J. Anim Sci. 2007. 85:1136-1143. doi:10.2527/jas.2006-371
© 2007 American Society of Animal Science
Adiposity of calf- and yearling-fed Brangus steers raised to constant-age and constant-body weight endpoints
S. B. Smith1,
A. A. Chapman,
D. K. Lunt,
J. J. Harris and
J. W. Savell
Department of Animal Science, Texas A & M University, College Station 77843-2471
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Abstract
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We tested the hypothesis that fatty acid biosynthesis and adipocyte diameter and volume would be greater in s.c. and i.m. adipose tissues of calf-fed steers than in yearling-fed steers at a constant BW, due to the greater time on feed for the calf-fed steers. Conversely, we predicted that the capacity for s.c. and i.m. preadipocytes to divide, as estimated by 3H-thymidine incorporation into DNA, would be greater in the less mature adipose tissues of calf-fed steers and in yearling-fed steers at 16 mo of age than in yearling-fed steers fed to 18 mo of age. Brangus steers were fed a corn-based finishing diet as calves (calf-fed; n = 9) or yearlings (n = 4) to 16 mo of age (CA yearling-fed); another group of yearlings (n = 5) was fed to a constant-BW end point of 530 kg (CW yearling-fed). Both groups of yearling-fed steers had free access to native pasture until 12 mo of age. At slaughter, the fifth to eighth thoracic rib section of the LM was removed, and fresh s.c. and i.m. adipose tissues were removed for in vitro incubations. There were no differences in the number of s.c. adipocytes/g or mean peak volumes of adipocytes across production groups (P 
0.14). However, s.c. adipose tissue of CA yearling-fed steers contained greater proportions of smaller adipocytes (<1,500 pL) than calffed or CW yearling-fed steers, and similar results were observed for i.m. adipose tissue. Acetate incorporation into total lipids was greater (P = 0.02) in s.c. adipose tissue of CA yearling-fed steers than in calf-fed or CW yearling-fed steers, and tended to be different (P = 0.10) across production groups in i.m. adipose tissue. The production system x cell fraction interaction was significant (P = 0.03) for s.c. adipose tissue DNA synthesis, which was greatest in adipocytes from CA yearling-fed steers, whereas there were no differences across production system in stromal vascular (SV) DNA synthesis. For i.m. adipose tissue, DNA synthesis was greatest in adipocytes and SV cells from CA yearling-fed calves, and was greater in SV cells than in adipocytes (both P = 0.01). Therefore, stage of adipose tissue development more strongly influenced fatty acid synthesis, adipocyte volume, and DNA synthesis than age at sampling, final BW, or time on the finishing diet.
Key Words: adipose metabolism • preadipocyte proliferation • steer
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INTRODUCTION
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Calf-fed steers are fed high-concentrate finishing diets at weaning, whereas yearling steers typically are fed native pasture until approximately 12 mo of age. Thus, calf-fed steers typically are younger at slaughter, which may affect their ability to deposit s.c. and i.m. adipose tissues. Whereas Dikeman et al. (1985)
and Huffman et al. (1990) reported no difference in carcass quality between calf-fed and yearling-fed cattle, Lunt and Orme (1987) reported that yearling-fed cattle produced carcasses with lower yield and quality grades than calf-fed cattle raised to a constant BW. Similarly, yearling-fed Brangus steers had lower USDA yield grades than calf-fed steers (3.9 vs. 4.5), even though both groups were fed to a constant BW (Harris et al., 1997). Thus, s.c. adipose tissue development may be depressed in yearling-fed steers due to the additional time necessary to achieve the same BW as that of the calf-fed steers.
We previously demonstrated that between 16 and 18 mo of age in British-type cattle there are dramatic increases in the capacity of s.c. adipose tissue to synthesize fatty acids (Smith and Crouse, 1984; Martin et al., 1999). In the current investigation, we tested the hypothesis that rates of de novo fatty acid biosynthesis and adipocyte diameter and volume would be less in s.c. and i.m. adipose tissues of yearling-fed steers than in calf-fed steers at a constant BW because of the greater time on the finishing diet for the calf-fed steers. Also, the capacity for s.c. and i.m. preadipocytes to divide should be greater in the less mature adipose tissues of calf-fed steers and in yearling-fed steers at 16 mo of age than in yearling-fed steers fed to 18 mo of age. To test these hypotheses, adipose tissues were collected at slaughter from 16-mo-old, calf-fed and 18-mo-old, yearling-fed Brangus steers raised to a constant BW (530 kg) and from another group of Brangus steers raised to the same age end point as that of the calf-fed steers (i.e., 16 mo).
