THE INTEGUMENTARY SYSTEM (or the integument)

Two components:

1) The cutaneous membrane (the skin) an organ composed of two tissues: the epidermis (stratified squamous epithelial tissue) and the dermis (dense irregular connective tissue-a type of connective tissue proper)

2) Accessory structures consisting of the hair, the nails, and exocrine glands.

***Note that below dermis is a layer of connective tissue called the hypodermis or superficial fascia. It is composed of   adipose and areolar connective tissue. It is not part of the integumentary system, but functions in storing fat (subcutaneous fat), and anchoring the skin to the underlying muscles. It is attached loosely so that the skin can slide over our muscles so that a blow to the body will just glance off and do little harm to underlying structures. It acts as a shock absorber and heat insulator by storing fat molecules. Distribution of fat changes with age.  When one gains weight as we get older, it thickens in certain areas of the body. In females it thickens first in the thighs and breast and then the buttocks. In males it collects in the neck, upper arms, lower back and over the buttocks. Both accumulate fat in the anterior abdomen forming a "pot belly".    Men are apples, women are pears.  Also the layer into which a "hypodermic needle" is inserted for injections.  Infants and children contain a larger layer of body fat.  It reduces excessive heat loss, stores more energy for growth, absorbs shocks from tumbles.   

THE INTEGUMENT

I) Five major functions:

a) Protection of underlying organs and tissues from infection, impact, chemicals, and loss of body fluid.

b) Temperature maintenance (control of body temperature)

c) Storage of nutrients: lipids in the dermis

d) Excretion and secretion: excretes salts, excess water, and organic wastes and secretes milk (mammary glands)

II) The Epidermis 

a) Thickness: varies in different body regions. Thin layers (0.8mm) covers most of the body while thicker layers (6x thicker) cover the palms of the hands and soles of the feet.

b) The Epidermis is a keratinized (keratin is a protein produced by the most common cell in the epidermis) stratified squamous epithelium consisting of four layers or strata:

1) The stratum Basale (Basal layer) also called the Stratum Germinativum,                                                                              a)deepest layer firmly attached to the dermis by the basement membrane. It consist of a single row of the youngest keritonocytes.                                                             

b) It forms epidermal ridges extending into the dermis for an increase in surface area.  The ridges increase the gripping ability in the soles and palms while increasing skin sensitivity (palms and soles of feet are very sensitive). Dermal papillae occur between the ridges which increase the surface area for diffusion of oxygen, carbon dioxide, and glucose.  The ridges make up ridge patterns (whorls) like treads on a tire in our palms and soles of our feet increasing friction for a better grip. The complex whorl patterns from the ridge patterns form our finger prints. Fingerprints result from films of sweat that occur due to sweat pores opening  along their crests.

c) Most cells present are large stem cells which divide and replace cells shed at the epithelial surface. These cells are constantly in different mitotic stages rapidly dividing to replace cells lost above on the surface.

d) Contain melanocytes which synthesize melanin which is a pigment that colors the epidermis. It also functions in receiving information concerning touch (cytoplasm of cells called Merkel cells extend between the cells of the epidermis) 

2) The Stratum Spinosum (or prickly layer)

a) They are formed from the underlying dividing stem cells. They continue to divide to form several layers. 

b) They are bound together by the cell junctions called desmosomes and appear shrunken and spiny under the microscope due to the desmosomes and cytoskeletal structures.

c) Vitamin D is produced in the stratum spinosum and in the stratum germinativum .

d) Scattered among the kertinocytes are malanin granules and Langerhans' cells which are most abundant in this layer. Langerhan's cells arise from bone marrow and migrate to the epidermis. They are macrophages also called epidermal dendritic cells  and are involved in our immune system. 

3) The Stratum Granulosum (granular layer) 

a) Cells three to five layers thick are formed from the stratum spinosum and are no longer dividing.

b) Cells begin to flatten and their nuclei and organelles begin to disintegrate and they accumulate keratohyaline granules which help to form keratin in the upper layers  and lamellated granules which contain a waterproofing  glycolipid which slows water loss from the epidermis. As the cells move further away from the dermis, they begin to die being too far from the dermal capillaries for oxygen and glucose.

