Maternal omega-3 polyunsaturated fatty acid supplementation on offspring hip joint conformation.

Unsaturated omega-3 fatty acids, especially docosahexaenoic acid (DHA), when fed to dogs improves cognitive and neurological development. Supplementation with omega-3 fatty acids such as DHA and eicosapentaenoic acid (EPA) has also been associated with lipid peroxidation, which in turn has been implicated in reduced body weight and altered bone formation. To assess the impact of omega-3 fatty acid supplementation on skeletal growth, diets containing three levels of DHA and EPA (0.01 and 0.01%, 0.14 and 0.12%, and 0.21 and 0.18%, respectively) were fed to bitches during gestation and lactation with puppies also supplemented through weaning. Thus, the subjects studied were the puppies supplemented with DHA and EPA through gestation and early postnatal life. The hip joint conformation of the puppies (n = 676) was recorded at adulthood using two radiographic, non-invasive evaluations. In this population, females had higher hip distraction indices (DI) than males. Males from the lower two levels of DHA and EPA supplementation had significantly smaller hip DI than all females and males from the highest DHA and EPA supplementation. In contrast, there were no diet effects on anatomical indicators of hip joint conformation and no visible arthritic changes. These data suggest that dietary supplementation of DHA and EPA during gestation and the perinatal period to weaning does not adversely influence hip joint formation of dogs.

Long-term genetic selection reduced prevalence of hip and elbow dysplasia in 60 dog breeds.

Canine hip dysplasia (CHD) and elbow dysplasia (ED) impact the health and welfare of all dogs. The first formally organized assessment scheme to improve canine health centered on reducing the prevalence of these orthopedic disorders. Phenotypic screening of joint conformation remains the currently available strategy for breeders to make selection decisions. The present study evaluated the efficacy of employing phenotypic selection on breed improvement of hips and elbows using the Orthopedic Foundation for Animals complete database spanning the 1970-2015 time period. Sixty breeds having more than 1000 unique hip evaluations and 500 elbow evaluations (1,056,852 and 275,129 hip and elbow records, respectively) were interrogated to derive phenotypic improvement, sex and age at time of assessment effects, correlation between the two joints, heritability estimates, estimated breeding values (EBV), and effectiveness of maternal/paternal selection. The data demonstrated that there has been overall improvement in hip and elbow conformation with a reduction in EBV for disease liability, although the breeds differed in the magnitude of the response to selection. Heritabilities also differed substantially across the breeds as did the correlation of the joints; in the absence of a universal association of these differences with breed size, popularity, or participation in screening, it appears that the breeds themselves vary in genetic control. There was subtle, though again breed specific, impact of sex and older ages on CHD and ED. There was greater paternal impact on a reduction of CHD. In the absence of direct genetic tests for either of these two diseases, phenotypic selection has proven to be effective. Furthermore, the data underscore that selection schemes must be breed specific and that it is likely the genetic profiles will be unique across the breeds for these two conditions. Despite the advances achieved with phenotypic selection, incorporation of EBVs into selection schemes should accelerate advances in hip and elbow improvement.

Ten inherited disorders in purebred dogs by functional breed groupings.

BACKGROUND: Analysis of 88,635 dogs seen at the University of California, Davis Veterinary Medical Teaching Hospital from 1995 to 2010 identified ten inherited conditions having greater prevalence within the purebred dog population as compared to the mixed-breed dog population: aortic stenosis, atopy/allergic dermatitis, gastric dilatation volvulus (GDV), early onset cataracts, dilated cardiomyopathy, elbow dysplasia, epilepsy, hypothyroidism, intervertebral disk disease (IVDD), and hepatic portosystemic shunt. The objective of the present study was to ascertain if disorders with higher prevalence in purebreds were restricted to particular breed group classifications within the purebred population, specifically the American Kennel Club breed grouping or groups with genomic similarities based upon allele sharing. For each disorder, healthy controls seen at the hospital during that same time period were matched for age, weight, and sex to each affected dog to determine risk of disease presentation in the purebred group as compared to that of the mixed-breed population. To enhance reliability of the analyses, sampling of matched healthy to affected dogs was repeated 50 times. For each comparison, the purebred subgroups to mixed-breed odds ratio was determined as was the mean P value used to test this ratio. RESULTS: For aortic stenosis, GDV, early onset cataracts, dilated cardiomyopathy, elbow dysplasia, epilepsy, and portosystemic shunt, most purebred groups were not statistically distinct from the mixed-breed population with higher prevalence in purebreds restricted to distinct subsets of purebred dogs. The conditions of atopy/allergic dermatitis, hypothyroidism, and IVDD were more pervasive across the purebred population with many groups having higher prevalence than the mixed-breed population. The prevalence of IVDD in purebred terrier groups was statistically lower than that observed for mixed-breed dogs. CONCLUSIONS: The results offer an assessment of the distribution of inherited disorders within purebred dogs and illustrate how mixed-breed and subpopulations of purebred dogs do not differ statistically in prevalence for certain disorders. Some disorders appear linked to common ancestors providing insight into disease allele origin whereas others may be due to selection for common structural morphology. Knowledge of the origin of a condition may aid in reducing its prevalence in the dog population as a whole.