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MATERIALS AND METHODS
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Animals and Experimental Procedures
The cattle and experimental protocol for this study were described previously (Harris et al., 1997), and this research was approved by the Texas A & M University Laboratory Animal Care Committee. Nuclear transfer-cloned Brangus steers were produced as described by Bondioli et al. (1990) and Westhusin et al. (1992). All recipient cows were managed under the same conditions, and the calves were kept with their cows, with free access to forage, until 8 mo of age. The calves were transported to and raised at the Texas A & M University Research Center, McGregor. During the feeding periods described below, the steers were housed in pens (2 to 4 steers per pen) of approximately 6 x 8 m in size and equipped with automatic watering devices.
A first group of 8 steers was born in November 1991 and was slaughtered in February 1993, at 16 mo of age. All calves were from the same sire, but there were 4 calves each from embryos recovered from 2 donor cows. Two calves from each dam were assigned to each of the treatment groups: 1) calf-fed and 2) yearling-fed steers raised to a constant age (CA yearling-fed)]. A second set of 10 steers was born in February and was slaughtered in June (calf-fed steers) or in September at a constant BW (530 kg; CW yearling-fed steers). All calves were from the same sire and the same donor cow. Recipient cows and both groups of calves were raised under similar production conditions, although gestation and subsequent parturition occurred at different times of the year for the 2 groups of calves (early and late winter, respectively).
For 2 wk, the calf-fed steers were fed a starter diet composed of 34.5% ground sorghum, 10% cottonseed meal, 50% cottonseed hulls, 1.5% mineral premix, and 4% molasses; DM basis. Subsequently, the steers were fed a series of 3 diets (10 d for each diet), with the percentage of corn increasing in each diet (20.25, 37.88, and 50.5%; DM basis). The cattle then were fed an 85% concentrate diet (NEm = 1.89 Mcal/kg; CP = 12.3%) free choice until slaughter. The CA yearling-fed steers were placed on Bermudagrass pasture for 123 d before adaptation to the grain-based finishing diet. The CA yearling-fed steers were fed the finishing diet for 93 d, whereas the calf-fed steers were fed the finishing diet for an average of 220 d (Figure 1). The calf- and CW yearling-fed steers were approximately 530 kg of BW at slaughter. The CW yearling-fed steers grazed central Texas native pasture and oat pasture for 120 d before adaptation to the finishing diet. The CW yearling-fed steers subsequently were fed the finishing diet for 182 d and were 18 mo of age at slaughter.
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Figure 1. Change in BW as a function of time on study for calf-fed (n = 9) or yearling-fed Brangus steers raised to a constant age (n = 5; CA yearling-fed; 16 mo) or a constant BW (n = 5; CW yearling-fed; 530 kg). Steers were adapted to the high-energy, corn-based diet on d 0 (calffed) or on d 112 (CA yearling-fed steers) or d 120 (CW yearling-fed steers). The CA yearling-fed steers were weaned in June and the CW yearling-fed steers were weaned in September; data for the calf-fed steers from both groups were pooled. Pooled SEM for each production group are attached to the symbols. The data were derived from Harris et al. (1997).
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Source of Chemicals
All biochemicals, unless otherwise indicated, were purchased from Sigma Chemical Co. (St. Louis, MO) and Gibco BRL (Gaithersburg, MD).
Sampling
All steers were transported by commercial carrier to the Rosenthal Meat Science and Technology Center, Texas A & M University, College Station. The steers were slaughtered following all appropriate humane slaughter methods. A portion of the fifth to eighth thoracic rib-section of the LM was removed immediately after removal of the hide, placed in oxygenated, 37°C Krebs-Henseleit buffer (KHB), pH 7.4, containing 5 mM glucose, and transported to the laboratory within 20 min of exsanguination. Ether-extractable lipid was measured in a portion of the LM section as described by AOAC (1990).
