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Patricia Forcinito, Anenisia C Andrade, Gabriela P Finkielstain, Jeffrey Baron, Ola Nilsson and Julian C Lui

The mammalian growth plate undergoes programed senescence during juvenile life, causing skeletal growth to slow with age. We previously found that hypothyroidism in rats slowed both growth plate chondrocyte proliferation and growth plate senescence, suggesting that senescence is not dependent on age per se but rather on chondrocyte proliferation. However, one alternative explanation is that the observed slowing of growth plate senescence is a specific consequence of hypothyroidism. We reasoned that, if delayed senescence is a general consequence of growth inhibition, rather than a specific result of hypothyroidism, then senescence would also be slowed by other growth-inhibiting conditions. In this study, we therefore used tryptophan deficiency to temporarily inhibit growth in newborn rats for 4 weeks. We then allowed the animals to recover and studied the effects on growth plate senescence. We found that structural, functional, and molecular markers of growth plate senescence were delayed by prior tryptophan deficiency, indicating that the developmental program of senescence had occurred more slowly during the period of growth inhibition. Taken together with previous studies in hypothyroid rats, our findings support the hypothesis that delayed senescence is a general consequence of growth inhibition and hence that growth plate senescence is not simply a function of time per se but rather depends on growth.

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JM Kindblom, O Nilsson, T Hurme, C Ohlsson and L Savendahl

Indian Hedgehog (Ihh) has been reported to control the rate of cartilage differentiation during skeletal morphogenesis in rodents through a negative feedback loop involving parathyroid hormone related protein (PTHrP). The role of Ihh and PTHrP in the regulation of human epiphyseal chondrocytes is unknown. The aim of the current study was to examine the expression and localization of Ihh and PTHrP in the human growth plate at various pubertal stages. Growth plate biopsies were obtained from patients subjected to epiphyseal surgery and the expression of Ihh and PTHrP was detected by immunohistochemistry. We show that Ihh and PTHrP are expressed mainly in early hypertrophic chondrocytes in the human growth plate. The levels of expression of Ihh and PTHrP are higher in early stages of puberty than later. Our results suggest that Ihh and PTHrP are present in the human growth plate and that Ihh and PTHrP may be involved in the regulation of pubertal growth in humans.

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Rosa Chung, Bruce K Foster and Cory J Xian

Growth plate injuries often result in undesirable bony repair causing bone growth defects, for which the underlying mechanisms are unclear. Whilst the key importance of pro-angiogenic vascular endothelial growth factor (VEGF) is well-known in bone development and fracture repair, its role during growth plate bony repair remains unexplored. Using a rat tibial growth plate injury repair model with anti-VEGF antibody, Bevacizumab, as a single i.p. injection (2.5 mg/kg) after injury, this study examined the roles of VEGF-driven angiogenesis during growth plate bony repair. Histology analyses observed isolectin-B4-positive endothelial cells and blood vessel-like structures within the injury site on days 6 and 14, with anti-VEGF treatment significantly decreasing blood-vessel-like structures within the injury site (P<0.05). Compared with untreated controls, anti-VEGF treatment resulted in an increase in undifferentiated mesenchymal repair tissue, but decreased bony tissue at the injury site at day 14 (P<0.01). Consistently, microcomputed tomography analysis of the injury site showed significantly decreased bony repair tissue after treatment (P<0.01). RT-PCR analyses revealed a significant decrease in osteocalcin (P<0.01) and a decreasing trend in Runx2 expression at the injury site following treatment. Furthermore, growth plate injury-induced reduced tibial lengthening was more pronounced in anti-VEGF-treated injured rats on day 60, consistent with the observation of a significantly increased height of the hypertrophic zone adjacent to the growth plate injury site (P<0.05). These results indicate that VEGF is important for angiogenesis and formation of bony repair tissue at the growth plate injury site as well as for endochondral bone lengthening function of the uninjured growth plate.

