Integumentary System - Hair Development

Notice - Mark Hill

Currently this page is only a template and is being updated (this notice removed when completed).

Introduction

Hair formation, or follicle development, is an excellent example of two distinct developmental processes: epithelio-mesenchymal interactions and pattern formation. The differentiated hair follicle will eventually contain 20 or more different cell types. Melanocytes, which provide the hair colour, have a neural crest origin, and with ageing their numbers decline leading to whitening (grey) of the hair process.

Hair follicle development in humans begins as an epithelial-mesenchymal interaction at week 9 - 12. This initial lanugo hair is replaced in the late fetal or early neonate by vellus and terminal hairs. A second round of development occurs during puberty under the influence of steroidal hormones.

The hair follicle is also a site for stem cells, allowing replacement of the follicle.

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Some Recent Findings

Incomplete development of the nail of the hallux in the newborn. "Between March and October 2008, the nails of 541 (252 females, 289 males) consecutively born neonates with an average age of 3.2 days were examined in the Neonatology Unit. Of these newborns with nail disorders, 36 were re-examined after a period that ranged from seven days to six months. The most frequent nail alteration was the incomplete development of the hallux nail, which was triangular - sometimes trapezoidal - shaped. This alteration, which had been previously reported in the literature as congenital hypertrophy of the lateral folds of the hallux, spontaneously regressed within one to three months in the infants re-examined. There was no associated inflammation or onychocryptosis at any time. The apparent hypertrophy of the nail folds seemed to be secondary to the lack of pressure of the nail lamina." Lyonization pattern of normal human nails. "To examine the X-inactivation patterns of normal human nails, we performed the human androgen receptor gene assay of DNA samples extracted separately from each finger and toe nail plates of nine female volunteers. The X-inactivation pattern of each nail was unique and constant for at least 2 years. ...These findings suggest that the composition of precursor cells of each nail is maintained at each site at least through several cycles of regeneration time, and that the nail plate has a longitudinal band pattern, each band consisting of cells with only one of the two X-chromosomes inactivated."

Textbooks

• Human Embryology (2nd ed.) Larson Chapter 14 p443-455
• The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud Chapter 20: P513-529
• Before We Are Born (5th ed.) Moore and Persaud Chapter 21: P481-496
• Essentials of Human Embryology Larson Chapter 14: P303-315
• Human Embryology, Fitzgerald and Fitzgerald
• Color Atlas of Clinical Embryology Moore Persaud and Shiota Chapter 15: p231-236

Development Overview

Embryonic

• follicle forms in stratum germinativum of epidermis
• hair bud -> hair bulb
• hair bulb forms hair
• mesenchyme forms hair papilla
• germinal matrix cells become keratinized to form hair shaft
• week 12 - lanugo hair (Latin, lana = wool) - first hair formed replaced postnatally, role in binding vernix to skin

Puberty

Coarse hair in pubis and axilla in both male and female (in males also on face and other body regions chest, etc) (More? Puberty)

Lanugo Hair

From about the third month lanugo hair (Latin, lana = wool) hiar is initially formed and it has a role in binding vernix to skin.

Hair grows over the entire body at the same rate, so the hairs are the same length, and is shed abut 4 weeks before birth. Premature infants can still be covered with these hairs.

Neonatal Hair

Newborn infants have two types of hair:

Vellus Hairs

short hairs, only a centimetre or two long, and contain little or no pigment

follicles that produce them do not have sebaceous glands and never produce any other kind of hairs

Terminal Hairs

long hairs that grow on the head and in many people on the body, arms and legs

produced by follicles with sebaceous glands

the hairs in these follicles gradually become thinner and shorter until they look like vellus hairs

File:Neonatehair.jpg

On the head, there are also two periods of hair development in which hair growth begins at the forehead and then extends to the back of the neck. Then at 2- 3 months old, the first hairs may be shed naturally over an area on the back of the head. This is often mistakenly thought to be due to head rubbing.

Hair Follicle

This image shows a histological section through the adult skin showing "slices" through 4 hair follicles and their associated glands in different planes. The hair shaft is the yellow structure extending from the bulb within the hair follicle. From the top down the three major skin layers shown are: epidermis (red upper region) surface ectoderm in origin, epithelium containing keratinocytes dermis (dark blue middle region) mesoderm in origin, connective tissue containing many different cell types and collagen fibers. hypodermis (pale lower region) mesoderm in origin, connective tissue containing many adipose cells.

File:Hair.jpg Hair follicles in the skin

Hair Follicle Phases

There are two main hair follicle phases.

Anagen Phase: active

Catagen Phase: apoptosis-driven involution

Molecular Hair Development

Signaling pathways in follicle formation:

Shh and its effectors

antagonists for the Wnt (Dkk4)

BMP (Sostdc1)

variant NF-kappaB-signaling cascade, based on lymphotoxin-beta (LTbeta)/RelB.