Acetate Incorporation into Total Lipids
In vitro incubations (2 h) were performed with approximately 100-mg tissue explants as described previously (Smith et al., 1996). The explants were placed in 3 mL of KHB (pH 7.4) containing 10 mM glucose, 10 mM HEPES buffer, and 1 µCi of [1-14C]acetate (Amersham, Arlington Heights, IL). The vials were gassed for 1 min with 95% O2, 5% CO2, capped, and incubated for 2 h in a shaking water bath at 37°C. After 2 h, the reactions were stopped by the addition of 3 mL of 5% trichloroacetic acid. The samples were removed from the incubation medium and rinsed with KHB and 0.154 M NaCl to remove free lipid and unincorporated acetate.
The neutral lipids were extracted using a modification of the Folch et al. (1957) procedure. Rinsed tissue was transferred into individual 50-mL, screw-cap centrifuge tubes containing 5 mL of chloroform:methanol (CHCl3:CH3OH, 2:1, vol/vol), and the lipids were extracted as described previously (Smith et al., 1996). Lipid extracts were transferred to scintillation vials, where they were evaporated to dryness and resuspended in 10 mL of scintillation cocktail, and the radioactivity was counted on a liquid scintillation counter (Model LS3800, Beckman Instruments, Palo Alto, CA).
Adipose Tissue Cellularity
Adipose tissue cellularity (size and number of cells/ 100 mg of tissue) was determined as described previously (Etherton et al., 1977; Smith et al., 1996). Subcutaneous and i.m. adipose tissue samples were frozen at –25°C and sliced into 1-mm-thick sections to facilitate tissue fixation. Adipose slices were placed into 20-mL scintillation vials with 1.5 mL of 50 mM collidine-HCl buffer (pH 7.4) and 2.5 mL of 3% osmium tetroxide in collidine. The samples were incubated for 96 h at 37°C. The osmium tetroxide solution was removed, the tissue was rinsed with 0.154 mM NaCl, and the samples were incubated in 10 mL of 8 M urea at 22°C for 96 h. The fixed cells were filtered through nylon mesh screens (VWR International, Bristol, CT) with 240-, 64-, and 20-µm pore sizes using 0.1% Triton in 0.154 M NaCl. Cell fractions from the 62- and 20-mm mesh screens were collected to determine the cell size, volume, and cells per gram of tissue with a Coulter counter (model ZM) equipped with a channelizer (model Z56, Coulter Electronics, Hialeah, FL).
DNA Synthesis
Incorporation of 3H-thymidine into DNA was measured in cultured adipose tissue pieces as described previously (May et al., 1994). Fresh pieces of s.c. and i.m. adipose tissue (2 to 3 pieces weighing approximately 100 mg in total) were transferred to 5-mL culture plates, 5 mL of culture media was added, and the plates were incubated for 24 h at 37°C in a humidified atmosphere containing 5% CO2. The culture media consisted of Dulbecco’s modified Eagle’s medium with 25 mM glucose, 10% fetal bovine serum, 0.0584 mg/mL of L-glutamine, 1.7 µM insulin, 1.0 µM dexamethasone, 0.5 µM 3-isobutyl-1-methylxanthine, and 2 µCi [3-3H]thymidine (Amersham). The medium was refreshed after 12 h of incubation.
After the 3H-thymidine incubations, the tissue was collagenase-treated using a modification of the procedures described by May et al. (1994). The tissue was rinsed with 150 mM NaCl and 1 mM HEPES buffer. The samples were placed into vials with 2.5 mL of incubation medium containing KHB, 10 mM HEPES, 5 mM glucose, 3% BSA (fatty acid-free), 1 mM CaCl2, 1.67 mg/ mL of collagenase, 0.3 mg/mL of elastase, and 0.5 mg/ mL of hyaluronidase collagenase, C-2139 hyaluronidase, H-3506 elastase, E-7885 (Sigma Chemical Co.). The samples were incubated for 1 h in a shaking water bath at 37°C. At the end of the incubation period, the vial contents were transferred to centrifuge tubes and centrifuged at 21,000 x g for 5 min. The top layers containing the fat cells were transferred to new tubes, leaving the stromal-vascular (SV) fraction behind. To lyse the cells, 0.5 mL of 20% TCA was added to both fractions, the samples were centrifuged at 21,000 x g for 5 min, and the top layer was aspirated. The resulting pellet was redissolved in 1 mL of 0.5 M NaOH, and 0.25 mL was transferred to a clean scintillation vial. To this, 0.1 mL of 5 N HCl and 10 mL of scintillation cocktail were added, and the radioactivity was counted on a liquid scintillation counter (Model LS3800, Beckman Instruments).