Bovine and murine tissue expression of insulin like growth factor-I.

The genomic architecture and expression of the Igf-1 gene are complex yielding multiple IGF-I transcript isoforms with putative functional contributions to growth and metabolism. Using RNA-seq on different tissues, physiological states, and species, the breadth of transcripts expressed was determined. Tissues from pre- and post-pubertal heifers and mature mice were collected and the transcript isoforms characterized. Three different IGF-I isoforms were detected in heifers with Class 1 transcripts most abundantly expressed. The pituitary reduced IGF-I expression post-pubertally whereas the uterus increased expression. Murine IGF-I transcript expression was more diverse utilizing multiple exons, start sites, and 3'UTRs. The RNA-seq methodology to characterize expression profiles permits assessment of the transcript isoforms yielding insight into functional roles of each transcript.

The Regulation of IGF-1 Gene Transcription and Splicing during Development and Aging.

It is commonly known that the insulin-like growth factor-I gene contains six exons that can be differentially spliced to create multiple transcript variants. Further, there are two mutually exclusive leader exons each having multiple promoter sites that are variably used. The mature IGF-I protein derived from the multiplicity of transcripts does not differ suggesting a regulatory role for the various transcript isoforms. The variant forms possess different stabilities, binding partners, and activity indicating a pivotal role for the isoforms. Research has demonstrated differential expression of the IGF-I mRNA transcripts in response to steroids, growth hormone, and developmental cues. Many studies of different tissues have focused on assessing the presence, or putative action, of the transcript isoforms with little consideration of the transcriptional mechanisms that generate the variants or the translational use of the transcript isoforms. Control points for the latter include epigenetic regulation of splicing and promoter usage in response to development or injury, RNA binding proteins and microRNA effects on transcript stability, and preferential use of two leader exons by GH and other hormones. This review will detail the current knowledge of the mechanical, hormonal, and developmental stimuli regulating IGF-1 promoter usage and splicing machinery used to create the variants.

Determining the heritable component of dairy cattle foot lesions.

Lameness and hoof health affect dairy cows as an animal welfare issue, in decreased milk production, and in premature culling. Selection schemes for dairy cattle focus on sire contribution to milk production, with little consideration of the cow's physical structure or disease probability. On 3 commercial California dairies, 6 phenotypic binary hoof traits that contribute to lameness were recorded: white line disease, sole ulcer, other claw horn lesions, foot rot (interdigital phlegmon), foot warts (digital dermatitis), and other lesions. Monthly lactation records were collected from December 2006 to April 2009 with weekly observations of hoof lesions for lame and dry cows. In addition to hoof lesion information, data on cows (n=5,043) included parentage, birth date, freshening date, lactation number, and date of lameness diagnosis. The prevalence of hoof lesions ranged from a low of 2.2% (foot rot) to a high of 17.1% (foot warts). The farm environment increased the odds ratio depending upon the lesion. Lameness was more common in early lactation and as lactation number increased. Using a threshold model, heritabilities and repeatabilities were estimated for each binary trait. The heritability for risk varied by lesion, with the higher estimates being 0.40 (95% confidence interval: 0.20-0.67) for digital dermatitis and 0.30 (95% confidence interval: 0.08-0.63) for sole ulcer. Including terms to account for cow productivity on either a 305-d mature-equivalent basis or a per-lactation basis had minimal effect on the heritability estimates, suggesting that selection for hoof health is not correlated with response to selection for greater milk production and that improvement could be made for both traits. The genetic component lends support for further genetic studies to identify loci contributing to some of the lesion phenotypes such as foot warts or sole ulcers, 2 of the top 3 causes of lameness in dairy cattle.

Growth hormone enhances arachidonic acid metabolites in a growth hormone transgenic mouse.

