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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. Animal growth Starts with single cell

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physiology of growth and senescence chapter 12

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
the phenomenon of growth

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

the phenomenon of growth3

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)
growth and development of humans

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

growth and development of humans5

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

the cell is the unit of growth

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

the cell is the unit of growth7

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
the cell is the unit of growth8

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
growth and development of muscle fat and bone

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
growth and development of muscle fat and bone10

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
growth and development of muscle fat and bone11

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)
growth and development of muscle fat and bone12

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

periods of growth

Periods of Growth

  • Growth generally divided into two periods:
    • Prenatal – prior to birth
    • Post natal – after birth
prenatal growth

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

prenatal growth15

Prenatal Growth

  • Differentiation into various structures and organs begins early in pregnancy
  • Morphogenesis (organogenesis)
    • Organization of cells into specialized organs
prenatal growth16

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

prenatal growth17

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
postnatal growth

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
hormonal control of growth

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

hormonal control of growth20

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
hormonal control of growth21

Hormonal Control of Growth

  • Thyroid hormone
    • Mostly thyroxine, some triiodothyronine
    • Controlled by TSH (thyroid stimulating hormone)
    • Deficiency disrupts metabolism, development and growth
hormonal control of growth22

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

hormonal control of growth23

Hormonal Control of Growth

  • Thyroid hormone
    • Hyperthyroidism – increased thyroid activity
      • Increased metabolic rate
      • Muscle catabolism (breakdown)
hormonal control of growth24

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
hormonal control of growth25

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
hormonal control of growth26

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)
hormonal control of growth27

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
nutrition and growth

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

heredity mechanisms in growth

Heredity Mechanisms in Growth

  • Growth influenced by:
    • Genetics
    • Environment
    • Generally 20 to 40% of variation in growth due to genetics
heredity mechanisms in growth30

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
heredity mechanisms in growth31

Heredity Mechanisms in Growth

  • Growth from birth to weaning
    • Affected by genetic makeup of offspring
    • Affected by maternal environment
      • care of offspring
      • milk production
heredity mechanisms in growth32

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

genetic control of growth mechanisms

Genetic Control of Growth Mechanisms

  • Growth is heritable
  • Elements of growth also heritable
    • Nutrient requirements
    • Hormonal control
    • Metabolic rate
association between growth and other traits

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)
senescence aging

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”
senescence aging36

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)

some hypotheses about aging

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
some hypotheses about aging38

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)
some hypotheses about aging39

Some Hypotheses about Aging

  • Biochemical hypotheses
    • Rare, irreparable non-genetic metabolic accidents occur
      • products accumulate in cells to interfere with metabolism
some hypotheses about aging40

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
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