Anatomy of endolymphatic sac
The endolymphatic duct and the endolymphatic sac are non sensory components of the endolymph filled, closed membranous labyrinth. The endolymphatic duct leads from the utricular and saccular ducts wthin the vestibule through the vestibular aqueduct to the endolymphatic sac.
Endolymphatic duct and endolymphatic sac system consists of components both inside and outside the otic capsule connected by a narrow passageway through the capsule.
Traditional anatomical description:
Traditional anatomical texts have depicted the endolymphatic duct and endolymphatic sinus system as a single lumen tubular structure, with a thin long endolymphatic duct ending in a short, blunt pouch like endolymphatic sinus.
Computer aided reconstructions of histologic sections have revealed a system which is far different in appearance. The endolymphatic duct is in fact a short single lumen tube, only for about 2mm. The endolymphatic sac is a much larger and highly complex structure of interconnecting tubules, cisterns and crypts. The endolymphatic sac is highly variable in size and irregular in outline that too distally.
The endolymphatic duct forms from the confluence of the utricular and saccular ducts. The proximal portion of the endolymphatic duct is mildly fusiform. This segment is known as endolymphatic sinus. This sinus lies in a groove on the posteromedial surface of the vestibule, while its major portion is contained within the short, slilghtly upwardly arched horizontal segment of the vestibular aqueduct. On entering the vestibular aqueduct, the sinus tapers to its intermediate segment within the horizontal segment of the vestibular aqueduct and narrows at its isthmus within the isthmus of the vestibular aqueduct.
Distal to the isthmus the endolymphatic duct begins the endolymphatic sac. It has two portions i.e interosseus and extraosseus portions. The interosseus portion lies proximal within the transversely widening vertical segment of the vestibular aqueduct. This portion is covered by a thin scale of bone known as the operculum.
The distal portion is extraosseus rests on a fovea on the posterior wall of petrous bone between the layers of dura. The mid portion of the sac can lie in or out of the aqueduct depending in the length of the vestibular aqueduct.
The main arterial supply of endolymphatic duct and endolymphatic sac is the occipital artery.