In a transgenic growth hormone (GH) mouse model, highly elevated GH increases overall growth and decreases adipose depots while low or moderate circulating GH enhances adipose deposition with differential effects on body growth. Using this model, the effects of low, moderate, and high chronic GH on fatty acid composition were determined for adipose and hepatic tissue and the metabolites of 20:4n-6 (arachidonic acid) were characterized to identify metabolic targets of action of elevated GH. The products of Δ-9 desaturase in hepatic, but not adipose, tissue were reduced in response to elevated GH. Proportional to the level of circulating GH, the products of Δ-5 and Δ-6 were increased in both adipose and hepatic tissue for the omega-6 lipids (e.g., 20:4n-6), while only the hepatic tissues showed an increase for omega-3 lipids (e.g., 22:6n-3). The eicosanoids, PGE2 and 12-HETE, were elevated with high GH but circulating thromboxane was not. Hepatic PTGS1 and 2 (COX1 and COX 2), SOD1, and FADS2 (Δ-6 desaturase) mRNAs were increased with elevated GH while FAS mRNA was reduced; SCD1 (stearoyl-coenzyme A desaturase) and SCD2 mRNA did not significantly differ. The present study showed that GH influences the net flux through various aspects of lipid metabolism and especially the desaturase metabolic processes. The combination of altered metabolism and tissue specificity suggest that the regulation of membrane composition and its effects on signaling pathways, including the production and actions of eicosanoids, can be mediated by the GH regulatory axis.

Certification testing as an acute naturalistic stressor for disaster dog handlers.

USA Federal Disaster Canine Teams, consisting of a handler and a dog, are essential for locating survivors following a disaster. Certification, required by the Federal Emergency Management Agency Urban Search and Rescue organization, requires two successful mock searches. Confirmation of the certification testing process as an acute stressor might offer further opportunities to consider stress effects on handlers and dogs in a controlled environment. This study used a pretest-posttest design to evaluate relationships between salivary hormone concentrations (cortisol and testosterone) and subjective stress ratings in handlers and controls, handler assessments of stress in their dogs, and posttest temperature and pulse rate in dogs. Posttest, both subjective stress ratings and salivary cortisol concentration were higher in handlers than controls with both correlated to handlers' assessment of stress in their dogs. Handlers' posttest salivary cortisol concentration was associated with posttest dog pulse and temperature. Posttest cortisol concentration was lower in handlers who were successfully certified compared with those who failed, and was also lower in handlers whose primary occupation was "firefighter". Salivary testosterone concentrations increased from pretest to posttest in handlers but decreased in controls, and higher posttest handler testosterone concentration was negatively associated with posttest dog pulse rate. These findings confirm certification testing as an acute stressor, suggest a relationship between stress and performance moderated by occupation, and demonstrate an interaction between handler stress and dog physiological responses. This certification testing offers a controlled environment for targeted evaluation of effects of an acute naturalistic stressor on disaster dog handlers and dogs.

Biological markers of neonatal calf performance: the relationship of insulin-like growth factor-I, zinc, and copper to poor neonatal growth.

Raising a heifer calf to reproductive age represents an enormous cost to the producer. Poor neonatal growth exacerbates the costs incurred for rearing, and use of blood variables that may be associated with poorly growing calves may offer predictive value for growth and performance. Thus, the principal objective of the present study was to describe changes in serum IGF-I, zinc, and copper from birth to 90 d in Holstein calves, while accounting for sex and twin status, in poorly growing calves and calves growing well. A second objective was to test the hypothesis that an association exists between these serum variables and morphometric indicators of growth. Measurements of BW, length, and height were recorded at birth and at 30, 60, and 90 d of age. Jugular blood (12 mL) was collected from each calf on d 1 to determine serum total protein, serum IgG, packed cell volume, serum zinc, serum copper, serum IGF-I, and CD18 genotype for bovine leukocyte adhesion deficiency; serum zinc, serum copper, and serum IGF-I (predictor variables) were also determined for each calf on d 2 through 10 and on d 30, 60, and 90. Stepwise multiple regression and logistic regression analyses were used to examine the relationships between the predictor variables and the dependent variables (BW, height, and length at d 30, 60, and 90 of life). Birth weight, sex, serum IGF-I (at all ages), serum copper, and the serum copper-to-zinc ratio were associated, to varying degrees, with the dependent growth variables. Birth weight was consistently the dominant predictor. In conclusion, these results suggest that lighter birth weight, reduced serum IGF-I, and inflammation may be important causes of poor growth in neonatal Holstein dairy calves.

