Transcript Chapter 1: Animal Agriculture

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
• 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
• 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
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
• 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 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
• 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 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 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 Bone
• 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
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
• Differentiation into various structures and
organs begins early in pregnancy
• Morphogenesis (organogenesis)
• Organization of cells into specialized organs
Prenatal Growth
• 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
• 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
• 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
• 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 Growth
• Thyroid hormone
• Mostly thyroxine, some triiodothyronine
• Controlled by TSH (thyroid stimulating hormone)
• Deficiency disrupts metabolism, development and
growth
Hormonal Control of Growth
• 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
• Thyroid hormone
• Hyperthyroidism – increased thyroid activity
• Increased metabolic rate
• Muscle catabolism (breakdown)
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 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 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 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
Heredity Mechanisms in Growth
• Growth influenced by:
• Genetics
• Environment
• Generally 20 to 40% of variation in growth due to
genetics
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 Growth
• Growth from birth to weaning
• Affected by genetic makeup of offspring
• Affected by maternal environment
• care of offspring
• milk production
Heredity Mechanisms in Growth
• 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
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
• 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)
• 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 (Aging)
• 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
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 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 Aging
• Biochemical hypotheses
• Rare, irreparable non-genetic metabolic accidents
occur
• products accumulate in cells to interfere with
metabolism
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