Additional Notes for Brief Histology Overview and Intro to Integumentary System:

Cell Types:

 

Fibroblast – Cell of connective tissue that produces and secretes fibers (e.g. collagens, reticular and elastic fibers) and other ground substance in order to maintain the extracellular matrix (especially during wound healing or tissue repair), and to provide a structural framework for many tissues.

“Nerve” cells – neurons, but there are also many times more support cells, or glia, in the brain which provide support to the neurons.  These include astroglia, or star-shaped cells, that provide support to the neurons, provide nutrition, and clean up debris; Schwann cells that provide insulation (myelin) to the neurons; etc.

Smooth muscle - an involuntary, non-striated type of muscle cell.  There are two primary types.  It is found in the organs and also in various other areas, such as the arrector pili of the skin; human reproductive tracts; iris of the eye; gastrointestinal tract; respiratory tract, etc.

Skeletal muscle - a long, cylindrical, multi-nucleated cell formed from myoblasts (embryonic progenitor cells that give rise to muscle cells).  Tissue looks striated (long threads or transverse stripes or lines seen in tissues).

Cardiac muscle – third type of muscle tissue (smooth and skeletal are other two) – also striated; involuntary; found in the walls of the heart; extremely fatigue resistant.  Myocardiocytes (muscle cells that make up the cardiac muscle) are long and cylindrical, like skeletal muscle cells, but contain only one nucleus.

Cartilage cells – chondrocytes are the only cells found in healthy cartilage; they reproduce very, very slowly.  Do not contain blood vessels but are supplied by diffusion, which is why they grow and repair so slowly.

Fat cells – 85% fat and only 15% cytosol.  Also called adipose tissue.  The average lean adult has 40 billion fat cells.  An obese person has two to three times that many cells.  They safeguard your vital organs, direct hormones, reserve energy, and release chemicals that direction brain function and metabolism.  Some scientists are beginning to think of fat as an endocrine organ, like the pituitary or thyroid glands because of the hormones they secrete.

The Integumentary System:

Just as each individual cell of our body is protected by a cell membrane, so our entire body is enwrapped in a protective covering. 

Our skin, nails, hair and associated glands are all part of our integumentary system – our most visible organ (the largest) and one of the most complex. 

This system protects the body from the outside world and many of its harmful substances.

It utilizes the sun’s rays while at the same time shielding the body from its harmful effects (creates vitamin D).  In the lower layers of the epidermis, cells contain a form of cholesterol (fatlike substance produced by the liver that is an essential part of cell membranes and body chemicals). When exposed to UV radiation, that cholesterol changes into vitamin D, which the body uses to absorb calcium and phosphorus from food in the small intestine. Those two minerals are then used to build and maintain bones and teeth, among other functions.

Its helps regulate body temperature (maintains homeostasis), serves as a minor excretory organ (through sweating), and makes the inner body aware of its outer environment through sensory receptors.  (More on these three functions shortly….)

It receives about one third of the blood pumped through the body from the heart every minute.

Keratin is abundant in the outer layer of the skin and provides some degree of waterproofing (it also helps keep water in, to prevent excess evaporation).

The thickness of the outer layer of the skin, combined with its keratin content, helps prevent microorganisms and viruses from entering the skin.  Also, sebum, a waxy or oily substance produced by the sebaceous glands of the skin helps to create a slightly acidic environment on the skin on which many organisms cannot live.  Sebum also helps keep the skin and hair moist to prevent cracking that could allow in other organisms.

If the protective outer layer of the skin is broken because of an injury and microorganisms enter the body, the many blood vessels in the dermis help prevent the microorganisms from reaching internal tissues. As an immune response, the vessels dilate or expand. This increases the amount of blood flowing to the area, which in turn brings in more white blood cells and other protein factors to battle the infection.