4) The Stratum Lucidium (clear layer) 

a) Found only in the thick skin of our palms and soles of our feet.

b) It appears as  a thin transparent band above the stratum granulosum and is composed of a few rows of flat dead keritonocytes.

c) It is here where the protein called keratin is produced. The keratohyalin granules cling to the keratin produced by the stratum lucidium arranging them in parallel arrays.

5) The Stratum Corneum (Horny Layer)  

a) Most superficial layer consisting of 20 to 30 cells of thick flattened keratinized cells and forms about 75% of the thickness of the epidermis.

b) They contain a large amount of keratin  which coats the surface of the skin protecting the underlying tissue from water loss, abrasion and penetration.  Keratin forms hair, calluses, nails, cow horns, hooves, reptile scales, and porcupine quills. 

c)  The skin cells on the surface of the stratum corneum are called cornified or horny cells (more familiar as dandruff and flakes of dry skin).

d) It takes 2-4 weeks to produce the cells in the stratum corneum (cells originated in the stratum germinativum). They stay of the surface for two weeks and are washed or shed away. 

III) The Dermis

a) Consists of two layers:

1) The papillary layer which is thin and the most superficial contains areolar connective tissue (a type of connective tissue proper classified as loose connective tissue)  in which the collagen and elastin fibers form a web like mat which contains blood vessels.

 The superior surface contains the dermal papillae in which some contain capillary loops, or free nerve endings (pain receptors), and touch receptors called Meissner's corpuscles.

2) The reticular layer 

a) It accounts for about 80% of the dermis is dense irregular connective tissue (a type of connective tissue proper classified as dense connective tissue)  consisting of irregularly arranged collagen fibers with some elastic fibers.  The major cell of both layers of the dermis is the fibroblast

b) Contains lines of cleavage or tension lines in the skin. They are places where there exists less dense areas between the bundles of collagen fibers. They are not visible externally but run longitudinally in the skin of the head and limbs and in circular patterns around the neck and trunk. Surgeons make incisions parallel to these lines rather than across them because the skin gapes less and heals more rapidly.

c) Collagen fibers give the skin strength making it difficult to penetrate.  Collagen fibers also absorb  and bind water hydrating the skin.

d) Flexure lines are also present in the dermis (creases in your palms, wrists, fingers, soles and toes) and represent dermal folds that occur at or near joints .  The skin cannot slide to accommodate joint movement in these areas.  

ACCESSORY STRUCTURES (APPENDAGES OF THE SKIN)

1) SWEAT GLANDS (sudiferuous glands)

a) They are found all over the body except for the nipples and parts of the external genitalia at more than 2.5 million per person. 

b) Two types of sweat glands :

1) Eccrine (merocrine) sweat glands                                          

a) They are the most numerous and found more on the palms,  soles of the feet, and the forehead.

b) Consists of a secretory part which lies coiled in the dermis, and a duct which extends  to open in a funnel shaped pore in the surface of the skin.

c) Eccrine gland secretions are called sweat or perspiration and are made up of 99% water with some salt, vitamin C, antibodies, metabolic wastes (urea, uric acid, ammonia) and lactic acid (the chemical which attracts mosquitoes).

d) The major role of sweating is to prevent overheating of the body. Heat induced sweating begins at the forehead and spreads inferiorly to the rest of the body. Emotionally induced sweat or the "cold sweat"  brought on by fright, embarrassment, or nervousness begins in the palms, soles, and armpits and then spreads to other areas. 

e) We can lose up to a gallon of water in an hour when all glands work together such as in the hot sun.

2) Apocrine sweat glands 

a) They are found in the armpits, nipples and anogenital or groin  areas. 

b) They are larger than eccrine glands and empty into hair follicles. They contain the same components as sweat described above plus fatty substances and some protein and appears as a very viscous milky or yellowish substance. The secretion is odorless but when decomposed  by bacteria produce a musky smell of body odor. The unpleasant odor is produced from the formation of butyric acid (mayonnaise left in the sun produces this same odor).  We mask the odor with deodorants and use antiperspirants which contracts  the skin and closes  or narrows the sweat gland openings.

c) Apocrine glands begin to function at puberty (when one needs deodorant) and has very little to do with thermoregulation (maintenance of body temperature).

d) Ceruminous glands are modified apocrine sweat glands which are found in the external ear canal and secret cerumin or earwax.

e) Mammary glands are also specialized or modified sweat glands.  

