Bone Tissue

Location of sesamoid bones

patellae (kneecaps) In the upper limbs, sesamoid bones usually occur only in the joints of the palmar surface of the hands.

Protection:

Protects the most important internal organs from injury.

Fossa

Shallow depression Coronoid fossa of the humerus

Bone formation occurs in four principal situations:

(1) the initial formation of bones in an embryo and fetus, (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached, (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life), and (4) the repair of fractures (breaks in bones) throughout life

Medullary cavity

(marrow cavity), is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. Minimizes the weight of the bone by reducing the dense bony material where it is least needed. The long bones' tubular design provides maximum strength with minimum weight.

Processes that form attachment points for connective tissue:

-Crest -Epicondyle -Line -Spinous process -Trochanter -Tubercle -Tuberosity

Sesamoid Bones

-Develop in certain tendons where there is considerable friction, compression, and physical stress. -Protect tendons from excessive wear and tear, and they can alter the direction of pull of a tendon, which improves the mechanical advantage at a joint.

Articular cartilage

-A thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. -Reduces friction and absorbs shock at freely movable joints. -Lacks a perichondrium and lacks blood vessels, repair of damage is limited.

Periosteum

-A tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. -Composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. -Some of the cells enable bone to grow in thickness, but not in length. -Protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons.

Circumferential lamellae

-Arranged around the entire outer and inner circumference of the shaft of a long bone -They develop during initial bone formation. -Directly deep to the periosteum are called outer circumferential lamellae. They are connected to the periosteum by Sharpey's fibers. The circumferential lamellae that line the medullary cavity are called inner circumferential lamellae

Spongy bone tissue

-Bone tissue that consists of an irregular latticework (thin columns) of thin plates of bone called trabeculae; spaces between trabeculae of some bones are filled with red bone marrow; found inside short, flat, and irregular bones and in the epiphyses (ends) of long bones. -Always located in the interior of a bone, protected by a covering of compact bone. -Don't contain osteons

Osteroblasts

-Bone-building cells. -Synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification. -As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes.

Long Bones

-Greater length than width and consist of a diaphysis (shaft) and a variable number of epiphyses or extremities (ends). -They are slightly curved for strength. -Consist mostly of compact bone tissue, which is dense and has smaller spaces, but they also contain considerable amounts of spongy bone tissue, which has larger spaces

Interstitial lamellae

-In the areas between neighboring osteons -Have lacunae with osteocytes and canaliculi. -Fragments of older osteons that have been partially destroyed during bone rebuilding or growth.

Trabeculae

-Irregular latticework of thin plates of spongy bone tissue. Fibrous cord of connective tissue serving as supporting fiber by forming a septum extending into an organ from its wall or capsule. -Each trabecula consists of concentric lamellae, osteocytes that lie in lacunae, and canaliculi that radiate outward from the lacunae.

Osteocytes

-Mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. -Like osteoblasts, osteocytes do not undergo cell division.

Nutrient arteries

-Near the center of the diaphysis; enters the compact bone at an oblique angle through a hole called the nutrient foramen -On entering the medullary cavity, the nutrient artery divides into proximal and distal branches that course toward each end of the bone. -These branches supply both the inner part of compact bone tissue of the diaphysis and the spongy bone tissue and red bone marrow as far as the epiphyseal plates (or lines). -Most bones, like the tibia, have only one nutrient artery entering the diaphysis; others, like the femur (thigh bone), have several

Processes that form joints:

-Projections or outgrowths on bone that form joints or attachment points for connective tissue, such as ligaments and tendons -Condyle -Facet -Head

Canaliculi

-Small channels or canals, as in bones, where they connect lacunae. -Filled with extracellular fluid. Inside the canaliculi are slender fingerlike processes of osteocytes. -Neighboring osteocytes communicate via gap junctions. -Connect lacunae with one another and with the central canals, forming an intricate, miniature system of interconnected canals throughout the bone. -This system provides many routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes.

