The Adult Human Rhomboid Lip: 9 + 2 Ciliary Ultrastructure and Surgical Anatomy of the Cerebellopontine Angle

The rhomboid lip (RL) is a neuroepithelial structure extending from the lateral recess of the fourth ventricle into the cerebellopontine angle (CPA). Although cilia have been described on its surface, their ultrastructural characteristics in adults remain unclear. We characterized the histological and ultrastructural architecture of the adult RL and evaluated its anatomical relevance in CPA surgery.

RL specimens were obtained from 2 patients undergoing surgery for vestibular schwannoma. Histological and immunostaining analyses were performed in both cases, including double immunofluorescence for acetylated α-tubulin and aquaporin-4, whereas scanning electron microscopy and transmission electron microscopy were performed in one case. Two formalin-perfused cadaveric heads were examined to assess the anatomical relationships of the RL with CPA structures.

Histology demonstrated a ventricular monolayer of ependymal-like cells overlying a glial-rich layer. Immunofluorescence demonstrated acetylated α-tubulin-positive cilia along the apical surface of the RL. Scanning electron microscopy revealed dense ciliary coverage, whereas transmission electron microscopy demonstrated a characteristic 9 + 2 axonemal arrangement consistent with the ultrastructural features of motile cilia. Cadaveric dissection demonstrated the spatial relationship of the RL with the choroid plexus, foramen of Luschka, and lower cranial nerves, particularly cranial nerve IX.

The adult human RL retains organized neuroepithelial architecture. In the ultrastructurally examined specimen, cilia exhibited a characteristic 9 + 2 axonemal arrangement. Although ciliary motility and its effects on cerebrospinal fluid dynamics were not directly assessed, these findings provide new insight into lateral recess microanatomy. The RL may serve as a useful anatomical reference for microsurgical orientation in CPA surgery.