SEARCH · PubMed Health

Results for “Bone Development”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,009 records · Page 56Linked to original sources

PubMed1999

The effects of anorexia nervosa on bone metabolism in female adolescents.

Osteopenia is a frequent, often persistent, complication of anorexia nervosa (AN) in adolescent girls and occurs during a critical time in bone development. Little is known about bone metabolism in this patient population. Therefore, we measured bone density (BMD) and body composition by dual energy x-ray absorptiometry, nutritional status, bone turnover, calcium, and hormonal status in 19 adolescent girls with AN (mean +/- SEM, 16.0+/-0.4 yr) and 19 bone age-matched controls. The mean duration of AN was 19+/-5 months. Spinal (L1-L4) osteopenia was common in AN. Lumbar anterioposterior BMD was more than 1 SD below the mean in 42% of patients, and lateral spine BMD was more than 1 SD below in 63% of patients compared with controls. Lean body mass significantly predicted lumbar bone mineral content (r = 0.75; P < 0.0001) in controls only. In AN, duration of illness was the most significant predictor of spinal BMD (lumbar: r = -0.44; P = 0.06; lateral: r = -0.59; P = 0.008). AN adolescents with mature BA (15 yr and greater) were hypogonadal [estradiol, 16.2+/-1.9 vs. 23.3+/-1.6 pg/mL (P = 0.01); free testosterone, 0.70+/-0.17 vs. 1.36+/-0.14 pg/mL (P = 0.01)] although dehydroepiandrosterone sulfate and urinary free cortisol levels did not differ. Leptin levels were reduced in AN (2.9+/-2.1 vs. 16.5+/-1.8 ng/mL; P < 0.0001). Insulin-like growth factor I (IGF-I) was reduced in AN to 50% of control levels (219+/-41 vs. 511+/-35 ng/mL; P < 0.0001) and correlated with all measures of nutritional status, particularly leptin (r = 0.80; P < 0.0001). Surrogate markers of bone formation, serum osteocalcin (OC) and bone-specific alkaline phosphatase (BSAP), were significantly (P = 0.02) reduced in AN vs. controls (OC, 39.1+/-6.4 vs. 59.2+/-5.2 ng/mL; BSAP, 27.9+/-4.0 vs. 40.6+/-3.4 U/L). The majority of the variation in bone formation in AN was due to IGF-I levels (OC: r2 = 0.72; P = 0.002; BSAP: r2 = 0.53; P = 0.01) in stepwise regression analyses. Bone resorption was comparable in patients and controls. These data demonstrate that bone formation is reduced and uncoupled to bone resorption in mature adolescents with AN in association with low bone density. Lean body mass was a significant predictor of BMD in controls, but not AN patients. The major correlate of bone formation in AN was the nutritionally dependent bone trophic factor, IGF-I. Reduced IGF-I during the critical period of bone mineral accumulation may be an important factor in the development of osteopenia in adolescents with AN.

Absorptiometry, Photon↗

PubMed2003

PTH/PTHrP activity and the programming of skeletal development in utero.

There is increasing evidence that nutritional deficiency in utero adversely affects bone development and the risk of developing osteoporosis in later life. Although the mechanisms involved are unknown, circumstantial evidence points to an important role of PTH/PTHrP activity. It is recognized that PTH and PTHrP are critically involved in regulating fetal calcium homeostasis, actions that are mediated at least in part by PPR. As well as playing a central role in the maintenance of calcium homeostasis in the fetus, studies in transgenic mice show that PTH, PTHrP, and PPR exert similar effects on skeletal development in utero, acting to increase the size of the trabecular envelope and decrease that of the cortical envelopes. Taken together, these observations raise the possibility that stimulation of PTH/PTHrP activity in the fetus in response to calcium deficiency acts to increase the size of the trabecular envelope but to reduce that of the cortical envelope. Although any increase in trabecular bone at birth is likely to be relatively transient, a decrease in size of the cortical envelope may have a persistent effect on the trajectory of bone growth in subsequent childhood. Consistent with this proposal, preliminary findings from birth cohort studies suggest that maternal calcium intake and cord blood calcium levels are positively related to bone mass of the offspring as assessed later in childhood. Further studies are justified to determine whether alterations in fetal PTH/ PTHrP activity caused by calcium stress lead to a reduction in size of the cortical envelope at birth that persists into childhood and later adult life and to identify modifiable maternal factors that are responsible for these changes.