Difference between compact bone tissue and spongy bone tissue

-Spongy bone tissue is light, which reduces the overall weight of a bone. This reduction in weight allows the bone to move more readily when pulled by a skeletal muscle. -The trabeculae of spongy bone tissue support and protect the red bone marrow. -Spongy bone in the hip bones, ribs, sternum (breastbone), vertebrae, and the proximal ends of the humerus and femur is the only site where red bone marrow is stored and, thus, the site where hemopoiesis (blood cell production) occurs in adults.

Resorption

-The osteoclast releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying extracellular matrix of bone. -Breakdown of the extracellular matrix of bone; part of the normal development, growth, maintenance, and repair of bone.

Metaphyses

-The regions between the diaphysis and the epiphyses. -In a growing bone, each metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length -When bone growth in length stops, the cartilage in the epiphyseal plate is replaced by bone and the resulting bony structure is known as the epiphyseal line.

Perforating (Sharpey's) fibers

-Thick bundles of collagen that extend from the periosteum into the bone extracellular matrix. -Attaches the periosteum to the underlying bone

Osteoprogenitor cells

-unspecialized bone stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. -The only bone cells to undergo cell division; the resulting cells develop into osteoblasts. -Found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels

Growth in thickness

1. At the bone surface, periosteal cells differentiate into osteoblasts, which secrete the collagen fibers and other organic molecules that form bone extracellular matrix. The osteoblasts become surrounded by extracellular matrix and develop into osteocytes. This process forms bone ridges on either side of a periosteal blood vessel. The ridges slowly enlarge and create a groove for the periosteal blood vessel. 2. Eventually, the ridges fold together and fuse, and the groove becomes a tunnel that encloses the blood vessel. The former periosteum now becomes the endosteum that lines the tunnel. 3. Osteoblasts in the endosteum deposit bone extracellular matrix, forming new concentric lamellae. The formation of additional concentric lamellae proceeds inward toward the periosteal blood vessel. In this way, the tunnel fills in, and a new osteon is created. 4. As an osteon is forming, osteoblasts under the periosteum deposit new circumferential lamellae, further increasing the thickness of the bone. As additional periosteal blood vessels become enclosed as in step 1, the growth process continues.

Six steps of endochondral ossification

1. Development of cartilage model: mesenchymal cells develop into chondroblasts, which from the cartilage model 2. Growth of cartilage model: growth occurs by cell division of chondrocytes 3. Development of primary ossication center: in this region of the diaphysis, bone tissue has replaced most of th cartilage 4. Develpment of the medullary cavity: bone breakdwon by osteoclasts forms the medullary cavity 5. Development of secondary ossification centers: these occur in the epiphyses of the bone 6. Formation of articular cartilage and epiphyseal plate: both structures consist of hyaline cartilage

Four steps of intramembranous ossification:

1. Development of the ossification center. At the site where the bone will develop, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification center. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it. 2. Calcification. Next, the secretion of extracellular matrix stops and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification). 3. Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue that is associated with the blood vessels in the trabeculae differentiates into red bone marrow. 4. Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme at the periphery of the bone condenses and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape.

Functions of bone tissue

1. Supports soft tissue and provides attachment for skeletal muscles 2. Protects internal organs. 3. Assists in movement along with skeletal muscles 4. Stores and releases minerals. 5. Contains red bone marrow, which produces blood cells 6. Contains yellow bone marrow, which stores triglycerides (fats), a potential chemical energy reserve

Central canal

A circular channel running longitudinally in the center of an osteon of mature compact bone, containing blood and lymphatic vessels and nerves. Also called an haversian canal.

Red bone marrow

A highly vascularized connective tissue located in microscopic spaces between trabeculae of spongy bone tissue.

Sutural bone

A small bone located within a suture between certain cranial bones. Also called Wormian bone.

Lacunae

A small, hollow space, such as that found in bones in which the osteocytes lie. Between the concentric lamellae

Endosteum

A thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.

Osteoporosis

Age-related disorder characterized by decreased bone mass and increased susceptibility to fractures, often as a result of decreased levels of estrogens. a condition of porous bones

Ossification center

An area in the cartilage model of a future bone where the cartilage cells hypertrophy, secrete enzymes that calcify their extracellular matrix, and die, and the area they occupied is invaded by osteoblasts that then lay down bone.

