. Bone in the medial third of the femoral head is relatively normal-appearing endochondral bone, but most bone centrally and in the lateral third is intramembranous bone from the new tissue ingrowth. Tissue from the box at left is shown at higher magnification in B, and tissue from the box at right is shown at higher magnification in C (paraffin-embedded section, hematoxylin and eosin stain). B: The characteristic fibrovascular invasion tissue (*) is seen centrally with large and small blood vessels prominent. Below, cartilage tissue persists, while, above (+), intramembranous bone synthesis from the fibrovascular component has occurred (paraffin-embedded section, hematoxylin and eosin stain). C: Tissue at the upper right showing endochondral bone formation emanating not from a physeal structure but as differentiation with vascularization of preceding cartilage repair tissue. Osteoclasts are prominent in the remodeling repair tissue (paraffin-embedded section, hematoxylin and eosin stain). D: Magnetic resonance imaging along the same plane and orientation as A, highlights the low-signal-intensity tissue (upper arrow), which is dense woven and lamellar intramembranous bone. Immediately below (inferior), the fibrovascular tissue has intermediate signal intensity (lower arrow). Note also the oval shape of the femoral head and its diminished height (T2-weighted sequence). E: Magnetic resonance image at a different depth but still in the coronal plane showing high-signal-intensity (arrow) accumulations, which represent fibrovascular tissue in greater accumulation at this site (T1-weighted sequence after gadolinium enhancement).