(See Cui CY, Hashimoto T, Grivennikov SI, Piao Y, Nedospasov SA, Schlessinger D. Ectodysplasin regulates the lymphotoxin-{beta} pathway for hair differentiation. Proc Natl Acad Sci U S A. 2006 May 31)

Signaling inhibitors of hair/follicle formation: EGF, FGF5

Sonic Hedgehog Pathway

SHH Knockout mice

• Hair germs comprising epidermal placodes and associated dermal condensates were detected in both control and Shh -/- embryos, but progression through subsequent stages of follicle development was blocked in mutant skin. The expression of Gli1 and Ptc1 was reduced in Shh -/- dermal condensates and they failed to evolve into hair follicle papillae, suggesting that the adjacent mesenchyme is a critical target for placode-derived Shh. Despite the profound inhibition of hair follicle morphogenesis, late-stage follicle differentiation markers were detected in Shh -/- skin grafts, as well as cultured vibrissa explants treated with cyclopamine to block Shh signaling.
• Our findings reveal an essential role for Shh during hair follicle morphogenesis, where it is required for normal advancement beyond the hair germ stage of development.
• Chin etal Developmental Biology, v 205, n 1, Jan 1999, p 1-9

SHH Activation (inappropriate)

• shh transduction cascade in human epidermis can cause basal cell carcinoma. Here we show that during normal development of avian skin, Shh is first expressed only after the responsiveness to this protein has been suppressed in most of the surrounding ectodermal cells. Forced expression of Shh in avian skin prior to this time causes a disorganized ectodermal proliferation. However, as skin begins to differentiate, the forced expression of Shh causes feather bud formation. Subsequently, expression of Shh in interfollicular epidermis has little or no morphological effect. Restricted responsiveness to Shh in developing skin has functional consequences for morphogenesis and may have important implications for cutaneous pathologies as well
• Morgan etal. Developmental Biology, v 201, n 1, September 1, 1998, p1-12

Pattern Formation Skin

• In birds, the main appendages are the feathers and the foot scales. Their formation results from a series of inductive events between ectoderm (later epidermis) and subectodermal mesoderm (later individualized dermis).
• Morphogenetically, the mesodermal (mesenchymal) component of skin is the predominant tissue, insofar as it controls most morphological and physiological features of developing skin and appendages, notably transformation of ectoderm into epidermis, polarization, proliferation and stratification of epidermal cells, initiation, site, size and distribution pattern of epidermal placodes, species-specific architecture of appendages, regional specification of keratin synthesis.
• The ectodermal (epithelial) component is able to respond to the mesodermal inductive instructions by building feathers and scales in conformity with the specific origin of the dermis. In these epithelial-mesenchymal interactions, extracellular matrix and the microarchitecture of the dermal-epidermal junction appear to play an important role.

Extracellular matrix components

• (primarily collagens, proteoglycans and adhesive glycoproteins) and dermal cell processes close to the epidermal basement membrane become distributed in a microheterogeneous fashion, thus providing a changing substratum for the overlying epidermis. It is assumed that the latter is able to somehow sense the texture and composition of its substratum, and by doing so to appropriately engage in the formation of glabrous, feathered or scaly skin.
• Sengel P Int J Dev Biol 1990 Mar;34(1):33-50

Puberty Hair Development

The appearance of pubic hair occurs along with ther secondary sexual characteristics (being also [#TannerTable staged] similarly) and is under endocrine control.

Estrogens- (1 beta-estradiol, E2) involved in skin physiology and are potent hair growth modulators.

Testosterone- Face, trunk and extremities increases hair follicle anagen phase (active) and increases also hair growth rate, thickness, medullation and pigmentation. Effects due to high hormone levels and target organ conversion to 5 alpha-dihydrotestosterone. Pubic hair develops even in absence of 5 alpha-reductase effect. ([#3521958 Ebling FJ., 1986])

Tanner Stages

Stage	Pubic Hair Development
1	None
2	Few darker hairs along labia or at base of penis
3	Curly pigmented hairs across pubes
4	Small adult configuration
5	Adult configuration with spread onto inner thighs
6	Adult configuration with spread to linea alba

Table based upon the Tanner stages of secondary sexual development. (Tanner JM. Growth at Adolescence. 2nd ed. Oxford: Blackwell Scientific, 1962.)

Hair Colour

Melanocytes produce melanin which influences hair colour.

Arrector Pili Muscle

Develop in mesenchyme and form the muscles that move hair.

Hair Loss

In development the initial hair is lost. In the adult hair loss, or "alopecia", has three distinct forms:

androgenetic alopecia - male- and female-pattern hair loss.

telogen effluvium - alteration of the normal hair cycle, due to many different stress stimuli (severe stress, chemotherapy, childbirth, major surgery, severe chronic illness, rarely occurance in vaccination)

alopecia areata - autoimmune disease, form antibodies against some hair follicles, distinct circular pattern of hair loss.

References

Reviews

<pubmed>16568906</pubmed> <pubmed>16382678</pubmed>

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Additional Images

Category:Integumentary

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Cite this page: Hill, M.A. (2024, April 26) Embryology Integumentary System - Hair Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Integumentary_System_-_Hair_Development

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G