A&P 1: module 3

bones and skeletal tissue

skeletal system

includes the bones of the skeleton in addition to cartilages, ligaments, and other connective tissues (help to hold the bones together and allow for movement of the bones at joints)

skeleton system functions

supports the body, facilitates movement (leverage), protects internal organs, produces blood cells, stores and releases minerals and fat

cartilage

type of connective tissue, but matrix is flexible; has a unique appearance due to polysaccharides called chondroitin sulfates; is avascular

chondroitin sulfates

polysaccharides; bind with ground substance proteins to form proteoglycans

chondrocytes

cartilage cells; embedded within the cartilage matrix; found inside spaces called lacunae

perichondrium

layer of dense irregular tissue; encapsulates the cartilage

avascular

no blood vessels supplying nutrients and removing metabolic wastes; all these functions are carried out by diffusion through the matrix

3 types of cartilage tissues

hyaline, fibrocartilage, elastic

hyaline cartilage

most common type; found in rib cage and nose

articular cartilage

has structure of hyaline cartilage but no perichondrium surrounding it; found esp at joint articulations to help cushion joints and enable them to move freely; helps reduce friction when a bone articulates (connects in a joint) with another bone

fibrocartilage

tough because it has thick bundles of collagen fibers dispersed through its matrix (examples: menisci in the knee joint and the intervertebral discs)

elastic cartilage

contains elastic fibers, as well as collagen and proteoglycans; gives rigid support as well as elasticity (ex: tug gently at ear and lobe returns to initial shape)

cartilage development occurs via 2 main methods

interstitial growth and appositional growth

interstitial growth

occurs mostly during childhood and adolescent development and begins early in embryonic tissue; occurs as chondrocytes in the matrix grow and divide; the daughter cells then produce additional matrix, so this enlarges the cartilage from the inside

appositional growth

occurs during childhood and adolescent development; new layers of cartilage are added on the cartilage surface as the inner layer of the perichondrium divides and grows; the cells that are closet to the inner layer then develop into immature chondrocytes (chondrocytes), which are found near the perichondrium; chondroblasts then start to secrete cartilage matrix around themselves, eventually becoming chondrocytes; eventually they secrete enough matrix that chondrocytes become fixed within their own matrix and begin to mature'; slowly adds the outer surface of the cartilage; in very unusual circumstances can occur in adults to repair a minor surface cartilage injury, if the injury is more severe it will be replaced with fibrous tissue and the cartilage will not retain its original function

skeleton

made up of a network of bones held together at joints; protects vital internal organs; has 2 main divisions: axial and appendicular

axial skeleton

lies on the midline of the body and consists of the skull, vertebral column, sternum, laryngeal skeleton and thoracic (rib) cage

appendicular skeleton

consists of the bones within the pectoral and pelvic girdles and the attached limbs

the pectoral (shoulder) girdle and upper limbs (arms) specialized for

flexibility and increased range of motion

lower limbs and pelvic girdle are specialized for

strength

bone types

classified by their structure and shape; 6 basic types of bones, each with a distinct shape and form; the shape of a bone determines its function

long bones

long and thin; have a structure that supports body weight and enables movement (humerus, ulna, radius, tibia, fibia, metacarpals, metatarsals)

flat bones

very thin and have surfaces that are parallel with each other; flat bones in the cranium form the root of the skull to protect the brain (sternum, ribs, scapula)

sutural bones

small and flat; found in between the flat bones of skull; the number of sutural bones varies from person to person

short bones

small and cube shaped (carpals in the hand and tarsals in foot)

irregular bones

varied in structure with ridges or complex shapes (vertebrae - designed to protect the spinal cord as well as enable spinal movements; pelvic bones (ilium, ischium, pubis))

sesamoid bones

small and round; develop inside tendons to help reinforce the tendon and give it strength; (can be found near the joints of hands and fee; patella) - every person has a patellae but not all individuals have sesamoid bones in other joint locations

bone (osseous tissue)

a hard, dense connective tissue that forms most of the adult skeleton; facilitate movement by serving as points of attachment for your muscles; some bones transmit the forces produced when your muscles contract; unless a muscle spans a joint and contracts a bone is not going to move; protect internal organs; serves as site for fat storage and blood cell production

bone matrix

stores many critical minerals important to the functioning of the body (calcium and phosphorus), these minerals are incorporated into bone tissue and can be released back into the bloodstream to maintain homeostatic levels needed to support physiological processes (calcium ions are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses)

bone marrow

softer connective tissue that fills the interior of most bone; 2 types: yellow marrow and red marrow

yellow marrow

contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy; is fat storage tissue found mainly in long bones

red marrow

where hematopoiesis takes place; RBCs, WBCs, and platelets are all produced in the red marrow; found primarily in short and flat bones; newborns have all red barrow and over time it is converted to yellow bone marrow in long bones; contains blood vessels that supply nutrients and remove wastes to and from the osteocytes in the trabeculae by diffusion

