Adenoid cystic carcinoma of the salivary gland (SACC) is a rare malignant tumors of the head and neck region, but it is one of the most common malignant tumors that are prone to perineural invasion (PNI) of the head and neck. The prognosis of patients with SACC is strongly associated with the presence of perineural spread (PNS). Although many contributing factors have been reported, the mechanisms underlying the preferential destruction of the blood-nerve barrier (BNB) by tumors and the infiltration of the tumor microenvironment by nerve fibers in SACC, have received little research attention. This review summarizes the current knowledge concerning the characteristics of SACC in relation to the PNI, and then highlights the interplay between components of the tumor microenvironment and perineural niche, as well as their contributions to the PNI. Finally, we provide new insights into the possible mechanisms underlying the pathogenesis of PNI, with particular emphasis on the role of extracellular vesicles that may serve as an attractive entry point in future studies.

BACKGROUND: Retromolar canal (RMC) arises from the mandibular canal (MC) behind the second or third molar and travels anterosuperiorly to a retromolar foramen (RMF). RMCs and RMFs have generally been ignored in anatomical textbooks and have rarely been reviewed or studied in the anatomical and dental literature until the last decades. OBJECTIVE: This study aimed to characterize RMF in a Chinese population concerning its incidence, origin, and classification via anatomical study and periapical radiography. METHODS: 123 dry adult Chinese mandibles were collected to observe the incidence of RMFs. RMFs were determined using a steel wire 0.5 mm in diameter. The passways or origins of the retromolar canal (RMC) were determined and classified via periapical radiography. For each RMF, two dentists independently measured the diameter and its distances to the lingual cortex, the buccal cortex, and the distal edge of the last tooth (or the alveolar fossa) using a vernier caliper. RESULTS: The incidence of RMFs was 31.71%. The average RMF diameter was 0.78 ± 0.27 mm. From RMF, the distance was 4.27 ± 1.87 mm to the lingual cortex, 8.61 ± 2.23 mm to the buccal cortex, and 7.84 ± 3.87 mm to the distal edge of the last tooth (or the alveolar fossa). RMCs were classified into MC type originating from the mandibular canal and AF type originating from the alveolar fossa. The diameters of MC ones were more significant than those of AF ones. There was no apparent correlation between the existence of the third molar and the presence of an RMF. CONCLUSION: The incidence of RMFs in Chinese may be about one-third, which is a potential factor in the onset of surgery accidents. RMCs can be classified into two types by their origins. One of them is MC, which originates from the mandibular canal, and the other is AF, which originates from the alveolar fossa.