Bone Development↗

PubMed2005

Pro-alpha3(V) collagen chain is expressed in bone and its basic N-terminal peptide adheres to osteosarcoma cells.

The third alpha-chain of type V collagen, alpha3(V) chain, was initially identified in the placenta more than 20 years ago, but was poorly characterized with regard to its expression and function. We generated a specific monoclonal antibody against the N-terminal domain of the pro-alpha3(V) chain and examined gene expression using immunohistochemical methods combined with in situ hybridization. The pro-alpha3(V) chain was seen in funis and amnion, but not chorionic villi and deciduas of mouse placenta. In mouse embryo, the transcripts of the pro-alpha3(V) gene were seen in tissues that were related to bone formation as well as developing muscle and nascent ligament previously reported. However, immunohistochemistry showed that pro-alpha3(V) protein accumulated rather in the developing bone of mouse embryo. On the other hand, the N-terminal globular domain of the pro-alpha3(V) chain has a unique structure that contains a highly basic segment of 23 amino acids. The peptide derived from the basic segment showed a specific adhesive feature to osteosarcoma cells but not to chondrosarcoma cells. The four heparin binding sites in the basic segment equally contribute toward adhesion to the osteosarcoma cells. Our data suggested that N-terminal globular domain of the pro-alpha3(V) chain influence bone formation of osteoblasts through proteoglycan on the cell surface during development or regeneration.

Amino Acid Sequence↗

PubMed1995

Expression of interstitial collagenase during skeletal development of the mouse is restricted to osteoblast-like cells and hypertrophic chondrocytes.

We determined the expression pattern of the matrix metalloproteinase interstitial collagenase (MMP-1) during mouse embryo development using in situ hybridization and immunohistochemistry. Localized MMP-1 mRNA was first detected at 14.5 days postconceptus. The spatial and temporal expression was restricted to areas of endochondral and intramembranous bone formation, such as in the mandibula, maxilla, clavicle, scapula, in the vertebrae, and in the dorsal, but not the ventral part of the ribs. The highest levels of MMP-1 transcripts and MMP-1 protein were found in the metaphyses and diaphyses of the long bones. MMP-1 was expressed by hypertrophic chondrocytes and by osteoblastic cells localized along the newly formed bone trabeculae. No expression was detected in osteoclasts. Two other related members of the MMP family, stromelysin-1 (MMP-3) and stromelysin-2 (MMP-10), were not expressed during days 7.5 and 16.5 of mouse embryogenesis. The tissue-specific expression of MMP-1 and the exclusive ability of interstitial collagenase to digest native collagen of types I, II, III, and X, the major components of bone, cartilage, and tendon, strongly suggests an important and specific function of this enzyme in bone development and remodeling.

Animals↗

PubMed1993

Microvascular events in bone marrow related to development of and recovery from bone atrophy in thiotepa-treated rats.

To clarify the relationship of microvascular events in the bone marrow to the development of and the recovery from bone atrophy, we examined quantitatively and qualitatively trabecular bone, sinusoids and haematopoietic tissue in the metaphysis of the tibia of rats given thiotepa, an antitumour drug derived from nitrogen mustards. The administration of thiotepa rapidly reduced the number of haematopoietic cells causing significant dilatation of the sinusoids and atrophy of the trabecular bone 1 day after the injection. However, these changes were transient; regeneration of haematopoietic cells and normalization of the dilated sinusoids started 3 days after the injection, and the atrophied trabecular bone gradually returned to normal starting 6 days after the injection. These results imply that an extreme decrease in velocity and margination of sinusoidal blood flow following marked dilatation due to a decrease in haematopoietic cells cause atrophy of the trabecular bone, and that normalization of the sinusoidal blood flow following regeneration of haematopoietic cells leads to recovery from atrophy of trabecular bone.

Animals↗

PubMed2005

Limited nutritional energy supply differentially impairs growth and bone mineralization of the developing lumbar vertebrae in minipigs.