marrow cavity/medullary - structure of long bone

extends throughout the diaphysis

diaphysis - structure of long bone

shaft or center length of bone

compact bone

solid outer layer of bone; usually found on surface of the bone and surrounds an interior of spongy bone

proximal and distal epiphysis

ends of each long bone; widens at each end and is made mostly of spongy bone

metaphysis

where epiphysis and diaphysis meet

cortex

two layers of compact bone

diploe

layer of spongy bone in cranium

osteons - compact bones

formerly called Haversian systems; osteocytes (bone cells) can be found in tiny open chambers called lacunae; compact bone contains many

canaliculi

small, narrow channels found in between the lacunae; allow for osteocytes to receive nutrients, remove wastes, and allow for communication between cells

lacunae/lacuna

small, open pocket surrounded by hard, dense matrix; bone cells found here (can only ever have one osteocyte); separated by a matrix that contains the protein fibers of collagen and mineral deposits, primarily of calcium and phosphorus salts

lamellae

layers of matrix

central canals

contain blood vessels and nerves; the blood vessels bring the nutrients that allow the bone to renew itself

perforating canals (volkmanns canals)

run perpendicular to the bone surface and carry blood vessels deep into the bone and bone marrow

periosteum

covers the superficial layer of compact bone, except within joint cavities; is a protective membrane that has a tough fibrous outer and inner cellular layer; functions to separate the bone from the surrounding structures and provides a structure for the vast amount of blood vessels that go into bone tissue

perforating fibers (sharpeys’ fibers)

made of strong collagen fibers that help connect the periosteum to the bone

endosteum

lines the marrow cavity and is a membrane lining that lines all the internal cavities inside the bone

spongy bone

does not contain osteons, instead it contains numerous bony bars and plates separated by irregular spaces; is an open network called trabeculae, which lacks capillaries and venules; lighter than compact bone but designed for strength; found in locations where stress comes from many different directions or where the bones do not have much stress applied; the spaces between trabeculae is where red bone marrow and yellow marrow are stored

what gives bone their hardness and strength and their flexibility

hydroxyapatite crystals, while collagen fibers give them flexibility so that they are not brittle

bone

living tissue that continually renews itself; contains a relatively small number of cells established in a matrix of collagen fibers that provide a surface for inorganic salt crystals to adhere; these salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which also incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes or calcifies on the collagen fibers; if a bone does not have a calcified matrix it will have a normal appearance but will be too flexible and unable to support the weight of the body

bone cells

compose a small amount of the bone volume but are crucial to the function of bones; four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts; in adults bone is constantly being broken down and built up again by the work of four specialized cells

osteoclasts

bone-absorbing cells; found on bone surfaces, multi-nucleated, and originate from monocytes and macrophages, two types of WBCs, not from osteogenic cells; secrete acids that break down bone, remove worn cells, and deposit calcium in the blood which has been removed from the bone matrix

osteoblasts

immature bone cells that work to form new bone matrix, repairing the destruction caused by the work of osteoclasts; as they form new bone they take calcium from the blood and deposit it into bone; secrete osteoid; once the calcium binds to the osteoid, it hardens the matrix that eventually entraps osteoblasts to form osteocytes; cannot divide to form new cells

osteoid

unmineralized collagen matrix that binds calcium

osteocytes

mature bone cells that eventually form as some of the osteoblast cells get caught in the matrix they secrete and are converted to osteocytes, the cells founds within the lacunae of osteons; maintain the mineral concentration of the matrix via the secretion of enzymes; lack mitotic acitivty; can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi channels within the bone matrix; thus the process of old bone tissue is replaced by new bone tissue

osteogenic cells (osteoprogenitor cells)

undifferentiated cells with high mitotic activity and they are the only bone cells that divide; immature ones are found in the deep layers of the periosteum and the marrow; they differentiate and develop into osteoblasts

bone formation

most of the bones of the human skeleton are cartilaginous during prenatal development; they are shaped like the future bones so they are like models of these bones; throughout fetal development and into childhood growth and development bone forms on the cartilaginous matrix; by a time a fetus is born, most of the cartilage has been replaced with bone; some additional cartilage will be replaced throughout childhood, and some cartilage remains in the adult skeleton

ossification

the process of converting tissue to bone; two methods: endochondral and intramembranous

calcification

the process of depositing calcium salts into the matrix; occurs during ossification

endochondral ossification

ossification of bones via replacement of the mini hyaline cartilage models into bones; long bones develop by replacing hyaline cartilage does not; cartilage does not become bone, instead, it serves as a template to be completely replaced by new bone; takes much longer than intramembranous ossification; bones at the base of the skull and long bones form via endochondral ossification

chondrocytes

cartilage cells

perichondrium

membrane that covers the cartilage

step 1 of bone formation

chondrocytes enlarge and then die as the matrix calcifies: as more matrix is produced the chondrocytes in the center of the cartilaginous model grow; as the matrix calcifies, nutrients can no longer reach the chondrocytes; this results in their death and the disintegration of the surrounding cartilage

step 2 of bone formation

blood vessels surround the outside of the cartilage: blood vessels grow around the outside of the cartilage, which converts the perichondrium into osteoblasts; this develops the periosteum, which gives the shaft a superficial layer of bone

