Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, PA 19104, USA.
Matrix Biol. 2011 May;30(4):267-74. doi: 10.1016/j.matbio.2011.03.009. Epub 2011 Apr 12.
Cross bridges are radial structures within the highly organized lamellar structure of the annulus fibrosus of the intervertebral disc that connect two or more non-consecutive lamellae. Their origin and function are unknown. During fetal development, blood vessels penetrate deep within the AF and recede during postnatal growth. We hypothesized that cross bridges are the pathways left by these receding blood vessels. Initially, the presence of cross bridges was confirmed in cadaveric human discs aged 25 and 53 years. Next, L1-L2 intervertebral discs (n=4) from sheep ranging in age from 75 days fetal gestation to adult were processed for paraffin histology. Mid-sagittal sections were immunostained for endothelial cell marker PECAM-1. The anterior and posterior AF were imaged using differential interference contrast microscopy, and the following parameters were quantified: total number of distinct lamellae, total number of cross bridges, percentage of cross bridges staining positive for PECAM-1, cross bridge penetration depth (% total lamellae), and PECAM-1 positive cross bridge penetration depth. Cross bridges were first observed at 100 days fetal gestation. The overall number peaked in neonates then remained relatively unchanged. The percentage of PECAM-1 positive cross bridges declined progressively from almost 100% at 100 days gestation to less than 10% in adults. Cross bridge penetration depth peaked in neonates then remained unchanged at subsequent ages. Depth of PECAM-1 positive cross bridges decreased progressively after birth. Findings were similar for both the anterior and posterior. The AF lamellar architecture is established early in development. It later becomes disrupted as a consequence of vascularization. Blood vessels then recede, perhaps due to increasing mechanical stresses in the surrounding matrix. In this study we present evidence that the pathways left by receding blood vessels remain as lamellar cross bridges. It is unclear whether the presence of cross bridges in the aging and degenerating intervertebral disc would be advantageous or detrimental, and this question should be addressed by future studies.
交叉桥是连接两个或多个不连续的板层的椎间盘纤维环高度组织化的板层结构内的放射状结构。它们的起源和功能尚不清楚。在胎儿发育过程中，血管穿透 AF 深部，并在出生后生长过程中退缩。我们假设交叉桥是这些退缩血管留下的途径。最初，在 25 岁和 53 岁的尸体人类椎间盘标本中证实了交叉桥的存在。接下来，对来自绵羊的 L1-L2 椎间盘（n=4）进行石蜡组织学处理，其年龄从胎儿妊娠 75 天到成年不等。对内皮细胞标志物 PECAM-1 进行中矢状切片免疫染色。使用相差对比显微镜对前、后纤维环进行成像，并对以下参数进行量化：明显板层的总数、交叉桥的总数、PECAM-1 染色阳性的交叉桥的百分比、交叉桥穿透深度（%总板层）和 PECAM-1 阳性交叉桥穿透深度。交叉桥最早在胎儿妊娠 100 天时观察到。总数在新生儿时达到峰值，然后相对保持不变。PECAM-1 阳性交叉桥的百分比从妊娠 100 天的近 100%逐渐下降到成人的不到 10%。交叉桥穿透深度在新生儿时达到峰值，随后在随后的年龄保持不变。出生后 PECAM-1 阳性交叉桥的穿透深度逐渐下降。在前部和后部均发现了相似的发现。纤维环的板层结构在发育早期建立。随后，由于血管化而被破坏。血管然后退缩，这可能是由于周围基质中的机械应力增加所致。在这项研究中，我们提供了证据表明，退缩血管留下的途径仍然作为板层交叉桥存在。在老化和退化的椎间盘中存在交叉桥是否有利或有害尚不清楚，这个问题应该通过未来的研究来解决。
