Membranes and Skin

     Epithelium that covers broad surfaces in the body are attached to connective tissue that provides physical and metabolic support. The epithelium together with its underlying connective tissue forms a membrane. There are three basic membranes:
  Mucous Membranes, e.g. Oral Cavity
     Mucous membranes line all body cavities that open to the exterior of the body. They line the digestive, respiratory, urinary and reproductive tracts. The epithelium varies depending upon the location. The underlying connective tissue is areolar and is called lamina propria.
     Mucous membranes are kept wet either by secretions or the fluids which they contain. Their name is derived from thick, viscous secretion, mucus, that is secreted onto the surface of most mucous membranes.
             Hagfish Video
  Serous Membranes e.g. Visceral Peritoneum of Liver; YouTube Video: Laparoscopic Cholecystectomy
     Serous membranes line most body cavities that are closed. Unlike mucous membranes, serous membranes are consistent in structure. They consist of simple squamous epithelium that rests on a thin layer of areolar connective tissue.
     Serous membranes are continuous but when the membrane covers the wall of the body cavity it is called parietal and when the serous membrane lines an organ within the body cavity it is called visceral (viscus is Latin for organ). 
     The word "serous" means watery. The simple squamous epithelium of serous membranes allows water to easily diffuse through it. This produces a thin layer of a watery solution on the surface and this gives the surface a slippery quality that reduces friction as organs move against one another and the inner wall of the body cavity.
     Serous membranes have specific names in three locations of the body:
  Peritoneum - the serous membrane that lines the abdominal cavity and the organs within it.
  Pleura - the serous membrane that surrounds the lungs.
  Pericardium - the serous membrane that surrounds the heart.
  Cutaneous Membrane (Skin)
     The skin serves a number of important functions and is associated with a number of structures that are derived from it. The skin forms the integumentary system. This will be the first organ system we will examine.
  Functions of the skin
  1. Protection
     The skin protects us in a variety of ways:
  Mechanical - The skin physically protects the tissues that it covers.
  Chemical - The skin offers some protection against chemical damage.
  Invasion of disease agents - The skin presents a barrier against invasion by a variety of pathogens.
  Ultraviolet radiation - YouTube video: Tanning is out, You Skin is in 
 The skin protects against the damaging effects of ultraviolet radiation.  (Xeroderma pigmentosum demonstrates the destructive energy of uv radiation)
  Thermal damage - The skin provides protection against the extremes of temperature.
  Desiccation - The skin provides waterproofing that protects against excessive water loss.
  2. Thermoregulation
     The skin contributes to our ability to maintain a constant internal body temperature by mechanisms that retain heat in cold environments and lose heat in hot environments.
  3. Excretion
     The skin aids in the secretion urea and uric acid in sweat.
  4. Synthesis of Vitamin D
     Exposure to sunlight enables the skin to participate in the conversion of cholesterol into vitamin D.
  5. Sensation
     The skin contains a variety of sensory receptors that provide information about touch, heat, cold and tissue damage.
  Structure of skin
     The bulk of the skin consists of two types of tissue: epidermis, the epithelium on the surface; and dermis, the underlying connective tissue. Subcutaneous tissue (a.k.a. hypodermis) attaches the skin to the underlying organs. We will consider it with the skin.
  Epidermis Thick skin
     The epidermis is an epithelium classified as stratified squamous. It is unique in that the cells that form this epithelium, called keratinocytes, programmed to die as they are pushed to the surface. This happens because the cells produce an intermediate filament, called keratin, that fills the cell and with other molecules gives the epidermis its properties of durability and water proofing.  
     The stages of this process, called keratinization, that takes place over time is reflected in the different appearance of the strata (layers) that form this epithelium. The epidermis can be divided into four strata:
Stratum basale
     The stratum basale (a.k.a. stratum germinativum) is the deepest layer and is only one cell thick. These cells are the closest to the nutrients provided by the underlying connective tissue and are constantly dividing to replace the cells that are lost on the surface. 
