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University of Toronto Scarborough
Health Studies
R Song

Lecture 2 –Patterns of Human Growth January 16, 2012 Human Life Stages 1. Prenatal 5. Juvenile 2. Neonatal 6. Adolescence 3. Infancy 7. Reproductive adulthood 4. Childhood 8. Old age and senescence 1. Prenatal Growth and Development Cellular Growth:  Hyperplasia: new cell growth (division) by mitosis  Hypertrophy: enlargement (growth) of already existing cells  First Trimester o Period of greatest cell production (multiplication of millions of cells and differentiation of cells into hundreds of different body parts)  Second Trimester o Completion of cell differentiation into tissues and organs o Embryo now a fetus o See increase in rate of growth of length  Third Trimester o Growth rate in weight increases o See development and maturation of circulatory, respiratory, digestive systems  Prenatal Life o Especially during first trimester (greatest metabolic activity/ cell growth) –is a period of high susceptibility to growth pathology caused by either  Inheritance of genetic mutations (congenital)  Exposure to harmful environmental agents that disrupt normal course of development  Importance of maternal nutrition and lifestyle 2. Postnatal Life Stressful  Fluid to gaseous environment, constant temp. To variable  From reliance on mom’s oxygen and nutrients to self-reliance for digestion, respiration, and waste elimination  Nearly ½ of all neonatal deaths occur during first 24 hours  Most neonatal mortality due to LOW BIRTH RATE resulting from growth retardation during fetal life (prematurity, congenital defects, maternal nutrition/lifestyle)  Importance of Socioeconomic Status (SES) ultimately o (ie. low birth weight in HIGH SES: 5.9%, Low SES: 23.6%) Brody (1945)  Grand unifying theory of growth: all animals exhibit the same curve of growth, but some mammals have a growth spurt soon after birth (cow/mouse), while others delay the spurt until later in life (humans)  All organisms exhibit “self-accelerating” and “self inhibiting” phases of growth  But note: humans actually have more than one phase of self-acceleration and self-inhibition o (3 phases: gestation, mid-growth spurt, and adolescent growth spurt) Bogin 1997: Figure 1  Five stages of life  Shows rate of growth  Infancy: steep decline in growth  Significant growth: adolescence Measures of Growth at Birth  Weight at birth  Recumbent length (lying down)  Circumference of head, arm, chest  Skinfold thickness 3. Infancy  Period (up to age 3) of most rapid postnatal growth (though decelerated compared to prenatal)  Principle reliance on mother’s breast milk  Deciduous dental eruption (baby teeth)  Rapid development of motor and cognitive skills (skeletal/muscular/ nervous systems)  Importantly: BRAIN GROWTH (brain grows more rapidly in infancy than almost any other tissue/ organ) Infant Brain Growth and Hypothalamus  Brain structure responsible for neurological and endocrine control; vital for reproductive system  During fetal and early infancy; hypothalamus produces high levels of gonadotropin –releasing hormone (GnRH)  GnRH responsible for release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary gland  LH and FSH travel in bloodstream to sex organs, stimulating production and release of estrogen or androgen hormones  These gonadal hormones partly responsible for rapid rate of growth during early infancy  By late infancy, hypothalamus is inhibited o GnRH stops, sex hormones decrease, suspending reproductive maturation  Hypothalamus not reactivated until just before puberty/onset of sexual maturation Human Brain Growth and Function  Newborn uses 87% of its resting metabolic rate (RMR) (body energy expenditure during rest) for brain growth and function o Under 5 years: 40-85% RMR o By 5 years: 44% of RMR for brain growth/ function o Adult human: 16-25% of RMR for brain function Normal Birth Weight Infants  Amount and rate of growth in length and weight during first six months of life similar  Rather than variation due to hereditary and environmental factors, including ethnic and socioeconomic status, often compare developed vs. developing nations  Likely that breastfeeding, which supplies all nutritional, immunological and psychological needs of infant overrides the effect of variations in other aspects of environment  After 6 months: with reduced breast milk nutrition (since infants’ need increase), can see significant differences in growth and size depending on diet/health (maternal SES) Weaning  Transition to solid food that marks end of infancy, usually 24-36 months in pre-industrialized society  Notably early compared to other mammals –generally when 3.2-4.9 times birth weight o Humans before 3x birth weight  Coincides with emergence of last deciduous tooth (usually m2); thus have all deciduous teeth to process solid foods  Period of great morbidity & mortality (to age 5) o Synergy of malnutrition-infection  Hard tissue evidence: Enamel Hypoplasia (generalized stress) o Defects of enamel o Reflect age of period of stress “Weaning Stress  Dietary transition  Food and utensil contamination  Underdeveloped GI/ immune system  Period of significant growth  Greater social interaction  Natural curiosity (exposure) Factors influencing nursing and weaning  Developmental factors: 1 molar eruption, digestive  Maternal responsibilities maturity, immunity  Gender inequality  Timing of onset of lactation  Epidemiological conditions  Tradition/ cultural perceptions of food appropriateness  Social climate (within families, ie. fathers and external)  Resource availability (food production/ seasonal  Female body image, sexuality instability)  Child (health, development, appetite, refusal/initiation)  Maternal education 4. Childhood  The period following infancy, when then youngster is weaned from nursing, but still depends on older people for feeding and protection (Bogin, 1997); ends at weaning  3-7 years; defined by its own pattern of growth, feeding behaviour, motor development (including, adult walking gait by 7) and cognitive  Growth rate now levels off at ~5 cm per year (unusual for mammals, who experience continued deceleration in growth velocity after infancy)  Kids weaned but still reliant on adults/ older kids for food and protection  IMPORTANTLY: 1. Replacement of deciduous teeth with permanent dentition (M1 =5-6 years) 2. Completion of brain growth (in weight) (see correlation of M1 eruption with brain growth completion ~7 years)  Dependence of others due to: 1. Need for diet in energy, lipids, and protein to support brain growth 2. Having immature dentition (deciduous), within enamel and shallow roots, which cannot process adult-type diet 3. Relatively small body size –thus small digestive system, which limits total food intake, furthering requirements for (low volume) nutrient-dense foods  Combines with: motor and cognitive immaturity, social inexperience and vulnerability to disease and accidents requiring protection Childhood Growth  Great predictability within individuals and between populations (of normal children)  Generally see similar distance and velocity curves for height, weight, and body composition of normal children over time  Factors influencing amount and rate of growth: heredity, nutrition, illness, SES, psychological well-being  “5-7 year old shift” and changing cognitive, social abilities (capable of own feeding and care –independence Long Bone Lengths of infants and children  N.B. wide range of variation even within a single relatively homogenous population  Hand bone changes from newborn to juvenile/adult Postcranial Epiphyseal Union  Epiphyseal closure of human long bones as in indicator of age of development  Determine the age of a ch
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