What is the classification system for bones

Anatomists recognize five types of bones in the skeleton on the basis of shape

Hemopoiesis

Blood cell production, which occurs in red bone marrow after birth. Also called hematopoiesis

Why is bone resorption important?

Bone resorption is necessary for the development, maintenance, and repair of bone.

Mineral storage and release:

Bone tissue stores about 99 percent of total body calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances and to distribute the minerals to other parts of the body. Stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone.

Foramen

Opening through which blood vessels, nerves, or ligaments pass Optic foramen (canal) of the sphenoid bone

Location of short bones

Carpal (wrist) bones and most tarsal (ankle) bones.

Endochondral ossification

Cartilage is replaced by bone Examples: most bones of the body Involves a 6-step process Long bones

Cocentric lamellae

Circular plates of mineralized extracellular matrix that form the rings of bony material in the osteon.

Irregular Bones

Complex shapes and cannot be grouped into any of the three categories just described. They also vary in the amounts of spongy and compact bone they contain.

Location of flat bones

Cranial (skull) bones, which protect the brain; the sternum (breastbone) and ribs, which protect organs in the thorax; and the scapulae (shoulder blades).

Calcification

Deposition of calcium salts, primarily hydroxyapatite, in a framework formed by collagen fibers in which the tissue hardens. Also called mineralization

Parts of a long bone

Diaphysis Epiphyses Metaphyses Articular cartilage Periosteum Medullary cavity Endosteum (the only type of bone with a diaphysis and a medullary cavity)

epiphyseal arteries

Enter the epiphyses of a long bone and supply the red bone marrow and bone tissue of the epiphyses

Metaphyseal

Enter the metaphyses of a long bone and, together with the nutrient artery, supply the red bone marrow and bone tissue of the metaphyses.

Nutrient foramen

For the entrance of the nourishing blood-vessels

Sulcus

Furrow along a bone surface that accommodates a blood vessel, nerve, or tendon Intertubercular sulcus (groove) of the humerus

Flat Bones

Generally thin and composed of two nearly parallel plates of compact bone enclosing a layer of spongy bone. The layers of compact bone are called external and internal tables. In cranial bones, the spongy bone is referred to as diploë. Flat bones afford considerable protection and provide extensive areas for muscle attachment.

Osteoclast

Huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell), are concentrated in the endosteum. The plasma membrane is deeply folded into a ruffled border on the side of the cell that faces the bone surface. Where resorption occurs. -Help regulate blood calcium level in response to certain hormones. They are also the target cells for drug therapy used to treat osteoporosis.

Location of long bones

Humerus (arm bone), ulna and radius (forearm bones), femur (thigh bone), tibia and fibula (leg bones), metacarpals (hand bones), metatarsals (foot bones), and phalanges (finger and toe bones).

Where is red bone marrow found?

It is present in developing bones of the fetus and in some adult bones, such as the hip bones, ribs, sternum, vertebrae, skull, and ends of the humerus and femur. In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow.

Condyle

Large, round protuberance with a smooth articular surface at the end of a bone Lateral condyle of the femur

Types of bones

Long bones, Short bones, Flat bones, Irregular bones, sesamoid bones

Line

Long, narrow ridge or border (less prominent than a crest) Linea aspera of the femur

Assistance in movement:

Most skeletal muscles attach to bones; when they contract, they pull on bone to produce movement

Fissure

Narrow slit between adjacent parts of bones through which blood vessels or nerves pass Superior orbital fissure of the sphenoid bone

Bone remodeling

Ongoing replacement of old bone tissue by new bone tissue.