Stratum spinosum
     The stratum spinosum is immediately superficial to the stratum germinativum. These cells are still alive and are in the process of developing stronger connections to one another. The number of layers of cells in this stratum vary according to the thickness of the epidermis.
Stratum granulosum
     The cells of the stratum granulosum are characterized by the presence of granules. These granules contain molecules that help to organize the keratin as it builds up in the cell. 
Stratum corneum
     The cells of this layer are dead! However, they remain strongly attached to one another and have within them a dense concentration of keratin which imparts the durability and toughness that the skin possesses. The protective qualities that the epidermis provides is primarily due to the presence of this layer. 
     The dermis is the connective tissue to which the epidermis is attached. It is primarily responsible for the elasticity and the resistance to stretching, tensile strength, that the skin possesses. The properties of this connective tissue is demonstrated to some degree by the properties of leather which is the treated dermis of animal skins.  
     The connective tissue that forms the dermis can be subdivided into two layers:
Papillary layer
     This layer is directly under the epidermis and is relatively thin. In places subject to severe shearing forces such as the palms of the hands, this layer contains numerous bumps that indent the epidermis above. This pumps are called dermal papillae. The dermal papillae contain loops of capillaries that provide nourishment for the epidermis and sensory receptors that detect touch, temperature and tissue damage (pain). 
     On the palms and soles the dermal papillae are arranged in definite patterns. These patterns are reflected by the ridges of epidermis that form the whorls and loops give us our distinctive fingerprints.
Reticular layer
     The reticular layer is the deeper layer and it makes up most of the dermis. It consists of dense irregular connective tissue that contains both elastic and collagen protein fibers. The elastic fibers give the skin its resilience after being stretched and collagen fibers gives the dermis its tensile strength or its resistance to stretching. 
  Subcutaneous Layer (Hypodermis)
     The subcutaneous layer is not part of the skin but is often discussed with the skin. This layer consists of a type of loose connective tissue called adipose tissue because of the predominance of fat cells, or adipocytes, in this layer. This layer serves to provide padding and to insulate and attaches the skin to the underlying organs. 
  Skin Color
     In the stratum basale of the epidermis amongst the keratinocytes another cell called a melanocyte is found. This cell is responsible for the production of melanin, a pigment that ranges in color from yellow to brown to black. The melanin produces by the melanocytes is distributed to the keratinocytes by long processes. Inside the keratinocyte the melanin forms a protective shield over the nucleus and protects the genetic material from the harmful effects of ultraviolet radiation. 
     The skin also takes on a yellow-orange cast (carotenodermia) from an orange-yellow pigment called carotene found in certain foods such as carrots. Carotene is deposited in the stratum corneum and the dermis. 
     Finally, normal skin color is due to the amount of oxygen-rich blood in the dermal blood vessels. This gives the skin its rosy hue. 
  Appendages of the Skin
     The appendages of the skin include cutaneous glands that include oil and sweat glands, hair with their associated follicles, and nails. We will only examine one of these:
Cutaneous glands
     Cutaneous glands are all exocrine glands that develop from epithelial cells that push down into the underlying connective tissue. There are two types of cutaneous glands:
 Sebaceous (oil) glands
  Sebaceous glands are usually associated with hair follicles but may stand alone. They produce an mixture of oily substances and fragmented cells called sebum. Sebum keeps the skin and hair in good condition and contains chemicals that kill bacteria. 
Sudoriferous (sweat) glands
  Sweat glands are widely distributed in the skin and their are two types:
1. Eccrine sweat glands
  These glands are the most numerous and produce sweat, a clear aqueous solution containing salts and traces of metabolic waste. Sweat is slightly acidic and this inhibits bacterial growth. Sweat reaches the surface by means of pores on the surface. 
  Sweat makes an important contribution to the thermoregulatory function of the skin. When the external temperature is too hot sweat helps to cool the body by evaporating on the surface.
2. Apocrine sweat glands
  These glands are located in specific regions of the body including the armpits and genital areas and their duct open into hair follicles. Their secretions are more viscous than that of eccrine sweat glands because of the addition of fatty acids and proteins. Bacteria metabolize these components to produce body odor.