Bone development during puberty and adolescence may influence the later risk of osteoporosis. The aim of this study was to investigate whether the three-dimensional development of the lumbar vertebrae is impaired in terms of bone mineral density and temporal control during the pubertal growth spurt when caloric intake is limited. Two groups of prepubertal female Gottingen minipigs (each n = 6) were fed either ad libitum (free fed) or with reduced food intake (controlled fed) for 12 months using a standard diet. Apparent volumetric bone mineral densities (vBMD) and size of the 4th vertebra were measured by quantitative CT in monthly intervals. The weight of free-fed animals was 29.5 +/- 0.9 kg compared to 25.9 +/- 3.5 kg in the controlled-fed group at the end of the observation period (mean +/- SD; p < 0.05). The maximal cross-sectional area of the 4th vertebra was 148.2 +/- 3.4 mm(2) in free-fed animals compared to 143.1 +/- 6.3 mm(2) in controlled-fed animals (P < 0.05) as both, the cortical and the medullary area were greater in the free-fed group. In contrast, cancellous and cortical bone mineral densities were not different between the two groups; however, maximal densities were reached earlier in the free-fed group. As regards the height of the vertebra, no difference was observed between the groups after 12 months (25.46 +/- 0.54 mm in controlled fed vs. 25.56 +/- 0.45 mm in free fed), although the peak height was reached earlier in the free-fed group. In both groups, the maximal growth rate in the cross-sectional direction was reached earlier compared to the longitudinal direction (both P < 0.05). Growth in the longitudinal direction that occurs in the primary and secondary epiphysis does not appear to depend on caloric intake contrary to the periosteal and endocortical bone generation in the cross-sectional direction. When limited energy is available, the organism seems to preserve the development of normal body height, while accepting reduced strength of the vertebra.

Age Factors↗

PubMed1990

The occurrence of neuropeptides at different stages of DBM-induced heterotopic bone formation.

In developing heterotopic bone in the rat, induced by allogeneic bone matrix, we immunohistochemically detected nerves containing substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) and tyrosine hydroxylase (TH). After 10 days they were dicernible amidst differentiating chondroblastoid cells in fibrous tissue around and within the implants. Over the next 3 weeks, the nerves increased in number and gradually attained a shape and distribution resembling normal osseal nerves; varicose fibres frequently occurred in periosteum-like fibrous tissue and bone marrow adjacent to newly formed bone. At 8 weeks, NPY-fibres increased, particularly in the marrow and this abundance of NPY fibres remained at 16 weeks. VIP-immunoreactive fibres were only observed in the surrounding periosteum-like fibrous tissue 4-6 weeks after implantation. These observations, in combination with recent findings of receptors to neuropeptides on bone cells, suggest a neurogenic influence on physiological processes in bone tissue.

Animals↗

PubMed2002

Treatment of bone complications in advanced prostate cancer: rationale for bisphosphonate use and results of a phase III trial with zoledronic acid.

Prostate cancer often metastasizes to bone during disease progression. Patients who develop bone metastases have a high risk of developing skeletal complications, including pathologic fractures, spinal cord compression, and severe bone pain. Bisphosphonate therapy is widely used for the prevention of skeletal complications in patients with bone lesions from multiple myeloma and breast cancer. Until recently, however, no bisphosphonate had ever shown objective clinical benefit in patients with prostate cancer and osteoblastic bone lesions. A recent multicenter, randomized, placebo-controlled trial found zoledronic acid (4 mg) to be a safe and effective therapy in patients with bone metastases from hormone-refractory prostate cancer. Zoledronic acid significantly reduced the proportion of patients who experienced skeletal complications and extended the time to first skeletal complication. Further, zoledronic acid significantly reduced the risk of skeletal complications over this 15-month study and provided consistent reductions in bone pain that were significant at the 3- and 9-month time points compared with placebo. These results suggest that zoledronic acid may become an important advancement in the care of patients with prostate cancer metastatic to bone. The role of zoledronic acid in the treatment of patients with prostate cancer continues to evolve.

Bone Neoplasms↗

PubMed2000

Fra-1 replaces c-Fos-dependent functions in mice.

Structure-function analysis as well as studies with knock-out and transgenic mice have assigned distinct functions to c-Fos and Fra-1, two components of the transcription factor AP-1 (activator protein-1). To test whether Fra-1 could substitute for c-Fos, we generated knock-in mice that express Fra-1 in place of c-Fos. Fra-1 rescues c-Fos-dependent functions such as bone development and light-induced photoreceptor apoptosis. Importantly, rescue of bone cell differentiation, but not photoreceptor apoptosis, is gene-dosage dependent. Moreover, Fra-1 fails to substitute for c-Fos in inducing expression of target genes in fibroblasts. These results show that c-Fos and Fra-1 have maintained functional equivalence during vertebrate evolution.

