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“Introduction to the Human Skeletal System – Bone Histology”
Laboratory Exercise “Introduction to the Human Skeletal System – Bone Histology”
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Human Skeletal System - Introduction
In this exercise, you will examine the histology of bone, describe and identify bones according to their shapes, describe the gross anatomy of a typical long bone and learn the markings found on bones. These terms should be familiar to you: spongy or compact bone, trabeculae, osteons, central canal, lamellae, lacunae, [bone types] – (long, short, flat, irregular, sesamoid, sutural), diaphysis, epiphyses, epiphyseal line, marrow, [bone markings] – foramen, fissure, meatus, sinus, fossa, alveolus, sulcus, head, facet, condyle, epicondyle, process, trocanter, tubercle, tuberosity This laboratory exercise correlates with Chapter 6 of your textbook.
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What procedures are we doing?
Examine the histology of compact bone Observe different bone shapes Study the anatomy of a long bone Examine different bone markings
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Resources available… Anatomy and Physiology Laboratory Study Pages; submenu: Skeletal
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Laboratory Lecture
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An Introduction to the Skeletal System
The Skeletal System includes: Bones and Cartilage Ligaments and Connective Tissues Adipose (fat) Vasculature (blood) Nervous tissue
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Functions of the Skeletal System
The Skeletal System has six primary functions Physical support (framework using long bones) Storage of minerals (calcium and phosphate) Storage of fat (yellow marrow) Blood Cell production (red marrow – hematopoetic) Protection (organ guardianship using flat bones) Leverage (force of motion using long bones)
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Bone (Osseous) Tissue Bone is a dense, supportive connective tissue that is mostly composed of a (98%) solid extracellular matrix with several types of (2%) specialized cells
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Components of the Solid Matrix
Calcium Salts are an inorganic component of the solid matrix Two thirds of the bone matrix is calcium phosphate Calcium phosphate reacts with calcium hydroxide and calcium carbonate to form crystals of hydroxyapatite, Ca10(PO4)6(OH)2 other minerals are also reactive in this matrix (fluoride, potassium, magnesium), which can pose health-related concerns (radioactive isotopes) The crystals are very hard and resist compression, but they are inflexible and can be broken with a side-way impact Collagen fibers are the organic component of the solid matrix One third of the bone matrix is collagen They are flexible and tough; they can be twisted and bent But they offer little resistance to compression Together, calcium salts and collagen fibers interlock with each other, giving bone its strength, flexibility and shatter-resistant properties…
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Bone-specific Cell types
Bone Cells make up only 2% of bone mass Bone contains four types of cells Osteoprogenitor (osteogenic) cells Osteoblasts Osteocytes Osteoclasts
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Osteoprogenitor cells are mesenchymal stem cells that divide and differentiate to produce osteoblasts They are located in endosteum, the inner cellular layer of periosteum (which is the outer cellular layer of bone) These cells assist in fracture repair
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Osteoblasts are immature bone cells that secrete compounds which form the osteoid matrix (osteogenesis) The osteoid matrix is fluid; it is not yet calcified to form bone Osteoblasts that are surrounded by bone become osteocytes
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Osteocytes are mature bone cells that maintain bone matrix
Osteocytes reside in lacunae between layers (lamellae) of the matrix; they are connected by cytoplasmic extensions through canaliculi They do not divide, but help maintain and repair damaged bone
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Osteoclasts secrete acids and protein-digesting enzymes that dissolve the bone matrix and release stored minerals (osteolysis) They are giant, multinucleate cells (many nuclei per cell) Derived from stem cells that produce macrophages
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the great balance: homeostasis
Bone building (by osteoblasts) and bone recycling (by osteoclasts) must balance homeostasis! More breakdown than building, bones become weak Exercise, particularly weight-bearing exercise, causes osteoblasts to build bone
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Classification of Bones
Bones are classified by their: Shape Bone Markings (surface features) Internal tissue organization (compact or spongy)
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1. There are six types of bone shape
Short bone: small & thick Carpal and tarsal bones Flat bone: thin & plate-like Bones of the cranium, sternum, scapula and ribs Long bone: long and thin Bones of arm, leg, hand, feet, fingers and toes Irregular bone: complex shapes Bones of the spinal vertebrae and face Sesamoid bone: small & flat Bones wrapped inside of a tendon – near joints of the hand, knee (patella) and feet Sutural bone: small & irregular Found between cranial bones
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…a little more about the structure of long bones
Structure of a Long Bone Diaphysis (pr. di-AF-i-sis) Is the “shaft” of a long bone Consists of a heavy wall of compact bone (or dense bone), and A central cavity that is filled with yellow bone marrow in adults (medullary, or marrow, cavity) Epiphyses (pr. ē-PIF-i-ses) Are the two wide parts at each end that articulate with other bones They are covered with a compact bone cortex, and A central cavity that is mostly spongy bone (cancellous or trabecular) with a red bone marrow filling OF NOTE: Articular cartilage covers parts of the epiphyses that articulate with other bones Metaphyses (pr. me-TAF-i-ses) Are where diaphysis and epiphyses meet It has a cartilagenous epiphyseal plate (pre-pubescent) which is the site of expansive bone growth (genesis)
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…a little more about the structure of flat bones
Structure of a Flat Bone Resembles a sandwich of spongy bone that is between two layers of compact bone Rich in red bone marrow Within the cranium, the layer of spongy bone between the compact bone is called the diploë (pr. dip-low-ë) An example is the parietal bone of the skull
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A word about bone marrow: soft connective tissue
Adult Bone Marrow Distribution Red bone marrow Contains hematopoietic (hemopoietic) stem cells for the production of red blood cells, white blood cells and platelets It is used in bone marrow transplantation (commonly pulled from the hip bone of the donor) Children contain mostly red bone marrow in the medullary cavity & epiphyses of long bones and spongy bone of flat bones In adults, red bone marrow is restricted to mostly flat bones – the skull, vertebrae, ribs, sternum and ilium
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Yellow bone marrow is not hematopoetic
Adult Bone Marrow Distribution Yellow bone marrow Is widespread in adults and consists of fat (adipose) cells that store energy reserves: It is found in the medullary cavity and epiphyses of long bones and in the spongy bone of flat bones It is not found in skull flat bones, the vertebrae, ribs, sternum and ilium, the proximal epiphysis of the humerus and the femur Yellow bone marrow is not hematopoetic
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2. Bone Markings Bone markings refer to surface features of bone.
