The Anatomy of Tattoos

tattoo

The word tattoo comes from the Tahitian word tattau, which means “to mark”; despite loving tattoos myself, I’m aware they are not everyone’s cup of tea. However, over the last several years there has been somewhat of a resurgence in body art, to the point that its now considered fashionable. In the last few years people have even started tattooing their eyes! Tattooing in some form or another has been practised by humans for thousands of years, reasons include tribal association, artistic expression and now medical applications, such as fiducial markers in radiotherapy, replacement of the areola removed during mastectomy and in camouflage treatment in hyper/hypo-pigmentation disorders. Tattooing can also be used to describe a pattern of traumatic injury, such as a patient suffering an abrasion injury resulting in particulate matter, small bits of grit etc. embedding into the skin.

In this post I’ll be reviewing the anatomy of the skin and also briefly the eye, discussing both in the context of tattoos. For example, have you ever considered how deep a tattoo is? Why is the ink not shed with the skin? Where is the ink deposited in tattoos involving the eye? Why do tattoos bleed? Why are some areas more painful to tattoo than others? Does the immune system react? Are there any long-term consequences? Is it safe to have an MRI if you have a tattoo? To answer these questions let’s start by reviewing the histology and anatomy of the skin.

The Epidermis, no place for tattoos

The skin consists of 3 main layers, superficial to deep:

  • Epidermis
  • Dermis
  • Subcutaneous tissue

The epidermis can be subdivided into 5 layers (Figure 1), superficial to deep:

  • Stratum corneum
  • Stratum lucida (thick skin only)
  • Stratum granulosum
  • Stratum spinosum
  • Stratum basale

The predominant cell in the epidermis are keratinocytes. They arise from the stratum basale and migrate towards the stratum corneum, differentiating as they do so. In fact, the five layers can be thought of as representing the different stages of maturation of the keratinocyte. So in the stratum basale, the cells are in a germinal phase and have a rounded appearance, while in the spinosum layer they have begun to synthesise keratin. In this layer the cells are tightly bound to adjacent cells by desmosomes when the tissue is fixed, the cells shrink but remain connected via their desmosomes. However, the desmosomes now have the appearance of spines.

In the granulosum layer, the cells begin to lose their nuclei and organelles and start to flatten into keratinised squames. The cells also contain lipid (water sealant) filled granules, hence the histological name of this stratum. The stratum corneum contains dead and dying flattened cells, the remnants of which are filled with mature keratin. These dead squames will eventually be lost (desquamation) and become house dust. The rate of mitosis in the stratum basale usually match the rate of loss from the stratum corneum. The turnover of cells from basale to corneum is approximately 20 to 50 days. Hence a tattoo which is too shallow and deposited into the epidermis, will not last as the layers containing the ink will eventually all be replaced.

epidermis

Figure 1 Epidermis – Dermis interface, Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

The epidermis can be therefore described as a stratified (multiple layers) squamous (surface cell are flattened) keratinising (surface specialisation – the presence of keratin ) epithelium. Other cells found in the epidermis include melanocytes, Langerhans, lymphocytes and Merkel cells.

The size of the epidermis varies depending on how the site is affected by desiccation and abrasion. Areas exposed to greater abrasive forces are usually thicker, for example, the skin on the palms and soles of the feet are 1.5mm thick as opposed to the eyelid 0.7 mm (see Figure 2). Remember thick skin (fingertips, palms and soles of the feet), actually has a thinner dermis and contains no hair follicles, sebaceous or apocrine sweat glands, they do contain merocrine (aka eccrine) glands though. The epidermis has no vascular supply and thus receives its nutrients by diffusion from the dermis, through the basal lamina. The basal lamina is a very thin layer that connects the epidermis and dermis, it’s not visible on light microscopy but becomes evident on electron microscopy. In terms of innervation, the epidermis does contain free nerve endings and Merkell cells.

The Dermis, home of the tattoo

The dermis is a connective/supporting tissue as opposed to the epidermis which is epithelial tissue. The dermis is a fibroelastic tissue that contains a rich supply of nerves and blood vessels. The dermis also contains sweat glands and hair follicles. However, these are both actually invaginations of the epidermis into the dermis (remember glands arise from epithelial tissues). The dermis sits on the subcutaneous tissue layer, this is also is a type of connective tissue and in gross anatomy is referred to as superficial fascia.

skin2

Figure 2 Comparing Thick hairless skin vs thin hirsute skin, Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

The dermis is the layer that a tattoo artist injects the insoluble ink into, as this layer is relatively stable and doesn’t have the cellular turnover of the epidermis, the tattoo stays in place. The rich neurovascular supply of the dermis, explains why secondary to trauma from the tattoo artist’s needle, bleeding occurs and also why free nerve endings are stimulated causing pain. The level of pain can vary from person to person due to indivdual thresholds, but there is agreement that certain areas are more painful than others, can you think why this might be the case? Another way to think about this, is to ask yourself if superficial nerve density is homogeneously distributed throughout human skin? A quick look at the sensory homunculus should help you determine areas where tattooing might be particularly painful.

What about tattooing the eye?

