The aim of this study was to describe the histological specificities of posterior lumbar epidural fat (PLEF) considered a "semi-fluid" tissue. We performed axial histological sections of posterior lumbar epidural fat through the abdomen of two foetuses, as well as histological sections on several planes in 11 adults. For comparison, we simultaneously cut sections in subcutaneous fat of adults (SCF). In the foetuses the epidural fat was arranged around the dural sac and extended along the entire lumbar spine, whereas in adults the epidural fat was discontinuous and the fat pads were located at intervertebral disc level, in a trianglular space with posterior apex, limited by the ligamenta flava laterally and by dural sac anteriorly. The PLEF was a homogeneous tissue in both size and shape and consisted of regular adipocytes and little connective tissue. In contrast, the SCF was made of adipocytes which varied in size and shape and were accompanied by numerous connective fibers (fibrous cones) subdividing the hypodermis into lobules. But the main specificity of the PLEF was the oriented empty spaces or slits found and observed in all samples with a special arrangement. These slits subdivided the fat into several layers enabling them to slide and could be regarded as "sliding spaces". They were never observed in the SCF samples. PLEF, therefore, is not a simple filling tissue. Its histological features (homogeneity, scarcity of connective tissue and oriented slits) explain the "semi-fluid" characteristic and confirm the specialization of this fat tissue. Its location at the level of the mobile segment of the lumbar spine suggests that it plays a role of sliding structure between the posterior surface of the thecal sac and the anterior surface of the vertebral arch.