Maxillary sinus floor elevation has been documented as a safe and predictable procedure for gaining vertical bone height in the atrophic posterior maxillae. Conversely, there is a lack of basic research on the characteristics of the union between the sinus membrane (SM) and the bone. Clinical implications of an impaired union in healthy or pathologic membranes remain unknown. The objective of this study was to present a comprehensive histologic and morphologic description of the sinus membrane-lateral bone wall complex. In 14 fresh cadaver heads, 28 lateral wall sinus augmentation procedures were performed to obtain SM samples. Samples were assessed using hematoxylin-eosin, Masson trichrome, and toluidine blue staining and immunofluorescence and immunohistochemistry procedures. Specimens were coded and studied by a trained examiner using an optical microscope at ×4, ×10, ×40, and ×100 objectives. Thickness and inflammation status were assessed in these samples. Overall SM thickness of the samples was 0.40 ± 0.12 mm and was positively correlated to the inflammatory condition of the membranes. Such low values are the consequence of limited inflammation. Most of the fibers and cells in the deeper layers of the SM ran in a horizontal direction, oriented parallel to the underlying bone wall. In the immunohistochemistry study, 3 out of 7 samples showed a certain degree of nestin expression, suggesting osteogenic potential in spite of the elderly specimens. Large variations in thickness across the SM were found. These were noted to be partially correlated to the SM inflammatory status. The vast majority of the fibers were oriented parallel to the maxillary lateral wall, and only a few isolated areas showed a stronger perpendicular attachment. This might indicate the surpassing importance of the SM inflammatory status, operator skill, and other anatomical factors over the sinus membrane-maxillary lateral wall complex interface. Moreover, about half of the SM investigated were positive for nestin, indicating their osteogenic potential.