15.7 - Growth in Humans and Animals

The Concept of Growth

Growth is a permanent, irreversible process that involves an increase in the number of cells, size, volume, and dry mass of an organism.

Parameters to Measure Growth

• Wet Mass: Easy to measure but less accurate because water content fluctuates.

• Dry Mass: The most accurate parameter because it measures the organic matter of the organism. However, it requires killing the organism (dehydration in an oven), which is unsuitable for studying continuous human or animal growth.

• Height / Length: Easy to measure and non-destructive. Commonly used to track human growth.

• Volume: Measures the space occupied by the organism.

Sigmoid Growth Curve

The growth of most multicellular organisms (humans and animals) over time shows a sigmoid (S-shaped) curve.

Phases of the Sigmoid Growth Curve

• Lag Phase:

  • Growth rate is slow.
  • Organism is adapting to the environment, and cell division rate is low.

• Exponential Phase / Log Phase:

  • Growth rate is rapid and at its maximum.
  • Cell division and cell elongation are highly active.

• Decelerating Phase:

  • Growth rate slows down.
  • Limiting factors (e.g., nutrients, space, accumulation of waste) begin to restrict growth. Cell division rate equals the rate of cell death.

• Stationary Phase:

  • Growth ceases (zero growth rate).
  • The size or mass of the organism remains constant. Cell division is only for tissue repair and replacement of dead cells.

• Senescent Phase / Senescence:

  • Negative growth rate.
  • Physical degeneration of cells and organs occurs, leading to aging and eventually death.

Growth in Organisms with Exoskeletons

Insects and other arthropods have a hard outer skeleton called an exoskeleton made of chitin, which does not expand. Therefore, they have an intermittent growth curve (staircase-shaped curve).

Ecdysis (Moulting)

• To grow, insects must shed their old exoskeleton in a process called ecdysis (moulting).

• During ecdysis, the insect takes in air or water to expand its body while the new, soft cuticle beneath is still flexible.

• Once the new cuticle hardens, growth stops until the next ecdysis.

• The stages between successive ecdysis are called instars.

Metamorphosis

Insects undergo metamorphosis to transition from larval stages to adults.

Complete Metamorphosis

• Involves four distinct stages: Egg $\to$ Larva $\to$ Pupa $\to$ Adult (Imago).

• The larva (e.g., caterpillar, maggot) looks entirely different from the adult (e.g., butterfly, housefly).

• The pupa is a non-feeding, transitional stage where tissue reorganization occurs.

Incomplete Metamorphosis

• Involves three stages: Egg $\to$ Nymph $\to$ Adult (Imago).

• The nymph (e.g., young cockroach, young grasshopper) looks like a miniature, wingless version of the adult.

• The nymph undergoes several stages of ecdysis (instars), gradually developing wings and reproductive organs until it becomes an adult.