Even though the skin forms a protective barrier, it is still slightly permeable or allows certain substances to pass through it. Vitamins A, D, E, and K all pass through the skin and are absorbed in the capillaries in the dermis. Steroid hormones such as estrogen and chemicals such as nicotine also pass through and are absorbed. With this in mind, medical researchers have developed therapeutic patches that are attached to the skin to deliver chemicals or medication (nicotine patches for those individuals trying to quit smoking are an example).

Nails protect the exposed tips of fingers and toes from physical injury. Fingernails also aid the fingers in picking up small objects.

Hair serves a protective function, although it is limited. On the head, hair protects the scalp from damaging ultraviolet (UV) radiation from the Sun, cushions the head from physical blows, and insulates the scalp to a degree. On the eyelids, eyelashes prevent airborne particles and insects from entering the eyes. Hairs in the nostrils and the external ear canals perform a similar function.

When stimulated by cold or an emotion such as fear, the arrector pili muscles contract, pulling hair follicles upright. In animals (and in our evolutionary ancestors, who had much more body hair), this action adds warmth by adding a layer of insulating air to the fur. In present-day humans, who have very little body hair, this action seems to serve no purpose other than to create dimples or "goose bumps" in the skin.

The body is protected against the Sun's harmful UV radiation by melanin, produced by melanocytes in the epidermis. Melanin accumulates within the cells of the epidermis. It then absorbs UV radiation before that radiation can destroy the cells' DNA. Increased exposure to the Sun causes melanocytes to increase their production of melanin. The temporary result is that the skin becomes darker or tanned and is able to withstand further exposure to UV rays.

The protection afforded by melanin, however, is limited. Prolonged or excessive exposure to UV radiation eventually damages the skin. It causes elastic fibers in the dermis to clump, and the skin takes on a leathery appearance. Overexposure can also result in melanoma, a tumor composed of melanocytes.

Temperature:

Normal internal body temperature averages approximately 98.6°F (37°C). The heat-regulating functions of the body are extremely important. If the internal temperature varies more than a few degrees from normal, life-threatening changes take place in the body.

Eccrine glands play an important part in maintaining normal body temperature. When the temperature of the body rises due to physical exercise or environmental conditions, the hypothalamus (region of the brain containing many control centers for body functions and emotions) sends signals to the eccrine glands to secrete sweat. When sweat evaporates on the skin surface, it carries large amounts of body heat with it and the skin surface cools.

Because blood carries heat (a form of energy), blood flow is another regulator of body temperature. Under warm conditions, the hypothalamus signals blood vessels in the dermis to dilate or expand. This increases blood flow (and carries excess heat) to the body's surface. Like a radiator, the skin then gives off heat to the surrounding environment.

During cold conditions, the hypothalamus signals eccrine glands to stop secreting sweat. It also signals blood vessels in the dermis to constrict or close, which reduces blood flow to the skin surface. As a result, heat is kept within the core of the body.

Secretion:

Excretion is a very minor function of the skin. Sweat does contain salt and urea (a compound produced when the liver breaks down amino acids), but the amounts of these wastes are slight. The kidneys are mainly responsible for removing waste products from the blood.

Sensory Reception:

The main function of the sensory receptors in the dermis is to provide the brain with information about the external world and its effect on the skin. Thus, they alert the body to the possible tissue-damaging effects of extreme heat or cold or something that is pressing hard against the skin. They also transmit pleasant sensations, such as a gentle breeze blowing across the face or the soft caress of a loved one.

The receptors differ in their sensitivity. Touch receptors are the most sensitive, responding to the slightest contact. Found mainly in the fingers, tongue, and lips, they number about 500,000. Pain receptors, however, do not react unless the stimulus is strong enough. Located all over the body, pain receptors number between three and four million. Their high numbers indicate their importance to the body.

Receptors send their information to the brain to be interpreted. The brain then directs the body to respond, whether to remove itself from the situation or remain. Sensation, therefore, is a function of the brain and the nervous system.

 

(Don't know why the spacing is wonky, sorry!)

Regena