Determinants of Entheseal Anatomy: Body Size, Activity Level, or Developmental Vestige?
Authors: Ruth Aidan 
Published: June 2021

Entheses are sites of extreme mechanical stress, as soft, collagenous ligament inserts on to stiff bone. Fibrocartilaginous entheses display a four‐layered structure that progresses from ligament through unmineralized and mineralized fibrocartilage, and finally bone. This structure is hypothesized to minimize tension and compression at the insertion site by incorporating a stiffer material into the ligamentous midsubstance. Yet, little is known about the genetic (evolutionary) or environmental (plastic) entheseal responses of mineralized fibrocartilage to mechanical stimuli, including those associated with body size.We assessed differences in the microscopic anatomical structure of the anterior cruciate ligament insertion in seven species, including five primates (Gorilla, Pan, Pongo, Loris, Lemur, Leontopithecus, and Cercopiethecus) and two non‐primates (Panthera, Suricatta). We prepared decalcified histological sections at 7 micrometers and stained them using Safranin O and Fast Green. Mineralized fibrocartilage thickness was measured using scaled photomicrographs in ImageJ.Results of a standard linear regression of fibrocartilage thickness and body size indicate that there is no relationship between body size and average mineralized fibrocartilage thickness at either the femoral (R2= 0.04; F= 0.29; p= 0.61; eta2= .04) or tibial (R 2 =0.16; F = 1.283; p = 0.29; eta2= 0.16) insertion sites of the ACL. Additionally, there was no relationship between body size and maximum mineralized fibrocartilage thickness at either the femoral (R2= 0.01; F = 0.07; p = 0.8; eta2= 0.01) or tibial (R2= 0.28; F= 2.7; p = 0.14; eta2= 0.28) insertion sites.We also assessed regional differences in mineralized fibrocartilage thickness within the same insertion site, In order to compare fibrocartilage thickness between different regions of the femoral ACL enthesis, we conducted one‐way ANOVAs within each species. Significance was set at p= 0.05 The ligament was divided into anterior and posterior segments, as well as medial and lateral segments. We divided the ligament both based on a percentage of overall size (e.g., We took measurements at 0, 25%, 50%, 75%, and 100% of the antero‐posterior length of the ligament), as well as based on the division of the ligament into different anatomical portions (e.g., in the anteromedial bundle insertion vs. the posterolateral insertion). We were unable to detect differences in mineralized FC thickness in any of these areas.These data indicate that, despite the potential biomechanical advantage of substantial mineralized fibrocartilage thickness, in reality it is highly conserved relative to body size and mechanical environment. This suggests that this tissue may 1) be limited in thickness by avascularity; 2) exist in order to prevent communication between bone and ligamentous tissue; or 3) exist as a developmental consequence of unmineralized fibrocartilage at the ligament insertion site.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.