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Published 2021 in Aesthetic Dermatology

ABSTRACT

Skin anatomy The skin of the face provides crucial functions of temperature regulation, reparative functions following injury, facial expression, protective mechanisms against infection and ultraviolet radiation, as well as sensory perception. The integument consists of two principle layers, the epidermis and dermis. Four principle cells make up the stratifi ed squamous keratinised epithelium of the epidermis, of which keratinocytes are the most abundant [1]. The stratum basale comprises a single layer of basophilic column shaped keratinocytes with large nuclei [2], which are fi rmly attached to the basement membrane with desmosomes [3]. Some of the proliferated cells remain adherent to the basal lamina as stem cells, while others differentiate to spinal keratinocytes of the stratum spinosum and push the overlying cells towards the surface [4]. Renewal of the epidermis takes 15-30 days and is infl uenced by age as well as the anatomical body location [5]. Melanocytes are melanin-producing cells of the skin, and are distributed among the cells of the stratum basale. Melanocytes do not keratinise but produce melanin pigment, which serves to protect the DNA of the cells by absorbing and scattering the harmful ultraviolet rays [5]. The exposed quantity of UV radiation stimulates the production of melanin and accelerates its transfer to keratinocytes (by a process of pinicytosis), as well as darkening existing ones [6], and has a pivotal role in determining the colour of skin, as a darker skin colour depends on the amount of melanin production in the melanocytes [6]. Merkel cells are intraepidermal mechanoreceptor cells and can also be found in the stratum basale [4]. The stratum spinosum is the thickest layer of the epidermis with layers of polyhedral-shaped keratinocytes, which fl atten as they approach the surface [1]. Langerhans cells are located in this layer and derived from monocytes of bone marrow [7]. They are morphologically and functionally similar to macrophages and have no role in keratinisation or pigmentation of the epidermis. Langerhans cells initiate an immune response against foreign antigens and are signifi cant in allergic responses and other cellmediated skin reactions [7]. The stratum granulosum comprises large, fl attened polygonal granulated cells which still have nuclei and undergo fi nal cell differentiation of the uppermost keratinocytes of the stratum spinosum. The stratum corneum is made up of fl attened and highly keratinised cells lined with keratins, which are shed from the epidermis as dead skin cells [1]. The dermis adheres the epidermis to the hypodermis and comprises a tough supporting fi broelastic tissue with two distinct layers. The fi rst is the papillary layer located immediately under the epidermis, which covers the dermal papillae and connective tissue matrix, which holds type III collagen [2]. The reticular layer is also a mixture of dense connective tissue with coarse elastic fi bres as well as collagen type I. The dermis encompasses sebaceous glands, blood vessels, nerves and sensory receptors [8], as well as some facial musculature. Subcutaneous tissue is located beneath the reticular layer of the dermis, connecting the deep fascia [5], as well as providing a gliding plane, which is a key mechanism for facial expression. The thickness of facial skin varies at different anatomical sites [9]. Reported skin thicknesses vary signifi cantly within the literature [10]. Chopra et al. [9] describe the dermis to be thickest at the inferior nasal wall and thinnest on the upper medial eyelid and the epidermis to be thickest on the upper lip and thinnest at the posterior auricular skin. The skin of the face has a rich network of blood vessels in the dermis and hypodermis with two principle blood plexuses, which nourish the skin. The deeper resides between the hypodermis and dermis, and the superfi cial plexus is located between the papillary and reticular layers of the dermis, which supplies blood to a large network of capillaries in the dermal papillae [4]. There are many anastomoses between the two plexuses to allow for thermoregulation to maintain a constant temperature in the body by regulating the amount of blood fl ow in the capillaries of the papillary layer [4]. A Baker

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