Common bone fractures

Open (compound) fracture Comminuted fracture Greenstick fracture Impacted fracture Pott's fracture Colles' fracture Stress fracture

Histology of compact bone

Osteons Central canal Concentric lamellae Lacunae Canaliculi Interstitial lamellae Circumferential lamellae

Crest

Prominent ridge or elongated projection Iliac crest of the hip bone

Difference between red and yellow bone marrow

Red, or hematopoietic -Produces red blood cells, white blood cells and platelets -Gets its red color from the hemoglobin in the erythroid cells -Hematopoietic cells mature and migrate into sinusoids to enter the circulation when they are formed. -Highly vascular Yellow, or stromal -Produces fat, cartilage, and bone -Gets its yellow color from the carotenoids in the fat droplets in the high number of fat cells -Paucity of vasculature

Spinous process

Sharp, slender projection Spinous process of a vertebra

DEPRESSIONS AND OPENINGS:

Sites allowing the passage of soft tissue (nerves, blood vessels, ligaments, tendons) or formation of joints -Fissure -Foramen -Fossa -Sulcus -Meatus

Periosteal arteries

Small arteries accompanied by nerves, enter the diaphysis through numerous perforating (Volkmann) canals and supply the periosteum and outer part of the compact bone

Facet

Smooth, flat, slightly concave or convex articular surface Superior articular facet of a vertebra

Short Bones

Somewhat cube-shaped and nearly equal in length, width, and depth. They consist of spongy bone except at the surface, where there is a thin layer of compact bone.

Functions of the skeletal system

Support, Protection, Assistance in movement, Mineral storage and release, Blood cell production, Triglyceride storage

Osteons

The basic unit of structure in adult compact bone, consisting of a central (haversian) canal with its concentrically arranged lamellae, lacunae, osteocytes, and canaliculi. Also called a haversian system.

Diaphysis

The bone's shaft, or body—the long, cylindrical, main portion of the bone

Open (Compound)

The broken ends of the bone protrude through the skin. Conversely, a closed (simple) fracture does not break the skin.

Endochondral growth

The cartilage model grows in length by mitosis and by the deposition of extracellular matrix In contrast, growth of cartilage in thickness is due mainly to the addition of more extracellular matrix to the periphery

As people age, some central (haversian) canals may become blocked. What effect would this have on the osteocytes?

The central (haversian) canals are the main blood supply to the osteocytes of an osteon (haversian system), so their blockage would lead to death of the osteocytes.

Role of the Epiphyseal Plate in Bone Growth

The growth in length of a long bone involves Interstitial growth Replacement of cartilage with bone by endochondral ossification on the diaphyseal side of the epiphyseal plate

Intramembranous ossification

The method of bone formation in which the bone is formed directly in membranous tissue. Simpler process than endochondral ossification Examples: flat bones of the skull Involves a 4-step process

Ossification

The production of a special mineralized extracellular matrix by bone cells.

Epiphyses

The proximal and distal ends of the bone.

Support:

The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles.

Trochanter

Very large projection Greater trochanter of the femur

What is the significance of bone surface markings?

They are structural features adapted for specific functions.

Bone tissue

Tissue containing an abundant extracellular matrix (mineral salts, collagen fibers, and water) that surrounds widely separated cells (osteocytes, osteoblasts, and osteoclasts). Also called osseous tissue.

Meatus

Tubelike opening External and internal auditory meati of the temporal bone

Epicondyle

Typically roughened projection above a condyle Medial epicondyle of the femur

Head

Usually rounded articular projection supported on the neck (constricted portion) of a bone Head of the femur

Tuberosity

Variable sized projection that has a rough, bumpy surface Ischial tuberosity of the hip bone

Tubercle

Variable sized rounded projection Greater tubercle of the humerus

Blood cell production:

Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets in a process called hemopoiesis or hematopoiesis

Triglyceride storage:

Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve

Nutrient veins

accompany the nutrient artery and exit through the diaphysis

Metaphyseal/epiphyseal veins

accompany their respective arteries and exit through the epiphyses

Periosteal veins

accompany their respective arteries and exit through the periosteum.

What does red bone marrow consist of?

consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers.

Which cells are responsible for making and maintaining bone

osteoprogenitor cells, osteoblasts, osteocytes, and osteoclast

Bone deposition

the addition of minerals and collagen fibers to bone by osteoblasts bone deposition results in its formation.

Bone resorption

the removal of minerals and collagen fibers from bone by osteoclasts results in the breakdown of bone extracellular matrix

Where do periosteal arteries enter bone tissue?

through perforating (Volkmann) canals.

Location of irregular bones

vertebrae (backbones), certain facial bones, and the calcaneus (heel bone).