Association of a dog leukocyte antigen class II haplotype with hypoadrenocorticism in Nova Scotia Duck Tolling Retrievers.

Canine hypoadrenocorticism (Addison's disease) is due to a deficiency of corticosteroids and mineralocorticoids produced by the adrenals. Although this is a relatively uncommon disease in the general dog population, some breeds, including the Nova Scotia Duck Tolling Retriever (NSDTR), are at increased risk for developing hypoadrenocorticism. A prior study has shown that the increased risk is due to a heritable component. This potentially lethal disorder is hypothesized to have an autoimmune etiology, thus the aim of this study was to determine whether genetic susceptibility to hypoadrenocorticism in NSDTRs is associated with genes of the canine major histocompatibility complex [MHC; dog leukocyte antigen system (DLA)]. Samples were collected from NSDTRs diagnosed with hypoadrenocorticism and healthy siblings or country-matched controls. The DLA class II alleles and haplotypes were determined and compared between cases and controls. We found seven different haplotypes of which the haplotype DLA-DRB1*01502/DQA*00601/DQB1*02301 was significantly more prevalent in the diseased dogs (P = 0.044). In addition, these affected dogs also were more likely to be homozygous across the DLA class II region than the control dogs (OR = 6.7, CI = 1.5-29.3, P = 0.011). We also found that homozygous dogs, regardless of their haplotype, tended to have earlier disease onset compared with heterozygous dogs. These data indicate a limited MHC diversity in North American NSDTRs and suggest that the MHC may play a role in the development of hypoadrenocorticism in the NSDTR, supporting the autoimmune origin of the disease.

Alendronate inhibits VEGF expression in growth plate chondrocytes by acting on the mevalonate pathway.

Bisphosphonates decrease chondrocyte turnover at the growth plate and impact bone growth. Likewise vascular endothelial growth factor (VEGF) plays an important role in endochondral bone elongation by influencing chondrocyte turnover at the growth plate. To investigate whether the action of bisphosphonate on the growth plate works through VEGF, VEGF protein expression and isoform transcription in endochondral chondrocytes isolated from growing mice and treated with a clinically used bisphosphonate, alendronate, were assessed. Alendronate at 10microM and 100microM concentrations decreased secreted VEGF protein expression but not cell associated protein. Bisphosphonates are known to inhibit the mevalonate intracellular signaling pathway used by VEGF. Addition of the mevalonate pathway intermediates farnesol (FOH) and geranylgeraniol (GGOH) interacted with the low concentration of alendronate to further decrease secreted VEGF protein whereas FOH partially restored VEGF protein secretion when combined with the high alendronate. Similar to the protein data, the addition of alendronate decreased VEGF mRNA isoforms. VEGF mRNA levels were rescued by the GGOH mevalonate pathway intermediate at the low alendronate dose whereas neither intermediate consistently restored the VEGF mRNA levels at the high alendronate dose. Thus, the bisphophonate alendronate impairs growth plate chondrocyte turnover by down-regulating the secreted forms of VEGF mRNA and protein by inhibiting the mevalonate pathway.

Pamidronate alters the growth plate in the oim mouse model for osteogenesis imperfecta.

Bisphosphonates alleviate bone pain and fractures associated with osteogenesis imperfecta (OI). Using the oim mouse model to simulate variations in OI severity, the effect of pamidronate on bone growth was assessed. Homozygous (oim/oim) and heterozygous (oim/wt) mice from 4 to 12 weeks of age were given pamidronate at 0 mg/kg/wk (control), 1.25 mg/kg/wk (low) and 2.5 mg/kg/wk (high). Humerus and ulna lengths were reduced in oim/oim mice relative to those of the oim/wt. Further, the oim/oim genotype exhibited a 23.5% prevalence of fractures in these bones as compared to the 2.8% prevalence observed in the oim/wt mice. Pamidronate tended to reduce fracture prevalence in a dose dependent manner for the oim/oim genotype (p<0.08) but had no effect on the low fracture prevalence in oim/wtmice. The high dose of pamidronate reduced bone length in females of both genotypes but not males when compared to control (p<0.01). Pamidronate increased growth plate area (p<0.05) by increasing growth plate height, particularly the proliferative and hypertrophic zones, in both genotypes indicating reduced growth plate cell turnover. The increased area coincided with increased osteoclast numbers in the metaphyseal region (p<0.05) though when corrected for the greater mineralized surface area that accompanies bisphosphonate treatment, osteoclasts per surface area were reduced indicating reduced resorptive capacity. This study demonstrated that the effects of pamidronate were independent of the degree of collagen deficit and fracture prevalence was improved in the most severe OI model, the oim/oim genotype.

