Computer image analysis of intramuscular adipocytes and marbling in the longissimus muscle of cattle

The deposition of fat in muscle, recognized by the consumer as marbling, is an important meat quality trait. The objective of the study was to provide additional insights into the quantitative extent of marbling by means of computer image analysis. Fifty-one F₂ generation German Holstein and Charolais crossbreed cattle, 18 mo of age, were used to determine relationships among marbling traits, adipocyte size, and the amount of adipose tissue in different depots. Differences were recorded among the size of i.m. adipocytes in different groups of marbling flecks, divided according to the location in the muscle cross-section and to the size of the marbling flecks. The results showed positive correlation between i.m. adipocyte size and the weight of s.c. fat, intestinal fat, omental fat, and perirenal fat (r = 0.50, 0.61, 0.70, and 0.63, respectively, P < 0.001). The i.m. adipocyte size was correlated with i.m. fat content, number of marbling flecks, proportion of marbling fleck area, and total length of marbling flecks (r = 0.71, 0.44, 0.62, and 0.55, respectively, P < 0.01). The number of marbling flecks was also correlated with i.m. fat content, proportion of marbling fleck area, and total length of marbling flecks (r = 0.58, 0.62, and 0.91, P < 0.01, respectively). The ventral marbling flecks had a 5-fold larger fleck area, 4-fold more adipocytes, and larger adipocytes (P < 0.001). Larger marbling flecks contained larger adipocytes (P < 0.001). Moreover, compared with the small marbling flecks, there was a 48-fold larger fleck area and 26-fold more adipocytes in the large marbling flecks. The results indicate that i.m. fat deposition increases concurrently with the other fat depots but is still independent. Furthermore, the i.m. fat is preferentially deposited in the ventral area of LM. Although the i.m. adipocyte size has an important effect on the traits of marbling flecks, cell number plays a greater role in i.m. fat deposition than cell size.