Final answer:
Long bones grow from the epiphysis and lengthen through a process called ossification. The growth plate, or epiphyseal plate, allows for this lengthening until it is fully ossified. Skeletal maturity occurs around age 20 when further lengthening ceases.
Step-by-step explanation:
Growth and Development of Long Bones: Long bones, such as those in the arms and legs, grow from their ends, which are called the epiphysis. The process of these bones getting longer is called ossification, a term that describes the gradual replacement of cartilage by bone tissue. During fetal development and into childhood and adolescence, the epiphyseal plate, or growth plate, allows for this growth in length. This cartilage plate is located between the bone's diaphysis (shaft) and the epiphyses (ends). As ossification progresses from the primary ossification center in the diaphysis toward the ends of the bone, new cartilage is continuously formed at the epiphyseal plate, which is later replaced by bone, contributing to the bone's lengthening. This cycle of growth continues up until adolescence. After the period of growth, the epiphyseal plate is completely ossified, meaning it is replaced entirely by bone, signifying the end of the bone's growth in length. This is indicated by the presence of an epiphyseal line. This process of growth and subsequent ossification results in bones reaching their maximum length around the age of 20, when skeletal maturity is typically achieved. Bone growth in diameter continues through a process called appositional growth, which can occur even after the bone has finished lengthening. This involves osteoblasts laying down new bone tissue beneath the periosteum while osteoclasts resorb old bone tissue within the medullary cavity.