Statistical Analyses
The a priori comparison of interest was the feeding system; i.e., calf-feeding vs. CA yearling-feeding vs. CW yearling-feeding. Therefore, the data for both groups of calf-fed steers were pooled, and the data were analyzed independently as a completely randomized, single-factor ANOVA (SuperAnova, Abacus Concepts Inc., Berkeley, CA) with feeding system as the main effect. The data for DNA synthesis were analyzed as a 2-factor ANOVA, with the main effects of feeding system and adipose tissue fraction (lipid-filled adipocytes vs. SV cells). The feeding system x adipose tissue fraction interaction also was tested. Means were separated with Fisher’s LSD method, with significance at P < 0.05.
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RESULTS
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There were no differences in the number of adipocytes/100 mg or mean peak volumes across production groups for s.c. adipose tissue (P 0.14). Peak volume is the cell volume that contributes the most to total adipocyte volume. Although mean peak volumes did not differ for s.c. adipose tissue among production groups, s.c. adipose tissue of CA yearling-fed steers contained greater proportions of smaller adipocytes (<1,500 pL) than calf-fed or CW yearling-fed steers (Figure 2). Also, s.c. adipose tissue of CW yearling-fed steers had a greater proportion of very large adipocytes (2,150 to 2,600 pL) than s.c. adipose tissue of calf-fed and CA yearling-fed steers.
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Figure 2. Volume distributions of s.c. adipocytes from calf-fed and yearling-fed Brangus steers raised to a constant age (16 mo; CA yearling-fed) or a constant BW (530 kg; CW yearling-fed). a–cRelative volume proportions of s.c. adipocytes without common superscripts are different (P < 0.05).
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There were more adipocytes/100 mg i.m. adipose tissue and correspondingly lesser mean peak volumes in CA yearling-fed steers than in calf-fed or CW yearling-fed steers (both P = 0.01; Table 1). Consistent with their lesser peak volumes, i.m. adipose tissue from CA yearling-fed steers contained a greater proportion of smaller adipocytes (<500 pL) than i.m. adipocytes from the other production groups (Figure 3). However, the concentration of lipid in the LM was not different among production groups (P = 0.13).
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Table 1. Cellularity and lipogenesis for s.c. and i.m. adipose tissues from calf-fed and yearling-fed steers fed to a constant age (CA; 16 mo) or a constant BW (CW; 530 kg)1
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Figure 3. Volume distributions of i.m. adipocytes from calf-fed and yearling-fed Brangus steers raised to a constant age (16 mo; CA yearling-fed) or a constant BW (530 kg; CW yearling-fed). a,bRelative volume proportions of s.c. adipocytes without common superscripts are different (P < 0.05). a*,b*Relative volume proportions <300 pL are all greater in i.m. adipose tissue of CA yearling-fed steers than in calf-fed or CW yearling-fed steers.
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Acetate incorporation into total lipids was greater (P = 0.02) in s.c. adipose tissue of CA yearling-fed steers than in calf-fed or CW yearling-fed steers and tended to be different (P = 0.10) across production groups in i.m. adipose tissue (Table 1
). Acetate incorporation into lipids in i.m. adipose tissue was less than 10% of the rate observed in s.c. adipose tissue.
The production system x cell fraction interaction was significant (P = 0.03) for 3H-thymidine incorporation into DNA (i.e., DNA synthesis) in s.c. adipose tissue. The DNA synthesis was greatest in adipocytes from CA yearling-fed steers and least in adipocytes from CW yearling-fed steers, whereas there were no differences across production systems for SV cells (Figure 4). In contrast, the main effects of production system and cell fraction were significant for i.m. adipose tissue, whereas the interaction was not significant (Figure 5). The DNA synthesis was greatest in adipocytes and SV cells from i.m. adipose tissue of CA yearling-fed steers than in the other production groups (P = 0.01) and was greater in the SV cell fraction than in the adipocyte fraction (P = 0.01).
