HLTC23H3 Lecture Notes - Weaning, Birth Weight, Sex Steroid

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9 Aug 2013
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Lecture 2 Patterns of Human Growth January 16, 2012
Human Life Stages
1. Prenatal
2. Neonatal
3. Infancy
4. Childhood
5. Juvenile
6. Adolescence
7. Reproductive adulthood
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
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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)
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