Artlabeling Activity Hair Follicles Arrector Pili Muscles and Associated Structures

Introduction

The structural, or pilosebaceous, unit of a hair follicle consists of the hair follicle itself with an attached sebaceous gland and arrector pili muscle.

The hair follicle begins at the surface of the epidermis. For follicles that produce terminal hairs, the hair follicle extends into the deep dermis, and sometimes fifty-fifty subcutis. Meanwhile, follicles producing vellus hairs extend just to the upper reticular dermis. There are 3 important segments of hair follicles constitute on the head: the infundibulum, the isthmus, and the lower follicle/junior segment (which includes the bulb).

The infundibulum segment is the upper portion of the follicle. It begins at the surface of the epidermis and extends to the opening of the sebaceous duct. The isthmus is the expanse between the sebaceous duct opening and the bulge. The bulge is an area of the follicle marked by the insertion of the arrector pili muscle. Also, the burl contains several epidermal stem cells that are part of the outer root sheath and stain with CK19, CK15, and CD200. Finally, the inferior segment of the hair follicle extends from the bulge to the base of the follicle. This segment includes the seedling, which contains the follicular matrix surrounding the sides and top of the dermal papilla. The dermal papilla contains capillaries. The papilla interacts with the matrix, which has the highest mitotic rate of any organ. The matrix is the part of the hair follicle where matrix keratinocytes proliferate to form the hair shaft of growing hair. Melanocytes are mixed amongst the matrix cells to provide the hair shaft with color.

The pilus shaft consists of an inner core known as the medulla. This is surrounded by the cortex, which makes up the bulk of the hair. Moving outwards, there is a single layer of cells making upwards the shaft cuticle. The shaft cuticle is then encased in three layers that course the inner (internal) root sheath. The inner sheath is important in shaping the hair shaft as information technology grows upwards from the matrix. The inner sheath keratinizes from the exterior-in, and will eventually disintegrate mid-follicle, around the level of the isthmus. Finally, the outer (external) root sheath encases the entirety of the hair shaft. This layer undergoes trichilemmal keratinization effectually the level of the isthmus. [i] [ii] [3] [4]

Sebaceous glands are holocrine glands closely associated with pilus follicles, particularly in certain areas of the skin such as the face. These glands open up onto the hair follicles, except in areas such every bit the lips, where they empty directly onto the mucosa surface considering lips practice non contain hair follicles. When stimulated by hormones such as androgens, sebaceous glands secrete a lipid-rich sebum that protects the pilus and provides the skin with a hydrophobic barrier that can serve as protection. [3] [five] [six]

Arrector pili muscles insert at the level of the burl and also attach to the papillary layer of the dermis. In cold climates, sympathetic stimulation causes these muscles to contract. This raises the level of the skin slightly and causes the pilus to stand cock, which is commonly referred to as "goose-bumps." [4]

Structure and Function

The hair follicle cycles through iii different growth phases: anagen, catagen, and telogen.

1. The anagen phase is the proliferation phase, and it occurs when the hair follicle is growing a new hair shaft. The length of this phase tin vary. For hair on the scalp, the growth stage tin can last two to six years, whereas, for eyebrows and eyelashes, hair may only need a few months to grow. This is the merely phase during which the inferior segment of the pilus follicle is present. To indicate the beginning of the anagen phase, the dermal papilla signals to the multipotent epithelial stalk cells in the bulge. Once these stem cells are stimulated, the junior segment of the hair follicle can at present abound downwards, forming a bulb around the dermal papilla. At present, the dermal papilla tin signal matrix cells in the bulb to proliferate, differentiate, and grow upwards, forming a new hair.

2. The catagen stage is too known as the transition or regression stage. Information technology is the shortest of all three phases, and may merely terminal a few weeks. During this phase, cell sectionalization in the matrix ceases, and the inferior segment of the pilus follicle begins to regress. Eventually, the inferior segment of the follicle no longer exists, and the dermal papilla has moved upwardly to contact the burl once over again. During this process, a club pilus is formed with a white, hard node on the end.

3. Finally, the telogen stage ensues, which is referred to as the resting phase. Social club hairs, which are essentially dead, are held. On the scalp, these club hairs are typically held for almost 100 days. Eventually, these hairs are released and shed then that the anagen stage can begin again with a new hair.

4. At birth, all hairs begin in the anagen average; the human scalp contains over 100,000 hairs. At any one point in time, nearly hairs (85% to 95%) are commonly establish in the anagen phase. These hairs can grow approximately i cm per month. Meanwhile, pilus loss is continuous, with people losing virtually 100 hairs per day on average. With shampooing, the number of hairs lost can double.  [7] [viii]

Embryology

Around nine weeks gestation, the fetal epidermis gives rise to pocket-size buds of specialized cells that will course the pilus follicle and its associated appendages. Mesenchymal cells that accept accumulated inside the dermis help drive this process past secreting substances such as epimorphin that signal the epithelial cells to proliferate and migrate in a down (craniocaudal) management towards the dermis. In one case this procedure is complete, the fetus is left with a pilus follicle containing a matrix derived from ectoderm and an underlying dermal papilla-derived from mesoderm. Additionally, an arrector pili muscle and a sebaceous gland form around each follicle. In utero, maternal androgens will stimulate the activation of fetal sebaceous glands, so that the glands can secrete a lipid-rich sebum that combines with desquamated stratum corneum cells to form the vernix caseosa.

