Identification of a stem cell niche in the zone of Ranvier within the knee joint

Abstract

A superficial lesion of the articular cartilage does not spontaneously self-repair and has been suggested to be partly due to lack of progenitor cells within the joint that can reach the site of injury. To study whether progenitor cells are present within the joint, 3-month-old New Zealand white rabbits were exposed to bromodeoxyuridine (BrdU) for 12 consecutive days and were then sacrificed 4, 6, 10, 14, 28 and 56 days after the first BrdU administration. Presence of BrdU and localization of progenitor markers were detected using immunohistochemistry. After 10 days of BrdU exposure, BrdU-positive cells, i.e. proliferating cells, were abundantly detected in the epiphyseal plate, the perichondrial groove of Ranvier, and in all zones of the articular cartilage. After a wash-out period, BrdU-positive cells were still present, i.e. those considered to be progenitor cells, in these regions of the knee except for the proliferative zone of the epiphyseal plate. Cells in the perichondrial groove of Ranvier were further positive for several markers associated with progenitor cells and stem cell niches, including Stro-1, Jagged1, and BMPr1a. Our results demonstrate that a small population of progenitor cells is present in the perichondrial groove of Ranvier as well as within the articular cartilage in the knee. The perichondrial groove of Ranvier also demonstrates the properties of a stem cell niche.

Fig. 1

Schematic picture showing the experimental set-up of BrdU exposure and sacrifice of animals.

Fig. 2

Histological figures showing localization of the three areas of interest in the joint: (A) perichondrial groove of Ranvier, (B) articular cartilage, (C) growth plate. Detection of cells labelled with BrdU using immunohistochemistry. Samples after 10 days of consecutive exposure to BrdU (D–F) and after a wash-out period of 44 days (G–I). n = 3. Magnifications: (A,B) ×20, (C,F,H,I) ×100, (D,E,G) ×200.

Fig. 6

Positive (A,C,E,G,I,K,M) and negative (B,D,F,H,J,L,N) controls for the antibodies used to detect progenitor cells in the rabbit joint. As controls, either rabbit intestine (A,B,G–L) or rabbit dermal tissue (C–F, M and N) was used.

Fig. 5

Immunohistochemical localization of Stro-1 (A) and N-cadherin (B) in the epiphyseal plate in New Zealand white rabbits. n = 3. Magnifications: (A) ×100, (B) ×200.

Fig. 4

Immunohistochemical localization of Stro-1 (A), N-cadherin (B), BMPr1a (C) Integrin β1 (D), Notch1 (E) and Delta4 (F) in articular cartilage in New Zealand white rabbits. n = 3. Magnifications: (A,B,E) ×200, (C,D) ×100, (F) ×20.

Fig. 3

Immunohistochemical localization of Stro-1 (A), Patched (B), Jagged1 (C), BMPr1 a (D), N-cadherin (E) and Delta4 (F) in the perichondrial groove of Ranvier in New Zealand white rabbits. n = 3. Magnifications: (A,D,E) ×200, (B,C,F) ×100.

Fig. 7

Schematic overview of the localization of the BrdU-positive cells in the knee. (A) perichondrial groove of Ranvier, (B) growth plate, (C) articular cartilage. Label-retaining cells were localized in the perichondrial groove of Ranvier as well as the articular cartilage. Of special interest are the cells in the perichondrial groove of Ranvier, in which label-retaining cells were detected as well as cells positive for progenitor markers. Cells above the perichondrial groove of Ranvier further proved to be a distinct population of cells compared to the surrounding tissue, and are thus likely transient amplifying cells that later will mature into terminally differentiated cells. We also propose that it is these progenitor cells that have the ability to migrate (arrows).