The Dermis and Hypodermis

Lesson Goals:

  • Describe the structure of the dermis and hypodermis.

  • Identify the accessory structures of the dermis.

  • Describe the functions of the accessory structures of the dermis.

  • Describe how the integument works to help regulate body temperature.

Key Vocabulary:

  • Dermis: The middle layer of skin, contains collagen, blood vessels, glands, hair follicles, and nerve endings support and protect the skin and deeper layers, assist in thermoregulation, and aid in sensation.

  • Hypodermis (subcutaneous): The innermost layer of skin, storing energy, connecting the dermis layer of your skin to your muscles and bones, insulating your body, and protecting your body from harm.

  • Dermal papillae: Protrusions of dermal connective tissue into the epidermal layer.

  • Melanocytes: Produce and distribute melanin, which is a skin pigment.

  • Sebaceous glands: Microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum

  • Sudiferous (sweat glands): Found in the dermis, the sweat duct that transports the secretion to the surface of the epidermis (eccrine glands) or into a hair follicle (apocrine glands).

  • Eccrine: Sweat glands that are the most numerous and widely distributed over the body.

The Dermis and Hypodermis

The dermis is the middle layer of the integument. The dermis consists of loose connective tissue and houses a number of accessory structures of the skin. The dermis connects to the epidermis by means of wavy structures called dermal papillae.

The dermis contains a variety of accessory structures of the integument. These include:

  • Hair follicles

  • Arrector pili muscles

  • Sweat glands

  • Sebaceous glands

  • Sensory receptors

  • Blood vessels

Hair Follicles/Sebaceous Glands

The human body has approximately 2.5 million hairs on its surface. Hair is not found on the palms of the hands and soles of the feet as well as on the lips, parts of the external genitalia, and sides of the feet and fingers. Hair is not alive and develops from old dead cells pushed outward by new cells. The cells contain keratin for hardness and melanin for color. Hairs can be very sensitive. This is due to a tiny plexus of nerves that surround each hair follicle. Hair is so sensitive that you can feel the movement of even a single hair.

A band of smooth muscle connects to each hair follicle. This structure, called an arrector pili muscle, is capable of moving each follicle causing it to stand up in times of sympathetic nervous system activity such as emotional stress.

Hair begins to grow at the base of the hair follicle in a structure called the hair bulb. The hair bulb is surrounded by a hair papilla that contains blood vessels and nerves. The cells of the hair bulb divide and push the cells toward the surface along the hair root and shaft.

Hair grows at a rate of about .33 mm per day. Normal adults lose about 50 hairs per day. A loss of over 100 hairs per day will cause a net loss of hair. This can happen especially in males due to changing levels of sex hormones (male pattern baldness).

There are 2 types of hair. Vellus hairs are the fine hairs located on much of your body’s surface. Terminal hairs are thicker, more pigmented and are found on your head as well as genitals and axillary region.

A small sebaceous gland surrounds each hair follicle. The sebaceous glands secrete an oily substance known as sebum. Sebum is secreted in response to contraction of the arrector pili muscle. Sebum contains triglyceride, protein, cholesterol and some electrolytes. Sebum makes the hair more flexible and hydrated.

Sweat Glands

Sweat glands (aka sudoriferous glands) are also located in the dermis. There are 2 types of sweat glands. Apocrine sweat glands secrete their substances into the hair follicles. The secretions of apocrine glands can develop odor. The odor can increase because the secretion acts as a nutrient for bacteria that enhance the odor.  Apocrine glands begin to secrete substances at puberty and are located in the axilla and genital regions.

Eccrine sweat glands secrete their substances directly onto the surface of the skin. They are coiled tubular glands that secrete a substance that mostly consists of water with a trace of some electrolytes and a peptide with antibiotic properties. The eccrine sweat glands primary function is to help to regulate body temperature. The sweat can evaporate and carry away heat. The sweat also excretes water and electrolytes.

Nails

The nails exist at the distal portions of the fingers and toes. The nail body is the visible portion of the nail sits over the nail bed. The nail begins deep in the skin proximal to where it is seen at the nail matrix. The nail begins to grow at the nail root which is close to the bone. A portion of the superficial epidermis (stratum corneum) extends over the proximal portion of the nail forming the eponychium or cuticle. Blood vessels deep to the nail give it a pink color.

Temperature Regulation

The skin is very important in regulating body temperature. The skin helps keep in heat produced by skeletal muscles and liver cells. When the body gets too hot the skin opens up the sweat pores so that the sweat can carry the heat away by evaporation.

Heat can be lost by the body in a number of ways. Heat always moves along a gradient from warmer to cooler temperatures. Heat can radiate from the body to the surrounding areas at lower temperatures. In conduction, heat moves via molecules from the warmer body to cooler objects. An example of conduction would be to lean against a cooler concrete wall. The heat flows from your body into the wall.  In convection, heat moves via air molecules circulating around body. In evaporation fluid on the surface of the body carries heat away.

Body temperature is primarily regulated by an area in the brain known as the hypothalamus. The hypothalamus sets the body’s temperature and controls it by opening and closing sweat glands and contracting muscles.

Let’s say that it is a hot summer day and you are working hard mowing the lawn. As your body’s temperature rises the hypothalamus senses this and sends a message to your sweat glands to open. The sweat evaporates off of your skin and you begin to cool down.

Now let’s say that you’ve finished mowing the lawn and you go inside of your air conditioned home. Your body’s temperature will begin to drop. The hypothalamus senses this and sends a message to your sweat glands to close. If your body’s temperature continues to drop the hypothalamus may send a message to your muscles to contract or shiver. The muscles will generate heat to help maintain your core temperature. In more severe cases of cold your blood vessels will constrict in your extremities in an attempt to conserve heat at the core of your body for survival.

If your core body temperature continues to drop you may develop a condition called hypothermia. You will progress from feeling cold to shivering, experiencing mental confusion, lethary, loss of reflexes and eventually loss of consciousness and shutting down of organs. Conversely if your core body temperature increases too much, you can develop hyperthermia. This can develop in humid conditions because of lack of evaporation. The signs of hyperthermia include light headedness, dizziness, headaches, muscle cramps, fatigue and nausea.