Homeostasis
David Sturgeon in Introduction to Anatomy and Physiology for Healthcare Students, 2018
It is estimated that the average adult has about 5 million hair follicles of which approximately 100,000 are on their head (Figure 3.4). This sounds an implausibly high number but there are two different types of hair, the most abundant of which are very difficult to perceive. The thick hair that we find on our heads, under our armpits and around the genital area is known as terminal hair. The short, colourless hair found (almost) everywhere else is known as vellus hair. In fact, the only places that hair is not found on the human body are the palms of the hand, the soles of the feet, the lips, the back of the ears, the very end of the penis (glans), the clitoris and the labia minora (inner lips of the female genitalia). In normal circumstances, vellus hair is very difficult to see and it is often further obscured by terminal hair (e.g. on the arms). Before birth, babies are completely covered in a special fur-like hair called lanugo which is thought to aid temperature regulation in the womb. By the time they are born, however, it has usually transformed into vellus hair. Interestingly, those who suffer from severe anorexia or anorexia nervosa may also develop lanugo (usually on the face, arms and back) as a result of profound malnutrition. Since they have lost a significant proportion of insulating body fat (adipose tissue) it may represent a heat conservation mechanism. In health, hair helps to trap warm air next to the skin but it is also a physical barrier to protect the skin from foreign material and UV light.
Skin
Pritam S. Sahota, James A. Popp, Jerry F. Hardisty, Chirukandath Gopinath, Page R. Bouchard in Toxicologic Pathology, 2018
Hair follicles in animals are classified on the basis of morphological appearance and organization in the skin. Primary hair follicles are characterized by a large diameter, deeply rooted in the dermis, and usually have sebaceous and sweat glands and arrector pili muscles. Secondary follicles have a smaller diameter, rooted more superficially in the dermis, and may have associated sebaceous glands but not sweat glands or arrector pili muscles. Single or simple follicles have only one emergent hair, whereas compound follicles have several hairs emerging from one opening at the skin surface. Each hair of a compound follicle arises from its own papilla but then fuse at the level of the opening of the sebaceous gland to form a single orifice with multiple hair shafts. The types of hair follicles differ with animal species. For example, in dogs, compound follicles are composed of a single large primary follicle surrounded by numerous smaller secondary follicles with as many as 15 hairs emerging from a single opening. To make this more confusing, mice have simple follicles but will retain old telogen hairs produced by the same hair bulb. Thus, one can often observe multiple hair shafts in the murine hair follicle.
Geriatric hair and scalp disorders
Robert A. Norman in Geriatric Dermatology, 2020
Hair follicle activity is intermittent and characterized by a growth phase (anagen), followed by a brief transition phase (catagen) and a resting phase (telogen). Normally 90% or more of scalp hair is in the anagen phase at any given time, 10% or less is in the telogen phase and less than 1% is in catagen. The anagen phase is determined genetically and differs in individuals. The length of the anagen phase determines the natural length an individual’s hair will attain and tends to remain stable unless pathologic alterations intervene. Telogen hairs are also retained for a defined period of time and then shed. On the scalp, telogen hairs are held 2–3 months before they are shed. The follicles in different anatomic sites vary in duration of anagen and telogen phase, which gives hair its growth characteristics by site. Those areas with long anagen cycles and a short telogen phase, such as scalp hair, produce a long fiber. A short anagen growth phase, coupled with a prolonged telogen phase, produces the shorter, stable hairs of the eyebrows and eyelashes, and the terminal hairs on the extremities. Scalp hair grows at approximately one centimeter a month, although the growth rate slows down with advanced age.
A Child Presenting with Recurrent Corneal Ulcers: Hereditary Sensory and Autonomic Neuropathy IV (HSAN IV)
Published in Neuro-Ophthalmology, 2019
Beena Suresh, Vaishnavi Reddy, Ingo Kurth, Sujatha Jagadeesh
TM is the second child of a nonconsanguineous couple. He was brought to us at 10 months for evaluation of suspected ectodermal dysplasia. TM was a healthy boy born at term by LSCS with a birth weight of 3 kg with a good birth cry. On day three of life neonate had refusal of feeds, high temperature and lethargy. He was treated for fever and discharged on day nine of life. Subsequently his milestones and development were age appropriate. Child has recurrent respiratory infections, recurrent corneal ulcers and several febrile episodes. He was also noted to have decreased sweating and skin biopsy from scalp was done elsewhere which revealed small adnexal glands and absence of sebaceous glands. Hair follicles contained small hair shafts. Hence the diagnosis of anhidrotic ectodermal dysplasia was reported.
Eyelash hair transplantation with strip composite eyebrow graft: an enhancing technique in tarsoconjunctival flap eyelid reconstruction procedure
Published in Orbit, 2019
Ángel Nava-Castañeda, José Luis Tovilla-Canales, Paolo Solano-Leal, Lilia Garnica-Hayashi
Twenty-two patients were included in this study. There were fourteen female and eight male patients. All the patients were above 60 years old. Eighteen patients had lower eyelid basal cell carcinoma, one patient had upper eyelid Merkel cell carcinoma (Figure 1) and three had lower eyelid squamous cell carcinoma. In all cases the tumor was entirely removed with pathologist confirmation of tumor free margins. In all cases, the eyelash grafts showed a good follicle orientation and no cases of eyelashes misdirection toward the eyeball were reported. At the first postoperative day, we found a median of 23 (range 15–30) hair follicles in the graft. At 12 months an average of 17 (range 15–24) survival follicles were observed (Figure 1). No statistical differences (p > 0.05) were found in Wilcoxon signed rank test meaning that the vast majority of follicles survived at the end of the study. No cases of graft ischemia, necrosis or infection were found. No complications were seen at the eyebrow donor site. No tumor recurrence was appreciated during the study period.
Adnexal squamous cell carcinoma: incidence of eyelid margin involvement
Published in Orbit, 2023
Alison H. Watson, Sabah Akbani, Natalie Homer, Marie Somogyi, Vikram Durairaj
The eyelash follicle differs from other hair follicles in many ways. Notably, they are terminal hair follicles unlike the surrounding vellus follicles that are able to produce fully pigmented and medullated hair shafts.9 Unlike vellus hairs, they do not have arrector pili muscle and they are independent of sex hormones. Therefore, they are the darkest in the human body and the last to turn gray.9 There are limited identified studies of the lash follicle cycle in comparison to vellus hairs.9,13 One assessment postulated that the turnover cycle of these follicles is much longer than the vellus hairs.13 The predilection of SCC to develop along the marginal versus non-marginal eyelid may be a consequence of differences in the duration of the lash follicle life cycle, as this can influence the likelihood of malignant transformation.6,7
Related Knowledge Centers
- Hair Growth
- Neuropeptide
- White Blood Cell
- Dermis
- Skin
- Cell Type
- Hormone
- Hair
- Terminal Hair
- Lanugo