This actually isn’t a recent phenomenon, Galen was using corneal tattooing for the treatment of corneal leukoma. However, over the last several years, people are getting eye tattoos for purely aesthetic reasons. So anatomically speaking what exactly is being done here? Eye tattoos are either scleral or corneal. In a scleral tattoo, the ink is deposited between the bulbar conjunctiva and sclera. In a corneal tattoo, the ink is deposited within the stromal layers of the cornea itself. A quick review of the anatomy of the globe of the eye is a good idea (see Figure 3).

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Figure 3 The anatomy of the eye, adapted from Clinical Anatomy by Regions, Snell

The globe consists of three layers:

  • Sclera
  • choroid
  • Retina

The sclera is the tough outer coat, which gives the eye it’s shape (crucial for the eye to maintain its ability to refract light), provide protection and for the attachment of the extraoccular eye muscles. The choroid is a highly vascular layer and includes the ciliary bodies and iris. When any of the choroidal structures are inflamed the patient develops a painful condition known as uveitis. The retina is the innermost layer consisting of a pigmented and neural layer.

Back to the sclera, at the front of the eye, the sclera becomes transparent and is known as the cornea. The conjunctiva (see Figure 4) is an epithelial layer that sits in front of the sclera and has two parts, a palpebral (lines the inside of the eyelids) and a bulbar part (lines the anterior sclera up to the junction between the sclera and cornea – known as the limbus), remember the conjunctiva does not cover the cornea. A tattoo artist injects ink between the bulbar conjunctiva and sclera when applying a scleral tattoo; in theory this should not affect vision as the key refractive structures remains untouched. Corneal tattooing will in most cases impair vision, as the ink will interfere with the refraction of light; therefore this form of tattooing is used in medical treatment to cosmetically correct an opacity (caused by cataract, keratitis etc.) where vision has already been lost, and there is no likelihood of recovery or correction of vision.

conju

Figure 4 The conjunctiva, adapted from Clinical Anatomy by Regions, Snell

Some people have commented that eye tattooing is not painful, pain though can be a very subjective experience and the reality is, that these structures do have pain receptors and sensory fibres. The cornea has some of the greatest density of pain receptors and is innervated by the branches of the long ciliary nerve. The conjunctiva is innervated by the lacrimal, supratrochlear, infratrochlear, supraorbital, infraorbital and in the cirumcorneal region by the long ciliary nerves too. All of these nerves are branches of either the ophthalmic or maxillary divisions of the Trigeminal nerve CN V, their afferents projecting into the ventral trigeminothalamic tract.

Health risks associated with Tattoos

Tattooing is also associated with an increased risk of bacterial infections (impetigo, cellulitis, erysipelas and even sepsis) and transmission of blood borne viruses (BBVs) such as hepatitis B & C and HIV. Any skin infection including BBVs are more likely to occur if the person is tattooed in a poorly regulated environment such as a prison, using multiple use non sterilised equipment. This can include needles being reused as well as leftover pigments being shared. In one particular case, a prisoner had melted rubber from the soles of plimsolls to use as black ink! In contrast to that, modern tattoo parlours use single-use needles and pigments and containers. High standards of hygiene make the risks of any infection unlikely.

What about the immune response?

The process of applying a tattoo involves breaching the skin, hitting nerve endings and blood vessels, bleeding (relatively minor) triggers haemostasis mechanisms and the coagulation cascade, and the formation of a fibrin scab externally. With in the dermis a hypersensitivity reaction may rarely be triggered – leading to an allergic response, this can typically present as a contact or photosensitive dermatitis and is more likely to occur with red and yellow dyes respectively. Most people develop a typical acute inflammatory response, this initially involves, dilation of blood vessels, increased permeability, movement of neutrophils from blood vessels into the tissues. After 24 hours these neutrophils are usually replaced by monocytes, which eventually differentiate into macrophages. Macrophages are able to remove small particles of ink, but the larger particles making up the tattoo remain. The acute inflammatory response explains why after a tattoo, we see erythema, swelling and the person may complain of pain and tenderness. This normally settles down, however in some individuals they can go on to develop chronic reactions weeks to years afterwards, these include granulomatous reactions, lichen planus and psoriasis.

Long term consequences

As discussed granulomatous disease can develops within days to months of a tattoo, these can be associated with preexisting sarcoidosis, or as a result of a foreign body or even an allergic reaction to the tattoo ink. Another interesting complication documented in the literature, includes cases of cutaneous pseudolymphoma. The proliferation of either T or B cell lymphocytes is thoughts to occur due to chronic antigenic stimulation; though only a few cases (linked to tattoos) have been published, the lesions always seems to occur in the area of red ink usage. The published literature also contains case reports of basal cell, squamous cell carcinoma and melanoma occurring within tattoos. However, there is insufficient evidence to suggest a linkage.

Is MRI safe in patients with tattoos?

There is enough literature to suggest that MRI can cause transient symptoms in patients with tattoos, this ranges from minor skin irritation, swelling and a heating sensation. This has been attributed to metallic compounds within the ink, such as iron oxides. Patients should be advised to make radiographers aware of any tattoos; in the literature it has been suggested that a cold compress be applied to a tattoo during the imaging procedure.

References

  • Wheater’s Functional Histology, Paul Wheater
  • Clinical Anatomy by Regions, Richard Snell
  • Gray’s Anatomy, The Anatomical Basis Of Clinical Practice, Susan Stranding

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