Bisphosphonate treatment in the oim mouse model alters bone modeling during growth.

Osteogenesis imperfecta (OI) is a heritable disease, which results from an abnormal amount or structure of Type I collagen. Bisphosphonates, a class of synthetic antiresorptive drugs, used in osteoporosis management, are also used to decrease fracture incidence and improve quality of life in children with OI. In this study, we used the oim mouse to test the hypotheses that pamidronate treatment during active growth (1) produces larger, stronger, stiffer long bone diaphyses without altering bone material properties, and (2) negatively impacts longitudinal bone growth. Our results indicate that femoral cross-sectional moment of inertia in the distal metaphysis tended to increase with pamidronate treatment and that the treated bones are thicker and structurally stiffer, but shorter than their control-dose counterparts.

Long Term Cyclic Pamidronate Reduces Bone Growth by Inhibiting Osteoclast Mediated Cartilage-to-Bone Turnover in the Mouse.

Bisphosphonates, used to treat diseases exhibiting increased osteoclast activity, reduce longitudinal bone growth through an as yet undefined mechanism. Pamidronate, an aminobisphosphonate, was given weekly to mice at 0, 1.25, or 2.50 mg/kg/wk beginning at 4 weeks of age. At 12 weeks of age, humeral length, growth plate area, regional chondrocyte cell numbers, chondrocyte apoptosis, TRAP stained osteoclast number, and osteoclast function assessed by cathepsin K immunohistochemistry were quantified. Humeral length was decreased in pamidronate treated mice compared to vehicle control mice, and correlated with greater growth plate areas reflecting greater proliferative and hypertrophic chondrocyte cell numbers with fewer hypertrophic cells undergoing apoptosis. Pamidronate treatment increased TRAP stained osteoclast numbers yet decreased cathepsin K indicating that pamidronate repressed osteoclast maturation and function. The data suggest that long term cyclic pamidronate treatment impairs bone growth by inhibition of osteoclast maturation thereby reducing cartilage-to-bone turnover within the growth plate.

Enhanced skeletal growth of sheep heterozygous for an inactivated fibroblast growth factor receptor 3.

Normal fibroblast growth factor receptor 3 (FGFR3) acts as a negative bone growth regulator by restricting chondrocyte proliferation and endochondral bone elongation. In sheep, a heritable mutation that inactivates FGFR3 produces skeletal overgrowth when homozygous, this condition is commonly referred to as spider lamb syndrome (SLS). We hypothesized that sheep heterozygous for the inactivated FGFR3 mutation (FGFR3(SLS/+)) would exhibit enhanced long bone growth and greater frame size; additionally, the isolated effects of increased bone growth would translate into greater BW and larger LM area relative to normal lambs at harvest. The current study investigated bone length and LM area of FGFR3(SLS/+) sheep at maturity and during growth. At maturity, FGFR3(SLS/+) ewes exhibited a larger frame size and longer bones than normal FGFR3(+/+) ewes (P < 0.05). Similarly, FGFR3(SLS/+) lambs had greater frame sizes than normal FGFR3(+/+) lambs, as indicated by increased metacarpal III length and height at withers (P < 0.05). The FGFR3(SLS/+) lambs took longer than the normal FGFR3(+/+) lambs to reach the 60-kg common BW harvest end point (P < 0.05). The FGFR3(SLS/+) lambs showed no difference in BW, ADG, or LM area at any age compared with normal FGFR3(+/+) lambs (P > 0.2). A similar LM area produced in the context of a greater frame size and skeletal length produces a greater muscle volume, thereby potentially increasing meat yield. The results of this study suggest that FGFR3(SLS/+) animals exhibit a relaxation of the normal inhibition of chondrocyte proliferation, resulting in an increase in the overall frame size. The sheep industry could utilize the naturally occurring genetic mutation in FGFR3 to potentially increase meat yields with enhanced skeletal growth as an alternative to exogenous growth promotants.

Genetic evaluation of Addison's disease in the Portuguese Water Dog.

