Physiology of Growth and Senescence Chapter 12

1. Physiology of Growth and Senescence Chapter 12 • Each organism starts as a single cell • Process of development and differentiation • Results in mature individual with many trillion cells

2. Animal growth • Starts with single cell • By birth – individual has most of same physical characteristics as an adult • True growth – increase in amount of protein and mineral in the body • Fat (adipose) and water accumulation not part of true growth The Phenomenon of Growth

3. The Phenomenon of Growth • Wide variation within species • e.g. Clydesdale and Shetland pony are very different in size • Different parts of body grow at different rates after birth (see Figure 12.2)

4. Early in gestation (period of pregnancy) • Embryo stage • At 2 months – 1.5 inches but similar form to adult • Third month – called a fetus • Seventh month - ~ 15 inches long and 2 lb • Parturition – time of birth – 6 to 8 lb, 19 to 21 in • growing very rapidly near birth Growth and Development of Humans

5. Childhood – rapid growth continues to ~ 2 yr • Starts to level off until growth spurt in adolescence • Growth generally stops by age 18-20 • Maximum height generally at ~ age 26 • Gradual decrease in height afterward • Due to decrease in cartilage pad thickness Growth and Development of Humans

6. Growth results from increase in: • Cell number – hyperplasia • Cell size – hypertrophy • Hyperplasia – results in increase in DNA • Number of cells increases so amount of chromosomal material increases • Hypertrophy – results in increase in protein • From increase in amount of cytoplasm The Cell is the Unit of Growth

7. The Cell is the Unit of Growth • Three different types of cells are found • Permanent cells – cease dividing at embryo stage • Stable cells – continue to divide during growth but cease division at adult stage • Labile cells – continue to divide and differentiate throughout life

8. The Cell is the Unit of Growth • Cell division at maturity • Cell number remains relatively constant • each cell division results in one viable daughter cell and one daughter cell which is lost • Cancer Cells • Restraints on cell division are largely removed so uncontrolled growth

9. Growth and Development of Muscle, Fat and Bone • Muscle cells – form through unique series of events • Cells which will become muscle cells (myogenic cells) divide many times until becoming a myoblast • Myoblasts fuse to form myotubes • Further growth of muscle due to hypertrophy

10. Growth and Development of Muscle, Fat and Bone • Fat – consists of adipose cells and connective tissue • Adipocyte – mature adipose cell • Adipocyte – results from maturation of immature cell called adipoblast. • Fat tissue increases and decreases by changing size of adipocytes

11. Growth and Development of Muscle, Fat and Bone • Two types of fat tissue • White fat – most fat in mature individuals • depot of stored energy • Brown fat – found in newborn animals or hibernating animals • very active and helpful in maintaining body temperature (very important in newborns)

12. Bone - ~50% mineral: 50% organic material and water • Bone formed by interaction of three cell types • Chondrocytes – cells that produce cartilage • Osteoblasts – produce bone collagen • Osteoclasts – break down bone during resorption • Bones grow by ossification at the epiphysial plate • Bones stop growing when completely ossified Growth and Development of Muscle, Fat and Bone

13. Periods of Growth • Growth generally divided into two periods: • Prenatal – prior to birth • Post natal – after birth

14. Between fertilization and birth • Fertilization – union of sperm and egg • Single cell – diploid number of chromosomes • Two complete sets of chromosomes • One set of chromosomes from each parent • Fertilized egg begins to divide • Rate varies widely among species Prenatal Growth

15. Prenatal Growth • Differentiation into various structures and organs begins early in pregnancy • Morphogenesis (organogenesis) • Organization of cells into specialized organs

16. Time for prenatal growth • Varies widely among farm animal species • 110-115 days in pigs 335-345 days in horses • Degree of maturity during prenatal growth • Similar among farm animal species • Among non-farm species, some give birth to offspring which are very helpless Prenatal Growth

17. Prenatal Growth • Size of offspring at birth • Controlled by genes supplied by both parents • Also controlled heavily by uterine environment • small mothers will have small offspring even if genes from sire would promote large birth weight

18. Postnatal Growth • Growth follows a sigmoid (S-shaped) curve in virtually every animal and plant species • Growth very rapid to about 1/3 to 1/2 of mature weight • Starts to level off until mature size is reached

