Growth & Allometry Flashcards
Hyperplasia vs Hypertrophy
Hyperplasia: increase in cell number due to cell proliferation (cell division or mitosis)
Hypertrophy: increase in the size of cells
Factors Determine Animal Growth: Genes GH1 Growth Hormone GHRH Somatostatin GH IGF-1 HG1
GH1 gene: provides instructions for making growth hormone protein
Growth hormone produced in growth-stimulating somatotropic cells of pituitary gland
Protein encoded by this gene is a member of the somatotropin/prolactin family of hormones (play important role in growth control)
PRL and GH family ligands expressed in anterior pituitary and uteroplacental compartment
GHRH: stim pituitary gland to produce GH (somatotropin)
Somatostatin (secreted by cells of hypothalamus and stomach, intestine, and pancreas) inhibits GH production
GH: production of IGF-1 in liver and also secreted locally at tissue level
IGF-1: causes growth of bones and other tissues of the body
HG1 gene, along w 4 other related genes, located at growth hormone locus on chromosome 17 in humans –mutations or deletions of this gene leads to growth hormone deficiency and short stature
Factors Determine Animal Growth: Endocrine
Production of growth hormone is triggered when 2 other hormones are turned on (activated):
- Ghrelin which is produced in the stomach
- GHRH which is produced hypothalamus
Release of growth hormone into body peaks during puberty and reaches low point at ~age 55
Factors Determine Animal Growth: Nutrients
Nutrient requirements greater for younger animals
Protein requirements also higher for young animals b/c gaining more muscle than fat
Consumption of high-quality animal protein=improve human nutrition, growth, development, and health
Most important aa: methionine, lysine, isoleucine, threonine and leucine
- Deficiency in any of these aa=slowing of growth and delayed onset of maturity
Factors Determine Animal Growth: Environment
Birds and mammals attain greater body size when in cold regions than in warm regions
Bergmann’s rule: predicts body size is positively correlated with latitude and elevation, and hence, with decreasing environmental temp
Populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions
Growth Divided into 4 Phases in Humans
- Fetal: where predominant endocrine factors controlling growth are insulin and insulin-like growth factors
- Infancy: where growth mainly dependent upon nutrition
- Childhood: where GH-IGF-I axis and thyroid hormone are most important
- Puberty: where along w GH-IGF-I axis, activation of hypothalamo-pituitary-gonadal axis to generate sex steroid secretion becomes vital to growth completion
Growth Hormone
Synthesized and secreted by anterior pituitary cells called somatotrophs
Somatotrophs release between 1-2 milligrams of the hormone each day
Vital for normal physical growth in children
Levels rise progressively during childhood and peak during puberty (growth spurt)
Released under control of: GHRH, Gherkin, Somatostatin, IGF-1
Insulin-like Growth Factor-I Research
Animals are born with an inherited rate of synthesis of IGF-I and this can be related to its subsequent growth rate.
Standard (large) dogs have higher blood levels of IGF-I than do miniature ones. Mice w high circulating conc of IGF-I grow at greater rate than do mice w low levels of IGF-I
Plasma IGF-I conc can be related to:
- dietary energy and protein supply and are thought to mediate “anabolic status”
- IGF-I synthesis can be related to animal’s environment
EX: IGF-I conc are depressed in cold conditions when energy must be directed toward heat production rather than growth (acute response)
***Factors Activating of the Pituitary-Gonadal Axis during Puberty
Gonadotrophin releasing hormone (from hypothalamus)
Kisspeptin (released by neurons in hypothalamus): stim release of GnRH
Leptin (from adipose tissue): regulate energy balance by inhibiting hunger & stim release of GnRH
Tachykinin (neuropeptides): induce contraction of gut tissue & stim release of GnRH
ALL lead to increase of GH secretion=pubertal growth and fusion of growth plates cartilage
Metabolic Scaling
relation b/w metabolic rate and body size
- Rhino requires more food than vole
- If we pile up grass for both to eat in a week, vole’s pile is far larger than vole itself and rhino’s pile is much smaller than rhino itself
The energy needs for the species are NOT proportional to their body size
Resting Metabolic Rate (BMR)
allometric function of body in related species
Weight-specific metabolic rate decreases as weight increase
Max Runner (1882) “Rubner surface law” or "surface hypothesis"
stated metabolic rate of a given animal is roughly proportional to their body surface area
Based on Rubner’s theory:
Mammals maintain high, relatively constant body temp and thus tend to lose heat to environment
Rate of heat loss is proportional to animal’s body surface area
Small mammals have more surface area per unit of weight than do large mammals and thus lose heat more rapidly
Heat lost must be replaced metabolically to stay warm –small mammals must produce heat greater per unit of weight than large ones
Growth Hormone Secretion in Response to Stress in Man
Emotional and physical stimuli result in increased adrenocortical activity
Plasma cortisol-levels rise in response to trauma, surgery, fever and hypo-glycaemia
Increased secretion of growth hormone has been shown during insulin and tolbutamide induced hypo-glycaemia, and during exercise and starvation
Medical student told he had received large dose of insulin and would experience severe hypo-glycaemic symptoms (in fact injected w saline)
Plasma sugar remained unaltered but increase in plasma cortisol and growth hormone
Saline injection in 4 other control subjects, w/o prior suggestion, produced no changes in plasma sugar, cortisol or growth hormone
THUS due to stress
Pyrogen (produces fever) injection in 2 male and 2 female subjects –increase in growth hormone secretion & adrenocortical activity
Stim of growth hormone secretion occurred in response to emotional stress and to the injection of pyrogen
Stim of cortisol secretion by lysine 8-vasopressin was accompanied by rise in plasma growth hormone levels in women but not men
Plasma sugar-levels were unchanged in men and women
The results suggest that stress stimulates both pituitary growth hormone GH and ACTH secretion which is NOT due to a measurable hypo-glycaemia
Epidemiologist Dr Scott Montgomery et al. (UK) found significant relationship b/w parents who had argued and slower growth rate in their children
If they are taken out of that unhappy situation, hormone levels recover
Believes reason for link b/w stress and growth can be put down to evolution
At a time of stress, in an evolutionary way to switch off everything that isn’t essential, children don’t put energy into growing when might need it to defend themselves. Once they are out of danger, they can afford to use your energy on growing again
Allometric growth vs Isometric growth
Allometric growth: unequal growth rates showed by diff body parts and body as a whole
- Growth rates of body parts differ from growth rate of whole body
Isometric Growth: equal growth rates shown by diff body parts with that of the body as a whole
- Body parts grow at sim rates as growth rate of whole body