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E J Mackie, L Tatarczuch and M Mirams

Endochondral ossification is the process that results in both the replacement of the embryonic cartilaginous skeleton during organogenesis and the growth of long bones until adult height is achieved. Chondrocytes play a central role in this process, contributing to longitudinal growth through a combination of proliferation, extracellular matrix (ECM) secretion and hypertrophy. Terminally differentiated hypertrophic chondrocytes then die, allowing the invasion of a mixture of cells that collectively replace the cartilage tissue with bone tissue. The behaviour of growth plate chondrocytes is tightly regulated at all stages of endochondral ossification by a complex network of interactions between circulating hormones (including GH and thyroid hormone), locally produced growth factors (including Indian hedgehog, WNTs, bone morphogenetic proteins and fibroblast growth factors) and the components of the ECM secreted by the chondrocytes (including collagens, proteoglycans, thrombospondins and matrilins). In turn, chondrocytes secrete factors that regulate the behaviour of the invading bone cells, including vascular endothelial growth factor and receptor activator of NFκB ligand. This review discusses how the growth plate chondrocyte contributes to endochondral ossification, with some emphasis on recent advances.

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Richard Eastell

The rise in oestrogen levels at menarche in girls is associated with a large reduction in bone turnover markers. This reduction reflects the closure of the epiphyseal growth plates, the reduction in periosteal apposition and endosteal resorption within cortical bone, and in bone remodelling within cortical and cancellous bone. Oestrogen promotes these changes, in part, by promoting apoptosis of chondrocytes in the growth plate and osteoclasts within cortical and cancellous bone. The period of early puberty is associated with an increased risk of fracture, particularly of the distal forearm, and this may be related to the high rate of bone turnover. A late menarche is a consistent risk factor for fracture and low bone mineral density in the postmenopausal period; models that might explain this association are considered.

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JENNIFER M. DEHNEL and D. L. HAMBLEN

SUMMARY

Somatomedins are the intermediaries through which growth hormone acts on the epiphyseal growth plate to effect linear skeletal growth.

Rat epiphyseal chondrocytes were isolated and cultured in vitro in the presence of somatomedin. Two sources of somatomedin were used, foetal calf serum and rat liver perfusates. The chondrocytes proliferated and synthesized sulphated glycosaminoglycans when grown in the presence of somatomedin from either source, but were not metabolically active in chemically defined medium alone. Some differences in the growth patterns in response to serum or liver somatomedins are reported and discussed.

Chondrocyte metabolic activity in the presence of somatomedin in vitro showed a graded response to alterations in the atmospheric oxygen, being greatest at low oxygen pressure, and almost completely inhibited at 95% oxygen. A gradient of local oxygen tension has been reported to exist across the epiphyseal plate in vivo. The effects of somatomedin combined with changing oxygen levels may help to explain the divergence of cell proliferation and matrix synthesis seen in the various regions of the growth plate.

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O Nilsson, V Abad, D Chrysis, EM Ritzen, L Savendahl and J Baron

Estrogen regulates skeletal growth and promotes epiphyseal fusion. To explore the mechanisms underlying these effects we investigated the expression of estrogen receptor-alpha (ERalpha) and -beta (ERbeta) in rat and rabbit growth plates during postnatal development, using immunohistochemistry. Immunoreactivity for ERalpha and ERbeta was observed in resting zone and proliferative zone chondrocytes at all ages studied for both rat (7, 14, 28 and 70 days of age) and rabbit (1, 7, 28 and 120 days of age). In the rat distal humerus and the rabbit proximal tibia, expression of both receptors in the hypertrophic zone was minimal at early ages, increasing only at the last time point prior to epiphyseal fusion. Expression was rarely seen in the hypertrophic zone of the rat proximal tibia, a growth plate that does not fuse until late in life. Therefore, we conclude that ERalpha and ERbeta are both expressed in the mammalian growth plate. The temporal and anatomical pattern suggests that ER expression in the hypertrophic zone in particular may play a role in epiphyseal fusion.

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K. W. Kan, R. L. Cruess, B. I. Posner, H. J. Guyda and S. Solomon

ABSTRACT

In order to assess which hormones may exert direct effects on skeletal growth at the epiphysial growth plate, the specific binding of hormones to the epiphysial cartilage of growing dogs and rabbits was studied. Membrane fractions obtained by centrifugation of homogenates prepared from dog and rabbit growth plate cartilage at 600, 15 000 and 105 000 g showed significant specific binding of serum insulin-like activity and insulin. Binding of growth hormone and prolactin by the three membrane fractions was negligible. Saturable binding sites for triiodothyronine could be demonstrated in nuclei from the dog growth plate. Nuclear binding showed an apparent K d of 11 ±3·6 nmol/l and a maximum binding capacity of 4·1 ± 1·6 pmol/mg DNA, a level comparable to dog liver. Using a viable chondrocyte suspension prepared from dog epiphysial cartilage, specific steroid binding in the cells could be demonstrated for [3H]dexamethasone but not 17α-methyltrienolone, oestradiol-17β or 1α,25-di-hydroxycholecalciferol. Scatchard analysis of dexamethasone binding showed high affinity binding sites having a K d of 1·2 ± 0·35 nmol/l and a capacity of 1700 sites/cell, and a low affinity binding with a K d of 109 ± 57 nmol/l and a capacity of 24 000 sites/cell. Steroid competition for the specific binding showed the following sequence of affinity: dexamethasone > corticosterone > 11-deoxycortisol > testosterone > oestradiol-17β. Although all of the hormones examined except prolactin have well-established physiological effects on skeletal growth, our present results suggest that some of the hormonal effects observed in intact animals are secondary and do not involve receptor–hormone interaction in cartilage as such.

