Pigmented lesions on the skin can often look similar – brown in colour and usually flat. A common question is what are freckles? This blog investigates.
Freckle is the common term encompassing both ephelides (freckles) and lentigines (often referred to as liver spots). The two similar in many ways but are effectively different diagnoses.
Where do they arise from?
The main pigment producing cell of the skin is the melanocyte which produces melanin in an intra-cellular organ known as a melanosome. During embryotic development, melanocytes arise from the neural crest as melanoblasts and migrate into the hair follicle as well as populating along the basal layer of the epidermis where they mature into melanocytes, developing the ability to produce melanin in the melanosomes. As we grow in childhood there is a gradual migration of melanoblasts into the skin as its surface area increases with growth.
Along the basal layer, a single melanocyte serves approximately 30-40 cells via branch like arms known as dendritic processes [1]. That’s roughly 1200 per mm2 or, 1 in 10 cells of the basal layer. Along these dendritic arms, melanosomes can pass from the melanocyte into the keratinocyte. Here they determine skin colour as well as protecting the nucleus from UV damage. Keratinocytes exposed to UV light produce reactive oxygen species [ROS] (highly active oxygen based molecules) that can damage intra-cellular structures and processes [2]. Melanin acts as an absorber and soaks up ROS to limit intra-cellular damage.
What is a freckle?
As mentioned previously, freckle is a layman’s term which denotes two sub-types of lesion : ephelides (ephelis – singular) and lentigines (singular – lentigo). Ephelides represent a discrete accumulation of excess melanin round or oval in shape with ill-defined borders around 1 – 3 mm in diameter. These are mainly seen in fairer skin individuals, but they can arise in skin of colour. The colour can range from reddish, to light brown to dark brown. They are most prominent in the summer but uniquely fade in winter. Histologically, the basal layer remains normal with no increase in melanocytes.
Their origin is thought to be related to genetics rather than any environmental factors [3] with a number of genes identified that may predict freckling. They appear in the first few years of life, due to sun exposure, leading to an accumulation of melanin pigmentation. As the person ages they may fade and disappear. Freckles are a recognised risk factor for the development of melanoma [4] often denoting a fairer skin type susceptible to UV-B damage (accompanied with a red hair, blue eyes phenotype).
Dermatoscopically, freckles present with a uniform pigmentation and a moth-eaten border.
What is a lentigo?
A lentigo (plural lentigines), although similar in appearance to ephelides, do not fade after UV exposure. Their distinction from freckles (ephelides) is made on their comparatively darker colour, more scattered distribution and stability in colour throughout the year.
Lesions are less demarcated than ephelides with epidermal proliferation and replacement of local basal cells with melanocytes. Unlike ephelides, where melanocytes numbers remain the same. Various types exist such as the simple lentigo seen in childhood in fair skin individuals. They arise in the early years and increase in numbers until the age of 40. They represent increased melanocyte numbers at the dermo-epidermal junction.
Solar lentigo (liver spot) is brown macule arising from accumulated photodamage. They are typically on sun exposed sites such as the face and hands, larger than simple lentigo with an irregular border and occasional scaling on the surface.
Cases of multiple lentigines arising on patient’s skin undergoing PUVA have been reported. Occasionally, multiple lentigines can be associated with various recognised syndromes such as Peutz-Jeghers, Carney complex and LEOPARD syndrome. Lentigines can be treated effectively with lasers when required.
Table adapted from Praetorius et al., [5]
References
1. Cichorek M, Wachulska M, Stasiewicz A, Tymińska A: Skin melanocytes: biology and development. Postepy Dermatol Alergol 2013, 30:30-41.
2. Li R, Jia Z, Trush MA: Defining ROS in Biology and Medicine. React Oxyg Species (Apex) 2016, 1:9-21.
3. Bataille V, Snieder H, MacGregor AJ, Sasieni P, Spector TD: Genetics of risk factors for melanoma: an adult twin study of nevi and freckles. J Natl Cancer Inst 2000, 92:457-463.
4. MacKie RM: Incidence, risk factors and prevention of melanoma. Eur J Cancer 1998, 34:3-6.
5. Praetorius C, Sturm RA, Steingrimsson E: Sun-induced freckling: ephelides and solar lentigines. Pigment Cell Melanoma Res 2014, 27:339-350.
