Dermoscopy Pigment Network
The pigment network is the most important structure in dermoscopy. It appears as a grid of thin brown lines over a diffuse light brown background.
The network pattern is the dermoscopic hallmark of benign acquired melanocytic nevi and of thin melanomas. A pigment network is also found in lentigo simplex, solar lentigo and in the center of dermatofibromas.
We must understand how this pattern is formed in order to interpret all pigmented lesion. The pigment network is a grid-like or honeycomb-like structure consisting of round, pigmented lines and lighter hypo-pigmented holes. This picture has also been described as a net like or reticular pattern.
Melanin is produced by melanocytes located in the basal layer of the epidermis. Most of this pigment is distributed to keratinocytes located along the basal layer at the dermoepidermal junction. Let us look at the microanatomy of the epidermis. Look at a vertical slice down through the epidermis and dermis. The epidermis looks like a wave with high and low points the so-called rete ridge pattern. Normally there is more melanin in the lower rete than in the upper rete near the skin surface.
Now look at a horizontal section of a bees honeycomb. The epidermis has a similar structure. This shows that the rete are actually continuous and appear as round walls with a hole in the middle. The walls or rete are vertically thick, project down into the dermis and contain melanin. They look like dark round lines when viewed from above. The round lines are connected with adjacent rete to give the characteristic net like pattern. The epidermis at the top of this wave-like structure is thin. This round area contains small amounts of melanin and appears as a light circle when viewed from above.
The presence of a pigment network usually implies that the lesion is melanocytic. The pattern may be subtle or present only in a small area. The network of common lesions such as lentigo and junctional nevi fades and thins at the periphery. The typical honeycomb-like pattern of the pigment network on the trunk and proximal extremities results from pigmentation along the rete ridges. A so called pseudo-network patterns on the face, palms, and soles result from junctional pigment outlining hair follicles, sebaceous glands, and eccrine ducts. Pigment on the palms and soles outlines linear skin markings along or across the skin furrows resulting in a parallel, lattice, or fibrillar pattern.
The pigment network can be either typical or atypical. Slowing growing benign pigmented lesions such as a lentigo or junction nevus produce uniform network patterns. The lines are uniformly populated with benign pigment cells that grow at a slow rate. The network is homogeneous in color and usually thins out at the periphery. Melanomas consist of malignant melanocytes that vary in size and degree of pigmentation. Malignant cells move through the epidermis in all directions at varying rates to produce structures with bizarre patterns. Cells are numerous in some locations and sparse in others.
Growing melanomas distort the normal skin anatomy and cause variation in pigmentation patterns and structures. Malignant cells multiply and may stuff the rete. The rete can become thick and distorted. Networks in melanomas are variable. Thin melanomas and melanomas in situ may have only very subtle pigmentation changes. Subtle thickening and darkening of pigment network lines may be seen near the periphery in early lesions. As Breslow thickness increases, the pigment network tends to become more variable in thickness and color density as malignant cells become more numerous. The lines become thick and dark. They also become darker near the periphery. The projections called pseudopods and radial streaming occur at the edge of a region of thickened, darkened network at the periphery. This represents streaming melanoma cells into the surrounding epidermis.