BACKGROUND: Addison's disease, also known as hypoadrenocorticism, has been reported in many individual dogs, although some breeds exhibit a greater incidence than the population as a whole. Addison's is presumed to be an autoimmune mediated hereditary defect but the mode of inheritance remains unclear. In particular, the heritability and mode of inheritance have not been defined for the Portuguese Water Dog although Addison's is known to be prevalent in the breed. RESULTS: The analyses present clear evidence that establishes Addison's disease as an inherited disorder in the Portuguese Water Dog with an estimate of heritability of 0.49 (+/- 0.16); there were no differences in risk for disease across sexes (p > 0.49). Further, the complex segregation analysis provides suggestive evidence that Addison's disease in the Portuguese Water Dog is inherited under the control of a single, autosomal recessive locus. CONCLUSION: The high heritability and mode of inheritance of Addison's disease in the Portuguese Water Dog should enable the detection of segregating markers in a genome-wide scan and the identification of a locus linked to Addison's. Though the confirmation of Addison's disease as an autosomal recessive disorder must wait until the gene is identified, breeders of these dogs may wish to keep the present findings in mind as they plan their breeding programs to select against producing affected dogs.

Transgene transmission to progeny by oMt1a-oGH transgenic mice.

Most studies utilizing transgenic technology focus on the impact to traits of interest, rather than propagation of the transgene to offspring. In animals containing growth hormone constructs, transgene transmission to progeny follows a Mendelian pattern of inheritance in the first few generations following generation of a founder animal, but decreases in subsequent generations. In the present study, the ovine metallothionein 1a-ovine growth hormone (oMt1a-oGH) transgenic mouse was used to determine whether transgene transmission rate to progeny was affected by overexpression of ovine growth hormone in the transgenic parent. The oMt1a-oGH mouse is a useful model for assessing transgene transmission, as the construct is easily regulatable and transgene inactivation results in a return of plasma GH to basal levels. Male and female hemizygous oMt1a-oGH mice were assigned to 1 of 3 treatment groups: (1) mice never actively expressing the transgene, (2) mice actively expressing the transgene from 3 weeks of age, and (3) mice actively expressing the transgene from 3 to 11 (males) or 3 to 8 (females) weeks of age. Transgenic mice were mated to wild type animals and the resulting progeny were genotyped. Males never actively expressing the transgene passed on the transgene to progeny in a Mendelian fashion, while males actively expressing the transgene transmitted the transgene to a smaller than expected number of progeny. However, following inactivation of the oMt1a-oGH construct in transgenic males, subsequent offspring demonstrated Mendelian inheritance of the transgene. In contrast, females expressing the transgene from 3 to 8 weeks of age were able to pass on the oMt1a-oGH construct in a Mendelian fashion, but females from other treatment groups were not. In oMt1a-oGH males, reduced transgene transmission appears to be due to selection against transgenic gametes. In females, however, selection against the transgenic genotype likely occurs at the embryonic level.

Spatial distribution of growth hormone receptor, insulin-like growth factor-I receptor and apoptotic chondrocytes during growth plate development.

Linear bone growth depends upon proliferation, maturation, and apoptosis of growth plate chondrocytes, processes regulated by growth hormone (GH) and insulin-like growth factor-I (IGF-I). To investigate the contribution of GH, IGF-I and apoptosis to growth plate function, the expression of GH receptor (GHR) and IGF-I receptor (IGF-IR) mRNA were evaluated by in situ hybridization in fractionated costochondral growth plates of growing rats (at 2, 4, and 7 weeks). Apoptosis was determined by TUNEL assay and morphology in histological sections. GHR mRNA was greatest in resting cells with hypertropic cells increasing GHR expression with increasing age. Hypertropic and resting cell IGF-IR mRNA declined over the ages studied. Receptor mRNA expression was altered by exposing cells to GH or IGF-I. GH and IGF significantly decreased GHR mRNA in proliferative cells. GH and IGF also decreased IGF-IR mRNA in resting cells and the 2- and 4-week-old proliferative and hypertropic cells. Treating cells in culture with GH increased the number of apoptotic cells across all ages and zones. Histologically, apoptotic cells were observed at the chondro-osseous junction and within actively proliferating chondrocytes but not in resting cells. Apoptosis was highest at 4 weeks of age with lateral regions displaying the greatest number of cells undergoing apoptosis. These data indicate that apoptosis plays a role in growth plate function, particularly spatial configuration as indicated by the preferential lateral cell apoptosis. The susceptibility of proliferative cells to GHR and IGF-IR down regulation during the period of greatest apoptosis supports a role for the GH-IGF axis in both proliferation and apoptosis during growth plate development.