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Figure 4. Incorporation of 3H-thymidine into DNA in adipocyte and stromal vascular (SV) cell fractions from s.c. adipose tissue from calf-fed and yearling-fed Brangus steers raised to a constant age (16 mo; CA yearling-fed) or a constant BW (530 kg; CW yearling-fed). Values are reported as dpm incorporated per 105 cells in each fraction. The P-values for the main effects of feeding system (calf- vs. CA yearling-fed vs. CW yearling-fed) and tissue fraction [adipocytes vs. stromal vascular (SV) cells] were 0.13 and 0.39, respectively. The P-value for the interaction of feeding system x tissue fraction was 0.03. The SEM for the feeding system x tissue fraction interactions are affixed to the bars. a–cMeans without common superscripts are different (P < 0.05).
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Figure 5. Incorporation of 3H-thymidine into DNA in adipocyte and stromal vascular (SV) cell fractions from i.m. adipose tissue from calf-fed and yearling-fed Brangus steers raised to a constant age (16 mo; CA yearling-fed) or a constant BW (530 kg; CW yearling-fed). Values are reported as dpm incorporated per 105 cells in each fraction. The P-values for the main effects of feeding system (calf- vs. CA yearling-fed vs. CW yearling-fed) and tissue fraction (adipocytes vs. SV cells) were both 0.01. The P-value for the interaction of feeding system x tissue was 0.56. The SEM for the feeding system x tissue fraction interactions are affixed to the bars.
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DISCUSSION
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The carcass and production data for the steers used in the current study were published previously (Harris et al., 1997), and selected carcass traits are reported in Table 2. Raising calf- and yearling-fed steers to a constant age predictably resulted in large differences in carcass BW and adiposity. Calf-fed steers had heavier carcasses, greater adjusted fat thickness, more KPH fat, higher marbling scores, and higher yield grades than yearling-fed steers when both were sampled at 16 mo of age. Correspondingly, s.c. and i.m. adipocytes from the CA yearling-fed steers had much smaller peak volumes (1,553 and 575 pL, respectively) than the calf-fed steers (2,308 and 1,157 pL, respectively). Neither marbling scores nor adjusted fat thickness were significantly different between calf-fed steers and yearling-fed steers raised to the same BW, and in the current study, no measure of adipose tissue cellularity, metabolism, or DNA synthesis was different between these 2 production groups.
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Table 2. Carcass characteristics of calf-fed and yearling-fed steers fed to a constant age (CA; 16 mo) or a constant BW (CW; 530 kg)1
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We originally had predicted that rates of fatty acid biosynthesis and adipocyte diameter and volume would be greater in s.c. and i.m. adipose tissues of calf-fed steers than in yearling-fed steers at a constant BW, due to the greater time on feed for the calf-fed steers. Increasing time on feed (Smith et al., 1984; Martin et al., 1999; Schoonmaker et al., 2004) or ME intake (Smith et al., 1992) profoundly increased s.c. lipogenic enzyme activities, the rate of incorporation of acetate into lipids, and adipocyte volume. Therefore, we predicted that s.c. adipose tissue, and perhaps i.m. adipose tissue of calf-fed steers would exhibit greater rates of fatty acid biosynthesis than adipose tissues from CW yearling-fed steers, and both would exceed rates observed in the younger, CA yearling-fed steers. Instead, s.c. adipose tissue of CA yearling-fed steers had substantially greater rates of fatty acid synthesis than calffed or CW yearling-fed steers, even though the CA yearling-fed steers were on the finishing diet for only 93 d. A similar pattern was observed for i.m. adipose tissue, although this only approached significance (P = 0.10).
The differences in cellularity and fatty acid synthesis between the CA and CW yearling-fed steers may have been due to the stage of adipose tissue development, rather than time on the finishing diet. There was a large decline in acetate incorporation into fatty acids and acetyl-CoA carboxylase and ATP-citrate lyase enzyme activities in bovine s.c. adipose tissue between 17 and 19 mo of age (Smith et al., 1984) and an 80% decline in acetate incorporation into fatty acids in s.c. and i.m. adipose tissues of corn-fed Angus steers between 525 and 650 kg of BW (Chung et al., 2007). Thus, at some point in development, capacity to synthesize fatty acids de novo declines in s.c. adipose tissue, particularly in adipose tissue of steers fed a corn-based finishing diet (Smith et al., 1984; Chung et al., 2007). The data of the current study suggest that this occurred between 16 and 18 mo of age in this group of Brangus steers.
