Moles, medically known as nevi, are common skin growths formed by clustered pigment cells (melanocytes). Since moles are composed of living tissue, they require a constant supply of oxygen and nutrients. This necessary biological support relies on the skin’s extensive vascular network. Moles can appear anywhere on the body and vary in size and color depending on the depth of the melanocyte cluster.
Understanding the Structure of a Mole (Nevus)
Moles are classified as benign growths resulting from a localized accumulation of nevus cells. The location of these clusters determines the mole’s type and appearance relative to the skin’s two primary layers: the epidermis and the dermis. The dermis houses most of the skin’s support structures, including blood vessels and nerves.
Moles are categorized based on where the nevus cells reside. A junctional nevus is flat because the cells cluster at the border between the epidermis and the dermis. A dermal nevus is usually raised because the cell cluster is pushed entirely into the dermis.
The most common type, a compound nevus, has cells located in both the dermis and at the junction. Since nevus cells are physically present within the vascularized dermis or at its border, they are intimately connected to the body’s circulatory system.
Vascularization and Nutrient Supply
All living cells, including the nevus cells that form a mole, require oxygen and nutrients delivered by the microvasculature of the surrounding dermis. Although mole cells do not typically form large, dense capillary beds within their structure, they are completely dependent on existing dermal blood vessels.
The stable nature of a common mole means its nutrient demand is low and consistent. This is supported by an organized vascular network, often presenting as regular comma or dotted vessels when viewed under magnification. The process of forming new blood vessels (angiogenesis) is stable and slow in a healthy mole.
This stable vascular pattern contrasts sharply with the chaotic and rapid angiogenesis observed in malignant melanomas. Melanomas are aggressive and require a high, unregulated nutrient supply, leading to the formation of disorganized vessels. The morphology and distribution of blood vessels are specific details dermatologists use to distinguish a benign mole from a concerning growth.
Why Moles May Bleed
The presence of blood vessels explains why moles can bleed, with causes falling into two categories: trauma and pathological changes. Bleeding is most frequently caused by simple physical trauma. Raised moles, such as dermal or compound nevi, protrude and are easily snagged, scratched, or rubbed by clothing or shaving.
When a mole is physically injured, the highly vascularized dermal tissue beneath the surface is nicked, causing it to bleed. This type of bleeding is usually a minor wound. However, spontaneous, persistent, or recurring bleeding without an apparent cause is a serious concern.
Pathological bleeding occurs when the mole’s internal structure changes, often due to malignant transformation. As a cancerous lesion grows, it can cause ulceration or break down the surrounding skin, making blood vessels fragile and prone to oozing. This bleeding is often accompanied by other changes, such as asymmetry, irregular borders, varied color, or a change in diameter, which warrant immediate medical evaluation.