Things to think about According to archaeological evidence, the first humans Homo habilis appeared in Africa about three million years ago. They used simple stone tools. Humans Homo erectus expanded their populations through Europe and Asia at least a million years ago. The modern human species Homo sapiens appeared in Africa about 1.
Play media Hair-follicle cycling Hair grows at different speeds and different lengths. Its composition causes different colors and textures, which influence how long the hair strands grow. Marianne Ernst, a German "Long hair model".
The three stages of hair growth are the anagencatagenand 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. Anagen phase[ edit ] The anagen phase is known as the growth phase.
This is the phase where the hair physically grows approximately 1 cm per month. The longer the hair stays in the anagen phase, the longer it will grow.
During this phase, the cells in the papilla divide to produce new hair fibers[ contradictory ], and the follicle buries itself into the dermal layer of the skin to nourish the strand.
Catagen phase[ edit ] 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.
While hair is not growing during this phase, the length of the terminal fibers increase when the follicle pushes them upward. Telogen phase[ edit ] 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.
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. Growth inhibitors and disorders[ edit ] 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. Chemotherapy[ edit ] Most chemotherapy drugs work by attacking rapidly dividing cells. Rapid cell replication is one of the hallmarks of cancer; however, hair follicle cells also grow and divide quickly.
Consequently, the chemotherapy drugs usually inhibit hair growth. The dose and type of medicine will determine the severity of hair loss. Once the course of chemotherapy has ended, new hair growth may begin after three to 10 weeks. Baldness Alopecia is a hair loss disease that can occur in anyone at any stage of life.
Specifically Alopecia areata is an autoimmune disease that causes hair to spontaneously fall out. It is mainly characterized by bald patches on the scalp or other parts of the body, and can ultimately cause baldness across the entire body.
This disease interferes with the hair growth cycle by causing a follicle to prematurely leave the anagenor active growth, phase and enter the resting, or telogen phase.Before reading about the adult stages of growth and development, consider the following two points: 1.
Stages of human growth and development come . development of this resource packet for human growth and development (HGD) education from its inception to its current form. It was through much collaboration and teamwork that this document reached.
This lesson will give you an overview of the major human development theories. You will briefly examine their strengths and weaknesses and meet a.
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