Animals↗

PubMed2001

Effects of cartilage-derived morphogenetic proteins and osteogenic protein-1 on osteochondrogenic differentiation of periosteum-derived cells.

Localization studies and genetic evidence have implicated cartilage-derived morphogenetic proteins-1, -2 (CDMP-1 and CDMP-2), and osteogenic protein-1 (OP-1) in the osteochondrogenic differentiation of mesenchymal progenitor cells during embryonic development and in postnatal life. Based on their expression pattern and the evidence that periosteum contains mesenchymal cells in the cambium layer that can undergo bone and cartilage formation, we hypothesized that CDMPs and OP-1 may be involved in long bone development and fracture healing. To test this hypothesis, periosteum-derived cells from young calves were cultured as monolayers under serum-free conditions with and without the addition of recombinant CDMP-1, CDMP-2 and OP-1. Phenotypic analysis indicate that periosteum-derived cell populations prepared, expanded, and cultured under the conditions described below, constitutively express messenger RNAs for the bone markers osteocalcin, osteopontin and collagen type I, and the chondrogenic markers collagen type II and aggrecan as determined by RT-PCR. Moreover, histologic examinations showed positive staining for alcian blue and alkaline phosphatase (AP). Treatment of periosteum-derived cells with CDMPs and OP-1 resulted in a dose-dependent increase of cell proliferation; CDMP-2 was less active in this regard. Furthermore, all growth factors enhanced osteogenic differentiation as assessed by a time- and dose-dependent stimulation of AP activity and OP-1 increased messenger RNA expression for osteocalcin and collagen type I. We further examined the effects of CDMPs and OP-1 on chondrogenic differentiation of periosteum-derived cells. Both CDMPs and OP-1 stimulated (35)S-sulfate incorporation into newly synthesized macromolecules with OP-1 having a more pronounced stimulatory effect when compared with CDMP-1 and CDMP-2. Our results indicate that distinct members of the BMP-family increase the mitotic and metabolic activity of periosteum-derived cells. The enhancement of both the chondrogenic and osteogenic differentiation suggests that these growth factors might contribute to the local regulation of bone formation and fracture repair.

Animals↗

PubMed2003

The expression of matrix metalloproteinase-13 and osteocalcin in mouse osteoblasts is related to osteoblastic differentiation and is modulated by 1,25-dihydroxyvitamin D3 and thyroid hormones.

Matrix metalloproteinase-13 (MMP-13), is a key protein of bone matrix degradation, and is highly expressed by osteoblasts. We used the osteoblast-like MC3T3-E1 cell line and compared the stimulatory effects of the bone resorptive agents 1,25-dihydroxyvitamin D3 (1,25-(OH)(2)D(3)) 3,3',5-triido-L-thyronine (T3) on the expression of MMP-13 mRNA. We showed that the stimulatory effects were time and dose dependent, and were also transduced to the protein level, with 1,25-(OH)(2)D(3)being more potent.MMP-13 expression in different mouse cells and its localization within developing bone from the onset of osteogenesis were also investigated. 1,25-(OH)(2)D(3)- and T3-regulated osteocalcin (Osc) expression in mouse osteoblasts was compared to hormonal effects on MMP-13 expression and activity. Here we show divergent and common roles of 1,25-(OH)(2)D(3)and T3 action on the expression of these marker proteins, depending on the stage of cell differentiation. In addition, we propose a role for MMP-13 in the bone collar of developing long bones. The results could help to more precisely characterize hormonal regulation in the developmental sequence of osteoblasts.

Animals↗

PubMed1996

Targeted overexpression of parathyroid hormone-related peptide in chondrocytes causes chondrodysplasia and delayed endochondral bone formation.

