Stimulation of skeletal stem cells in the growth plate promotes linear bone growth

Recently, skeletal stem cells were shown to be present in the epiphyseal growth plate (epiphyseal skeletal stem cells, epSSCs), but their function in connection with linear bone growth remains unknown. Here, we explore the possibility that modulating the number of epSSCs can correct differences in leg length. First, we examined regulation of the number and activity of epSSCs by Hedgehog (Hh) signaling. Both systemic activation of Hh pathway with Smoothened agonist (SAG) and genetic activation of Hh pathway by Patched1 (Ptch1) ablation in Pthrp-creER Ptch1fl/fl tdTomato mice promoted proliferation of epSSCs and clonal enlargement. Transient intra-articular administration of SAG also elevated the number of epSSCs. When SAG-containing beads were implanted into the femoral secondary ossification center of 1 leg of rats, this leg was significantly longer 1 month later than the contralateral leg implanted with vehicle-containing beads, an effect that was even more pronounced 2 and 6 months after implantation. We conclude that Hh signaling activates growth plate epSSCs, which effectively leads to increased longitudinal growth of bones. This opens therapeutic possibilities for the treatment of differences in leg length.

The Origin and Fate of Chondrocytes: Cell Plasticity in Physiological Setting

PURPOSE OF REVIEW

Here, we discuss the origin of chondrocytes, their destiny, and their plasticity in relationship to bone growth, articulation, and formation of the trabeculae. We also consider these processes from a biological, clinical, and evolutionary perspective.

RECENT FINDINGS

Chondrocytes, which provide the template for the formation of most bones, are responsible for skeletal growth and articulation during postnatal life. In recent years our understanding of the fate of these cells has changed dramatically. Current evidence indicates a paradoxical situation during skeletogenesis, with some cells of mesenchymal condensation differentiating directly into osteoblasts, whereas others of the same kind give rise to highly similar osteoblasts via a complex process of differentiation involving several chondrocyte intermediates. The situation becomes even more paradoxical during postnatal growth when stem cells in the growth plate produce differentiated, functional progenies, which thereafter presumably dedifferentiate into another type of stem cell. Such a remarkable transition from one cell type to another under postnatal physiological conditions provides a fascinating example of cellular plasticity that may have valuable clinical implications.

MMP14 cleaves PTH1R in the chondrocyte-derived osteoblast lineage, curbing signaling intensity for proper bone anabolism

Bone homeostasis is regulated by hormones such as parathyroid hormone (PTH). While PTH can stimulate osteo-progenitor expansion and bone synthesis, how the PTH-signaling intensity in progenitors is controlled is unclear. Endochondral bone osteoblasts arise from perichondrium-derived osteoprogenitors and hypertrophic chondrocytes (HC). We found, via single-cell transcriptomics, that HC-descendent cells activate membrane-type 1 metalloproteinase 14 (MMP14) and the PTH pathway as they transition to osteoblasts in neonatal and adult mice. Unlike Mmp14 global knockouts, postnatal day 10 (p10) HC lineage-specific Mmp14 null mutants (Mmp14ΔHC) produce more bone. Mechanistically, MMP14 cleaves the extracellular domain of PTH1R, dampening PTH signaling, and consistent with the implied regulatory role, in Mmp14ΔHC mutants, PTH signaling is enhanced. We found that HC-derived osteoblasts contribute ~50% of osteogenesis promoted by treatment with PTH 1-34, and this response was amplified in Mmp14ΔHC. MMP14 control of PTH signaling likely applies also to both HC- and non-HC-derived osteoblasts because their transcriptomes are highly similar. Our study identifies a novel paradigm of MMP14 activity-mediated modulation of PTH signaling in the osteoblast lineage, contributing new insights into bone metabolism with therapeutic significance for bone-wasting diseases.

Scabietic nodules: a dermatopathologic and immunofluorescent study

Mites and their eggs were thought to be rarely found in nodular lesions of scabies. However, serial sections from 27 scabietic scrotal papules or nodules revealed mite parts in 22% (6/27). This supports the contention that scabietic nodules may result from persisting antigens of mite parts. In contrast to previous studies, vasculitis with fibrinoid degeneration was uncommon (3/27) in our series. This discrepancy may be due to the timing of the biopsies: 2 of 3 cases with vasculitis were associated with diffuse dense infiltration, and vasculitis may be a late event in the development of scabietic nodules. The specificity of immunoreactant deposits along the epidermodermal junction (EDJ) in scabies is controversial. In our study, direct immunofluorescence (DIF) was performed on 13 scabietic nodules. Four (31%) showed immunoreactants at the EDJ and two on blood vessels. Because of the relatively low positive rate, the low intensity of fluorescence, and 3 of 4 cases with positive immunoreactants having only a single class of immunoglobulin, the deposition may only be secondary to inflammation instead of a specific type II immunologic reaction to scabies.

Malignant melanoma: a clinicopathologic study of 22 cases

To provide updated clinicopathologic information on malignant melanoma, we studied 22 cases of malignant melanoma registered at Tri-Service General Hospital, Taiwan from 1983 to 1988. About 60% (13/22) of the patients had a tumor thickness of more than 2.5 mm and 45% (10/22) of the patients of over 4 mm. No patient had a thin melanoma (tumor thickness less than 0.76mm) when first diagnosed. Forty-four percent of the patients, which was twice as many as those (22%) in a previous study done at the same hospital, were classified as stage III. Eleven patients (50%) had acral lentiginous melanoma, the most common type of malignant melanoma in Orientals as previously reported. The cumulative survival rate was 59% and 39% at 3 years and 5 years respectively. The age primary sites, clinical stage and tumor thickness were retrospectively evaluated. Due to the small sample size, the accuracy of the statistical analysis of survival is questionable. An extensive island-wide multicenter epidemiologic study is mandatory to clarify this issue.