A 26-year-old man presents with 5 months of insidious-onset low back pain that is worse in the mornings and at night, improves with activity, and is associated with 75 minutes of morning stiffness. He also notes bilateral heel pain worse with the first steps after waking and intermittent 'puffy swelling' of his right third toe. He denies recent infections, diarrhea, urethral discharge, skin rashes, or nail changes. On examination, he is afebrile. There is bilateral tenderness to palpation at the calcaneal insertions of the Achilles tendons and at the plantar fascia insertions; the right third toe is diffusely swollen. Joint examination of the wrists, MCPs, PIPs, DIPs, knees, and ankles reveals no joint-line tenderness, no effusion, and no palpable synovial swelling at any location. Laboratory studies: ESR 42 mm/hr, RF negative, anti-CCP negative, HLA-B27 positive.
Which of the following best explains the primary anatomic target of inflammation in this patient?