Postnatal development of collagen structure in ovine articular cartilage

Abstract

Background: Articular cartilage (AC) is the layer of tissue that covers the articulating ends of the bones in diarthrodial joints. Across species, adult AC shows an arcade-like structure with collagen predominantly perpendicular to the subchondral bone near the bone, and collagen predominantly parallel to the articular surface near the articular surface. Recent studies into collagen fibre orientation in stillborn and juvenile animals showed that this structure is absent at birth. Since the collagen structure is an important factor for AC mechanics, the absence of the adult Benninghoff structure has implications for perinatal AC mechanobiology. The current objective is to quantify the dynamics of collagen network development in a model animal from birth to maturity. We further aim to show the presence or absence of zonal differentiation at birth, and to assess differences in collagen network development between different anatomical sites of a single joint surface. We use quantitative polarised light microscopy to investigate properties of the collagen network and we use the sheep (Ovis aries) as our model animal.

Results: Predominant collagen orientation is parallel to the articular surface throughout the tissue depth for perinatal cartilage. This remodels to the Benninghoff structure before the sheep reach sexual maturity. Remodelling of predominant collagen orientation starts at a depth just below the future transitional zone. Tissue retardance shows a minimum near the articular surface at all ages, which indicates the presence of zonal differentiation at all ages. The absolute position of this minimum does change between birth and maturity. Between different anatomical sites, we find differences in the dynamics of collagen remodelling, but no differences in adult collagen structure.

Conclusions: The collagen network in articular cartilage remodels between birth and sexual maturity from a network with predominant orientation parallel to the articular surface to a Benninghoff network. The retardance minimum near, but not at, the articular surface at all ages shows that a zonal differentiation is already present in the perinatal animals. In these animals, the zonal differentiation can not be correlated to the collagen network orientation. We find no difference in adult collagen structure in the nearly congruent metacarpophalangeal joint, but we do find differences in the dynamics of collagen network remodelling.

Figure 1

Example of qPLM results. Example of qPLM results with regions of interest at 0 weeks and 72 weeks of age. The width of each region of interest is 101 pixels = 161 μm. The articular surface is at the top of the figures. With (a) retardance, and (b) azimuth.

Figure 2

Cartilage thickness. (a) Mean cartilage thickness D (solid) ± standard deviation (dashed) for all samples at each age. (b) Mean cartilage thickness D as a function of age for hind legs (solid) and fore legs (dashed). The model predicts significant differences for: 'x' - fore leg > hind leg. (c) Mean cartilage thickness D as a function of age for lateral site (solid) and medial site (dashed). The model predicts significant differences for: 'x' - lateral site > medial site. (d) Mean cartilage thickness D as a function of age for the caudal site (solid), distal site (dashed) and rostral site (dotted). The model predicts significant differences for: 'x' - rostral site > caudal site, '+' - distal site > caudal site, '*' - distal site > rostral site, and ' ◇ - rostral site > distal site.

Figure 3

Predominant collagen orientation per age. (a) Mean predominant collagen orientation as a function of age and cartilage depth. Colours represent age in weeks. (b) Overview of the results per age in a contour plot.

Figure 4

Orientation index. (a) Mean orientation index (solid) ± standard deviation (dashed) as a function of age. (b) Mean orientation index as a function of age for the caudal site (solid), distal site (dashed) and rostral site (dotted). The model predicts significant differences for: 'x' - rostral site > caudal site and '+' - distal site > caudal site.

Figure 5

Retardance per age. (a) Mean sample retardance as a function of cartilage depth. Colours represent age in weeks. (b) overview of the results in a contour plot. The dashed white line marks the retardance minimum.

Figure 6

Retardance maximum and minimum. (a) Mean of the maximum retardance near the articular surface Δ_max (solid) ± standard deviation (dashed) as a function of age. (b) Mean of the maximum retardance near the articular surface Δ_max as a function of age for the caudal site (solid), distal site (dashed) and rostral site (dotted). The model predicts significant differences for: '*' - distal site > rostral site, '◇' - rostral site > distal site, and 'o' - caudal site > rostral site. (c) Mean position of the retardance minimum near the articular surface d|Δ_min (solid) ± standard deviation (dashed) as a function of age. (d) Mean position of the retardance minimum near the articular surface d|Δ_min as a function of age for the caudal site (solid), distal site (dashed) and rostral site (dotted). The model predicts significant differences for: 'x' - rostral site > caudal site, '+' - distal site > caudal site, '*' - distal site > rostral site, and '◇' - rostral site > distal site.

Figure 7

Sketch of sample sites. Left: sketch of the bones in the lower fore leg of a sheep. The double arrow shows the distal metacarpus that was used in this study. Right: sketch of anatomical sampling sites with l - lateral, m - medial, c - caudal, d - distal, and r - rostral.

Figure 8

Example for data analysis. For each sample a region of interest (ROI) is extracted from the images. The width W of this region is 101 pixels, and it runs from d = 1 pixel at the articular surface to d = D pixels at the cartilage/bone interface. The predominant orientation φ is expressed relative to the articular surface. For sample pattern calculations, we used the 101 pixels over the width of the ROI at each depth d = 1, 2, 3, ..., D.