Final answer:
The growth spurt triggered by increased secretion of growth hormone from the pituitary gland signifies the beginning of adolescence.
Step-by-step explanation:
A growth spurt triggered by an increased secretion of the growth hormone from the pituitary gland marks the beginning of adolescence. During the adolescent growth spurt, individuals experience a period of rapid growth in height and weight, along with significant changes in body composition and proportions. This is controlled by several hormones, including growth hormone, thyroid hormone, and sex hormones.