Histological disorders related to the focal disappearance of the epiphyseal growth plate in rats induced by high dose of vitamin A

Effects of high-dose all-trans retinoic acid on longitudinal bone growth of young rats

OBJECTIVE

The signaling axis consisting of GH-IGF1-IGFBP3 is the primary signal taht acts prepubertally to influence height development. Growth plate thinning and even premature closure have been reported in children with tumors treated with retinoid chemotherapy, resulting in long bone dysplasia. Growth failure may occur despite received GH treatment, but the reason is unknown. This study investigate the effect of high-dose all-trans retinoic acid (ATRA) on the development of long bones in growing SD rats.

METHODS

A total of 20 three-week-old male SD rats were randomly divided into a control group and an experimental group (n = 10). Rats were treated by gavage with or without high-dose ATRA for 10 days. The body weights of the rats were recorded daily. At the end of the experiment, we measured the length of nose-tail and tibia, stained the tibia and liver for pathological tissue and RT-PCR reaction, and measured the levels of serum GH, IGF1 and IGFBP3, and so on.

RESULTS

Compared with controls, experimental rats exhibited reduced body weight and shortened nasal-tail and radial tibial length. Cyp26b1 enzyme activity in the liver was elevated, and histopathological staining revealed that the cartilaginous epiphyseal plate was narrowed, the medullary cavity of trabecular bone was sparse, the number of trabecular bones was decreased, trabecular separation was increased, bone marrow mineralization was enhanced, osteoclastic activity was increased, and circulating GH-IGF1-IGFBP3 levels were decreased. However, RT-PCR reaction results of localized proximal tibiae showed upregulation of IGF1 and downregulation of IGFBP3.

CONCLUSIONS

High-dose ATRA intake over a short period of time can reduce GH-IGF1-IGFBP3 levels, affect cartilage and bone homeostasis, and inhibit bone growth in developing animals.

Biochemical and histopathologic assessment of effects of acitretin on epiphyseal growth plate in rats

Introduction

Acitretin is a commonly used retinoid in dermatology. Although there are generally known side effects, the effects on the epiphyseal plaque and bone metabolism are not clear in the literature.

Aim

To histopathologically investigate the effects on the epiphyseal plate and assess variations in bone metabolism caused by acitretin.

Material and methods

Three groups were formed with 10 rats in each group. The 1^st group (n = 10, 5 male, 5 female) were administered 10 mg/kg/day oral acitretin solution and the 2^nd group (n = 10, 5 male, 5 female) were administered 3 mg/kg/day oral acitretin solution. The control group were given normal standard feed and water. Rats were sacrificed at the end of 4 weeks. The proximal tibias were excised and histopathologically and immunohistochemically assessed. Biochemical assessment was also carried out.

Results

Staining with haematoxylin-eosin found reductions in the epiphyseal plate in the 1^st and 2^nd group compared to the control group, though this situation was not statistically significant. Immunohistochemical studies did not encounter Type II collagen in the epiphyseal bone, proliferative zone and hypertrophic zone in the control group, low dose acitretin solution group and high dose acitretin solution group. Type II collagen was not observed in osteoids and osteoblasts. Type I collagen was not observed in the hypertrophic zone and proliferative zone of any group.

Conclusions

Our data show that though acitretin caused degeneration of the epiphyseal plate, it did not cause clear thinning and we identified no significant variations in bone metabolism markers.

Accumulation of type VI collagen in the primary osteon of the rat femur during postnatal development

In rodents, the long bone diaphysis is expanded by forming primary osteons at the periosteal surface of the cortical bone. This ossification process is thought to be regulated by the microenvironment in the periosteum. Type VI collagen (Col VI), a component of the extracellular matrix (ECM) in the periosteum, is involved in osteoblast differentiation at early stages. In several cell types, Col VI interacts with NG2 on the cytoplasmic membrane to promote cell proliferation, spreading and motility. However, the detailed functions of Col VI and NG2 in the ossification process in the periosteum are still under investigation. In this study, to clarify the relationship between localization of Col VI and formation of the primary osteon, we examined the distribution of Col VI and osteoblast lineages expressing NG2 in the periosteum of rat femoral diaphysis during postnatal growing periods by immunohistochemistry. Primary osteons enclosing the osteonal cavity were clearly identified in the cortical bone from 2 weeks old. The size of the osteonal cavities decreased from the outer to the inner region of the cortical bone. In addition, the osteonal cavities of newly formed primary osteons at the outermost region started to decrease in size after rats reached the age of 4 weeks. Immunohistochemistry revealed concentrated localization of Col VI in the ECM in the osteonal cavity. Col VI-immunoreactive areas were reduced and they disappeared as the osteonal cavities became smaller from the outer to the inner region. In the osteonal cavities of the outer cortical regions, Runx2-immunoreactive spindle-shaped cells and mature osteoblasts were detected in Col VI-immunoreactive areas. The numbers of Runx2-immunoreactive cells were significantly higher in the osteonal cavities than in the osteogenic layers from 2 to 4 weeks. Most of these Runx2-immunoreactive cells showed NG2-immunoreactivity. Furthermore, PCNA-immunoreactivity was detected in the Runx2-immunoreactive spindle cells in the osteonal cavities. These results indicate that Col VI provides a characteristic microenvironment in the osteonal cavity of the primary osteon, and that differentiation and proliferation of the osteoblast lineage occur in the Col VI-immunoreactive area. Interaction of Col VI and NG2 may be involved in the structural organization of the primary osteon by regulating osteoblast lineages.