Skeletal Muscle Growth

200 400 600 800 Live Weight (Kg)

Postnatal skeletal muscle growth coincides with rapid body growth, and requires both DNA and protein accumu-

Fig. 2 Accretion of bone, muscle, and fat during postnatal live weight gain of cattle.

Kidney(+Pelvic Beef/Lamb)

Intramuscular Intermuscular Subcutaneous

Cattle

Fig. 3 Distribution of fat in various depots of pigs, sheep, and cattle.

Kidney(+Pelvic Beef/Lamb)

Intramuscular Intermuscular Subcutaneous

Cattle

Fig. 3 Distribution of fat in various depots of pigs, sheep, and cattle.

As adipogenesis proceeds, the lobules give rise to large lobes of proliferating preadipocytes, which are associated with an extensive capillary network within a connective tissue sheath. The preadipocytes differentiate into immature adipocytes and accumulate lipid droplets (multi-locular lipid). When lipid droplets become so numerous as to abut one another, they coalesce to form one large lipid globule (unilocular lipid), which characterizes mature adipocytes.1-4-1 These adipogenic events apply to all depots, but the rate and extent of development differs among depots, genders, and species (Fig. 3). The brown fat present at birth gives way to white adipose tissue during the first few weeks postnatally.

Preadipocytes are <20 mm in diameter, while mature adipocytes normally average from 120 to 150 mm in diameter, but have been found to be as large as 300 mm in diameter in very obese animals.[4] When adipocytes accumulate lipid so as to reach their average size, recruitment of new preadipocytes occurs. The newly recruited preadipocytes add further to the overall fatness of an animal.

Fat in adipose tissue of mammals is derived from dietary fatty acids present in the blood as triglycerides, as well as from de novo fatty acid synthesis and esterification to form triglycerides in adipose tissue depots. Triglyceride formation is called lipogenesis and it accounts for the bulk of the lipid present in adipocytes. Fatty acids are removed from triglycerides by enzymes within the adipocyte, pass into capillaries, and are carried to tissues where they provide energy. The process of fatty acid mobilization is called lipolysis. During fattening, lipogenesis exceeds lipolysis. However, lipolysis exceeds lipogenesis when the energy demand for maintenance and activity exceeds the dietary energy intake.[4]

Notable among the factors affecting fat deposition are the sex hormones. Estrogen increases deposition, while androgens limit fat deposition. Hence, females are fatter than males. Castration markedly increases fat deposition of both genders.

Factors Affecting Postnatal Growth

The proportion of dietary energy used for maintenance of body temperature and tissue mass influences body weight gain. Temperature extremes reduce the growth rate of animals due to additional energy expended to reduce or increase body temperature, or decreased appetite to reduce heat generated in digestion. Because of their larger surface area-to-volume ratio and lack of insulation, young animals have a higher critical temperature than older animals.[2]

Postnatal growth is dependent upon food intake. Appetite is regulated by nerve centers in the hypothalamus, which is influenced by leptin from adipose tissue and by insulin.1-6-1 The quantity and availability of dietary nutrients influence growth rate and body composition. Rapidly growing, muscular genotypes have greater amino acid requirements for maximal growth than slower-growing, less muscular animals of the same species.

Animals on a high plane of nutrition have greater organ weights, which increase energy expenditure. Increased energy expenditure is also associated with high lean genotypes. When feed is restricted to animals, organ weights and maintenance requirements decrease. Animals with restricted feed intake typically exhibit compensatory gain when shifted to a higher plane of nutrition because a higher proportion of dietary nutrients are available for tissue gain.[7]

Estrogenic and androgenic steroids are commonly used as growth promoters for cattle in the United States.[8] These steroids increase food intake, rate of body weight gain, and efficiency of growth, and the effects of estrogens and androgens are additive. Androgenic implants increase skeletal muscle and reduce fat accretion. More recently, ractopamine, a beta-adrenergic agonist, has been approved for finishing pigs and cattle. This compound increases animal growth rate, efficiency, and muscularity.

CONCLUSION

Postnatal body growth of animals follows a sigmoidal curve that reflects the differential growth of bone, muscle, and fat. Absolute rate of postnatal growth is species dependent, and is influenced by breed, body frame size, gender, nutrition, and environment. Understanding of food intake regulation and factors controlling expression of growth-related genes is rapidly improving. This information will enhance efforts to improve animal growth and body composition.

Weight Loss Enigma

Weight Loss Enigma

Finally Revealed The Revolutionary & Miraculous Weight Loss Secrets! Discover How to Command Those Unwanted Pounds To Take A Hike, So That You Can Get Into Shape & Lose Weight Easily Just Like You Dream Of In Just A Matter Of Weeks! You're About to Discover The Insider's Fat Burning Diet Tips to Easily Shed Off Those Extra Pounds And Obtain a Lovable Shape In No Time Flat!

Get My Free Ebook


Post a comment