Regulation of Growth
The complex process of growth is controlled by the interaction between genetic factors, endocrine system, and nutritional and environmental factors. Among these regulatory factors, endocrine system is considered as the primary system that controls the growth in animals.
Somatotroph axis (growth hormone, GH releasing hormone, somatostatin) and thyrotroph axis (thyrotropin releasing hormone, thyroid hormone stimulating hormone) regulate the growth in farm animals. During early embryonic period, most of the skeletal muscles develop from mesodermal progenitor cells and are controlled by a range of positive (e.g., Insulin-like growth factor-1 (IGF-1)) and negative (myostatin) signals. Growth hormone (GH), thyroid hormones, glucocorticoids, insulin, prolactin, and gonadal steroids influence growth in animals during various stages of life. Growth rate and feed conversion ratio increased in calves, lambs, and pigs subjected to exogenous growth hormone (GH) injection. In pigs, giving GH during early pregnancy increases fetal IGF and increases myofiber number at birth. During postnatal period, the growth rate is regulated by the genetic makeup of the animal and also by the environment. Hypothalamus controls the secretion of pituitary hormones which affects growth such as GH, thyroid stimulating hormone, gonadotropins, prolactin, and adrenocorticotropic hormone (ACTH). Hypothalamus is also a major regulator of feed intake which will influence the growth of an animal by increasing the intake of available nutrients.The GH is the major hormone regulating growth and its release is controlled by GH releasing factor and somatostatin, both are secreted from the hypothalamus. GH exerts its effects on the growth of muscles and connective tissues through IGF from the liver. The IGF-1 acts on muscle growth by stimulating the differentiation and proliferation of myoblasts and increase amino acid uptake and protein synthesis with the help of thyroid hormones, insulin, and anabolic steroids.
GH also exerts its direct metabolic effects on adipose tissues leading to lipolysis. Injection of purified chicken GH in the chicken increased the body weight in the treatment groups compared to the control; however, the increase in the body weight was transient and the chicken GH’s effect diminished at the end of the study. Chicken subjected to exogenous intravenous administration of human pancreatic GH releasing factor showed a significant increase in the body weight during early stages; however, the bodyweight diminished similar to the GH. On the contrary, the effect of thyroid hormones (T3 and T4) impact on the body weight of chicken was not transient and showed significant body weight gain. GH injection in pigs increased the GH receptors in the liver indicating the presence of higher GH in blood.Insulin increases growth hormone receptor in the adipose and hepatic tissue in dairy cows during periparturient stage. Insulin stimulates protein synthesis in the skeletal muscle of pigs, during the early postnatal period. Thus, insulin has an important role in protein deposition in the skeletal muscle of growing animals. Insulin exerts its action in the in vitro muscle cells by binding with IGF-I receptors. Infusion of balanced amino acid, which is the precursor of protein synthesis in the fast growing pigs showed increased protein synthesis in the skeletal muscle of pigs. Similarly, protein diet feeding also increased protein synthesis in the skeletal muscle of suckling lambs. In the skeletal muscle, GH treatment increased protein deposition, and thus enhancing the growth of muscles.
Under in vitro conditions in different cell lines GH and IGF-I increase the differentiation, proliferation, replication, and lipogenic enzyme activity of pre-adipocytes. Similarly, the T3 hormones under in vitro conditions increase lipogenic enzyme activity, fatty acid synthase synthesis, and increased pre-adipocyte proliferation and differentiation. Glucocorticoids accelerate the differentiation of adipocyte tissues.
Fetal growth rate is not affected by the size of uterus during early stages of pregnancy. However, there is a correlation between fetal growth and placental size during later stages of pregnancy. Placenta facilitates the transport of nutrients, gases, and metabolic waste products between mother and fetal circulations. Apart from this, placenta provides a large surface area for nutrient exchange and syntheses growth factors, nutrients, and immune modulators which are necessary for blood flow and delivery of nutrients to the fetus. Maternal nutrition is the most important factor that immensely contributes to growth and development of the fetus during pregnancy and maternal health. Nutrient availability during pregnancy decides the weight and size of the fetus thus affecting the growth of fetus. It is well known that maternal nutrient restriction during pregnancy leads to fetus that are smaller at birth than fetuses from normally fed animals.
26.2.1 Biochemical and Genetic Determinants of Growth
Normal body growth requires the integrated function of the endocrine system, metabolic, and other growth factors involved in the hypothalamo-pituitary growth axis. The growth of animals is mainly determined by genetic factors as well as non-genetic factors. The genetic factors include breed, feed conversion efficiency, and disease resistance. Several genes that control the growth and development of animals. Growth and body composition differ between breeds of all farm animals. The rate of maturation and body weight gain varies between different breeds. Feed conversion efficiency of individual animals influences their energy utilization and growth. Disease resistance traits of an animal reduce the impact of disease by preventing infection or by reducing the further growth of the pathogen in the host.
An adequate and continuous supply of protein is required for normal growth and maintenance of the animals. Proteins are made of various amino acids, including which are not synthesized in the animal body called as essential amino acids. Essential amino acids must be provided in diets for optimal growth and development. Inadequate supply of protein causes retardation of growth whereas excessive protein supply cannot increase protein synthesis beyond the genetic potential of the animal. Dietary fats are a good source of energy for animals. Some fatty acids are essential for growth. Dietary fats are digested into fatty acids, absorbed and deposited as body fat. Body fat composition varies between breeds and species.
26.3