The dermis is the middle layer of your skin. It provides support and elasticity to your body.
It contains sweat glands, hair follicles, sebaceous glands (oil glands), blood vessels, and nerves. It also has fibroblasts, which are cells that produce collagen and elastic fibers.
The papillary layer of the dermis has nipple-like extensions that interdigitate with epidermal ridges. These are known as dermal papillae.
The Epidermis
The epidermis, found all over the body, is the skin’s first layer of defense. It is a thin layer with multiple layers of cells, and it contains two main types of cells: keratinocytes and melanocytes. The keratinocytes produce a tough protective protein called keratin, which makes up 90 percent of the epidermis. Melanocytes, in turn, produce melanin, which gives the skin its color.
Cells in the epidermis are constantly generating new cells, pushing older ones up toward the surface of the skin, where they die and are shed. This process is known as keratinization.
At the bottom of the epidermis, in the stratum basale, are undifferentiated stem cells, cuboidal in shape, that continuously divide and give rise to keratinocytes. These cells are referred to as the basal cell layer, or stratum germinativum, because of their constant tendency to generate new cells.
Several other cell types are scattered throughout the basal cell layer. One is a modified epidermal cell called a Merkel cell, which serves a sensory function as mechanoreceptors, stimulating nerve endings in the skin to respond to light touch. Another is a group of round, oval-shaped cells called Langerhans cells, which are responsible for attacking foreign pathogens as they invade damaged skin.
The next layer of the epidermis is the stratum spinosum. This layer consists of keratinocytes that have matured into squamous cells, which are rounded and flattened. These cells, together with a lipid barrier produced by the keratinocytes, form a watertight covering that protects the skin.
The Papillary Dermis
The papillary dermis is a layer of loose connective tissue directly beneath the epidermis. It contains a network of thin haphazardly arranged collagen and elastic fibres with blood vessels and cellular infiltrations. It also forms dermal papillae (‘nipples’) which interdigitate with the epidermal ridges, increasing the surface area through which gases, nutrients and waste are exchanged. The papillae may contain terminal networks of blood capillaries or tactile Meissner’s corpuscles. The papillary dermis is more abundant and larger in thick skin which has been subjected to mechanical stress than in thin skin.
Underlying the papillary dermis is a much thicker, reticular layer of dense irregular connective tissue which is well vascularized and contains both sympathetic and sensory nerves. It is more pronounced on the hands and feet and has an appearance of being’reticulated’ due to a tight meshwork of fibres. This layer also contains hair follicles and mechanoreceptors that provide the sense of touch.
Some research has shown that fibroblasts from the papillary dermis differ from fibroblasts in the reticular dermis by producing different amounts of cytokines and growth factors, such as GM-CSF and keratinocyte growth factor. This may contribute to the difference in keratinocyte proliferation and differentiation that is observed between these layers. The papillary dermis contains blood vessels that can dilate to release heat when the body temperature rises and constrict to preserve it when the body cools.
The Reticular Dermis
The dermis (also called the corium) is a thick layer of connective tissue that contains blood vessels, hair follicles, glands, nerves, fat cells and lymphatic vessels. It supports the structure of your skin and allows it to stretch and move without tearing. It also contains proteins that give your skin its strength and flexibility. The dermis consists of two major layers, the papillary layer and the reticular layer.
The papillary layer has loose, finger-like projections that push into the epidermis and create the wavy lines you see on your hands and feet known as friction ridges or fingerprints. These projections are actually extensions of the dermis, and if your skin is stretched a lot, these projections can tear and form stretch marks. The reticular layer is dense, and is separated from the papillary layer by the vascular plexus, or rete subpapillare.
The reticular layer contains blood vessels, lymphatics and low numbers of immune system cells. The reticular layer is also surrounded by a network of elastin and collagen fibers that support the skin’s structure and allow it to move and stretch. The reticular layer is also rich in fat cells, and the adipose tissues in this layer are responsible for the formation of your body’s insulation, or fat. The adipose cells also secrete fatty acids that help to keep your skin smooth and supple.
The Subcutis
The subcutis, also called adipose tissue, provides thermal insulation and mechanical protection. It is rich in blood vessels and contains oil glands, sweat glands, and nerve endings. It is thicker under the hands and feet and softer in other areas of the body.
It contains mechanoreceptors that provide the sense of touch and thermoreceptors that sense temperature. The dermis is also a shock absorber and stores energy. It consists of dense irregular connective tissue containing anchoring filaments (type 7 collagen), fibronectin, and laminin – a glycoprotein. It has a clear appearance under electron microscope and it is tightly connected to the epidermis.
A fibrous layer, varying in thickness across body sites and individuals, separates the subcutis from a deeper adipose layer. The layer is called a fascia superficialis by English anatomists, a fascia tense by Italian and French anatomists, and straffenbindegewebe by German anatomists. The Federative Committee on Anatomic Nomenclature suggests simply using the term adipose tissue.
The subcutis is well supplied with blood vessels, a condition that allows it to expand in warm weather and after exercise. As a result, it brings body heat to the surface and cools us down. It is also responsible for the smoothness of our skin, for storing energy, and for protecting organs from injury and disease. It is important to keep in mind that conditions such as Raynaud’s phenomenon and sarcoidosis can affect your subcutis.