Development Of Intraorbital EOMs Pulleys In Late Trimester Of Pregnancy And Infants

ObjectivesTo investigate the possible inner mechanism between the intraorbitalEOMs Pulleys systems and the development of ocular motility and binocularvision, the morphology difference of EOMs Pulleys systems at various stages inlate trimester of pregnancy and infants were compared, and thedevelopment characteristic of the intraorbital EOMs Pulleys in latetrimester of pregnancy and infants were analyzed.Methods(1) Six human orbits(ages 27,30 and 33 weeks gestation and 1,6 and12 monthspostnatal, respectively) were fixed in situ within the cranial bones by immersion informalin, and dehydrated in graded solutions of alcohol and xylenes, embedded inparaffin, and serially sectioned in the coronal plane at 8 or 5μm thickness. Massontrichrome stain was used to show collagen, and Weigert stain to show elastin (8μmthickness). SM was confirmed using a monoclonal mouse antibody to human SMα-actin(5μm thickness)(SP Kit). The morphology and structure of the connectivetissues surrounding EOMs and their relationship were observed with lightmicroscopy.(2)The slides containing characteristic EOMs Pulleys and PulleyBands(PBs) in each orbit(ages 27,30 and 33 weeks gestation and 1,6 and12months postnatal, respectively) were stained with Massontrichrome to show thedistribution of collagen. The width and optical density(OD) of the blue stained rectusconnective tissue pulleys(orbital aspect) and PBs were measured with HMIAS-2000system. The difference of the development and distribution characteristics of EOMs Pulley systems at various stages in the late trimester of pregnancy and earlyinfants were compared and statistically analyzed.(3) Rectus extraocular muscles (EOMs) consist of orbital (OL) and global (GL)layers. All fibers of the rectus EOMs were counted(OL fibers and GL fibers werecounted respectively) with HMIAS-2000 system using light microscopy at midorbit infive orbits(30 and 33 weeks gestation and 1,6 and12 months postnatal, respectively).The difference of the development and distribution characteristics of GL and OLfibers at various stages in the late trimester of pregnancy and early infantswere compared and statistically analyzed.Results(1) At 27 weeks of gestation: the LR-IO PB was obvious while theLR-SR PB was quite thinner, and the LR was surrounded by the collagenouspulley ring, whereas the MR and SR were surrounded by very thin collagenouspulley rings. However, there was no MR-IO PB or MR-SR PB appeared, and neitherany distribution of black elastin fibrils or nonvascular SM cells. At 30 weeks ofgestation: the LR-IO PB and LR-SR PB were more developed and obvious,and the LR and SR were surrounded by obvious collagenous pulley rings,whereas the MR was surrounded by a thin collagenous pulley ring. Obvious blackelastin fibrils were scattered among the rectus Pulleys and PBs. However, there wasstill no MR-IO PB or MR-SR PB appeared, and neither any distribution ofnonvascular SM cells. At 33 weeks of gestation: the LR-IO PB and LR-SR PBwere even more developed and obvious, and the LR and SR weresurrounded by obvious collagenous pulley rings, and the collagenous pulley ringsurrounding MR become thicker and obvious. More black elastin fibrils werescattered among the rectus Pulleys and PBs. In the LR-IO PB, some nonvascularSM cells were distributed in a band form in the LR-IO PB. However, there was stillno obvious MR-IO PB or MR-SR PB appeared.(2) In 1 month postnatal infant, a thin MR-SR PB was present, the collagenous pulley ring surrounding MR was a bit thicker than that of LR, the MR-SRPB was more developed and become thicker, and a thin MR-IR PB could be noticed.More black elastin fibrils were scattered among the rectus Pulleys and PBs. In 6months postnatal infant, the MR-IR PB was obvious, Much more black elastin fibrilswere scattered among the rectus Pulleys and PBs. Some nonvascular SM cells weredistributed in a band form in the LR-IO PB. In 12 months postnatal, all the four rectuscollagenous pulley rings were well developed and obvious, while the MR pulley ringwas the thickest, and the development and thickness of the temporal PBs(LR-IO andLR-SR) and that of the nasal PBs(MR-IR and MR-SR) were equivalent. Thedistribution of black elastin fibrils among the rectus Pulleys and PBs become moredense.(3) Although there was still no distinctive boundary between GL andOL, the different morphology characteristic between GL and OL of rectusEOMs was already present in the 27-week fetus. In all six specimens, theOL of each rectus EOM inserts on its corresponding pulley, rather than on the globe.But the GL of the EOM inserts via its tendon on the sclera. In 30-week and 33-weekfetuses, the number of MR GL fibers were 6172±871 and 6954±926 respectively,and that of OL fibers were 3402±173 and 4134±301 respectively. And the numberof LR GL fibers were 5742±261 and 6198±333 respectively, and that of OL fiberswere 2982±142 and 3679±179 respectively. And the number of SR GL fibers were4236±511 and 4753±563 respectively, and that of OL fibers were 1932±86 and2630±78 respectively. And the number of IR GL fibers were 4749±246 and5223±272 respectively, and that of OL fibers were 2316±236 and 2937±129respectively. In 1,6 and 12 months infants, the number of MR GL fibers were8103±574, 9753±1170 and 10910±1168 respectively, and that of OL fibers were4715±487, 5831±371 and 8541±545 respectively. And the number of LR GLfibers were 6805±341, 7415±589 and 9095±1024 respectively, and that of OLfibers were 4356±172, 5476±497 and 6852±244 respectively. And the number of SR GL fibers were 5637±447, 6064±723 and 7230±539 respectively, and thatof OL fibers were 30524±161, 3743±264 and 4126±192 respectively. And thenumber of IR GL fibers were6154±265, 7280±567 and 8127±540 respectively,and that of OL fibers were 3735±175, 4697±206 and 5198±183 respectively.Conclusions(1) The development of the EOMs Pulley system in late trimester ofpregnancy and infants has temporal and spatial succession. The sequence of Pulleysystem development was possibly initiated from infratemperal Pulley Band,and thenLR pully ring, LR-SR PB, SR pulley ring, SR-MR PB, MR pulley ring, and finallyMR-IR PB. The elastin fibrils were scattered among the rectus Pulleys and PBs asearly as 30 weeks of gestation, and further development was present after birth. Themature time of the infranasal Pulley Band(MR-IR) development might be consistentwith the development of human binocular convergent movement.(2) Different morphology characteristic between GL and OL of rectusEOMs was already present in the 27-week fetus, however, there was nodistinctive boundary between GL and OL at this stage. The boundarybetween GL and OL become distinctive in the 30-week fetus, the numberof both GL and OL fibers were increased with age, with the MR has themost fibers and SR has the least(MR＞LR＞IR＞SR) in all five specimens (30and 33 weeks of gestation, 1,6 and 12 months postnatal, respectively). Inall five specimens, the OL contained 31% to 44% of the total number of musclefibers in the rectus EOMs, and the number of MR OL fibers was much higher thanthat of SR OL. The abundance of its fibers across ages is consistent with an importantrole for the OL in ocular motility: the anteroposterior translation of the active pulleymight be regulated by the contraction and relaxation of its OL fibers, which was oneof the key points of the active pulley hypothesis.