Structure of the Skin

Three distinct layers can be seen in the skin

• Epidermis - consists of keratinizing stratified squamous epithelium
  

• Dermis - consists of fibroelastic connective tissue
  

• Hypodermis - consists mostly of white adipose tissue (sometimes referred to as the subcutis)

The thickness of these layers varies depending on the specific location in/on the body and in a given location on the amount of exposure to wear and tear.  For example, in the buccal cavity, the epidermis consists of a moist stratified squamous epithelium which is relatively think and not highly keratinized whereas the epidermis of skin on the ball of the foot is thick and highly keratinized.  The skin covering the dorsum of the hand has a rather think hypodermis whereas the skin over the buttocks has a very thick hypodermis containing numerous fat cells.

Micrograph of skin of squirrel foot pad. 
(Lab slide 252)

  Layers of the Epidermis

 - in order from outermost (surface) to innermost (deepest)

• Stratum corneum - consists of remains of keratinocytes; mostly composed of the protein, keratin
    
• Stratum lucidum - present only in very thick skin as seen here; pale-staining layer of cells between the stratum corneum and stratum granulosum in which the dying keratinocytes contain a lot of keratin but are not completely replaced by it  
   
• Stratum granulosum - consists of keratinocytes containing large numbers of granules that contribute to the process of keratinization
  
• Stratum spinosum- consists of large, polyhedral keratinocytes that are actively synthesizing keratin which is inserted as tonofibrils into the area of the plasma membrane beneath desmosomes that connect adjacent cells together.  These "connections" or desmosomes between cells in this layer help hold them together and result in the "spiny" appearance of the cells that gives this layer its name.  
  
• Stratum basale - consists of keratinocytes undergoing mitosis to produce the constant supply of keratinocytes needed for replacement of the dead and dying cells in the more superficial layers of the epidermis

Micrographs of skin from squirrel foot pad. 
(Lab slide 252)

Two zones of the dermis:

• papillary zone - consists of loose areolar connective tissue containing collagen and fine elastic fibers; connects the epidermis to the thicker and denser reticular zone of the dermis
  

• reticular zone - contains dense, irregular and coarse collagen fibers and thick elastic fibers interspersed with fibroblasts and blood vessels and nerves

Micrographs of squirrel foot pad. 
(Lab slide 252)

Types of Glands and Modes of Secretion

• Sebaceous glands

• Apocrine sweat glands

• Merocrine (= eccrine) sweat glands [example not shown]

Sebaceous Glands: The epithelium of this gland is an outgrowth of the external root sheath of the hair follicle and the gland empties its oily product directly into the follicle itself.  The glands are of a branched acinar type and produce a lipid product called sebum that serves to reduce the entry of microorganisms into the body through the skin, to lubricate the hair and to preventing it from drying out.  The secretory cells die and become part of the product; a holocrine mode of secretion.  There are no sebaceous glands in foot pads, hooves, claws or horns.

Apocrine Sweat Glands:  These glands are coiled, tubular glands with a large lumen and a duct connecting it to an adjacent hair follicle.  These glands secrete a viscid, milky product and are analogous to odiferous glands of many mammals.  Once thought to use the apocrine mode of secretion, it is now known that thought that their mode of secretion is more like that of the merocrine sweat glands.  These glands are the primary sweat gland of domestic animals and are especially prominent in the horse.  

Merocrine Sweat Glands:  These glands are unbranched tubular in form and appear as a mass of tubules in cross section.  They are plentiful in the upper regions of the fatty hypodermis and open onto the surface of the skin directly.  They secrete a watery product that is hypotonic to the plasma.  It is the evaporation of this secretion on the surface of the skin that aids in thermoregulation. 

Micrographs of dog lip. 
(Lab slide 207)

General structure of hair and associated structures:

• Hair shaft: the part of the hair above the surface of the skin
  
• Hair root: the part of the hair below the surface
  
• Bulb: an enlarged, hollow portion at the base of the root
  
• Hair papilla: projection of dermis into center of the bulb
  
• Follicle: the indentation in the skin within which the root lies

Micrograph of dog lip. (Lab slide 39);
 A & B are from the same area, 
A is at the surface of the lip and B
 is farther down in the dermis.

Hair has three layers:

• Cuticle: the outermost layer. Single layer of flattened, keratinized cells. Overlap like shingles, with free edge distally.
• Cortex: the thickest, intermediate layer. Consists of several layers of keratinized cells containing hard keratin. If hair is colored, these cells contain pigment. Cells held together by desmosomes.
• Medulla: central core; loosely packed cuboidal cells.

The structure and organization of the cuticle and medulla cells are species-specific.

Micrograph of dog lip. 
(Lab slide 39)

The Hair Follicle 

The hair follicle is the structure that anchors the hair in the dermis.  It is composed of five layers of epithelial cells arranged concentrically.  The inner three layers form the hair shaft through a process of keratinzation while the outer two layers form the hair sheath.  

1. cells in the innermost layer form the medulla of the hair or core of the hair shaft

2. cells in the next layer form the cortex that makes up most of the hair 

3. cells in the third layer form the cuticle on the surface of the hair  

4. cells in the fourth layer make up the internal root sheath 

5. cells in the fifth or outermost layer form a layer called the external root sheath that does not take part in hair formation  

Micrograph of lip of dog to show a hair follicle. 
(Lab slide 39)

The external root sheath is separated from the surrounding connective tissue by a thick basement membrane known as the glassy membrane.