The amount of adipose tissue differs widely among carcasses. The amount of  fat stored in adipose tissue and the total quantity of adipose tissue increase rapidly as animals mature (if they are on a high plane of nutrition) and can be  affected by the amount and type of feed  fed to the animal, hormone balance and  genetics. As an animal “fattens? Fat is first deposited in the adipose tissues around the kidney and heart, and in the pelvic canal area; body cavity fat is more prevalent in dairy cattle than in beef cattle.

External or subcutaneous adipose tissue is the second site of fat deposition. Located just beneath the skin, the subcutaneous adipose tissue is often referred to as “finish? The third site in which fat is deposited is between muscles (intermuscular or seam fat).  The fourth area is within the muscles (as intramuscular fat or marbling).  Intramuscular fat (marbling) is deposited, if and only if, needs of the adipose tissue sites of higher priority for fat deposition have been satisfied (e.g., around the organs, under the skin, and between the muscles).

The amount of marbling is an important factor in determining the USDA quality grade for beef.   The muscle’s capacity to accumulate increasing quantities of intramuscular fat is partially related to animal age (increasing as the animal matures) but is almost entirely (given that the animal is on a high plane of nutrition) determined by genetics (hereditary predisposition to deposit fat as marbling).  Although there are differences among species, the general tendency is for the percentage of fat in the body to increase with age, particularly after the major stage of muscle growth has been completed.  In addition, site and amount of fat accumulation in adipose tissues differs among individual animals of the same species, depending substantially upon their genetic makeup. 

Source: Lessons on Meat