The human body reaches its full physical maturity, including the cessation of skeletal growth, typically by the early twenties. This marks the end of the growth phase defined by cell proliferation that increases the overall size of organs and structures. The question of which body parts continue to “grow” throughout life persists, often confusing true, size-increasing growth with the constant process of cellular replacement and changes due to aging. To understand continuous biological change, it is necessary to distinguish between actual cell division that adds mass and the maintenance cycle that replaces old cells with new ones.
The Myth vs. Reality: Cartilage and Gravity
The nose and ears are frequently cited as body parts that never stop growing, but this perception is largely an illusion created by the effects of time and gravity. The underlying structure of these features is primarily composed of cartilage, a strong yet flexible connective tissue. After adolescence, the cartilage itself does not continue to expand indefinitely through cell division.
The visible change in size or shape is mainly attributed to the weakening and degradation of the connective tissues over many decades. Cartilage is stabilized by structural proteins like collagen and elastin, and as the body ages, it becomes less efficient at producing new fibers. This loss of elasticity causes the cartilage structure to loosen and sag. Combined with the downward pull of gravity, this results in the appearance of longer earlobes and a droopier nasal tip.
Continuously Renewing Tissues
Certain external structures exhibit continuous production throughout a person’s life, specifically hair and nails. These structures are not living tissue once they emerge from the skin, but the machinery that creates them remains constantly active. Hair is formed by the hair follicle, where matrix cells divide rapidly, push upward, and undergo keratinization. This process fills the cells with the hard protein keratin, forming the visible hair shaft.
Nails grow from the nail matrix, which is the only living part of the nail structure, located beneath the skin at the base. This matrix continuously produces new cells that flatten and compress into the hard, protective nail plate, pushing the older, dead cells forward. Fingernails grow at an average rate of approximately 3.5 millimeters per month.
Internal Renewal and Cellular Lifecycles
Beyond external structures, the most significant and continuous form of “growth” in the adult body is the high rate of internal cellular turnover. The body constantly replaces old, damaged cells with new ones to maintain tissue function, a process distinct from increasing the size of an organ. This cellular replacement occurs at a high rate, with an estimated total cellular mass turnover of about 80 grams per day.
Blood cells, particularly red blood cells, are the largest contributor to this turnover, accounting for nearly 90% of the approximately 330 billion cells replaced daily. These cells have a relatively short lifespan of about four months, requiring the bone marrow to continuously generate replacements.
The epithelial cells lining the gut and the skin’s outer layer, the epidermis, also exhibit high turnover rates, with the entire epidermis being replaced roughly every 27 days. Even complex organs like the liver demonstrate regenerative capacity, with most of its cells being replaced within a few years.
This constant, rapid maintenance ensures that tissues remain fresh and functional, even though the organ’s overall size is not increasing. While some cell types, like most neurons in the central nervous system, are largely permanent, the vast majority of the body’s cells are part of a dynamic, perpetual renewal cycle.