anatomy of nail

7/28/2019 Anatomy of Nail

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ANATOMY OF NAILS


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Contents

Biology of Nails:

Nail Plate

Proximal Nail Fold

Nail Matrix

Nail Bed

Hyponychium

Basement Membrane Zone

Dermis

Blood and Nerve Supply

Chemical and Physical Properties

Nail Growth


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Biology of nails

Nail appratus consists of horny dead product

Nail plate

Proximal nail fold

Nail matrix

Nail bed hyponychium.

Develops during 9th gest.wk from epidermis of

dorsal tip of the digit.


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Biology of nails

Proximal border of nail field extends downwards

into dermis to form nail matrix primordium.

By 15th wk, nail matrix(NM)is completely

developedand starts to produce nail plate which continue

to grow until death.

Nail appratus lies above the periosteum of distalphalanx(responsible for occurance of bone

alterations in nail disorder and vice versa).


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Nail plate

Keratinised st. produced throughout life.

It results from maturation and keratinisation of Nm

epithelium,firmly attached to nail bed. Proximally and laterally-surrounded by nail folds


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At the tip of digit,nail plate(smooth,rectangular,

translucent and transparent) separates from underlying

tissues at hyponychium.

It shows mild longitudanal ridges which increases with

age (used for forensic identification).


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Nail plate..

Prox part of finger nail (esp. thumb)shows

whitish,opaque,half moon shaped area called

lunula (visible portion of nail matrix).

Onchocorneal band-thin distal transversewhite band marks the distal most portion of

firm attachment of nail plate to nail

bed(disruption produces nail plate detachment

with onycholysis)

This band is separated form nail plate white

free edge by 1-1.5mm pink band-onchodermal

band.


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Nail plate..

Consists of 3 portions

Dorsal

Intemediate

Ventral

The dorsal & intermediate portions --- the nail matrix,

ventral portion ----- nail bed. Above the lunula, the nail plate is thinner andconsists only of the dorsal and intermediate portions.

The nail plate progressively thickens from its emergence to its distalmargin, the mean toenail thickness at the distal margin is 1.65 0.43 mm inmen and 1.38 0.20 mm in women.

Fingernails are thinner, their mean thickness is 0.6 mm in men and 0.5 mm

in women. Nail thickness depends on the length of the nail matrix and nail bed.

Thinning of the nails is usually a sign of nail matrix disorders, whereas nailthickening is most commonly due to nail bed disorders.


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Proximal Nail Fold

The proximal nail fold is a skin fold that consists ofa dorsal and a ventral portion .

The dorsal portion is anatomically similar to theskin of the dorsum of the digit but thinner anddevoid of pilosebaceous units.

The ventral portion, cannot be seen fromexterior,which proximally is in continuity with thegerminative matrix, covers approximately one-fourth of the nail plate.

It closely adheres to the nail plate surface andkeratinizes with a granular layer.


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Proximal Nail Fold

The horny layer of the proximal nail fold forms the

cuticle, which is firmly attached to the superficial

nail plate and prevents the separation of the plate

from the nail fold. The integrity of the cuticle is essential for

maintaining homeostasis of this region.

The dermis of this contains numerous capillaries

that run parallel to the surface of the skin.

The morphology of proximal nail fold capillaries is

typically altered in connective tissue diseases.


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Nail Matrix

The nail matrix is epithelial structure that lies above the

mid-portion of the distal phalanx.

The matrix appears as a distally convex crescent with

its lateral horns extending proximally and laterally.

In longitudinal sections, the matrix has a wedge-shaped

appearance and consists of a proximal (dorsal) and a

distal (ventral) portion .

Nail matrix keratinocytes divide in the basal cell layer .


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Cornified onychocytes are composed mainly of keratin filaments,

high-sulfur matrix proteins, and the marginal band.

In histologic sections the site of keratinizationa of nail matrix

onychocytes appear as an eosinophilic area in which cells show

fragmentation of their nuclei and condensation of their cytoplasm. In this area, nuclear fragments are destroyed by DNAase and

RNAase enzymes.

In some conditions, nuclear fragments may persist within the

intermediate nail plate, producing leukonychial spots. These,

however, frequently disappear before reaching the nail free edge,due to persistence of active DNA and RNA lytic enzymes within the

horny nail plate


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Nail Matrix

Keratinization of the proximal nail matrix cells produces thedorsal nail plate and keratinization of the distal nail matrixcells produces the intermediate nail plate.

In some fingers, the distal matrix is not completely covered bythe proximal nail fold, but is visible through the nail plate as a

white half-moon-shaped area, the lunula. The white color of the lunula results from two main anatomic

factors: The keratogenous zone of the distal matrix contains nuclear

fragments that cause light diffraction.

The nail matrix capillaries are less visible than nail bedcapillaries due to the relative thickness of the nail matrixepithelium.


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NM Keratinocytes

The nail matrix cells synthesize both soft or skin-type and hard or hair-type keratins.

According to Kitahara and Ogawa the dorsal nailmatrix keratinocytes in vivo produce soft keratins,

while the ventral nail matrix keratinocytes producehard keratins.

When cultured in a chemically defined medium,nail matrix cells are larger than epidermalkeratinocytes and show a low nucleus/cytoplasmand high euchromatin/heterochromatin ratio.

The growth rate of cultured nail matrix cells ishigher than that of epidermal keratinocytes.


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NM Melanocytes

Nail matrix melanocytes are usually quiescent .

Two types melanocytes

DOPA-negative, dormant melanocytes localized in theproximal and distal matrix and in the nail bed

DOPA-positive, activatable melanocytes localized in thedistal matrix.

DOPA-positive melanocytes contain key enzymes that are

necessary for melanin production .