2) SEBACEOUS (OIL) GLANDS

a) They are found all over the body except for the palms and the soles. Larger ones are found on the face, neck and upper chest while smaller sebaceous glands are found on the body trunk and limbs.

b) They secrete an oily substance called sebum.  The cells become engorged with  lipids until they burst and release their contents lipids and cell fragments which we call sebum.  Functionally they are holocrine glands. 

c) Sebaceous glands release their secretions into hair follicles and occasionally through a pore in the skin. 

d) Sebum softens and lubricates the hair preventing it from drying  and breaking. It also has a low pH which inhibits  bacterial growth.  It also slows water loss from the skin.

e) Sebum is released when the errector pili muscles contract. The errector pili muscles elevate the hair and squeezes out the sebum.

f) Sebaceous glands are inactive during childhood but are activated in both sexes during puberty when they become stimulated by androgens or sex hormones. Sebaceous glands can become blocked  and the secretions can accumulate during puberty and promote bacterial growth resulting in acne and blackheads (if they have large sebaceous glands)  

3) HAIR AND HAIR FOLLICLES

a) Hair is found all over our bodies except for the sides and soles of our feet, our palms, sides of our fingers, toes, lips, nipples, and external genitalia 

b) Hair begins in a structure called the hair follicle which is present in the dermis and often in the subcutaneous layer. Walls of the follicles contain all the layers found in the epidermis. Stem cells begin dividing at the bottom of the follicle and a shaft of hair  is the end product.

c) The hair or pili consist of dead keratinized cells.  The keratin present in hair is hard keratin as opposed to the soft keratin  of the epidermis.  It has advantages over that found in the epidermis: 1) it is tougher and more durable  2) its individual cells do not flake off

d) The two parts of the hair consist of the shaft (above the skin) and the root ( the part below the skin).  The shape of the follicle determines the shape of hair shafts. A flat and ribbonlike shaft produces a kinky hair; if it is oval it produces hair that is silk and wavy; if it is perfectly round, hair is straight and tends to be coarse. 

e) Hair consist of three concentric layers of keratinized cells: the central core is called the medulla (absent in fine hairs) consist of large cells and air spaces; the cortex  which is a bulky layer of several cells  surrounding the medulla and the outermost layer called the cuticle is a single layer of cells that overlap like shingles on a roof.  The cuticle is the most heavily keratinized part of the hair . 

 f) Hair follicles

a) The deep end of the hair follicle is expanded and is called the hair bulb.  Around the hair bulb are nerve endings called the root hair plexus.  Bending the hair will stimulate the root hair plexus resulting in the hairs becoming sensitive touch receptors. 

b) A hair papilla (nipple like bit of dermal tissue) protrudes into the hair bulb. It contains capillaries and supplies nutrients to the growing hair.

c) A connective tissue root sheath derived from the dermis forms the outer wall of a hair follicle, a thickened glassy membrane forms the middle layer. The glassy membrane is the basement membrane of the inner epithelial root sheath which is derived from an invagination of the epidermis.

d) A bundle of smooth muscles called arrector pili muscles is associated with each hair follicle.  Hair emerges from the skin at an angle and in response to cold external  temperatures, or fear the arrector pili muscles are stimulated to pull the hair into an upright position forming goose bumps.  It is a mechanism for animals to make them look larger  to its enemy, and a way to stay warmer by trapping a layer of insulating air in their fur.

NAILS

a) They contain hard keratin and consist of the following parts the nail matrix which is the proximal portion of the nail bed and responsible for nail growth. The lunula is a white crescent which appears over the nail matrix .  The proximal and lateral borders of the nail are overlapped  by skin folds called nail folds. The proximal fold projects onto the nail body as the cuticle, or eponychium.