19. Many hormones involved in growth regulation • Growth hormone (GH) (somatotropin) • Secreted by anterior pituitary • Protein hormone • Removal of pituitary causes growth to stop • injection of pituitary extracts will cause growth to resume Hormonal Control of Growth

20. Hormonal Control of Growth • Growth hormone (GH) (somatotropin) • Acromegaly – caused by excess growth hormone • head, hands and feet enlarged • GH promotes protein accretion • GH reduces amount of fat stored in body

21. Hormonal Control of Growth • Thyroid hormone • Mostly thyroxine, some triiodothyronine • Controlled by TSH (thyroid stimulating hormone) • Deficiency disrupts metabolism, development and growth

22. Thyroid hormone • Hypothyroidism – low thyroid activity • Reduced intake • Low blood sugar • Lower liver glycogen storage • Lower nitrogen retention • Increased fat deposition Hormonal Control of Growth

23. Hormonal Control of Growth • Thyroid hormone • Hyperthyroidism – increased thyroid activity • Increased metabolic rate • Muscle catabolism (breakdown)

24. Hormonal Control of Growth • Androgens • Male hormones • Stimulate growth • Castration (removal of testes) • slows growth, accelerates fattening process • Anabolic steroids • synthetic hormones with growth promoting effects

25. Hormonal Control of Growth • Androgens • Anabolic steroids • used in beef industry • implanted into ear • ~90% of all feedlot cattle are implanted • regulated by FDA – kept at safe levels • currently banned by European Union

26. Hormonal Control of Growth • Estrogens • Produced by ovary • Increased with onset of puberty • Aid in regression and closure of plate of long bones (explains why girls generally stop growing after puberty)

27. Hormonal Control of Growth • Insulin • Protein hormone – secreted by pancreas • Stimulates growth – synthesis of RNA and protein • Glucocorticoids • Produced by adrenal glands • Inhibitors of growth • Cortisol – decreases synthesis of DNA and protein

28. Nutrients must by obtained by consumption • Effect of underfeeding • Depends on: • age at which underfeeding occurs • length of underfeeding period • type of deficiency (energy, vitamin etc) • Recovery from underfeeding • Rapid (compensatory) growth Nutrition and Growth

29. Heredity Mechanisms in Growth • Growth influenced by: • Genetics • Environment • Generally 20 to 40% of variation in growth due to genetics

30. Heredity Mechanisms in Growth • Prenatal Growth • If a genetic potential for large birth weight • may be inhibited by several factors • e.g. piglets from large litters may have diminished birth weight

31. Heredity Mechanisms in Growth • Growth from birth to weaning • Affected by genetic makeup of offspring • Affected by maternal environment • care of offspring • milk production

32. Postweaning growth • Maternal influence lessens • Selection projects have demonstrated genetic influence on postweaning growth • Late maturing animals generally leaner at market weight • Early maturing animals generally fatter at market weight Heredity Mechanisms in Growth

33. Genetic Control of Growth Mechanisms • Growth is heritable • Elements of growth also heritable • Nutrient requirements • Hormonal control • Metabolic rate

34. Association between Growth and Other Traits • Metabolic rate not directly related to weight • Brody – doubling body weight increases metabolic rate ~ 73 % • Basal metabolism varies to .73 power (W.73)

35. Senescence (Aging) • Less important in farm animals than humans • Farm animals generally culled for production reasons prior to old age • Performance usually peaks at some “middle age”

36. Some thought that life span related to total calorie expense per kilogram adult body size during life • Value is similar among many species (but not humans) • Rate of decline in velocity of growth with increasing age is generally inversely proportional to the length of life Senescence (Aging)

37. Some Hypotheses about Aging • Genetic hypotheses • Accumulation of mutations causes organ degeneration • Telomeres (ends of chromosomes) become shorter at each cell division • Shorter telomeres ultimately stop cell division

38. Some Hypotheses about Aging • Immunological hypothesis • Gradual loss of ability to form antibodies • Increases susceptibility to some infectious diseases • Developmental hypothesis • Aging results from over-differentiation (extreme cellular specialization)

39. Some Hypotheses about Aging • Biochemical hypotheses • Rare, irreparable non-genetic metabolic accidents occur • products accumulate in cells to interfere with metabolism

40. Some Hypotheses about Aging • Biochemical hypotheses • Free radical theory • lipids in cell membrane exposed to free radicals, leading to unstable cells • Glycosylation theory • results in a deterioration of organ function