The size and number of recognizable fat cells (diameters greater than 5 μm) were examined at several sites in 42 Hereford and Friesian steers. Four, two and 15 animals from each breed were slaughtered at 6, 13 and 20 months of age, respectively, after ad libitum feeding.

For the 15 animals of each breed slaughtered at 20 months, biopsy samples of adipose tissue were removed at 10, 13 and 17 months from the 12th rib, midloin, rump and perirenal sites. Samples from these sites and from the brisket (subcutaneous), prescapular (intermuscular) and omental sites were also removed from the carcasses of slaughtered animals. Fat cell diameter (microscopic technique) and dry matter (from which lipid content was predicted) were measured on all samples. Measurement of fat depth (ultrasound) and surface area at the subcutaneous sites allowed changes in relative number of cells to be estimated during growth.

From biopsied samples it was apparent that the perirenal depot grew almost exclusively through cell enlargement in both breeds. In contrast, the subcutaneous depot grew principally through cell enlargement to about 13 months of age and thereafter through both increase in number of cells and cell enlargement. The trigger for this increase in number of cells was possibly a critical average cell size, although this clearly did not operate in perirenal fat.

Slaughter samples showed that cellularity changes with growth were similar in the perirenal and omental depots (intra-abdominal) whereas changes in the prescapular (intermuscular) site tended to parallel those in the subcutaneous depot.

In relation to fat-free body weight (and therefore taking body size into account) both breeds had similar numbers of intra-abdominal and prescapular fat cells, but the Herefords had approximately double the number of subcutaneous fat cells compared with the Friesians.