Parathyroid hormone-related peptide (PTHrP) was initially identified as a product of malignant tumors that mediates paraneoplastic hypercalcemia. It is now known that the parathyroid hormone (PTH) and PTHrP genes are evolutionarily related and that the products of these two genes share a common receptor, the PTH/PTHrP receptor. PTHrP and the PTH/PTHrP receptor are widely expressed in both adult and fetal tissues, and recent gene-targeting and disruption experiments have implicated PTHrP as a developmental regulatory molecule. Apparent PTHrP functions include the regulation of endochondral bone development, of hair follicle formation, and of branching morphogenesis in the breast. Herein, we report that overexpression of PTHrP in chondrocytes using the mouse type II collagen promoter induces a novel form of chondrodysplasia characterized by short-limbed dwarfism and a delay in endochondral ossification. This features a delay in chondrocyte differentiation and in bone collar formation and is sufficiently marked that the mice are born with a cartilaginous endochondral skeleton. In addition to the delay, chondrocytes in the transgenic mice initially become hypertrophic at the periphery of the developing long bones rather than in the middle, leading to a seeming reversal in the pattern of chondrocyte differentiation and ossification. By 7 weeks, the delays in chondrocyte differentiation and ossification have largely corrected, leaving foreshortened and misshapen but histologically near-normal bones. These findings confirm a role for PTHrP as an inhibitor of the program of chondrocyte differentiation. PTHrP may function in this regard to maintain the stepwise differentiation of chondrocytes that initiates endochondral ossification in the midsection of endochondral bones early in development and that also permits linear growth at the growth plate later in development.

Animals↗

PubMed1987

[The value of follow-up bone scans in the management of postoperative breast cancer--when should it be performed?].

We have reviewed the bone scans of 106 patients with breast cancer taken after surgery, in order to determine when follow-up bone scans should be performed. The results obtained are as follows: 1. The incidence of bone metastasis was seen to be 11.6% in stage I, 39.4% in stage II, 43.5% in stage III, 57.1% in stage IV, and 30.2% overall; 2. Of 32 patients with bone metastasis, 28 patients (87.5%) developed bone metastases within the first 4 years; 3. The scan conversion rate was found to be 31.9% and the time interval to conversion ranged from 4.5 months to 28.5 months; and, 4. The period from single to multiple metastases ranged from 2 months to 37.5 months. Our findings suggest that postoperative follow-up bone scans for stage I breast cancer should be performed annually or when symptoms develop, and follow-ups for stages II, III and IV at six monthly intervals for the first 4 years after surgery and then annually.

Bone Neoplasms↗

PubMed1987

Tissue specificity and developmental expression of rat osteopontin.

Osteopontin is a 44 kd phosphoprotein abundant in bone matrix. We isolated a partial length cDNA for rat osteopontin and used it to examine its tissue specificity, its expression during bone development and its hormonal regulation. Osteopontin mRNA is most abundant in bone but is also found in considerable amounts in kidney. Osteopontin mRNA is regulated by the osteotropic hormones dexamethasone and 1,25(OH)2D3. Estimates of osteopontin mRNA levels indicate that the osteopontin gene is turned on relatively late in calvarial development.

Age Factors↗

PubMed2004

Epidermal growth factor receptor-deficient mice have delayed primary endochondral ossification because of defective osteoclast recruitment.

The epidermal growth factor receptor (EGFR) and its ligands function in diverse cellular functions including cell proliferation, differentiation, motility, and survival. EGFR signaling is important for the development of many tissues, including skin, lungs, intestines, and the craniofacial skeleton. We have now determined the role of EGFR signaling in endochondral ossification. We analyzed long bone development in EGFR-deficient mice. EGFR deficiency caused delayed primary ossification of the cartilage anlage and delayed osteoclast and osteoblast recruitment. Ossification of the growth plates was also abnormal resulting in an expanded area of growth plate hypertrophic cartilage and few bony trabeculae. The delayed osteoclast recruitment was not because of inadequate expression of matrix metalloproteinases, including matrix metalloproteinase-9, which have previously been shown to be important for osteoclast recruitment. EGFR was expressed by osteoclasts, suggesting that EGFR ligands may act directly to affect the formation and/or function of these cells. EGFR signaling regulated osteoclast formation. Inhibition of EGFR tyrosine kinase activity decreased the generation of osteoclasts from cultured bone marrow cells.

Animals↗

PubMed1996

Immunohistochemical and ultrastructural localization of CGRP-positive nerve fibers at the epiphyseal trabecules facing the growth plate of rat femurs.

