It is obvious that all human beings go through various cycles during their life, progressing from infant to child to adolescent to adult to senior citizen. Each of these stages builds upon previous biology and experience while evolving from one to the next. The concept that the body changes form as it ages has been around ever since early man sat around campfires and compared grandfather to a newborn baby boy. It is obvious change occurs, but no one knew how or why. They did know, even back then, that if that baby boy got fed regularly and stayed out of the mouth of large animals, chances were good that he would someday look like grandfather. Still, it was hard for early man to grasp the idea, and make much sense out of the fact that grandfather was once a tiny baby himself, and that he occupied all the various body shapes and ages in between the two extremes. That was one of life's mysteries that they just blindly accepted.
So the process of aging was known, early on, to involve noticeable and significant changes in body shape and size, hair color, skin texture, strength, weight, stamina, agility, appetite, thinking ability, hearing, vision, sleeping patterns, communication, and perhaps most notably, wellness. As the baby boy grew into adolescence, and then into manhood, and then into middle age, he no doubt suffered the "slings and arrows of outrageous fortune" as Shakespeare might have described it. Perhaps he wore a few scars to prove his more stressful experiences; and most assuredly, he probably grew to know many aches and pains, loss of energy, sickness and disease, weight fluctuations, and all the other curses and afflictions that accompany growing old.
During Shakespeare's time, the average life expectancy was a mere 35 to 40 years, and people thought the body entered a new "age" roughly every five or so years. According to Shakespeare's estimation, a person only had seven bodies. He wrote about this observation in his play, As You Like It, saying that "... man in his time plays many parts, His acts being seven ages."
The concept of seven-year body change cycles has been found in many sources including the Torah, Buddhist lore, Native American tradition, the New Testament, American folk wisdom, the philosophy of the Greek mathematician Pythagoras, traditional Chinese medicine, and the phases of the moon that change every seventh day, which influence women's reproductive rhythms and hormonal pulses.The seven-year body change cycle has its deepest roots in traditional Chinese medicine dating back to 1500 BC, which claims that natural and normal health changes occur at regular seven-year intervals in women and eight-year intervals in men. The two most significant changes in women's bodies occur at around 14 years, when menstruation begins, and around 49 years, when menstruation becomes less frequent and eventually ceases altogether. It is thought in this medical tradition as women age, much of their vital essence and nutrients are lost in their monthly periods and this transforms them eventually into old, stooped grandmothers. The consensus of understanding today, from all these various opinions, is that every seven years individuals grow a new body, but unfortunately; it is not nearly that routine, nor is it accurate.
Recent research reveals that this popular folk notion is not exactly true because, although most cells in the body do, in fact, reproduce and replace dying cells during the aging process, not all cells do. Certain cells replace themselves many times over during a seven-year period, while some cells never change at all. The truth is that different cells have different life spans and rates of regeneration, depending on the kind of tissue or fluid in which they are located. For example, white blood cells have the shortest life expectancy and only last several days, while neurons in the cerebral cortex of the brain are never replaced. Yes, there are no new neurons added to the brain after birth, and any that die during a person's life time are never replaced. Think about that next time you go out drinking. Every time you get intoxicated, you destroy irreplaceable brain cells!
Adults produce their body weight in red blood cells, white cells, and platelets every seven years, but the cells in the stomach lining only last five days. Liver cells regenerate in four to six weeks, but it takes two full years for all the cells of the liver to turnover. Tooth enamel is one of the hardest tissues in the body, and the cells that form permanent teeth, that replace the milk teeth, often last an individual their entire life. Similarly, fat cells are replaced in adults at the average rate of about ten percent each year, or in other words, humans replace all their fat cells roughly every decade.
Cardiomyocyte heart cells, on the other hand, are replaced in the body at a gradually reduced rate as a person ages. Around age 25, for example, about one percent of these cells are replaced annually. Replacement gradually slows to only about 0.5% at age 70. Even in people with much longer life spans, less than half of the cardiomyocyte cells are usually ever replaced by the body, and those that are not, have been in most people since their birth. In the heart, the cardiomyocyte cells comprise the heart muscle, but the heart is also made up of other connective tissues and other cell types that do indeed grow in size, even though some are never replaced.
Scientists from all around the world are currently studying all the major tissues and fluids of the body to determine turnover rates and the aging process as a whole. While it is obvious that skulls and brains and hearts grow larger after birth, how can it be that certain cells do not reproduce? Where then does all that extra mass come from? In the brain, even though no new neurons are replaced in the cerebral cortex as previously stated, research is still ongoing on other parts of the brain as well. It appears there are lots of other kinds of cells that do get added such as glial cells, which may possibly make up about 90% of the cells in the brain, and lest we forget, the brain is composed 78% of water.
