What is the dermis?

The dermis layer of the skin lies beneath the epidermis layer and above the subcutaneous tissue (hypodermis). It is made up of a thin, upper layer referred to as the papillary dermis, and a thick, lower layer called the reticular dermis.

Between the epidermis and the dermis is a membrane called the basement membrane, which tightly binds the two skin layers together.

The thickness of the dermal layer varies from 0.3 mm on the eyelids to 3 mm on the back. It is the thickest part of the human skin accounting for almost 90% of the skin thickness and is made up of fibrous and elastic tissue. The epidermis and dermis together form the cutaneous tissue.

Structures present in the dermis

The dermis of the skin is made of the following tissues:

  • Connective tissue: collagen and elastin
  • Nerve endings provide the skin with the feeling of touch and heat
  • Sweat glands (eccrine glands empty on the skin surface)
  • Hair follicles
  • Sebaceous glands (small glands that secrete an oily substance called sebum to lubricate the skin and hair are present all over the skin, more on the face and scalp but absent on palms and soles)
  • Blood capillaries
  • Lymphatic vessels
  • Apocrine glands. (tiny sweat glands that open into the hair follicles).

It is the blood vessels in the dermis that supply nourishment and remove waste from this layer as well as the epidermis.

Dermis tissue composition

The dermal tissues spread throughout the dermis and they are not spread in layers. The dermal tissues are of three types:

  • Elastic tissue. Elastic tissue makes up 10% of the dermal fibers. It is synthesized by the fibroblast cells of the dermis and made up of elastin protein and surrounded by microfibrils. They are present in the papillary dermal layer as thin fibers but become thicker in the reticular dermal layer.
  • Collagen tissue. Collagen is the major protein of the dermis and makes up 90 % of the dermal fibers. It is continuously being synthesized by the dermal fibroblast cells. The collagen bundles are arranged parallel to the surface in the reticular layer but are placed haphazardly in the papillary layer. This tissue provides tensile strength to the dermis.
  • Reticular fibers. The reticular fibers provide strength and elasticity to the skin. It supports the hair follicles, sweat glands, and the sebaceous glands

Cellular structure

The cells of the dermis are mainly derived from the somatic mesoderm. They include:

  • Fibroblasts are the main cells of the dermis. They secrete the connective tissue matrix and are responsible for the synthesis of collagen, elastic and reticular fibers.
  • Schwann cells are responsible for the formation of the perineural structures. They produce the myelin sheath around neuronal axons.
  • Mast cells of the dermis are found around the blood capillaries and in the periphery of subcutaneous tissues. These cells release inflammatory substances such as prostaglandins, leukotrienes, and platelet-activating agents.
  • Histiocytes are macrophages (a type of white blood cell) present within the connective tissue of the dermis and are part of our immune system.

Layers of dermis

The layers of the dermis are only two in number and their composition and function are explained below.

  • Papillary dermis. This dermal layer consists of loose connective tissue, which contains finger-like projections called papillae that extend upwards into the epidermis and anchor it to the dermis. Collagen is a thinly spread layer. The papillary layer contains a rich network of blood capillaries, which also nourish the epidermis. This layer also contains nerve endings.
  • Reticular dermis. This dermal layer is made up of thick collagen and elastic fibers that run parallel to the skin and is situated deeper below the papillary region. It has more dense connective tissue and gives the dermis its strength, elasticity, and extensibility. The roots of the hair, the sebaceous glands, blood vessels, sweat glands, and receptor nerve endings are situated in this reticular layer.

Functions

The dermis is the thickest part of the skin and possibly the most important because it serves vital key roles.

  • The dermis contains the apocrine and eccrine sweat glands, which are responsible for the production of sweat. This helps regulate body temperature.
  • The sebaceous glands in the dermis secrete an oily substance called sebum. Sebum prevents the growth of bacteria on the skin and the loss of water from the skin’s surface. It also conditions the hair and skin.
  • The hair follicles are present in the dermis from which the hair on the skin grows. The hair on the skin gives protection, facilitates the evaporation of perspiration, and helps in the regulation of body temperature. Hair also plays an important role in the body’s sensory apparatus.
  • The nerve endings in the dermis send sensory impulses to the brain when something hurts the skin, or there is itching, or even when something feels good.
  • The blood capillaries located in the dermis provide nourishment to the skin, remove toxins and even supply the epidermis with blood since it has no blood supply of its own. It derives its nourishment from the blood vessels present in the dermal layer.
  • The main function of the reticular dermal layer is to give firm support to the skin as a whole and is responsible for the turgor of the skin.
  • The main function of the collagen fibers of the dermis is to support the internal structure of the skin.
  • The principal function of dermal elastic fibers is to give elasticity to the skin in order to facilitate movements. Over-extension of these fibers causes stretch marks as seen during post pregnancy period.
  •  Because of its composition, the dermal layer also acts as a cementing substance keeping the layers of the skin together and the body as a whole.
  • The dermis holds water molecules to form an amorphous gel, which performs two functions of allowing the oxygen and nutrients to enter into the deeper tissues and protecting the dermal structure.