       BONES AND SKELETAL TISSUES

 SKELETAL CARTILAGES

1) Cartilage tissue:

a) contains no nerves or blood vessels

b) consists primarily of water (high water content is what gives cartilage its flexibility)

c) Cartilage is surrounded by  a layer of dense irregular connective tissue called the perichondrium which acts like a girdle to prevent outward expansion of the cartilage while it is being compressed.  The perichondrium is also the source of blood vessels from which nutrients diffuse through the matrix to the cartilage cells called the chondrocytes. 

d) Three types of cartilage exist in the skeletal system: hyaline, elastic, and fibrocartilage. All have basically the same structure. Their properties differ due to a difference in the elasticity of their matrix by the number of elastin fibers found (elastic cartilage has more elastin fibers), or to a difference in the ability o of their matrix to compress  as in fibrocartilage. The skeletal cartilages contain representatives from all three tissue groups (figure 6.1)

2) Growth of Cartilage

a) Appositional growth (growth from the outside)  is one type of cartilage growth in which cartilage-forming cells in the surrounding perichondrium secrete new matrix on the outside of the existing cartilage.

b) Interstitial growth (growth from the inside) occurs when the chondracytes inside the cartilage divide and secrete new matrix.

c) Cartilage growth ends during adolescence when the skeleton stops growing.

CLASSIFICATION OF BONE

1) LOCATION:  Bones  can be grouped as being part of the  the axial skeleton which includes the bones of the skull, vertebral column, and the rib cage or  the appendicular skeleton consisting  of the bones of the upper and lower limbs and the girdles (shoulder and hip bones).

2) SHAPE: 

a) Long Bones. These are bones that are longer than they are wide. They have a central shaft and two ends . Examples include all bones of the limbs except for the patella (kneecap) and those of the wrist and ankles.

b) Short Bones. Short bones are roughly cube shaped. The bones of the wrist and ankles are examples. Sesamoid (shaped like a sesame see) bones are special types of short bones which form within a tendon. An example is the patella or kneecap. Some seem to function in altering the direction of the pull of a tendon.

c) Flat Bones.  Flat bones are thin, flattened, and usually a bit curved. Examples include the sternum (breastbone), the scapulae (shoulder blades), ribs, and most skull bones. 

FUNCTIONS OF BONES

1) Support. Examples include the support of the thoracic wall by the rib cage, our lower limbs support our body trunk when we stand, while others "cradle" our delicate organs.

2) Protection. Examples include the bones of the skull which protect the brain, the vertebrate which protect the spinal cord, and the rib cage which protects our heart and lungs.

3) Movement. Bones are used as levers by our skeletal muscles to lift our limbs. Our skeletal system consist of 206 bones, many of which are not fused together to allow for movement.

4) Mineral Storage. Calcium and phosphorus  are stored within the bone matrix, and released into the bloodstream  as needed. The calcium and phosphorus also has to be continually replaced back into the matrix by proper diet.

5) Blood Cell Formation. Hematopoiesis or blood cell formation occurs within the marrow cavities of certain bones.

BONE  STRUCTURE

1) Bones are organs and organs are made up of several types of tissue. Each organ has a dominant type of tissue forming it which gives the organ and the system that it belongs to its name.  Bone (osseous) tissue forms most of the tissue of our bones, but included in a bone is nervous tissue (forming nerves innervating our bones), fibrous connective tissue in their cavities called marrow, cartilage tissue in their articular cartilages, and muscle and epithelial tissue in blood vessels within the bone.

2) Structural Levels of Bone

a

A) Gross Anatomy

1) Bone Textures. All bones have a dense outer layer called compact bone, and an inner layer called spongy bone. The spongy bone consist of a honeycomb of small needle-like pieces called trabeculae. There are open spaces between the trabeculae which are filled with red or yellow bone marrow.