Human hair typically grows at a rate of about half an inch per month consistently across the scalp, depending on diet, age, race, gender, and general health. Human hair goes through three stages of growth. The first stage is the anagen phase, where the hair is actively growing for a period of two to six years. During this phase, cells in the hair follicle actively divide, pushing the hair up and out of the skin layer on the scalp. The next phase is the catagen phase in which the rate of growth stops because the follicle grows dormant. The final stage is the telogen phase where the hair falls out to make way for new hair growth. Human hair has a "terminal length" which is the maximum length a hair will grow, and it varies according to the individual. Some people have a terminal length of only a few inches while other individuals may grow their hair several feet long before the hair follicle eventually dies. The average human head has roughly 150,000 single hairs on it and most hairs are in different stages of growth. If a person were to shave their head completely bald and then grow it back in one event, it would take anywhere from a few months to several years depending on their health and the personal characteristics mentioned here.
The most dramatic physical changes to the human body occur within the first two seven-year body cycles. The skeletal system takes, on average, ten years to renew as bone-dissolving and bone-rebuilding cells work in concert to constantly remodel itself over a typical life span. Bones are the primary structural component of a human body and determine the individual's stature. Generally, girls usually grow until a bone age of about 14 years and boys stop growing after a bone age of around 16 years, depending on when they reach puberty. Children grow at a rate of about 2 to 2 ½ inches per year in early childhood up until they begin puberty, at which time their growth will slow to about 1 ½ inches annually. As they reach their peak growth velocity in puberty, there is acceleration in growth to about 3 to 3 ½ inches per year for girls and 4 inches per year for boys. Growth slows down again after puberty to about 2 ½ to 3 inches per year in girls following menarche (the first period) until they reach their adult height. Girls often reach their growth spurt at puberty about two years earlier than boys, which explains why girls are often taller than boys during early adolescence.
Skin is the largest organ on the body and has the ability to constantly regenerate itself. Human skin consists of primarily two main layers: the epidermis, or surface layer, and the dermis, or deeper layer. There are several other smaller layers of skin located within these two main sections. These include the basal and the stratum spinosum layers of the epidermis, which are mostly responsible for skin regeneration. New skin cells are born all the time and rise into the epidermal layer as old skin cells die and fall away on the surface. Young skin regenerates its epidermal surface area about every two to three weeks. As a person ages, the cell turnover rate slows down, but never completely stops. Direct sunlight is a major reason for this slowdown due to several factors. The skin is comprised of collagen, which gives the skin elasticity. Sunlight lessens collagen production which makes the skin thinner and less resilient. This causes skin cells to become disorganized and malformed and ages the skin to eventually form wrinkles and spots.
The regeneration process, unfortunately, doesn't continue forever, nor always works efficiently, because it is influenced by individual lifestyle habits, choices, environmental factors, and behaviors; which all impact cell renewal. Poor lifestyle choices, harsh living conditions, along with heredity, lack of exercise, and improper diet can develop into chronic conditions such as heart disease, diabetes, high cholesterol, high blood pressure, liver disease, and even numerous forms of cancer.
Organs, tissues, and systems of the body are designed by nature to serve a specific purpose, and when not treated properly, they develop disease and dysfunction which impacts the body's homeostasis or metabolic balance and cell regeneration. The liver, for example, the second largest organ in the body, performs many critical functions such as producing immune agents to fight infection; it neutralizes toxins in the blood, and filters out germs and bacteria from the bloodstream. The liver also makes proteins that regulate blood clotting, produces bile to help absorb fats, and stores glucose for when the body requires energy. No one can live long, or well, without proper liver function because it is the metabolic factory of the entire body.
So with all this diversity in tissues and fluids within the human body, and all the corresponding differences in cell regeneration rates, not to mention all the numerous individual differences in the aging process due to influences from heredity, metabolism, digestion, personality, intelligence, sleep, diet, minerals, allergies, exercise, gender, race, disease, injuries, relationships, emotions, medical care, neighborhoods, weather conditions, and immune system function, in general. How would it ever be possible to conclude that the body changes completely every seven years? There are just too many variables and influences affecting a human body to be able to chop it up into nice, crisp intervals like this. It is just not that easy.
There is no disputing the fact that an infant baby transforms into an old man or woman over the course of a life time. However, rather than look at the normal human life span as one big continuum from birth to death, or divided into decade-long intervals, it is much more interesting, and more understandable, to view it as if it were composed of twelve individual and distinct body changes, each spanning seven years. This folk-notion viewpoint is a very useful tool and just makes better sense for understanding the human metamorphosis, even though it may not be 100 percent accurate. I prefer, and advocate, the seven year cycles because they provide more milestones and age groupings which can be viewed, and studied, as distinct separate bodies.