Macroscopic and microscopic assessment of zygapophyseal joints. (a) Macroscopic picture of a zygapophyseal joint from a patient with ankylosing spondylitis (AS). (b) Hematoxylin and eosin staining of a zygapophyseal joint from an AS patient: weakly eosinophilic fluid accumulation in the bone marrow interstitium (red arrows) neighboring dense interstitial infiltrates of mononuclear cells (yellow arrows). The black arrow indicates fat vacuoles. (c) Immunohistochemical analysis of CD3+ T cells in the zygapophyseal joint of an AS patient: dense formations of CD3+ T cells (red stain, yellow arrow) neighboring bone marrow edema (red arrow). (d) Immunohistochemical analysis of CD3+ T cells in a zygapophyseal joint of a non-AS control with only few loosely distributed CD3+ T cells in the bone marrow without edema.