2) Bone Structure: Long Bones

a) The diaphysis is the long axis of the bone. It consists of a "collar" of compact bone surrounding an inner medullary cavity or marrow cavity.  In adults the medullary cavity in long bones contain yellow marrow. It is also called the yellow bone marrow cavity.

b) The epiphyses are the expanded ends of long bones. They have compact bone on their exterior and internally they contain spongy bone. The joint surface of each epiphysis is covered with a thin layer of articular cartilage. Between the diaphysis and the epihyses is the epiphyseal line which is the remains of the epiphyseal plate which is a disc of hyaline cartilage  that grows during childhood to lengthen the bones as we grow taller. 

c) The periosteum is a double layered  membrane which covers the external surfaces of our long bones except for the joint surfaces. The outer fibrous layer is dense irregular connective tissue and the inner osteogenic layer consists of osteoblasts (bone-forming cells) and osteoclasts (bone destroying cells).  The periosteum has a rich supply of nerves, blood and lymphatic vessels which all enter into the shaft of the long bone through  openings called the nutrient foramen. Tufts of collagen fibers called Sharpey's fibers, attach the periosteum to the bone. Sharpey's fibers actually extend into the bone matrix. The periosteum also provides points of insertion or anchoring points for tendons and ligaments. Sharpey's fibers are exceptionally dense at these points.

d) The endosteum which contains both osteoblasts and osteoclasts line the internal bone surfaces covering the trabeculae of spongy bone and lines the canals that pass through the compact bone.

3) Bone Structure of Short, Irregular, and flat bones.

 a) They have the same structure as long bones in that they have periosteum covered compact bone and an inner endosteum covered spongy bone. They have no shaft of epiphysis because they are flat and not cylindrical and they have no marrow cavity. The internal layer of spongy bone is called the diploe.  Red bone marrow is found within the diploe of flat bones and in the marrow cavities of long bones. Red bone marrow is called hematopoietic tissue and is the tissue responsible for producing our blood. In infants all the bone marrow is red and as one becomes older some of our marrow turns to yellow marrow.  Yellow marrow does not actively produce blood cells, but instead has become a storage place for fat.  Red marrow is present in the adult in the ribs, the body of the vertebrate, the pelvis, the femur, and the ends of the humerous in the upper arm.

B) MICROSCOPIC ANATOMY

1) Compact Bone.                                                             a)Compact bone consists of structural units called osteons or Haversion systems. Each is a group of hollow tubes of bone matrix placed outside the next like the rings of a tree.  

b)Each matrix tube is called a lamella. The collagen fibers in a single lamella all run in the same direction, but in the opposite direction in adjacent lamellae. This design helps to reinforce one another to resist twisting. Even the bone crystals line up with corresponding collagen fiber in the lamella and alternate their direction as well giving additional support.

d) Volkmann's canals or perforating canals lie at right angles to the axis of the bone connecting the blood and nerve supplies of the periosteum to those of the central canals and the medullary cavity.  Both the central and Volkmann's canals are lined with endosteum.

e) Osteocytes (bone cells) are found within small cavities called lacunae at the junctions of the lamellae, and hairlike canals called canaliculi connect the lacunae to each other and to the central canal. Cytoplasmic extensions within the canaliculi connect osteocytes together with gap junctions and permits nutrients and wastes to be transferred between osteocytes.

f) Interstitial lamellae are found between osteons and fill the gaps between osteons, and circumferential lamellae are found just deep to the periosteum and extend around the entire circumference of the shaft. Circumferential lamellae resists twisting of the long bone as a whole.

2) Spongy Bone.

a) Spongy bone does not have osteons , but have irregularly arrange lamella containting osteocytes. Nutrients reach the ostecytes through canaliculi by diffusion from the capillaries in the endosteum.  The trabeculae  do offer some support to the bone structure.

C) CHEMICAL COMPOSITION OF BONE

1) Organic components include 

a) Cells:  osteoblasts, osteocytes, and osteoclasts

b) Osteiod which is the organic part of the matrix. It makes up about 30 % of the matrix and includes ground substance (composed of proteoglycans and glycoproteins) and collagen fibers. They are organic because they were made or secreted by osteoblasts. 

2) Inorganic components include

a) Hydroxyapatites form about 65% of the bone mass and consists of mineral salts, largely calcium phosphates. The salts are present in the form of crystals around the collagen fibers and are responsible for the "hardness" of bone which helps bone resist compression.

D) BONE MARKINGS. External features of bones have bulges, depressions, holes in them. These bone markings represent display a particular function of that bone Identifying marks are found in Table 6.1 on page 180.

