A distinct transition from cell growth to physiological homeostasis in the tendon.

Mor Grinstein,Luke D O'Connor,Heather L Dingwall,Jenna Lauren Galloway,Terence Dante Capellini,Ken Zou

doi:10.7554/elife.48689

Mor Grinstein, Luke D O'Connor + Show 4 more

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https://doi.org/10.7554/elife.48689

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Abstract

Changes in cell proliferation define transitions from tissue growth to physiological homeostasis. In tendons, a highly organized extracellular matrix undergoes significant postnatal expansion to drive growth, but once formed, it appears to undergo little turnover. However, tendon cell activity during growth and homeostatic maintenance is less well defined. Using complementary methods of genetic H2B-GFP pulse-chase labeling and BrdU incorporation in mice, we show significant postnatal tendon cell proliferation, correlating with longitudinal Achilles tendon growth. Around day 21, there is a transition in cell turnover with a significant decline in proliferation. After this time, we find low amounts of homeostatic tendon cell proliferation from 3 to 20 months. These results demonstrate that tendons harbor significant postnatal mitotic activity, and limited, but detectable activity in adult and aged stages. It also points towards the possibility that the adult tendon harbors resident tendon progenitor populations, which would have important therapeutic implications.

Highlights

Development, growth, and homeostasis rely on the precise regulation of cell proliferation and differentiation to generate and maintain a functioning organism
To characterize cell proliferation in the tendon, we used the doxycycline (Dox) inducible Histone 2Bgreen fluorescent protein reporter mouse model (Col1a1-tetO-H2B-GFP; ROSA:rtTA (H2B-GFP); ROSA-rtTA, referred to as H2B-GFP), which has been used to quantify cell proliferation and identify slowly cycling label-retaining cell populations based on the stability and dilution of H2B-GFP protein in each cell (Chakkalakal et al, 2014; Foudi et al, 2009)
After H2B-GFP expression is induced by Dox addition, Dox is removed for the chase period and H2B-GFP protein becomes diluted in proportion with each subsequent cell division (Figure 1A)

Summary

Introduction

Development, growth, and homeostasis rely on the precise regulation of cell proliferation and differentiation to generate and maintain a functioning organism. Scanning electron microscopy at postnatal stages (P0 and P42) has shown an increase in the diameter of the collagen fibrils rather than an increase in collagen fibril or cell number in the tail tendons of mice (Kalson et al, 2015) These observations have led to a model whereby tendon postnatal growth is primarily driven by expansion of the extracellular matrix (ECM), which results in a reduction in cell density across the whole tissue in growth and aging (Dunkman et al, 2013; Kalson et al, 2015). Using complementary methods of genetic pulse-chase labeling to trace the cell division history and BrdU/EdU incorporation to detect proliferation, we were able to identify changes in tendon cell turnover from birth to the early juvenile period (beginning around 3–4 weeks), with comparisons to adult and aged stages (!3 months and !18 months, respectively). Understanding which cell populations can continue to divide in adults and the mechanisms driving the switch from proliferative to more quiescent stages would greatly benefit clinical approaches to tendon injuries

Results

Discussion

Materials and methods