growth

Second Lecture Overview

  • Focus: Growth and Aging (Lectures 1 & 2)

    • Emphasis on growth in this session

    • Aging will be addressed in the next lecture

Learning Outcomes for Today's Lecture

  • Describe human growth across the life cycle

  • Understand variations in growth within populations

  • Explain hormonal and other mechanisms regulating human growth

Life Cycle Context

  • Importance of framing growth within the life cycle

  • Growth and aging are interlinked processes occurring simultaneously

  • Milestones in the life cycle are associated with key attributes related to growth and aging

  • Introduction to senescence and its connection to the inevitability of death

Rationale for Life Cycle

  • Physics and Biology:

    • Growth and aging are affected by the second law of thermodynamics (increasing disorder/entropy over time)

    • Humans are complex organisms with optimal functioning, existing as a temporary reversal of this law

    • Example: clouds as organized but unstable structures, emphasizing the transient nature of life

Genetic Influences on Growth

  • Growth is directed by genetic instructions (genome)

    • Analogy: genome as an instruction manual for complex outcomes (e.g., origami birds)

    • Environment affects the expression of genetic instructions

  • Continuous adaptations occur; there is no final product in human growth

Key Processes in Growth

  1. Hyperplasia:

    • Increase in the number of cells through mitosis and meiosis

  2. Hypertrophy:

    • Increase in cell size (e.g., muscle and fat cells)

  3. Extracellular Matrix Formation:

    • Generation of connective tissue, contributing to overall growth

Historical Documentation of Growth Patterns

  • Mont Belier's observations on growth:

    • Noted seasonal differences, with more pronounced growth in spring/summer due to food availability

    • Identified diurnal variation in height, with individuals being taller in the morning than evening due to gravitational effects on intervertebral discs

    • Documented changes in the velocity of growth across the lifespan:

    • Rapid growth from birth to about 2 years

    • Period of steady growth

    • Adolescent growth spurt

Detailed Growth Patterns

  • Growth Velocity: Influences

    • Early postnatal growth: initially rapid before declining

    • Inflection points in growth charts represent changes in growth rate

    • Peak height growth velocity during mid-pregnancy

    • Weight gain generally peaks later than height growth during prenatal development

Implications of Growth Data

  • Individual variability observed in growth charts

    • Comparison of plotted data against stylized average growth charts

    • Recognizing physiological challenges during pre and post-birth

    • Prenatally: limitations imposed by placental capability

    • Postnatally: adaptation from maternal support to self-sustaining life, leading to initial dip in growth

    • Catch-Up Growth: Compensating for initial challenges in growth postnatally

Sexual Dimorphism in Growth

  • Growth Patterns for Males and Females:

    • Males slightly taller than females early in life

    • Girls reach adolescent growth spurt before boys; boys typically experience a more significant growth spurt

  • Variability in timing for the onset of puberty and the adolescent growth spurt

    • Leptin levels marked influence on the timing of puberty and growth spurts

Challenges in Female vs. Male Growth

  • Developmentally different trajectories seen in weight and height growth charts:

    • Girls typically have higher peak weight gains correlated with earlier onset of puberty

    • Weight distribution differences attributed to gonadal sex steroids (testosterone for males, estrogen for females)

Growth Charts as Clinical Tools

  • Importance of growth charts in identifying pathologies:

    • Example: tracking a child’s growth pattern to identify potential health issues (like hyperthyroidism)

  • Understanding growth velocity indexed with height and weight across the lifespan

  • Predicting adult height based on early childhood height (50% of final height attained by age 2 concept)

Key Hormonal Regulators in Growth

  • Growth Hormone (GH):

    • Secreted from the anterior pituitary, promotes protein synthesis, cell division (hyperplasia), and cell enlargement (hypertrophy)

    • Affects both muscle and bone growth

    • Insulin-like Growth Factors (IGFs) stimulated by GH play significant roles in growth

Fetal Growth Regulation

  • Insulin: Critical for fetal growth through IGF actions

    • Unmanaged maternal diabetes leads to macrosomia due to fetal adaptations to excess glucose

  • Thyroid hormone: also crucial in bone and muscle growth during fetal life

Pubertal Growth Patterns

  • Pulsatile secretion of growth hormone initiates at puberty, linked to gonadal steroids

  • Reasons for cessation of the adolescent growth spurt include the closure of epiphyseal plates, which limits further height growth due to gonadal steroids' effects

Conclusion

  • Recap of growth patterns across the life cycle

  • Transition to aging and senescence in the next lecture, emphasizing the continuum of growth and aging processes

  • Encouragement for review and preparation for the next session on aging