Effect of aging on the tendon structure and tendon-associated gene expression in mouse foot flexor tendon

Abstract

To evaluate the biological changes in tendons during the aging process, the present study examined the effect of aging on the tendon structure, distribution of collagen types I and III, and expression of tendon-associated genes, using flexor tendons in a mouse model. Histological assessment of the tendon structure and distribution of collagen types I and III were performed, and the expression of tendon-associated genes was evaluated in flexor digitorium longus tendons of young (8 weeks) and aged (78 weeks) female C57BL/6 mice. The results indicated that the Soslowsky score, based on the analysis of cellularity, fibroblastic changes, and collagen fiber orientation and disruption, was significantly increased, or worsened, in the tendons of the aged group compared with those in the young group. Furthermore, in the aged group, the distribution of type I collagen was decreased and the distribution of type III collagen was relatively increased compared with the young group. Finally, the mRNA expression levels of collagen (type I and type III) and tenogenic markers (Mohawk homeobox, tenomodulin and scleraxis BHLH transcription factor) were significantly decreased in the aged group compared with the young group. The present observations demonstrated that the structure of the tendons, distribution of types I and III collagen and the expression of tendon-associated genes were modulated by aging in the flexor tendon, and that these changes may contribute to the degeneration of tendons in tendinopathy.

Keywords: aging; collagen; tendinopathy; tendon; tendon-associated gene.

Figure 1.

Histological assessment of tendon structure and Soslowsky score. (A and B) Sections of flexor digitorium longus tendons in the (A) young and (B) aged groups were stained by hematoxylin and eosin. Nuclei of tenocytes are stained in violet. Yellow arrowheads indicate the altered and disrupted orientation of collagen fibers in the aged group. Scale bar=50 µm. (C) The Soslowsky score was calculated, based on the cellularity, fibroblastic changes, and collagen fiber orientation and disruption. Data values are expressed as the means ± standard deviation. ^*P<0.05.

Figure 2.

Evaluation of the differential distribution of collagen types I and III. (A and B) Sections of flexor digitorium longus tendons in the (A) young and (B) aged groups were stained by picrosirius red, which allowed visualization of type I collagen (red) and type III collagen (yellow/green). Magnification, x40. (C and D) The total number of red and yellow/green pixels in these images were analyzed with the Nuance multispectral imaging system, and the distribution of (C) type I collagen (red/total pixels) and (D) type III collagen (yellow and green/total pixels) were calculated, and expressed as proportions. Values are expressed as the means ± standard deviation of the young (n=5) and aged (n=5) groups.

Figure 3.

mRNA expression of tendon-associated genes. mRNA expression of (A) Mohawk, (B) scleraxis, (C) tenomodulin, (D) type I collagen and (E) type III collagen in flexor digitorium longus tendons of the young and aged groups was evaluated by reverse transcription quantitative polymerase chain reaction. The mRNA expression levels are presented as relative quantities in comparison with that of GAPDH mRNA. Values are expressed as the means ± standard deviation, and comparisons were made between the young (n=8) and aged (n=8) groups. ^*P<0.05. Mohawk, Mohawk homeobox; scleraxis, scleraxis BHLH transcription factor.