Developmental Anatomy Of The Myodural Bridge In SD Rats

Objective:In 1995,Hack et al.(1995)first proposed the myodural bridge and described it was a dense fibrous tissue bridge connecting the rectus capitis posterior minor muscle to the spinal dura mater via the posterior atlanto-occipital membrane.Reportedly,the myodural bridge consists of not only the rectus capitis posterior minor muscle but also rectus capitis posterior major muscle fibers,the obliquus capitis inferior muscle,and the ligamentum nuchae,via both the posterior atlanto-occipital and posterior atlanto-axial interspaces.These structures together form a functional unit and we term this structure the ‘Myodural Bridge Complex’.Many researchers have been studying this particular structure.Our group has studied representative animals from five different mammalian orders and proves that the myodural bridge is a universal existing normal anatomical structure in mammals.This result indicates that the myodural bridge is an evolutionally conserved structure present in many mammals,suggesting that it has an important physiological role.Many researchers have also speculated and studied the function of the myodural bridge.Some scholars believe that the myodural bridge play vital roles in preventing in-folding of the spinal dura mater and keep the subarachnoid space and the posterior cerebellomedullary cistern patent.The myodural bridge is also speculated to be associated with the transmission of proprioception and may be related to cervicogenic headache.Our group have proposed a novel hypothesis regarding the physiological function of the myodural bridge that the mydural bridge is likely to play an important role in promoting the circulation of cerebrospinal fluid.The myodural bridge can transmit the strength of the suboccipital muscles,which pulls the spinal dura mater and then causes an increase or decrease at cerebrospinal fluid flow rate.At present,many scholars have conducted various researches on the myodural bridge.Nevertheless,there have been only few reports on the development and maturation of the myodural bridge.Thus,The purpose of this study is to clarify morphological evolution characteristics and compositional changes in the fibrous structures of the myodural bridge during its growth and development in Sprague-Dawley(SD)rats(age,E17 to adulthood).A comprehensive explanation of the myodural bridge’s developmental process can provide morphological basis for future studies on the function of this structure.Materials and methods:The head and neck specimens from 55 rats.Rat embryos aged 17,18,19,20,and 21 days post conception;newborn rats(within 12 hours after birth);postnatal rats aged 1,2,3,7,10,14,and 21 days(collected materials at the same time every day);and adult rats were used in the present study.The samples of rats were fixed in 10% formalin and were used for the histological slices and staining.Results:During the development of SD rats,the posterior aspects of the C1 and C2 were not closed at E17;however,they were completely closed at E18 and at this time,the muscle tissue and spinal dura mater could be clearly distinguished and they were tightly attached to each other.In rat embryos aged E18-E21,the rectus capitis dorsal minor muscle,rectus capitis dorsal major muscle and oblique capitis posterior muscle fibers appeared loose and fusiform,the posterior atlanto-occipital membranes and the atlanto-axial membranes appeared thin and lightly stained,and spinal dura mater appeared dense and intensely stained.In postnatal rats,suboccipital muscles fibers and collagen fibers appeared denser than rat embryos.We observed that the staining intensity of slices and the number of fibers increased in postnatal rats.During the embryonic period of the rat,the rat suboccipital muscle and the posterior atlanto-occipital membrane or the atlanto-axial membrane and the dura mater were closely attached together.There were two ways of connection between the deep muscles in the suboccipital region and the posterior atlanto-occipital membrane or the atlanto-axial membrane in rats after birth.One was that the muscles were directly connected to the posterior atlanto-occipital membrane or the atlanto-axial membrane,and the other was that fibers between the muscles merged into the posterior atlanto-occipital membrane or the atlanto-axial membrane.The spinal dura mater and the posterior atlanto-occipital membrane were always closely connected,and the connection between the spinal dura mater and the posterior atlanto-occipital membrane was not as tight as the posterior atlanto-occipital interspace.Occasionally,separation of the spinal dura mater and the posterior atlanto-occipital membrane was observed in the atlanto-axial interspace.Observation of Picrosirius red staining under a polarizing microscope showed that there were both type I and type III collagen fibers in the myodural bridge of embryonic rats.In postnatal rats,collagen fibers mainly comprised type I collagen.Observation of the suboccipital region of the rat found that the subdural space of the rat was small in the embryonic period,but after the birth,the rat had an obvious subdural space.Conclusion:The results of this study revealed the developmental progression of the myodural bridge.the myodural bridge fibers appeared to be already in their required anatomical location by E18;however,these fibers were thin and loose in rat embryos.However,in postnatal rats,from P0 onward,the myodural bridge fibers became significantly denser.With further development and maturation of rats,the myodural bridge also increased in density and further matured.In addition,we demonstrated that there were both type I and type III collagen fibers in the the myodural bridge of embryonic rats.In postnatal rats,collagen fibers mainly comprised type I collagen.Based on the results of this study,we speculate that the myodural bridge may have the function of maintaining the subdural space of the upper cervical segment during development.Moreover,the results of this study provide a morphological basis for future functional studies on the myodural bridge.