J. Endocr. (1984) 103, 125–131

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Thomas M Braxton, Dionne E A Sarpong, Janine L Dovey, Anne Guillou, Bronwen A J Evans, Juan M Castellano, Bethany E Keenan, Saja Baraghithy, Sam L Evans, Manuel Tena-Sempere, Patrice Mollard, Joseph Tam and Timothy Wells

Human Prader–Willi syndrome (PWS) is characterised by impairments of multiple systems including the growth hormone (GH) axis and skeletal growth. To address our lack of knowledge of the influence of PWS on skeletal integrity in mice, we have characterised the endocrine and skeletal phenotype of the PWS-ICdel mouse model for ‘full’ PWS and determined the impact of thermoneutrality. Tibial length, epiphyseal plate width and marrow adiposity were reduced by 6, 18 and 79% in male PWS-ICdel mice, with osteoclast density being unaffected. Similar reductions in femoral length accompanied a 32% reduction in mid-diaphyseal cortical diameter. Distal femoral Tb.N was reduced by 62%, with individual trabeculae being less plate-like and the lattice being more fragmented (Tb.Pf increased by 63%). Cortical strength (ultimate moment) was reduced by 26% as a result of reductions in calcified tissue strength and the geometric contribution. GH and prolactin contents in PWS-ICdel pituitaries were reduced in proportion to their smaller pituitary size, with circulating IGF-1 concentration reduced by 37–47%. Conversely, while pituitary luteinising hormone content was halved, circulating gonadotropin concentrations were unaffected. Although longitudinal growth, marrow adiposity and femoral geometry were unaffected by thermoneutrality, strengthened calcified tissue reversed the weakened cortex of PWS-ICdel femora. While underactivity of the GH axis may be due to loss of Snord116 expression and impaired limb bone geometry and strength due to loss of Magel2 expression, comprehensive analysis of skeletal integrity in the single gene deletion models is required. Our data imply that thermoneutrality may ameliorate the elevated fracture risk associated with PWS.

Free access

Rakefet Pando, Majdi Masarwi, Biana Shtaif, Anna Idelevich, Efrat Monsonego-Ornan, Ron Shahar, Moshe Phillip and Galia Gat-Yablonski

Growth stunting constitutes the most common effect of malnutrition. When the primary cause of malnutrition is resolved, catch-up (CU) growth usually occurs. In this study, we have explored the effect of food restriction (RES) and refeeding on bone structure and mechanical properties. Sprague–Dawley male rats aged 24 days were subjected to 10 days of 40% RES, followed by refeeding for 1 (CU) or 26 days long-term CU (LTCU). The rats fed ad libitum served as controls. The growth plates were measured, osteoclasts were identified using tartrate-resistant acid phosphatase staining, and micro-computed tomography (CT) scanning and mechanical testing were used to study structure and mechanical properties. Micro-CT analysis showed that RES led to a significant reduction in trabecular BV/TV and trabecular number (Tb.N), concomitant with an increase in trabecular separation (Tb.Sp). Trabecular BV/TV and Tb.N were significantly greater in the CU group than in the RES in both short- and long-term experiments. Mechanical testing showed that RES led to weaker and less compliant bones; interestingly, bones of the CU group were also more fragile after 1 day of CU. Longer term of refeeding enabled correction of the bone parameters; however, LTCU did not achieve full recovery. These results suggest that RES in young rats attenuated growth and reduced trabecular bone parameters. While nutrition-induced CU growth led to an immediate increase in epiphyseal growth plate height and active bone modeling, it was also associated with a transient reduction in bone quality. This should be taken into consideration when treating children undergoing CU growth.