INTERGUMENTARY SYSTEM

Skin or Cutis

• Covers the entire outer surface of the body.

• Consists of 2 layers which differ in function, histological appearance and embryological origin.

Epidermis

Outer layer formed by an epithelium of ectodermal origin.

Dermis

Underlying thicker layer which consists of CT and develops from the mesoderm.

Hypodermis or Subcutis

• Beneath the 2 layers is subcutaneous layer of loose CT.

• Binds skin to underlying structures

Appendages

Hair, nails, sweat glands and sebaceous glands are of epithelial origin.

Integumentary System

Skin and appendages

Epidermis

• Surface layer which is usually the darkest layer visible

• Sublayers visible in the epidermis

• At transition from epidermis to dermis, staining becomes lighter

Dermis

• The lighter stained layer

• Consists of dense irregular connective tissue

• Much thicker than the epidermis

Dermal papilla

In thick skin, it creates an irregular border between the epidermis and dermis.

Hypodermis

• Lightest visible layer and consists mainly of adipose tissue.

• Dense CT may extend from dermis into this and anchor the skin to the underlying structures.

Epidermis

• Keratinized stratified squamous epithelium

• Main function is to protect body from harmful influences in the environment and against fluid loss.

• Five structurally different layers.

Stratum corneum

• Cells completely filled with keratin filaments (horny cells) which are embedded in a dense matrix of protein individual cells difficult to identify.

• Cells are flat.

• Spaces between cells filled with lipids which cement cells together into continuous membrane.

• Under the EM, cell membranes appear thickened and interdigitate with neighboring cells.

• Closest to the surface of the epidermis and has a somewhat looser appearance.

• Horny cells are constantly being shed from it.

• Protection by the epidermis is essentially due to its functional features.

Stratum Lucidum

• Consists of several layers of flattened dead cells

• Nuclei begin to degenerate in the outer part of stratum granulosum

• Faint nuclear outlines are visible only in a few cells

• Cannot be identified in tick skin

Stratum Granulosum

• In thick skin, consists of a few layers of flattened cells

• Only one layer visible in thin skin

• Cytoplasm of cells contain numerous fine granules, keratohyalin

• Cells release the contents of the lamellar granules and the lipid contents fill the entire interstitial space which is important as a barrier from the external environment.

Keratohyalin

Not found in membrane bound organelles but found free in the cytoplasm of cells

Stratum Spinosum

• Cells become irregularly polygonal

• Cells separated by narrow, translucent clefts

• EM shows membrane-bound lamellar granules in the cytoplasm of spinous cells

Clefts

Spanned by spine-like cytoplasmic extension of the cells (spinous cells) which interconnect the cells.

Desmosome

Spines of cells meet end-to-end or side-to-side and are attached to each other by

Stratum Basale

• Deepest layer of epidermis, closest to the dermis

• Consists of single layer of columnar of cuboidal cells which rest on the basement membrane

Basal Cells

• Stem cells of the epidermis

• Mitotic activity replenishes the cells in the superficial layers as they are shed from the epidermis,

• Renewal of the human epidermis takes 3 to 4 weeks

• Variations in the thickness of the epidermis are mainly the results of variation in the thickness of the stratum corneum, although the other layers also vary in thickness.

Keratocytes

Cells of the epidermis that will keratinize.

Keratinization

Should not be used as synonym for the formation of the stratum corneum as other stratified squamous epithelia may become keratinized but may not form a stratum corneum in which cells join to form a horny cell membrane.

Red and Yellow Hues of the Skin

Due to hemoglobin in the red blood cells which pass through the capillaries beneath the epidermis and the carotene which accumulates in fat cells found in the dermis and hypodermis.

Melanocytes

• Produces melanin (brown color of the skin)

• They are located in the epidermis and send processes between other cells

• Difficult to distinguish in ordinary LM preparation are located in the processes of cells

• Can transfer melanin to basal cell keratinocytes

• Fine processes invade keratinocytes and bud-off part of their cytoplasm including the melanosomes within the keratinocytes.

• Disease of the endocrine organs results in changes of pigmentation of the skin.

Melanosomes

Where melanin is located in membrane-bound organelles inside melanocytes

Melanin

Protects the chromosomes of mitotically active basal cells against UV-induced damage.

Pituitary and Adrenal Glands

Produces pigmentation is not only under the control of light but also by hormones.

Melanocytes

Are ectodermal in origin but are derived exclusively from the neural crest of the embryo from where they migrate to all other parts of the body.

Cells of Langerhans

• Cells found within the epidermis

• Morphologically like the melanocytes but functionally more related to macrophages.

• Important in immune reactions of the epidermis; their fine processes form a network between cells of the epidermis and they phagocytose antigens which have entered the epidermis.

• May only be temporary residents of the skin.

• When they come in contact with an antigen, they migrate to regional lymph nodes when they initiate an immune response.

T-lymphocytes

• Group of cells associated with the immune system

• Some present in the epidermis

• Together with the cells of Langerhans, are sometimes referred to as SALT or skin-associated lymphoid tissue.

Dermis

• Thick layer of CT which the epidermis is attached

• Deepest part continues into the subcutaneous tissue without sharply defined boundary.

• Approximately 1-2mm thick

• Divided into 2 sublayers

Papillary layer

• Consists of loose, cell-rich CT which fills the hollows at the deep surface (dermal papilla) of the epidermis.

• Capillaries frequent, collagen fibers finer than in reticular layer.

Reticular layer

• Denser and contains fewer cells

• Thick collagen fibers that often aggregate into bundles

• Fibers from intercalating network though their predominant direction is parallel to the surface of the skin.

• Preferred orientation of collagen fibers not visible in sections but main orientation differs in skin from different parts of the body.

• Usually amin orientation follows the lines of greatest tension in the skin (Kraiss lines).

