Meikmeika said...I hope I don't sound too off but, what are speckles?
Stages of the hair cycle
Between starting to grow and falling out years later, each hair passes through three distinct stages. These are so important that they have been given special names: anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase).
We shall look at these three stages in turn.
Anagen (the growing phase)
The anagen phase of a new hair starts at the moment it begins to grow. At that time there is very active growth in the hair bulb. This usually lasts for some years, generally between three and seven, without interruption. Since human hairs grow at a rate of roughly 1 centimeter a month, hairs can grow to a length of a meter or so.
Hair growth varies with the season as a result of a change in the difference between hair follicles in the growing and shedding phases.
Pigment (melanin) is made in the hair bulb throughout this phase of the hair cycle. Less pigment is made in the hair of older people. This is why white hairs start to appear, even though the hair itself may still be growing strongly.
In some older people the hair cycle becomes shorter, the follicles gradually give up producing long, strong hair, and the hairs become thinner and shorter. The result may be a general thinning of the hair, or even a degree of baldness.
Catagen (the intermediate phase)
The anagen phase is followed by a short resting phase. This catagen phase lasts for between two and four weeks in the human scalp. No pigment is made during that time, and the follicle stops producing hair. The base of the follicle moves upwards towards the surface of the skin.
Telogen (the shedding phase)
The telogen phase lasts for three or four months. This is the time at which a new hair begins to grow from the hair follicle. As it grows upwards the old hair will be shed naturally or may be pulled out, which happens easily and painlessly with telogen hairs. These are the hairs that come out when you shampoo or brush your hair.
Shedding is part of the normal process of the replacement of old hair with new. At any one time, around one in ten of the follicles on an individual's head are in the shedding phase.
The new hair emerges from the same opening at the surface of the skin as the old one, and the hair cycle begins again.
Under normal circumstances hair growth in each hair follicle occurs in a cycle. There are three main phases of the hair growth cycle; anagen, catagen and telogen with anagen further subdivided into proanagen, mesanagen and metanagen. Anagen is the active growth phase when hair fiber is produced. Proanagen marks initiation of growth with RNA and DNA synthesis in a follicle which then quickly progresses through mesanagen to metanagen and maximum follicle length and girth. In this mature state of proliferation and differentiation the hair follicle consists of a total of eight concentric layers and melanogenesis occurs within pigmented hair follicles. Anagen is followed by catagen, a period of controlled regression of the hair follicle. Ultimately the hair follicle enters telogen, when the follicle is in a so-called resting state.
Anagen is the longest phase with up to 90% of follicles on a normal human scalp in this active hair growth state at any given time and correspondingly telogen hair follicles comprise up to 10% on the scalp (Kligman 1988). The average rate of hair fiber growth is around 0.35mm a day but this rate varies depending on the site of the hair follicle and the age and sex of the individual (Pelfini 1969). The length of the anagen growth phase for scalp hair is usually 6-10 years while telogen lasts just 30-90 days and catagen is best estimated at 14-21 days (Van Scott 1957, Orfanos 1990). In most young mammals the anagen growth phase occurs in a wave like pattern across the skin surface. But the hair follicles of humans and guinea pigs can run through the normal cycles of growth entirely independently of neighboring follicles.
2.4 General Physiology of Hair—An Overview
Robert Baratz, M.D., Ph.D., D.D.S.
To provide a foundation for subsequent discussions, Dr. Baratz described the general characteristics of hair and the underlying skin (e.g., structure, composition, growth patterns, growth cycles). Understanding the characteristics of hair, the temporal and spatial patterns of hair growth, and the factors that affect hair growth, for example, is important when collecting and interpreting hair analysis data. Dr. Baratz's presentation is summarized below.
|Anatomy of hair. Hair is encompassed in the follicle located below the skin surface in the dermis, the fiber-rich layer that makes up the bulk of the skin. The follicle has a connective tissue component (muscles) and glandular component (sebaceous glands). The muscles elevate the hair and the glands lubricate the hair. |
The primary components of the hair follicle are the dermal papilla and the follicle cells. The dermal papilla is the "generative zone" of hair (it contains blood vessels, nerves, and pigment-forming cells). The follicle cells generate the hair shaft; the hair shaft is composed of essentially dead cells, which are the outermost layers of the epithelium and form a solid cylinder in the dermis. Mitotic activity at the base of the hair follicle generates different layers that will "keratinize" (see below).
|Keratinization of hair. Hair is composed of hard keratin (a family of proteins ranging in size from 20,000 to 70,000 Daltons) and is chemically denser than other forms of keratin (e.g., calluses, dander flakes). Keratinized cells contain more than 85% protein. Where the hair shaft separates from the follicle it undergoes "disjunctive" keratizination, which involves the splitting of layers and exposing surfaces not previously exposed. |
Keratinized cells have a very distinctive appearance, and have tiny pores littering their surfaces. The cells are flattened and tightly bound to their neighbors in a very complex array. When they begin to split apart (by an unknown process), large "nooks and crannies" are formed. These types of anatomical features allow external environmental agents to be easily trapped in the outer surface of the hair.
