question: How do certain hairs on our bodies (for example, on legs) know what length to stop growing at?
comment: The cells in hair roots are not always active. When they are active, hair grows, and then the cells rest some time, which makes the hair "die" and drop. Then after some time they start growing a new hair. The length of the active time varies between different kinds of hair roots and the longer it is, the longer the hair.
passages: Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair.
   Hair grows in cycles of various phases: anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase (names derived using the Greek prefixes ana-, kata-, and telos- meaning up, down, and end respectively). Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase, while 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor. DLX3 is a crucial regulator of hair follicle differentiation and cycling. Specifically, colocalization of phosphorylated Smad1/5/8 complex and DLX3 regulate role for BMP signaling to Dlx3 during hair morphogenesis in animal models.
   In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet. Exceptions to this rule can be observed in individuals with hair development abnormalities, which may cause an unusual length of hair growth.
  
  Section::::Growth inhibitors and disorders.:Chemotherapy.
   The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year. 
  
  Section::::Growth cycle.:Anagen phase.
   Section::::Description.:Human hair growth.
  
  Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and lips.
  
  Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases; all three occur simultaneously - while one strand of hair may be in the anagen phase, another may be in the telogen phase. Each has specific characteristics that determine the length of the hair.
   Section::::Growth cycle.:Catagen phase.
   The catagen phase, also known as the transitional phase, allows the follicle to, in a sense, renew itself. During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests," cutting the hair strand off from its nourishing blood supply. Signals sent out by the body (that only selectively affect 1 percent of all hair of one's body at any given time) determine when the anagen phase ends and the catagen phase begins. The first sign of catagen is the cessation of melanin production in the hair bulb and apoptosis of follicular melanocytes. Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward. While hair is not growing during this phase, the length of the terminal fibers increase when the follicle pushes them upward.
   Section::::Hair growth.:Anagen phase.
  
  Anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years; this period is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
  
  Section::::Hair growth.:Catagen phase.
   Other common causes of hair damage include the use of relaxers and other harsh chemicals; use of a razor to cut curly hair, which weakens the hair cuticle and promotes frizz; and harsh shampoos that strip the hair of its natural oils.
  
  Having some hairs that are shorter than others is not necessarily a sign of breakage. Hair naturally sheds as it reaches the end of its growth cycle, and a new hair grows in its place; the average lifespan is about six years.
  
  Section::::Causes.:Humidity.
   The maximum hair length that is possible to reach is about 15 cm (6 in) for infants (below the age of 1), about 60 cm (24 in) for children, and generally 100 cm (40 in) for adults. Documentation for decrease of the maximum length with age cannot be found in the literature. Some individuals can reach excessive lengths. Lengths greater than 150 cm (59 in) are frequently observed in long hair contests. Xie Qiuping had the longest documented hair in the world, measuring 5.627 m (18 ft 5.54 in) in May 2004.
   At some point, the follicle will begin to grow again, softening the anchor point of the shaft initially. The hair base will break free from the root and the hair will be shed. Within two weeks the new hair shaft will begin to emerge once the telogen phase is complete. The process results in normal hair loss known as shedding.
  
  Section::::Growth inhibitors and disorders.
   Section::::Growth cycle.:Telogen phase.
  