Once the pilus follicle has developed in the fetus, lanugo hairs grow in utero. These hairs are thin and short. They eventually shed by about 36 to 40 weeks gestation and are replaced by vellus hairs that cover most areas of the body. Meanwhile, thicker, courser terminal hairs tin be found on the caput in certain areas such as the scalp, eyebrows, and eyelashes. Beard hairs will arise one time puberty is reached, with androgen hormones transforming the vellus hairs in this area into last hairs.

After birth, no new pilus follicles volition be made. However, new hair can grow from the existing follicles. Also, follicle size can still be altered after nascency. [9] [4]

Clinical Significance

Hair growth rate and pilus density decrease with age. Hair loss, known as alopecia, is a common occurrence. It can be due to drugs, diet, hormone imbalances, altered mitotic action, growth cycle abnormalities, among others. A thorough history, physical exam, hair pull examination, daily hair counts, part width, clip tests to examine the hair shaft, pilus growth windows, and hair pluck, and trichograms can all be used to diagnose hair disease. Scalp biopsies, hormone studies, and a potassium hydroxide examination for fungi may also need to be performed in certain cases. It is of import to diagnose hair disease correctly, as the treatment for pilus loss is dependent on the diagnosis.

Male pattern baldness, known equally androgenetic baldness, is a common condition in which hair loss occurs as men age. With increased dihydrotestosterone bounden to the follicle of hair receptors, each successive anagen cycle shortens in length. In these individuals, follicles shrink and gradually produce shorter, thinner, more vellus-like hairs. Possible treatments may include slowing the conversion of testosterone to dihydrotestosterone in men with this status. [10] [11]

Review Questions

Effigy

The Common Integument, Department of skin showing the epidermis and dermis; a hair in its follicle; the Arrector pili muscle; sebaceous glands. Contributed by Greyness'due south Anatomy Plates

Figure

The Common Integument, Transverse department of hair follicle, Hyaline Layer, Cortex of hair, Medulla of pilus, Huxley'due south Layer, Henle's layer, Outer or dermic coat. Contributed by Gray's Anatomy Plates

Figure

Cantankerous section of layers of the pare. Hair follicles, pilus roots and hair shafts, sweat glands, pores, epidermis, dermis, hypodermis. Papillary and reticular layer. Eccrine sweat gland. Arrector pili muscles, sebaceous oil glands. Contributed by Chelsea (more...)

References

1.

Glover JD, Wells KL, Matthäus F, Painter KJ, Ho W, Riddell J, Johansson JA, Ford MJ, Jahoda CAB, Klika V, Mort RL, Headon DJ. Hierarchical patterning modes orchestrate hair follicle morphogenesis. PLoS Biol. 2017 Jul;xv(7):e2002117. [PMC complimentary article: PMC5507405] [PubMed: 28700594]

2.

Hamada K, Randall VA. Inhibitory autocrine factors produced by the mesenchyme-derived hair follicle dermal papilla may be a key to male pattern baldness. Br J Dermatol. 2006 Apr;154(4):609-18. [PubMed: 16536801]

3.

Paniagua Gonzalez LM, Tschen JA, Cohen PR. Ectopic Sebaceous Glands in the Hair Follicle Matrix: Case Reports and Literature Review of this Embryogenic Anomaly. Cureus. 2018 Nov 17;ten(11):e3605. [PMC costless article: PMC6338400] [PubMed: 30680266]

4.

Poblet Eastward, Ortega F, Jiménez F. The arrector pili musculus and the follicular unit of measurement of the scalp: a microscopic beefcake study. Dermatol Surg. 2002 Sep;28(9):800-3. [PubMed: 12269872]

v.

Wortsman X, Carreño L, Ferreira-Wortsman C, Poniachik R, Pizarro K, Morales C, Calderon P, Castro A. Ultrasound Characteristics of the Hair Follicles and Tracts, Sebaceous Glands, Montgomery Glands, Apocrine Glands, and Arrector Pili Muscles. J Ultrasound Med. 2019 Aug;38(8):1995-2004. [PubMed: 30570163]

vi.

Poblet E, Jiménez F, Ortega F. The contribution of the arrector pili muscle and sebaceous glands to the follicular unit structure. J Am Acad Dermatol. 2004 Aug;51(two):217-22. [PubMed: 15280840]

7.

Chen R, Miao Y, Hu Z. Dynamic Nestin expression during hair follicle maturation and the normal hair bike. Mol Med Rep. 2019 Jan;xix(ane):549-554. [PubMed: 30483790]

8.

Dobreva A, Paus R, Cogan NG. Analysing the dynamics of a model for baldness areata as an autoimmune disorder of hair follicle cycling. Math Med Biol. 2018 Sep 11;35(3):387-407. [PubMed: 28992198]

nine.

Grubbs H, Nassereddin A, Morrison M. StatPearls [Cyberspace]. StatPearls Publishing; Treasure Island (FL): May eight, 2021. Embryology, Pilus. [PubMed: 30521215]

x.

Lolli F, Pallotti F, Rossi A, Fortuna MC, Caro G, Lenzi A, Sansone A, Lombardo F. Androgenetic alopecia: a review. Endocrine. 2017 Jul;57(one):9-17. [PubMed: 28349362]

11.

Piraccini BM, Alessandrini A. Androgenetic alopecia. Thou Ital Dermatol Venereol. 2014 Feb;149(1):fifteen-24. [PubMed: 24566563]

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Source: https://www.ncbi.nlm.nih.gov/books/NBK470321/

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