Papillary and Reticular Layers of Dermis

Structure/Morphology

The dermis can be subdivided into two indistinct layers. The superficial layer is the papillary layer and a deep layer called the reticular layer. The papillary layer is a thin layer of loose connective tissue found beneath the epidermis. This part of the dermis contributes to the dermal papillae that interdigitate with the epidermal ridges. It contains collagen type III and elastic fibers, and loops of capillaries supplying blood to the avascular epidermis and to regulate temperature.

The reticular layer of the dermis, the deep layer, is much thicker than the papillary layer, yet there is no distinct boundary between them. The reticular layer is composed of a dense irregular connective tissue of mostly collagen type I with interspersing elastic fibers. The reticular layer is also continuous with the underlying subcutaneous tissue (hypodermis). A deep plexus of large blood vessels lies at the border between the reticular layer of the dermis and the hypodermis. It supplies numerous branches of capillaries to the papillary layer (Arda, Goksugur and Tuzun, 2014).

The papillary layer of the dermis contains loops of capillaries supplying blood to the avascular epidermis. These capillaries extend into the dermal papillae, supplying a rich, nutritive vasculature to just below the epidermis. In addition, this vasculature acts as a thermoregulator. In cold conditions, blood flow is decreased in the papillary layer to minimize heat lost from the body, while blood flow is increased in the papillary layer in hot conditions (Mescher, 2013). Lymphatic afferent vessels arise at the dermal papillae and converge forming plexuses adjacent to the blood vessels.

The dermis is also richly innervated, with a network of sensory afferent nerves extending from the papillary dermis, ending at epidermal and dermal receptors.

Other structures found in the dermis include hair follicles, sebaceous and sweat glands, blood and lymph vessels, sensory receptors, and nerves.

Recent studies have suggested that the tissue arrangement of dermis is not a densely packed wall-like structure, commonly seen in alcohol fixed tissue. In fact, it is composed of a collagen lattice surrounded by fluid filled interstitial spaces, the interstitium (Benias et al., 2018). The fluid has been described as pre-lymph where it is collected by lymph capillaries to form lymph. This has also been described in other tissues, such as lung, renal, blood vessel adventitia, and fascia.

Function

The dermis has a number of key roles. Firstly, it acts as a support for the epidermis and hypodermis. Its elastic fibers give the skin elasticity and pliability. As the dermis contains a rich blood supply, it provides the avascular epidermis with nourishment. In addition, this vasculature acts as a thermoregulator. In cold conditions, blood flow is decreased in the papillary layer to minimize heat lost from the body, while blood flow is increased in the papillary layer in hot conditions (Arda et al., 2014).

The interstitium present in the dermis may serve as a shock absorber, due to the constant distension and compression of the skin and may also play a role in pathologies such as edema (Benias et al., 2018).

References

Arda, O., Goksugur, N. and Tuzun, Y. (2014) 'Basic histological structure and functions of facial skin', Clin Dermatol, 32(1), pp. 3-13.

Benias, P. C., Wells, R. G., Sackey-Aboagye, B., Klavan, H., Reidy, J., Buonocore, D., Miranda, M., Kornacki, S., Wayne, M., Carr-Locke, D. L. and Theise, N. D. (2018) 'Structure and Distribution of an Unrecognized Interstitium in Human Tissues', Scientific Reports, 8(1), pp. 4947.

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Mescher, A. (2013) Junqueira's Basic Histology: Text and Atlas. 13th edn.: McGraw-Hill Education.