Fetal Growth

Rapid growth of the fetal body occurs during the last trimester of pregnancy in domestic animal species (Fig. 1). Components of the fetal body exhibit differential growth and development, and the relative development of specific body portions and systems is species dependent. In general, growth rates of tissues and regions of the body peak in a regular sequence, which begins during fetal development, and continue through postnatal life.[1] Although the brain, limbs, and some internal organs have little function in utero, they are required to be functional at birth. Growth of some vital organs, such as the heart, liver, and kidney, generally parallels fetal weight gain, whereas the lungs and spleen have been shown to stop growing late in gestation.[2] Fetal growth of the head and brain of all species is proportionally greater than that of other body regions. Additionally, the legs of those species that nurse while standing undergo relatively greater fetal growth and are more highly developed than those of species that nurse dams that are lying down. This accounts for the disproportionately large head and long legs of newborn animals of species such as cattle, sheep, and horses. In many cases, development and growth of one tissue is dependent upon that of others. For example, skin or hide development is stimulated by whole body growth, just as skeletal muscles grow in length at a rate proportional to growth of the long bones with which they are associated.

The nature of fetal growth is somewhat tissue-specific, but typically begins as hyperplasic growth (increase in cell number), which is followed by cellular hypertrophy (increase in cell size). Accumulation of extracellular material such as collagen coincides with the cellular growth of a tissue. The following discussion will focus on fetal development and growth of bone, skeletal muscle, and adipose tissue, which are critical to the function of agricultural species and the composition and quality of the products derived from these species.

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