Elements found in hair. Because so many elements are ubiquitous in the environment and therefore found in the human body, merely finding a particular element in the hair does not prove that it got there via a specific route/source, or that finding it has clinical significance.
|Growth rates. Hair growth varies depending on body region. For example, average eyelash/brow growth rates have been reported at 0.16 millimeters (mm) per day, scalp hair at 0.34 to 0.36 mm/day, and beard hair at 0.38 mm/day. Growth rates also are affected by age, gender, hair color, and ethnicity. For example, scalp hair in a prepubescent, adolescent, adult, and older adult have been reported at 0.41, 0.30, 0.34, and 0.32 mm/day, respectively (Myers and Hamilton 1951). |
Interindividual variability also occurs. Scalp hair grows at an average rate of 1 centimeter (cm) per month, but can range from 0.6 to 3.36 cm/month (Harkey 1993). Thus, 12 cm can represent 3½ to 20 months of hair growth.
|Growth cycles. Hair grows in phases (see Figure 2-2). Usually, more than 90% of the hair is in the growing (or anagen) phase. The length of anagen varies from 2 to 6 years. The longer the hair, generally the longer the phases. For example, long hair tends to grow more slowly. Through apoptosis, the hair will begin to enter the relatively short catagen phase, during which the follicle will begin to regress and move toward the surface (the papilli will essentially disappear). During the next phase, telogen, the hair will actually fall out. If the cycle is complete, a resting phase will follow and then the follicle will resume the anagen phase. However, hair can "exit" the cycle and cease being a terminal hair. For example, it can become a vellus hair (non-pigmented "peach-fuzz" hair) or the hair follicle may permanently disappear, as is the case with male-pattern baldness. |
Events known to affect the hair follicle and its cycle include local signaling events (e.g., cytokines, hormones, adhesion molecules). However, no firm theory of cycle control exists. Hypotheses include the presence of (1) a morphogenesis clock, (2) a cycling inducer, (3) a desynchronizer, and (4) an actual cycle clock, but none of these are specifically known.
|Generation, cycling, and "patterning" of hair. The hair growth cycle changes throughout life and varies based on species and body location. Patterning of hair is important to the generation and cycling of hair, and to how it relates to its neighbors (e.g., signaling goes on in various regions to space follicles in even arrays). Because of similarities in hair growth patterns, studying sheep hair growth has been useful in understanding human hair growth patterns. Rodent hair growth models, on the other hand, may not be applicable to humans because rodents have regional variation in hair growth; the hair cycles, but in waves across the body.|
|Substances affecting hair growth. A great number of substances can affect hair growth. For example, some drugs, such as alkalating agents, are cytotoxic and can make hair fall out (e.g., cancer chemotherapeutic agents). Other agents drive hair into telogen (e.g., heparin, Vitamin A, ß-blockers, L-dopa, lithium, and some of the non-steroidals). Drugs that inhibit hair growth include parathyroid hormone (PTH) and PTH-related proteins. Variable agents also exist, such as Vitamin D. At low concentrations, Vitamin D may simulate hair growth, but at high concentrations hair growth is inhibited. |
Substances such as testosterone, danazol, adrenocorticotropin hormone, metyrapone, anabolic steroids, glucocorticoids, retinoids, and insulin can lead to hirsutism (growth of hair where it does not normally occur). Cyclosporin, minoxidil, diazoxide, and chromakalin increase the growth rate and size of hair (hypertrichosis). However, some regional variation may occur. For example, steroids will decrease the rate of growth of eyebrows, lashes, and hair on the extremities, but estrogen and testosterone will generally stimulate the growth of pubic and axillary hair.
Other factors can potentiate or inhibit hair growth by affecting the growth of the dermal papillae, hair, and follicle (see Table 2-1). (http://www.atsdr.cdc.gov/HAC/hair_analysis/)
Maybe more than you ever wanted to know... but for the person who really wants to know more then the general surface information, this is an adequate read.