Anatomy of the Skin
The epidermis is the most superficial layer of the skin and provides the first barrier or protection from the invasion of foreign substances into the body. The principle cell of the epidermis is called a keratinocyte. The epidermis is subdivided into five layers or strata, the stratum germinativum (SG), the stratum spinosum (SS), the stratum granulosum (SGR), the stratum lucidum (not seen in this photomicrograph) and the stratum corneum (SC) in which a keratinocye gradually migates to the surface and is sloughed off in a process called desquamation.
The condition of the epidermis determines how “fresh” your skin looks and also how well your skin absorbs and holds moisture. Spots, pores, evenness, porphyrins, and UV spots all manifest themselves in our epidermis.
Wrinkles, however, are formed from changes in the dermis.
The dermis assumes the important functions of thermoregulation and supports the vascular network to supply the avascular epidermis with nutrients. The dermis is typically subdivided into two zones, a papillary layer and a reticular layer. The dermis contains mostly fibroblasts which are responsible for secreting collagen, elastin and ground substance that give the support and elasticity of the skin. Also present are immune cells that are involved in defense against foreign invaders passing through the epidermis.
The papillary dermis (PD) contains vascular networks that have two important functions. The first being to support the avascular epidermis with vital nutrients and secondly to provide a network for thermoregulation. The vasculature is organized so that by increasing or decreasing blood flow, heat can either be conserved or dissipated. The vasculature interdigitates in areas called dermal papillae (DP). The papillary dermis also contains the free sensory nerve endings and structures called Meissner’s corpuscles in highly sensitive areas.
The reticular layer of the dermis (RD) consists of dense irregular connective tissue, this differs from the papillary layer, which is made up of mainly loose connective tissue (note the difference in the number of cells). The reticular layer of the dermis is important in giving the skin its overall strength and elasticity, as well as housing other important epithelial derived structures such as glands and hair follicles. This reticular layer is our target for collagen production.
Collagen is a member of the connective tissues: cartilage, bone, tendons, fascia, ligaments, and blood vessels. The character of any connective tissue is determined by the function of the specific cells that compromise that tissue. The major fibrous proteins are collagen and elastin. The predominant nonfibrous components are the complex carbohydrates mainly: hyaluronic acid, the proteoglycans, and glycoproteins. Collagens are a class of proteins, members of which have chemical and structural features in common, but each is a product of a specific gene (hence, the specific connective tissue disorders). One property of all collagen molecules is the unique triple helix, a particular conformation of the three component polypeptide (alpha) chains, each containing approximately 1000 amino acids. The conformation of the chains is determined by the amino acid content, with glycine constituting a third of the total and occurring at every third position in the amino acid sequence. Thare are two major classes of collagen: interstitial and pericellular. Interstitial collagens are the major collagen of the skin and essentially exclusive type of bone (type I); articular cartilage and nucleus pulposus (type II); and the collagen present in the skin, blood vessel walls, and the matrix of parenchymal organs (type III). Pericellular collagens are types of IV and V and predominate in basement membranes.
Biosynthesis of the collagen chains is a multistep process in which a precursor form (procollagen) is first synthesized, with peptide extensions at either end. During synthesis, several amino acids are uniquely modified post-translationally (after incorporation into the polypeptide chains). These post-translational modifications include hydrozylation of praline residues (hydroxyproline) and lysine residues (hydroxylysine) and addition of sugars (glucose and galactose) to the hydroxylysines, and formation of the hydroxylysing and lysine aldehydes. Specific proteases act to cleave off the extensions of the procollagens to produce the processed collagen molecules, which can then polymerize to form fibrils and fibers. Our lasers target the cell mediators which stimulate the production of procollagen I & III.
Cutaneous (skin) aging is a complex biological phenomenon. Disorders in aged skin are more the result of photoaging (UVR) than from chronological aging. There is gradual atrophy of the dermis and epidermis and a massive accumulation of abnormal elastic tissue with its associated microfibrils and proteoglycans. These elements replace the normal collagen rich dermal foundation, leading to a weakened and thinner dermal layer. UVR consists of UVA (320-400nm) + UVB (280-320nm) and UVC which at the present time does not penetrate our atmosphere. UVA light is the prime offending agent for collagen loss and subsequent wrinkles. UVB’s shorter wavelength only affects the epidermis. As the dermal layer “sags” so then follows the epidermis, and wrinkles appear. This decrease in collagen results more from increased deterioration (UVA) rather than from a significant decrease in production (age). The term solar elastosis is used to describe photodamaged skin.
The thickness of the skin changes with age: young skin gradually gets thicker until the age of about 20 years, after which there is a gradual atrophy of the dermis. At the level of the papillary dermis, a relatively nonechogenic band appears and develops with age. The exact reasons for the appearance of this shadow are unknown, but it corresponds to a homogenization of the papillary dermis, the local disappearance of elastic and collagen fibers, and their replacement with an undifferentiated matrix of hydrated gycosaminoglycans (elastotic tissue). The extent of this shadow steadily increases with age and is a reliable marker for skin aging (loss of collagen density). This shadow is found in both types of aging skin: photo and chronological. An increase in dermal thickness from laser therapy is found in the upper dermis and is due to an increase in the collagen fibers and not the other components of the matrix. The laser heat causes a partial denaturation of the older collagen and accelerates the new collagen synthesis process by the fibroblasts and the deposition of new glycosominoglycans and leads to a reabsorption of elastotic material. An increase in the echogenic band in the upper dermis is easily visualized with ultrasound and is directly attributable to an increase in collagen. Two possible benefits occur with laser collagen remodeling therapy. The thickness of the dermis increases and/or the density of the collagen increases.
Anti-aging modalities are classified by type:
Type I therapy is directed at the epidermis (microdermabrasion/chemical
Type II therapy is directed at the dermis (laser/chemical peels/ultrasound)
Type III therapy is surgical or injectable (facelift, botox, fillers).
Dr Barry Eppley