Comparison of Mangalica and Hungarian Large White pigs at identical bodyweight : 1 . Backfat histology ( Short Communication )

At 130 kg bodyweight Mangalica and Hungarian Large White pig adipose tissue was compared, by means of histology. Adipose cell diameter (117.2 ± 24.1 vs. 55.3 ± 17 μm, Mangalica vs. Hungarian Large White), size (118.9 ± 44.3 vs. 31.9 ± 0.63 μm2∙100) and volume (8.01 ± 0.67 vs. 2.18 ± 0.18 μm3 ∙1 000) was significantly higher in Mangalica, while the hydroxyproline content (0.04 ± 0.005 vs. 0.23 ± 0.014 weight% of wet tissue) and the connective tissue area in the microscopic images (%) were ca. 5-fold higher in Hungarian Large White. The present quantitative results underscore the experience that at identical bodyweight the fat (89.9 ± 1.19 vs. 78.9 ± 0.39 %) and dry matter content (97.8 ± 1.72 vs. 87.7 ± 0.38 %, Mangalica vs. Hungarian Large White), as well as the cellularity of Mangalica largely differs from that of Hungarian Large White.


Introduction
At present, divergently selected pig genotypes (from lean to fat) produce bacon products, Mangalica representing a special position over the meat-producing genotypes due to its superior bacon quality traits (ENDER et al. 2002).The preference of Mangalica is primarily bound to its fat production (EGERSZEGI et al. 2003).In this regard SZABÓ and FARKAS (2001) published detailed results on the fatty acid composition of adipose tissue with lower saturated (stearic acid) (CSAPÓ et al. 1999), and higher monounsaturated (oleic acid) proportions, as compared to meat-type genotypes, with fully identical feeding (VOLK et al. 2004).Thus, adipose tissue fatty acid profile may also play a role in shaping the consistency of Mangalica bacon.
In pigs, nutritional manipulation does not affect the number of extramuscular adipose cells (LEE et al. 1973b).Porcine adipocyte proliferation is limited to a short postnatal period (DESNOYERS et al. 1980), thus adipose tissue volume is primarily depending on the fat-filling (hypertrophy) of the existing preadipocytes or small adipose cells.To assess genotype-associated differences, comparison at identical bodyweight in the adulthood is the most widely applied strategy (LEE et al. 1973a,1973b, CORINO et al. 2005, FISCHER et al. 2006, VACLAVKOVA and BECKOVA 2007), when hyperplasia is not active.
However, there is lack of controlled studies on the adipose tissue, and in particular, on its connective tissue moiety of the highly important Hungarian Mangalica breed.Thus, the present study aimed to describe and compare some backfat histological characteristics of Mangalica and Hungarian Large White pigs, at identical bodyweight values.

Animals
Mangalica and Hungarian Large White castrates were compared (n=3-3).The bodyweight of pigs was highly similar (Table 1), while the keeping was extensive for Mangalica.Both pig genotypes were bedded on straw, had ad libitum access to water.Mangalica pigs had an access to free-range scavenging on a fence-limited paddock, while Hungarian Large White were kept closed.Adipose tissue samples were collected at the slaughter, by dissecting them from the halved bodies at the third cervical vertebra, at 10.5 cm off the carcass midline; backfat thickness was measured at this location.All samples showed a typical two-layered structure, divided with a connective tissue barrier.In all instances the subcutaneous layer was analysed.

Diets
The nutrient composition of the two fattening diets was basically similar, as assessed from the packages, and as shown in Table 2.For both genotypes dosed feeding was applied.Since Mangalica keeping was extensive, which is the conventional and characteristic keeping mode of this breed, the composition of the additional feed components was unknown.

