The Skin
Epidermis
o Most superficial layer
o Composed of stratified epithelium - varies in thickness in different parts of the body - thickest in hands and soles of the feet
o Surface - dead cells - non nucleated - cytoplasm replaced by keratin - stratum corneum
o No blood vessels
o No nerve endings
o Deeper layers - bathed in interstitial fluid - nutrients
The epidermis contains 5 layers.
From bottom to top the layers are named
1. stratum basale,
2. stratum spinosum,
3. stratum granulosum,
4. stratum lucidum, and
5. stratum corneum.
Stratum basale, has cells that are shaped like columns.
These cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die. The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.
Specialized cells in the epidermis.
o The Melanocyte produces pigment (melanin),
o Melanocytes are in the deep germinative layer
o The Langerhan's cells - the frontline defense of the immune system in the skin, and the
o The Merkel's cells - function is not clearly known.
o The projections of dermis into the epidermis are called the papillae
Dermis
The dermis also varies in thickness depending on the location of the skin.
It is 0.3 mm on the eyelid and 3.0 mm on the back.
composed of three types of tissue that are present throughout
collagen, elastic tissue, and reticular fibers.
o Tough
o Elastic
The structures in the dermis are :
o Blood vessels
o Lymph vessels
o Sensory (somatic) nerve endings
o The nerves transmit sensations of pain, itch, and temperature.
o Specialized nerve cells called Meissner's and Vater-Pacini corpuscles that transmit the sensations of touch and pressure.
o Hair roots, hair follicles and hairs
o The arrectores pilorum - involuntary muscles attached to the hair follicles
o Sweat glands and their ducts (the eccrine glands)
o Sebaceous glands
o Apocrine (scent) glands associated with the follicle.
The integumentary system is often the largest organ system comprising skin, hair,scales, nails, skin glands and their products (sweat). It distinguishes, separates, protects, and informs the animal with regard to its surroundings.