• Of surgical importance since incisions parallel to the line heals faster with less formation of scar tissue.

Hair

• Most of the skin is covered by it.

• Palms of the hands, soles of the feet, distal phalanges and sides of the fingers and toes and parts of the external genitalia lacks it.

• Free part - shaft

Vellus hair

Short, fine, and only lightly pigmented.

Hair follicle

• Root of hair is anchored here.

• A tubular invagination of the epidermis which extends down into the dermis and  a short distance into the hypodermis.

Bulb

Deepest end of hair follicle forms this enlargement.

Root Sheath

Cells in bulb mitotically active, their progeny differentiates into cell types that form  the hair and cells that surround its root.

Site where hair cells keratinize

Within the lower 1/3 of the hair follicle; above this level, it is not possible to identify individual cells within the hair.

Arrector pili muscle

• A bundle of smooth muscle in each hair follicle

• Inserts with one end to the papillary layer of the dermis and with the other end to the dermal sheath of the hair follicle.

Anagen

• Phase of hair growth

• Variable in various regions of the body.

• A few months for eyebrows and eyelashes

• 2-5 years for the hair of the scalp.

Telogen

Resting stage of hair growth

Sebaceous Glands

• Empty their secretory products into the upper parts of the hair follicle.

• Found in parts of the skin where hair is present

• Hair follicle and associated sebaceous gland for pilosebaceous unit.

• Also found in: lips, oral surface of cheeks and external genitalia.

• Simple and branched; secretory portion consists of alveoli

Sebaceous Glands

• Produce oily substance

• Helps waterproof the skin

Sebum

Oily substance produced by sebaceous glands

Acne

Infection of the sebaceous gland

Ewan q ulit ano to - Sebaceous Glands pa rin ata

• New cells replenish pool of basal cells while remaining cells are displaced towards the center of the alveolus as more cells are generated by basal cells

• Secretory cell gradually accumulate lipids and grow in size

• Nuclei disintegrate and cells rupture

• Lipid secretion has no softening effect on the skin and has limited antibacterial and antifungal activity

• Importance unclear but are liable to infections (acne)

• Present in all types of skin other than thick skin

• Number correlates with number of hair follicles

• Sebaceous glands embedded in dermis

• Empty into hair canal and hair follicle

• Deep in sebaceous glands, cells are smaller with intact nuclei

• Cell size increases with accumulation of sebum as cells gradually displaced towards the opening of gland into hair follicle

• Nuclei condense, become darker and irregularly shaped

Holocrine secretion

Secretory lipids and constituent of disintegrating cell

Sweat Glands

• Differ in detailed histological appearance and composition of sweat they secrete.

• Merocrine glands only glands on skin with clearly defined biological function

• Important in regulation of body temperature

• 300,00 all over the body with the exception of parts of the external genitalia.

• Simple tubular organs; secretory tubules and initial part of excretory duct coiled into a spherical ball at the border between the dermis and hypodermis

• Secretory epithelium cuboidal or columnar

Two types of Sweat Glands

Merocrine & Apocrine

Two types of cell sweat glands

1. Light type secretes watery eccrine sweat

2. Dark type may produce mucin-like secretion

• Cells may have slightly different shapes hence, may appear pseudostratified

• Myoepithelial cells found between secretory cells and basement membrane

Sweat Glands ata ulit

• Excretory duct has 2 layers of stratified cuboidal epithelium and empty directly onto the surface of the skin

• Apocrine sweat glands present in the axilla

• Stimulated by sexual hormones and not fully developed or functional before puberty

• Histological structure similar to merocrine sweat glands but lumen of secretory tubules much larger and secretory epithelium of only one major cell type which looks cuboidal or low columnar

• Glands much larger than merocrine sweat glands

• Excretory duct does not open directly into surface of skin but into upper part of hair follicle, part of pilosebaceous unit

Apocrine Sweat

Milky, proteinaceous and odorless but the odor is a result of bacterial decomposition; in mammals other than humans, of importance for sexual attraction.

Horse and Man 

Sweat glands are extensively developed.

Birds

Sweat glands are absent

Cats

Sweat glands present only in anal and oral regions, lower jaw and foot pads.

Dogs

Sweat glands is extensive but does not function to its maximum

Anal Gland (special glands)

Modified sweat glands in dogs, cats, and pigs. Lipid in dogs, mucoid in pigs.

Gland of the anal sac (special glands)

Present only in carnivores and many rodents. In dogs sweat glands, in cats sebaceous.

Circumanal gland

• In dog sebaceous, located in dermis around the anus

• Gives off a distinct odor and usually removed by surgical procedure

• Ranks 3rd frequent site of canine tumors

Mammary gland

• Compound tubuloalveolar modified sweat glands

• Secretes lipid component of milk (apocrine) and protein and CHO components (eccrine)

• Encapsulated by dense white connective tissue

Basic structure is compound gland except for duct system which from smallest to largest, consists of the following:

1. Secretory tubule

2. Lobular or interlobular duct

3. Lactiferous sinus

4. Teat sinus

5. Streak canal

• During early lactation, macrophages and leukocytes prominent and comprise part of colostrum, a laxative which in some species imparts initial passive immunity.

Avian Integumentary System

• Consists of skin, feathers and appendages (claws and beak)

• Skin covers majority of the body and contains glands in the outer ear canal and preen glands at the base of the tail used to preen feathers

Important in providing protection from potentially dangerous situations. Functions include: (In Avian Integumentary System):

1. Barrier between external environment and internal systems and organs, thus providing support and protection from infection by microorganisms and physical injury

2. Excellent thermal insulation to help regulate body temperature in variable environment

3. Numerous nerve endings enable birds to be aware of potentially harmful situations

4. Pigments for display and protection from elements