We performed immunohistochemical and ultrastructural studies to disclose a possible relationship between nerve fibers and bone metabolism. Immunohistochemical distribution of calcitonin gene-related peptide (CGRP)-positive nerve fibers during bone development was assessed in the femurs of rats. CGRP-positive nerve fibers were denser in the epiphysis than in the metaphysis. These nerve fibers particularly ran along the epiphyseal trabecules facing the growth plate and came in contact with osteoclasts. Many osteoclasts at the epiphyseal trabecules facing the growth plate contained abundant toluidine blue and periodic acid-Schiff-positive granules. Electron microscopy revealed that these osteoclasts have many membrane-bound, electron-dense granular structures and dilated cisterns of rough endoplasmic reticulum containing electron-dense material. They were often surrounded by clear cells displaying features of nerve fiber and had no ruffled border. Furthermore, ultrastructural observations revealed electron-dense structures coating the cytoplasmic side of plasma membranes of the nerve fibers. We also observed coated pits in the cytoplasm of the osteoclasts facing the nerve fibers. To further clarify the role of innervation, we compared trabecules of rats undergoing denervation of the sciatic nerve with those from unoperated rats. Denervation resulted in a significant increase in the number of cement lines on the epiphyseal trabecules facing the growth plate. These results suggest that the osteoclasts at the epiphyseal trabecules facing the growth plate are in part regulated by CGRP-positive nerve fibers. Thus, CGRP-positive nerve fibers could be a crucial element in bone metabolism during bone growth and development.

Animals↗

PubMed1996

Defective bone formation in Krox-20 mutant mice.

Endochondral ossification is the prevalent mode of vertebrate skeleton formation; it starts during embryogenesis when cartilage models of long bones develop central regions of hypertrophy which are replaced by bony trabeculae and bone marrow. Although several transcription factors have been implicated in pattern formation in the limbs and axial skeleton, little is known about the transcriptional regulations involved in bone formation. We have created a null allele in the mouse Krox-20 gene, which encodes a zinc finger transcription factor, by in frame insertion of the E. coli lacZ gene and shown that hindbrain segmentation and peripheral nerve myelination are affected in Krox-20-/- embryos. We report here that Krox-20 is also activated in a subpopulation of growth plate hypertrophic chondrocytes and in differentiating osteoblasts and that its disruption severely affects endochondral ossification. Krox-20-/- mice develop skeletal abnormalities including a reduced length and thickness of newly formed bones, a drastic reduction of calcified trabeculae and severe porosity. The periosteal component to bone formation and calcification does not appear to be affected in the homozygous mutant suggesting that the major role for Krox-20 is to be found in the control of the hypertrophic chondrocyte-osteoblast interactions leading to endosteal bone formation.

Animals↗

PubMed1998

Tissue hyperplasia and enhanced T-cell signalling via ZAP-70 in c-Cbl-deficient mice.

The c-Cbl protein is tyrosine phosphorylated and forms complexes with a wide range of signalling partners in response to various growth factors. How c-Cbl interacts with proteins, such as Grb2, phosphatidylinositol 3-kinase, and phosphorylated receptors, is well understood, but its role in these complexes is unclear. Recently, the Caenorhabditis elegans Cbl homolog, Sli-1, was shown to act as a negative regulator of epidermal growth factor receptor signalling. This finding forced a reassessment of the role of Cbl proteins and highlighted the desirability of testing genetically whether c-Cbl acts as a negative regulator of mammalian signalling. Here we investigate the role of c-Cbl in development and homeostasis in mice by targeted disruption of the c-Cbl locus. c-Cbl-deficient mice were viable, fertile, and outwardly normal in appearance. Bone development and remodelling also appeared normal in c-Cbl mutants, despite a previously reported requirement for c-Cbl in osteoclast function. However, consistent with a high level of expression of c-Cbl in the hemopoietic compartment, c-Cbl-deficient mice displayed marked changes in their hemopoietic profiles, including altered T-cell receptor expression, lymphoid hyperplasia, and primary splenic extramedullary hemopoiesis. The mammary fat pads of mutant female mice also showed increased ductal density and branching compared to those of their wild-type littermates, indicating an unanticipated role for c-Cbl in regulating mammary growth. Collectively, the hyperplastic histological changes seen in c-Cbl mutant mice are indicative of a normal role for c-Cbl in negatively regulating signalling events that control cell growth. Consistent with this view, we observed greatly increased intracellular protein tyrosine phosphorylation in thymocytes following CD3epsilon cross-linking. In particular, phosphorylation of ZAP-70 kinase in thymocytes was uncoupled from a requirement for CD4-mediated Lck activation. This study provides the first biochemical characterization of any organism that is deficient in a member of this unique protein family. Our findings demonstrate critical roles for c-Cbl in hemopoiesis and in controlling cellular proliferation and signalling by the Syk/ZAP-70 family of protein kinases.

Animals↗