E) BONE DEVELOPMENT.

1) Formation of the Bony Skeleton

a) The skeleton of an 8 week old human embryo is entirely made of fibrous membrane and hyaline cartilage.  The cartilage then is replaced by bone tissue by a process called ossification or osteogenesis. There are two types of ossification:

Intramembraneous ossification  occurs in the flat bones of the skull, the lower jaw and the clavicle. This type of ossifcation occurs when the bone develops from a fibrous membrane. The bone is called a membrane bone.  Four major steps are involved (figure 6.7 page 181)

Endochondrial ossification begins in the second month of development and forms all the bones from the skull down except for the clavicle.  Hyaline cartilage bones formed earlier are used as models.  The hyaline bones must be broken down as ossification proceeds. The stages of endochondrial ossification are illustrated in figure 6.8 on page 182.

1) Formation of a long bone begins in the primary ossification center which is formed when the perichondrium becomes invaded by many blood vessels changing it into a vascularized periosteum. Due to the change in nutrition underlying mesenchymal cells specialize into osteoblasts.   

2) A bone collar forms around the diaphysis  of the hyaline cartilage model. Osteoblasts in the newly formed periosteum secrete osteoid encasing it externally in a bone collar around the cartilage model

3) Cartilage in the center of the diaphysis calcifies and then cavitates. The chondrocytes begin to die and the cartilage matrix begins to deteriorate and cavities are formed within the long bone. The bone collar formed stabilizes the bone externally while the cavities form. This only occurs in the center of the bone and other cartilage cells will continue to make the bones grow longer.

4) The periosteal bud invades the internal cavities and spongy bone forms. This occurs during the 3rd month of development during which the cavity is invaded by a periosteal bud.  The periosteal bud consists of an artery, vein, lymphatics, nerve fibers, red marrow elements, osteoblasts, and osteoclasts.  The osteoclasts partially erode the calcified cartilate matrix, while the osteoblsts secrete osteoid around remaining cartilage forming  bone-covered cartilage trabeculae and begins forming spongy bone.

5) The diaphysis elongates and a medullary cavity forms. The primary ossification center enlarges spreading proximally and distally and osteoclasts break down the newly formed spongy bone and form the medullary cavity in the center of the bone. The formation of this caviy is the final step in the ossification of the shaft.

6) The epiphysis ossify. Secondary ossification centers appear in the epihysis and they too gain boney tissue. During secondary ossification the spongy bone in the interior is retained  and no medullary cavity forms. When secondary ossification is complete, hyaline cartilage only remains in two places: on the epiphyseal surface as the articular cartilages, and at the junction of the diaphysis and the epiphysis where it forms into the epiphyseal plates or growth plates.

BONE GROWTH

1)  In the growth zone are the cells at the top of the stack of cells on the side of the epiphyseal plate facing the diaphysis.  They divide quickly pushing the epiphysis away from the diaphysis. The older chondrocytes nearer to the the diaphysis form the transformation zone where cartilage cells are replaced by bone cells in the oseogenic zone.  The epiphyseal plaete maintains a constant thickness because the rate of cartilage growth on its epiphyseal side is balanced by its replacement with bony tissue on its diahyseal plate.    When growth of our long bones is over (at the age of about 21 years for males, 18 year for females), a process called epiphyseal plate closure occurs  in which the bone of the epiphysis and the diaphysis fuse together (they form the epihyseal line in adult bones).

2) Bones can still grow in diameter or thickness by oppositional growth if it is severely stressed by excessive muscle activity or weight.

BONE REMODELING

1) Two processes compose bone remodeling: bone deposit and bone resorption

a) Bone deposit occurs when bone is injured or added bone strength is needed. A healthy diet  rich in proteins, vitamin C (for collagen synthesis), vitamin D (essential for absorption of dietary calcium), vitamin A (needed for a balance between deposit and removal of bone), and several minerals (calcium,phosphorus, magnesium and magnesium) are essential.

b) Our bones are a storage place for calcium and it is from our bones, that our body will obtain the calcium that it will need for example for nerve conduction and muscular contraction.  This process is called resorption.   A balance between resorption and bone deposit occurs in healthy young adults.