Nail matrix melanocyte activation produces diffuse or banded

nail pigmentation and is morecommon in blacks andJapanese than in Caucasians.

Nail matrix melanocytes are frequently arranged in smallclusters among the suprabasal layers of the nail matrixepithelium.


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Langerhans Cells

Langerhans cells are more numerous in the

proximal than in the distal nail matrix.

As in normal epidermis, Langerhans cells are

predominantly found in the suprabasal layers.Merkel Cells

More than 20 Merkel cells per nail matrix

section in adults have been noted.


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Langerhans Cells

Langerhans cells are more numerous in the

proximal than in the distal nail matrix.

As in normal epidermis, Langerhans cells are

predominantly found in the suprabasal layers.Merkel Cells

More than 20 Merkel cells per nail matrix

section in adults have been noted.


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Nail Bed

Nail bed, contains sparse DOPA-negative melanocytes,extends from the distal margin of the lunula to theonychodermal band and is completely visible through the nailplate.

The nail bed epithelium is so adherent to the nail plate that it

remains attached to the undersurface of the nail when a nailis avulsed.

Its epithelium is thin and consists of two to five cell layers. Itsrete ridges, which are longitudinally oriented, interdigitate withthe underlying dermal ridges in a tongue-in-groovelikefashion.

Nail bed keratinization produces a thin horny layer that formsthe ventral nail plate, , the nail bed contribution to nail plateformation corresponds to about one-fifth of the terminal nailthickness and mass.


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HYPONYCHIUM

The hyponychium marks the anatomic area

between the nail bed and the distal groove, where

the nail plate detaches from the dorsal digit.

Its keratinization occurs through the formation of agranular layer.

The horny layer of the hyponychium partially

accumulates under the nail plate free margin.

The hyponychium is normally covered by the

distal nail plate, but it may become visible in nail

biters.


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THE DERMIS

The arrangement of the rete ridges varies in the different portions ofthe nail apparatus. The dermis beneath the proximal nail matrixconsists of condensed connective tissue that forms a tendon-likestructure connecting the matrix to the periosteum of the proximalphalangeal bone (posterior ligament) .

The rete ridges of the dermis underneath the nail matrix arecharacteristically long and root-likeshaped, running from lunula tohyponychium.

The dermis under the distal matrix consists of a loose network ofconnective tissue containing numerous blood vessels and rareglomus bodies.

The longitudinal orientation of the capillary vessels within the nailbed grooves explains the linear pattern of the nail bed hemorrhages(splinter hemorrhages).


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BLOOD AND NERVE SUPPLY

Blood supply provided by the lateral digitalarteries. These run along the sides of the digitsand give branches that supply both the matrix andthe proximal nail fold .

The nail bed is richly supplied (10 to 20/cm 2) byencapsulated neurovascular structures containingone to four arteriovenous anastomoses and nerveendings.

These glomus bodies are possibly involved in theregulation of the blood supply to the digits in coldweather.


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Chemical Properties

The nail plate, consists mainly of low-sulfurfilamentous proteins (keratins) embedded in matrixcomposed of (high-sulfur proteins rich in cystine).

Other nail constituents include water, lipids, and traceelements.

Nail keratins consist of

Hard hair-type keratins (80-90%)

acidic 44K/46K

basic 56K to 60K keratins

Soft skin-type keratins. (10-20%)

50K/58K

48K/56K keratin pairs.


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Chemical Properties

In normal conditions, the water content of nail

plate is 18%.

Dehydration is faster when the nails are kept

long. water content < 18 %, -- brittle;

when > 30 %, it becomes opaque and soft.


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The nail contains < 5 percent lipids, mainly

cholesterol, also contains traces of iron, zinc

and calcium.

Lipid composition of the nail plate varies withage and sex under the influence of sex

hormones.


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PHYSICAL PROPERTIES

The nail plate is hard, strong, and flexible.Hardness and strength are due to the nail plate'shigh content of hard keratins& cystine-rich, high-sulfur proteins,

while its flexibility depends on its water contentand increases with nail plate hydration.

The double curvature of the nail plate along itslongitudinal and transverse axes enhances nail

plate resistance to mechanical stress.

The strength of fingernails is with a maximumelastic stress varying from 420 to 880 kg/cm 2.


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Nail Growth

Fingernails grow faster than toenails, meangrowth being 3 mm per month for fingernailsand 1 mm per month for toenails.

Complete replacement of a fingernail requiresabout 100 to 180 days (6months). When thenail plate is avulsed, it takes approximately 40days before the new fingernail will first emerge

fromthe proximal nail fold. The total regeneration time for a toenail is 12

to18 months.


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Nail Growth

Nail growth rate varies among different individuals . It depends on the turnover rate of the nail matrix cells

and is influenced by several conditions.

Conditions that are associated with a slow growth rate

include systemic illness, malnutrition, peripheralvascular or neurologic diseases, and treatment withantimitotic drugs.

An arrest of nail growth is a typical feature of theyellow-nail syndrome .

A reduction in the longitudinal nail growth is usuallyassociated with nail thickening.

Accelerate nail growth may cause nail thinning.


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Nail Growth

Due to their slow growth rate, the nails may provideinformation on pathologic conditions ,nail clippingscan be used to detect previous exposure to drugs orchemicals.

The nail of the big toe is the best site for investigationbecause of its size and slow growth rate.

Nail clippings may also be used for genetic analysisand determination of blood groups.

DNA can, in fact, be extracted easily from fingernailclippings and used for enzymatic amplification andgenotypic or individual identification.


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References

Fitzpatrick Dermatology of General medicine

Rooks Textbook of Dermatology

Diagrams from google.com


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anatomy of nail

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