step 3 of bone formation

primary ossification center is formed as blood vessels invade and form the beginning of the medullary cavity: blood vessels invade the spaces inside the shaft; this enlarges the cavities and brings osteogenic cells, many of which will become osteoblasts; the blood supply to the shaft becomes the primary ossification center and the spaces combine to become the medullary cavity

step 4 of bone formation

the medullary cavity expands, bone increased in length: the medullary cavity is formed as growth and remodeling continues; while these deep changes are occurring, chondrocytes and cartilage continue to grow at the ends of the bone (the future epiphyses), which increases the bone’s length, at the same time bone is replacing cartilage in the diaphysis

step 5 of bone formation

secondary ossification centers are formed as more blood vessels invade the epiphyseal regions of the bone: secondary ossification centers are formed on each end of the bone as vessels penetrate the epiphyses regions these vessels deliver osteoblasts and develop bone in the ends of each bone

step 6 of bone formation

chondrocytes and osteoblasts work at the same rate to grow the bone via the epiphyseal cartilage: spongy bone is created in the epiphyses on each end of the long bone; in the metaphyseal region, the epiphyseal cartilage (epiphyseal plate/growth plate) is then formed between the diaphysis and the epiphysis; on the epiphyseal side, the cartilage continues to grow and divide, while on the diaphyseal side, the chondrocytes degenerate; during childhood, osteoblasts work at the same rate as the chondrocytes; osteoblasts move upwards towards the epiphysis and cartilage is gradually replaced by bone

step 7 of bone formation

cartilage activity slows, and osteoblast activity increases during puberty, eventually completely replacing the cartilage with bone, ending as the epiphyseal line in adulthood: at puberty, the rate of epiphyseal cartilage production slows down but osteoblast activity increases; this makes the epiphyseal cartilage get smaller and smaller until eventually there is no cartilage left; after growth is complete, there is a remnant of the epiphyseal cartilage in mature adult bones called the epiphyseal line; a thin covering of cartilage remains on the end of the bones as articular cartilage, which serves to prevent bone to bone contact

intramembranous ossification

the formation of flat bones form connective tissue that begins around week. of fetal development; compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue; the flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are formed via intramembranous ossification; also called dermal ossification because this occurs in the deepest layers of the dermis, forming dermal bones

step 1 of intramembranous ossification

process begins when mesenchymal cells in the embryonic skeleton gather and begin to differentiate into various specialized cells; some of these cells will differentiate into capillaries, while others will become osteogenic cells and then osteoblasts; although they will ultimately be spread out by the formation of bone tissue, early osteoblasts appear in a cluster called an ossification center; the osteoblasts secrete osteoid, uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby, entrapping the osteoblasts within

step 2 of intramembranous ossification

once entrapped, the osteoblasts become osteocytes; as osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts; spicules are the developing bone growing outwards from the ossification centers

step 3 of intramembranous ossification

blood vessels begin to become trapped in the bone as it grows and spicules connect

step 4 of intramembranous ossification

osteoid (unmineralized bone matrix) secreted around the inner capillaries results in a trabecular matrix

step 5 of intramembranous ossification

remodeling of spongy bone around blood vessels on the surfaces produces the typical osteon formation of compact bone on both sides of the inner spongy bone; osteoblasts on the surface of the spngy bone become the periosteum

blood and nerve supply

the spongy bone and medullary cavity receive nourishment and remove waste from arteries and veins that carry blood and pass through the compact bone; the arteries and vessels travel through the nutrient foramen; most bones only have one nutrient artery and vein, but some (like the femur) have more than one; branches come off the larger nutrient vessels to supply the shaft into the smaller perforating canals; the osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities; as the blood passes through the marrow cavities it is collected by veins which then pass out of the bone through the foramina; in addition nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone; the nerves sense pain and play roles in helping regulate blood supply and bone growth

nutrient foramen

small openings in the diaphysis

nutrient artery and vein

blood vessels that enter the diaphysis during endochondral ossification

metaphyseal vessels

supply the diaphyseal surface of the epiphyseal cartilage, as that cartilage is being replaced over time by bone

periosteal vessels

come from the periosteum and supply blood to the osteons in the outer layer of the shaft; branches enter the epiphyses during embryonic development; provide blood to the secondary ossification centers

bone remodeling

resorption of old or damaged bone takes places on the same surface where osteoblasts lay new bone to replace that which is resorbed; bones re constantly being resorbed, recycled, and placed back into the bone matrix through the process of remodeling; in normal adult bone, there is a balance of osteoblast and osteoclast activity so that it remains in homeostatic balance; as osteoclasts remove an osteon via osteolysis, an osteoblast forms another osteon to replace the lost one; due to continual remodeling the thickness of bones can change - physical use and homrone balance can also affect the thickness of bones, injury, exercise, and other activities lead to remodeling; undergoes remodeling as a result of forces or lack of forces placed on them

apposistional bone growth

when a bone grows in diameter and can continue even after longitudinal growth ceases