Growth

For this discussion growth is defined as the normal increase in size of animal tissues or organs. This may be accomplished by any of the three following processes:

hypertrophy - enlargement of existing cells
hyperplasia - multiplication or production of new cells
accretionary growth - an increase in non-cellular structural material.

Prenatal growth begins with the fertilization of an ovum and ends with birth. This gestation period is typically 113 days (16 weeks) for pigs , 147 days (21 weeks) for ewe, and 280 days (40 weeks) for cows. After birth, all animals undergo a period of slow growth which is then followed by a period of rapid, constant growth. As growth of muscle, bone, and vital organs slows, fattening accelerates until finally the mature size is reached.

Muscle Growth

Prenatal growth is the time of greatest hyperplasia (multiplication or production of new cells). Although muscles continue to grow after birth, the increase in muscle fibers is only about 10 to 20 percent. The majority of the muscle growth after birth is due to hypertrophy (enlargement of existing cells). As a mature animal ages, it enters senescence where muscles and tissue begin to degenerate without complete replacement or repair. During this period, the number of fibers may decrease and the remaining fibers increase in size to compensate for loss of function originally performed by the atrophied fibers.

Adipose Tissue Growth

In young animals, fat develops first in areas around the internal organs. Next fat is deposited beneath the skin (subcutaneous), between muscles (intermuscular), and within the muscle (intramuscular). With sufficient nutrient intake, these fat deposits continue to develop into adulthood. Fat cells are numerous and wide spread in the connective tissues of normal adult animals. In times of poor nutrition, the energy reserves of adipose tissue can be drawn on to supply energy to other tissues.

Intramuscular fat is the last to be deposited. Visible intramuscular fat in meat is called marbling and is an important quality of meat which improves its juiciness and flavor. Thus it is evident that proper feeding and nutrition plays an important role in the end quality of meat.

Bone Growth

Bone growth occurs both before and after birth by transformation of connective tissue. In some cases such as growth of limbs, bone is formed by ossification (hardening) of cartilage. Other bones such as those of the skull are formed directly. As bones grow in length and diameter, the marrow cavity within them also grows. Red bone marrow which is found in long bones, the sternum, ribs, and vertebrae is responsible for blood cell formation. Yellow bone marrow is primarily composed of adipose tissue.