Postnatalgrowth

The period of postnatal growth extends from birth until death of animal. Visible growth stops long before death and there is restricted growth that occurs in certain tis­sues only till death.

4. Senescence Phase

1. Pre-pubertal phase: This phase extends from birth of animal up to the period of acquisition of puberty of animal. During this phase, animal grows proportionately i.e. when growth rate of organs and the body is the same (Isometric pat­tern). There are 3 main tissues which develop very fast during this phase: Bones, Muscles and Adipose tissue.

Bones grow before birth as well as after birth. A bone grow in length through the ossification i.e. deposition of calcium at both the ends. Due to ossification, when epiphysis gets closer, the bone growth stops However, bones have the capability that they can repair themselves if broken. Bone elongation stops but bone tissue is constantly being deposited and resorbed.

Muscle fibers are formed during prenatal growth and there is no further increase in the number of muscle fibers after birth. Postnatal growth of mus­cle fiber is characterized by increase in length of muscles and diameter of muscles i.e. increase in the size of muscles (hypertrophy). As muscle fibers are mainly made up of proteins i.e. actin and myo­sin, therefore, the growth of muscles is decided by the rate of protein synthesis.

Growth of adipose tissue is totally depends upon the nutritional status of the animal. There are two types of fat tissue

a) W hite fat which stores energy.

b) Brown fat which maintain constant body tem­perature especially in young animals.

Fat is deposited in four different areas of the body

a) Intra abdominal fat i.e. Around kidneys and pelvic area in abdominal cavity

b) Subcutaneous fat i.e.

c) Intramuscular fat i.e. between muscles

The level of intramuscular fat deposition is called the degree of marbling. It has a importance in meat industry as marbling increases the palatability of meat. Bones, muscles and fat tissue growth occur differently throughout the dif­ferent phases of postnatal growth of animals. Majority of the muscle tissue grow during the pre pubertal periods and at peak level during the puberty. Thereafter, the growth of muscles slower down. Bone growth stops when the animal reaches a mature body size i.e. once puberty attain, bone growth stops but bone tissue deposition and resorption con­tinue till death. Growth in adipose tissue depends upon the diet of animal.

2. Pubertal phase: Phase of growth when the animal has attained puberty i.e. growth of reproductive organs and mammary tissue (in females) starts. Pubertal age of different animals:

Rabbit = 3-4 months

Sheep = 6-7 months

Goat = 6-7 months

Pig = 6-7 months

Dogs = 6-20 months

Cats = 5-12 months

Cattle = 12-18 months

Buffalo = 18-24 months

Horse = 15-24 months

Pubertal Phase: Phase of growth of reproductive organs

In females: Growth of ovary, uterus, mammary glands and other secondary sexual characters.

In males: Growth of testes, musculature and other secondary sexual characters. Prior to this phase, reproductive organs grow in pro­portionate to whole of the body (Isometric Growth) but, at the time of puberty, anatomical and physiological changes occur in hypothala­mus which causes release of gonadotropins and thus extensive reproductive growth takes place (Allometric Growth).

After puberty, bone elongation stops due to closure of epiphysis. Therefore, skeletal size does not increase, but, live body weight continuously increased due to Increase in deposition of proteins and fats.

3. Reproductive Phase: During this phase, repro­ductive organs become functional. Females start showing annual or rhythmic cycle of estrus.

4. Senescence/Aging Phase: It is a natural process and usually occurs during old age when cells are gradually lost and are not replaced. More catabo­lism than anabolism, Decrease in body weight and Irreversible change and ultimately death of animals

Three types of changes occur during Aging: Anatomical changes, Physiological changes and Degenerative changes.

Anatomical changes: Decrease in no. of mus­cle fibers, Decrease in muscle performance, Decrease in protein deposition, Decrease in fat deposition, Loss in CNS activity and Change in organs & glands.

Physiological changes: The metabolism and ther­moregulatory activity is reduced with increase in age, Decrease in Hb, RBC concentra­tion, WBC concentration, Blood supply and Bone marrow efficiency, Decrease in ACTH, Estrogen, Progesterone, testosterone, FSH & LH secretion and Increase in cholesterol & triglycerides.

Degenerative Changes: Degenerative changes start from the periphery of the organs i.e. the cells at the periphery degenerate first and then the cells in the middle portion. Irreversible loss of cells, Gradually functional loss and finally Death of animal.

Senescence: The process of aging or beginning of old age is known as senescence.

Longevity: It means how long an animal lives. It can be measured by median life span or how long an animal remains active.