  During the telogen or resting phase (also known as shedding phase) the follicle remains dormant for one to four months. Ten to fifteen percent of the hairs on one's head are in this phase of growth at any given time. In this phase the epidermal cells lining the follicle channel continue to grow as normal and may accumulate around the base of the hair, temporarily anchoring it in place and preserving the hair for its natural purpose without taxing the body's resources needed during the growth phase.
   The determination of a mechanical threshold using von Frey filaments requires a number of discrete tests using filaments with different bucking forces. There are two commonly used algorithms, the up-down method and the percent response method. The up-down method is most commonly used, usually requiring a minimum of four tests after the first response is detected. The first measurement should be made with a filament with a buckling force close to the expected mean of the population. Errors may result if testing is commenced a long way above or below the mean. These errors have been evaluated by a combination of experimentation and mathematical simulation 
   Hair length is most often measured (in centimeters or inches) from the line of the scalp on the forehead up over the highest point of the skull to its termination, and sometimes from the crown, the latter resulting in 10 centimeter lower values in average. In cosmetology, hair lengths are usually categorized according to the part of the body where the bulk of the longest hair terminates: "chin level", "shoulder length", "lower shoulderblade"/"mid-back level", "waist length", "hip-length", "classic length" (extends to almost "buttock-length", i.e. upper thigh-length, where the legs meet the buttocks), "thigh-length", "knee-length" and "ankle/floor length" hair.
   The auxological term "short-term growth" denotes growth characteristics that become evident when measurements are being performed within intervals of less than one year (e.g. monthly, weekly or even daily). Mainly knemometry has been used for this purpose, but also very frequent measurements using conventional devices for height measurements. 
   Though each plant grows according to a certain set of rules, each new root and shoot meristem can go on growing for as long as it is alive. In many plants, meristematic growth is potentially indeterminate, making the overall shape of the plant not determinate in advance. This is the primary growth. Primary growth leads to lengthening of the plant body and organ formation. All plant organs arise ultimately from cell divisions in the apical meristems, followed by cell expansion and differentiation. Primary growth gives rise to the apical part of many plants.
   In 1971 it was described that it was very difficult to find a CR cell in the adult cortex, because of the constant number of these cells and the fact that as the brain grows, the distance between these cells increases, required the observation of a great number of preparations to find one of these cells.
  
  In mice, CR cells are generated very early in the development, appearing between 10,5 and 12,5 embryonic days.
   Section::::Other techniques.
   Slow Adaption: The dominating model suggests that slow adaptation occurs when myosin-1c slides down the stereocilium in response to elevated tension during bundle displacement. The resultant decreased tension in the tip link permits the bundle to move farther in the opposite direction. As tension decreases, channels close, producing the decline in transduction current. Slow adaptation is most prominent in vestibular hair cells that sense spatial movement and less in cochlear hair cells that detect auditory signals.
  
  Section::::Neural connection.
   Section::::Structure, growth and development.:Factors affecting growth.
  
  The genome of a plant controls its growth. For example, selected varieties or genotypes of wheat grow rapidly, maturing within 110 days, whereas others, in the same environmental conditions, grow more slowly and mature within 155 days.
  
  Growth is also determined by environmental factors, such as temperature, available water, available light, carbon dioxide and available nutrients in the soil. Any change in the availability of these external conditions will be reflected in the plant's growth and the timing of its development.
   Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
   Affenpinschers often appear on lists of dogs that allegedly do not shed (moult). Every hair in the dog coat grows from a hair follicle, which has a three phase cycle, as do most mammals. These cycles are: "anagen", growth of normal hair; "catagen", growth slows, and hair shaft thins; "telegen", hair growth stops, follicle rests, and old hair falls off—is shed. At the end of the telegen phase, the follicle begins the cycle again. The length of time of the growing and shedding cycle varies by breed, age, and by whether the dog is an inside or outside dog.
   In the rod-shaped bacteria "E. coli", "Caulobacter crescentus" and "B. subtilis" cell size is controlled by a simple mechanisms in which cell division occurs after a constant volume has been added since the previous division. By always growing by the same amount, cells born smaller or larger than average naturally converge to an average size equivalent to the amount added during each generation.
  
  Section::::Cell division.
  
  Cell reproduction is asexual. For most of the constituents of the cell, growth is a steady, continuous process, interrupted only briefly at M phase when the nucleus and then the cell divide in two.
   The Dynamic Energy Budget theory provides a mechanistic explanation of this model in the case of isomorphs that experience a constant food availability. The inverse of the Bertalanffy growth rate appears to depend linearly on the ultimate length, when different food levels are compared. The intercept relates to the maintenance costs, the slope to the rate at which reserve is mobilized for use by metabolism. The ultimate length equals the maximum length at high food availabilities.
  
  Section::::Work.:Bertalanffy equation.
   The anagen phase is known as the growth phase. This is the phase where the hair physically grows approximately 1 cm per month. It begins in the papilla and can last from three to five years. The span at which the hair remains in this stage of growth is determined by genetics. The longer the hair stays in the anagen phase, the longer it will grow. During this phase, cells neighboring the papilla in a germinative layer divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand. About 85% - 90% of the hairs on one's head are in the anagen phase at any given time.
  
presuppositions: Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing.
corrections: Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive.
raw_presuppositions: Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing. Hairs on our bodies can recognize when to stop growing.
raw_corrections: Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive. Cells in hair roots are not always active, and cause hair to die and drop off when inactive.