Histological analysis
Fresh tissue samples were fixed in 10 % neutral buffered formalin.Sample preparation was performed with a tissue processor equipment (Thermo Shandon Pathcentre, Thermo Fisher Scientific Inc., Waltham, MA, USA).Sectioning was performed with a sledge microtome (3 μm, Reichert Optische Werke AG, Vienna, Austria) from the paraffinembedded blocks, and staining was carried out with a carousel-type slide stainer (Thermo Varistain 24-4, Thermo Fisher Scientific Inc., Waltham, MA, USA), with hematoxylin and eosin, at the Kaposi Mór Hospital, Department of Pathology, Kaposvár, Hungary.Histometrical measurements were performed on a standard PC with the ImageJ 1.41 (2002) software, on the digital images captured on a light microscope.For cell diameter calibrated line measurement was performed on 100 cells/ocular field (CORINO et al. 2005).Adipocyte size was measured with the polygon measurement method.Subsequently, cell volume was estimated according to HIRSCH and GALLIAN (1968), by the formula: 2 6( 3) where CV is the cell volume, σ is the variance of the cell diameter and x ¯ is the mean cell diameter.For the image-based quantification of connective tissue (area%), the green channel of images was selected, and a threshold value of 165 was applied for all images.

Chemical analysis
Adipose tissue collagen content was determined based on its hydroxyproline content (Mersmann et al. 1973), as determined with a photometric method (MSZ ISO 3496:2000).Fat content was determined by the Soxhlet method, while dry matter content with drying in an oven at 103 °C until constant weight.

Statistical analysis
Biometric data were compared by independent samples t-test, with the SPSS 10 (1999) software.

Results
Characteristic sections of Mangalica and Hungarian Large White adipose tissue are shown in Figure 1a and b, at identical magnifications.Image analysis results obtained for cellularity and connective tissue content are shown in Table 1.Adipose cell diameter, cross-sectional area and volume, and tissue fat and dry matter content was significantly higher in Mangalica pigs, while the connective tissue content, as assessed by direct chemical and image analysis was significantly higher in the Hungarian Large White pigs.This latter property can also be recognized in the microscope images (Figure 1 a, b).In the hydroxyproline content ca.5-fold higher values were found in the Hungarian Large White pigs.

Discussion
The propensity of pigs to fatten depends largely on genetical factors (HOOD and ALLEN 1977), as the final fat cell number is reached at a rather early age (LEE andKAUFFMAN 1974, DESNOYERS et al. 1980).Thus, at constant bodyweight total body adiposity is a correlate of adipose cell volume (HOOD and ALLEN 1977).More progressed cell hypertrophy and cellular fat content at identical weight refers to an excessive lipogenesis and a consequent fat accretion, as found in the Mangalica pigs in the present study, since in swine the site of de novo lipid synthesis is primarily the adipose tissue (STEELE et al. 1974).It is widely accepted that selection had a secondary influence on the lipogenic activity of adipocytes of lean and fat swine breeds, but altered the developmental stage at which maximal lipogenic ability is manifested.This is consonant with the large age and tissue dry matter content difference between the animals of the present study, meanwhile age basically determines the extent of adipose tissue development, manifesting also in prenatal differences among pre-obese and lean foetuses (TRUSTY and HAUSMANN 1990).
A basic aspect of investigating the connective tissue moiety in adipose tissue is that collagen deposition and lipid accretion are separately controlled in swine (HAUSMAN and KAUFFMAN 1986).Connective tissue fibers were more expressed in Hungarian Large White, and were less frequently occurring in Mangalica.Merely in Hungarian Large White, at some infrequent sites adipocyte groups, surrounded by loose connective tissue fibers were also recognized in the sections.These fat cells were spindle-shaped or smaller than the majority of adipocytes (Figure 1c, marked).MERSMANN et al. (1975) reported similar findings in newborn piglets, with higher frequency, suggesting an earlier developmental stage of adipose tissue Hungarian Large White pigs, as compared to Mangalica, where these were absent.
In conclusion, the comparison of adipose tissue at identical bodyweight provides quantitative evidence for the more expressed lipogenesis (larger cell size) of Mangalica and an earlier developmental status of Hungarian Large White (dry matter), with overt small cells and a higher connective tissue contribution.

Table 1
Biological characteristics of the pig genotypes and their adipose tissue samples Biologische Merkmale der Genotypen der Schweine und ihrer Fettgewebeproben