Life span of different animals:

23.8.1 Measurement of Postnatal Growth

Growth in postnatal animals can be measured by height, length, girth/circumference and body weight.

Height of an animal is the vertical distance from the ground to the highest point of withers.

Length of animal is measured in inches from the point of shoulder to the point of buttocks in case of horses and from the point of shoulder to pin bone in case of cattle, sheep and goat.

23.8.2 Factors Affecting Postnatal Growth

Genetics: Genetic composition of the animal decides the potential for growth and development of ani­mal. All animals have set genotype that determine the growth. However, phenotype is affected by external factors like nutrition, climate, disease sta­tus of animal etc.

Nutrition: Balanced supply of calories, amino acids, lipids, vitamins, minerals and water is necessary for proper growth of animal as these nutrients furnishes energy and raw material for growth. Nutrition is important aspect to determine the postnatal growth. Restriction of caloric intake and nitrogen retention greatly reduced intracellular DNA synthesis, although the cells may continue to grow. If the dietary protein is restricted but caloric intake and nitrogen retention is normal, DNA syn­thesis continues but the cell size is reduced. Thus both protein and total caloric intake are important. Vitamins and minerals are also essential for nor­mal growth of the organism. Severe deficiencies or excess may cause striking changes in metabo­lism and thus, growth and development of an ani­mal. e.g., Iodine deficiency causes hypothyroidism and also inhibits brain development during early phases of postnatal growth.

Climate: There are direct and indirect effects of cli­mate on animal growth as the living organisms respond to changing weather conditions i.e. tem­perature, humidity, rain etc. in that particular area. Change in Body Weight: Environment affect the food intake, energy utilization and protein metab­olism in animals. During cold environmental con­ditions, plenty of food is available and moreover, food intake is increased, that results in gain in body weight of animals.

Development: Change in body form: Animals living in the cold environment, typically appear short having thick rough coats, while those living in the warm conditions are long, lean and sleek coated.

Body Composition: Deposition of protein is less affected by environmental stress and it is the fat which is more affected. Animals will be fattest in a thermoneutral environment and those outside this zone will be leaner.

Weight of organs: Visceral organs are enlarged as a result of cold exposure. Hypertrophy of lungs, heart spleen, liver and kidneys occurs.

During cold→↑ in food intake →↑ metabolism → means more work for organs like liver/ Kidney → Hypertrophy results

Pathological status of animal Various disease conditions may cause change in body weight of animal (may increase or decrease) which adversely affect the growth of animal.

Hormones:

Hormones directly or indirectly influence the growth by altering the biochemical reactions. Hormones which have a direct effect on whole body growth include Somatotropin/Growth Hormone

Thyroxine Androgens Estrogens Corticosteroids

Somatotropin (Growth Hormone): GH causes a general increase in body mass. GH affects skeletal and protein metabolism. GH stimu­lates growth of epiphyseal plates of long bones in growing animals. GH administra­tion promotes nitrogen retention, decrease in plasma amino acid levels and urea excretion (due to increased transport of amino acids to skeletal muscles, heart, kidney and liver). GH also stimulates protein synthesis by directly increasing the ability of ribosomes to incor­porate amino acids into protein. GH can act alone to promote growth and nitrogen reten­tion, but its full effects are observed only in the presence of insulin and thyroxine. GH also increases lipid mobilization from adipose tis­sue and reduces lipid synthesis.

Thyroxine: Essential for the maintenance of basal metabolic rate (BMR). Hypothyroidism decreases the BMR, the ability to respond to GH secretion, appetite, food intake and thus slows down the growth. Thyroid hormones and goitrogens such as thiouracil or its deriva­tives given to swine, sheep, poultry, beef and dairy cattle showed a positive response on growth, milk/egg production wool production. Corticosteroids: Excessive corticosteroids have adverse effects on bone growth and results in growth retardation in young animals. Excessive amounts of corticoids, especially mineral corticoids, are catabolic to bone and produce a negative calcium balance. Glucocorticoids cause an increase in urinary nitrogen excretion and carbohydrate syn­thesis from amino-acids (gluconeogenesis). Glucocorticoids increase amino acid transport to the liver and decrease the same to skeletal muscles. As a result, there is decrease in pro­tein synthesis in the skeletal muscles. Thus affect the growth of animals.

Specific tissue growth factors affect prolifera­tion and development of specific type of cells. Important tissue growth factors are

- Epidermal growth factor (EGF)

- Nerve growth factor (NGF)

- Erythropoietin (EP)

- Retine and promine

- Chalone

23.9