As we reported previously (Smith and Crouse, 1984), the rate of fatty acid biosynthesis in i.m. adipose tissue was low and not strongly affected by production system. However, production system had a profound effect on i.m. adipocyte peak volume and DNA synthesis, which both were substantially greater in the CA yearling-fed steers than in calf-fed and CW yearling-fed steers. We had predicted that the capacity for s.c. and i.m. preadipocytes to divide, as estimated by 3H-thymidine incorporation into DNA, would be greater in the less mature adipose tissues of calf-fed and yearling-fed steers at 16 mo of age than in yearling-fed steers fed to 18 mo of age. Robelin (1981) and Cianzio et al. (1985) demonstrated no additional increase in total s.c. adipocyte number in cattle over 16 mo of age, suggesting a slowing or cessation of preadipocyte proliferation. Because 3H-thymidine incorporation into DNA was so much greater in adipocytes and SV cells of CA yearling-fed steers than in calf-fed steers, the lesser developmental stage of s.c. adipose tissue in the CA yearling-fed steers must have been more important than age of the animals for determining the rate of DNA synthesis.
Many reports have demonstrated apparent adipocyte hyperplasia in s.c. adipose tissue (Robelin, 1981; Cianzio et al., 1985; Schiavetta et al., 1990) and i.m. adipose tissue (Hood and Allen, 1973; Cianzio et al., 1985). We first reported DNA synthesis in lipid-filling s.c. and i.m. adipocyte and SV cell fractions in a comparison of Angus and Wagyu steers raised to the Japanese heavy-BW endpoint (May et al., 1994), indicating that preadipocyte proliferation persists even in long-fed cattle. More recently, we demonstrated DNA synthesis in s.c. adipose tissue of young, growing pigs (Adams et al., 2005). In heavy BW Angus and Wagyu steers, 23 to 35% of total DNA synthesis was associated with the adipocyte fraction of s.c. and i.m. adipose tissues (May et al., 1994). In the current study, the lowest percentage recovery of 3H-thymidine was in the adipocyte fraction of s.c. and i.m. adipose tissues of CW yearling-fed steers (10 and 6%, respectively) and was 5-fold greater in the adipocyte fraction of s.c. and i.m. adipose tissues of CA yearling-fed steers (66 and 36%, respectively). This indicates that, in the 2 groups of yearling-fed steers, developmental stage of the adipose tissues strongly in-fluenced their potential for hyperplastic adipose tissue growth. In contrast, rates of DNA synthesis in the adipocyte and SV cell fractions were similar for calf-fed and CW yearling-fed steers, as were peak volumes and adipocyte volume distributions. Thus, the s.c. and i.m. adipose tissues of calf-fed and CW yearling-fed steers were at similar developmental stages in spite of the differences in age and production conditions for these steers.
The adipocyte fraction is separated from the SV cell fraction by flotation, a process that is dependent on some degree of lipid filling in the adipocytes. We interpret the presence of DNA synthesis in the floating fraction to indicate the occurrence of cell proliferation in smaller adipocytes as well as in SV cells. The physical limitations imposed by a large, central lipid vacuole would restrict adipocyte proliferation to less mature cells that contain only small amounts of triacylglycerol. The data presented here and previously (May et al., 1994; Adams et al., 2005) support the possibility that adipocytes retain the ability to proliferate even after lipid filling has begun.
An alternative explanation for DNA synthesis in the adipocyte fraction is that some SV cells remained attached to the adipocytes after collagenase digestion. We cannot rule out this possibility, especially in i.m. adipose tissue in which the highest DNA synthesis was observed in the adipocyte and SV cells fractions of CA yearling-fed steers. However, the significant production system x adipose tissue fraction interaction for s.c. adipose tissue would seem to rule out the random attachment of SV cells to adipocytes because the adipocyte and SV cell fractions responded independently of each other. Thus, we interpret these data and those from our earlier study (May et al., 1994) as evidence for preadipocyte and immature adipocyte proliferation in s.c. and i.m. adipose tissues of growing steers. Furthermore, this process is less in adipocytes or preadipocytes from adipose tissues containing populations of very large adipocytes, in s.c. and in i.m. adipose tissues, regardless of age or time on a high-energy finishing diet.
1 Corresponding author: sbsmith{at}tamu.edu
Received for publication June 8, 2006.
Accepted for publication December 18, 2006.
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Abstract
INTRODUCTION