They help in naming parts of bone, and being landmarks for physicians and scientists to locate internal structures Depressions, grooves or tunnels run along and into bone These indicate conduits for vascular and nervous tissue Elevations or projections form out from the bone Where tendons and ligaments attach (very strong!) Where adjacent bones articulate (at joints)
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3. Internal Tissue Organization (compact or spongy)
Compact Bone composes 80% of bone It is dense, supporting body weight and movement The osteon is the basic unit of compact bone: Osteocytes reside in a lacunae cavity and are arranged in concentric lamellae, which surround a central canal that contains blood vessels and nerves Perforating canals are perpendicular to the central canal and carry blood vessels into the bone and the marrow Canaliculi (shown in later slide) are narrow passageways containing cytoplasmic extensions of osteocytes; they connect osteocytes to each other and to the vasculature Circumferential and Interstitial lamellae are wrapped around the long bone or between the osteons Circumferential lamellae Osteon Interstitial lamellae Concentric lamellae Central canal Perforating canals
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NOTE: the criss-crossed orientation of collagen fibers (strength) and the canaliculi networking of osteocytes from the central canal
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NOTE: Look at how few lacunae (the residence of osteocytes) there are in the osteon! Remember that mature bone only has a cellular content of about 2% (the rest is mineral and collagen)!
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Trabeculae of spongy bone Canaliculi opening on surface
Spongy Bone composes 20% of bone; it is lighter than compact bone Spongy bone does not have osteons, it is composed of an open network of trabeculae that can withstand stress from many directions Concentric lamellae are present, but are not arranged as osteons The trabeculae have no blood vessels, they are fed by canaliculi The space between the trabeculae is filled with red bone marrow, blood vessels and nerves Forms red blood cells by the process of hematopoesis Supplies nutrients to the osteocytes in the lamellae Trabeculae of spongy bone Canaliculi opening on surface Concentric Lamellae
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Except for joint cavities, compact bone is covered with a membrane of periosteum
The outer layer is fibrous, dense irregular connective tissue It provides protection and repair to bone Anchors blood vessels and nerves Serves as an attachment site for ligaments and tendons The inner layer is a cellular layer containing cells (osteogenic, osteoblasts and osteoclasts) Sharpey’s Fibers (perforating fibers) are collagen fibers that anchor the periosteum to bone and with fibers of joint capsules, tendons and ligaments
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The endosteum in an incomplete cellular layer that lines the medullary (marrow) cavity of compact bone It covers the trabeculae of spongy bone and lines central canals It contains osteoblasts, osteogenic cells and osteoclasts It is active in bone growth and repair
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Extra material…
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Types of Bone Fractures
Fractures are cracks or breaks in bones that are caused by physical stress, and are named according to their external appearance, location and the nature of the crack or the break in the bone The broadest general categories are: Closed (simple) fractures that are completely internal and can only be visualized by x-rays Open (compound) fractures that project through the skin, which can complicate the condition with secondary infection Many fractures fall into more than one category, because terms overlap…
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Transverse fractures break a bone shaft along its long axis
ulna
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Displaced fractures produce new and abnormal bone arrangements.
(Nondisplaced fractures retain the normal alignment)
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Compression fractures occur in vertebrae subjected to extreme stresses
Compression fractures occur in vertebrae subjected to extreme stresses. (like falling on your backside in an abrupt “sitting” manner)
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Spiral fractures are produced by twisting stresses that spread along the length of the bone.
tibia
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Epiphyseal fractures tend to occur where the bone matrix is undergoing calcification and the chondrocytes are dying. Clean transverse fractures heal well (for obvious reasons) if treated properly. femur
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Comminuted fractures shatter the affected area into many bony fragments.
femur
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Greenstick fractures are a partial fracture, where one side of the shaft is broken and the other side is bent. This happens frequently in children. radius
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Colles fractures are a break in the distal portion of the radius
Colles fractures are a break in the distal portion of the radius. (typically caused by extending your hand to cushion a fall)
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Pott’s fractures occur at the ankle and affect both bones of the leg.
(this is a common sports injury)
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