Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Developmental Aspects of Tissue Primary germ layers: ectoderm, mesoderm, and endoderm.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Developmental Aspects of Tissue

• Primary germ layers: ectoderm, mesoderm, and endoderm

• Three layers of cells formed early in embryonic development

• Specialize to form the four primary tissues

• Nerve tissue arises from ectoderm


Developmental Aspects of Tissue

• Muscle, connective tissue, endothelium, and mesothelium arise from mesoderm

• Most mucosae arise from endoderm

• Epithelial tissues arise from all three germ layers

Figure 4.13


Skeletal Cartilage

• Contains no blood vessels or nerves

• Surrounded by the perichondrium (dense irregular CT) that resists outward expansion

• Three types – hyaline, elastic, and fibrocartilage


BONES and SKELETAL TISSUE


Bones and Cartilages of the Human Body

Figure 6.1


Function of Bones

• Support – form the framework that supports the body and cradles soft organs

• Protection – provide a protective case for the brain, spinal cord, and vital organs

• Movement – provide levers for muscles

• Mineral storage – reservoir for minerals, especially calcium and phosphorus

• Blood cell formation – hematopoiesis occurs within the marrow cavities of bones


Reminder -Structure of Long Bone

Figure 6.3a, c


Reminder-Structure of Short, Irregular, and Flat Bones

Figure 6.4


Location of Hematopoietic Tissue (Red Marrow)

• In infants

• Found in the medullary cavity and all areas of spongy bone

• In adults

• Found in the spongy bone of flat bones, and the head of the femur and humerus


Chemical Composition of Bone: Organic

• Osteoblasts – bone-forming cells

• Osteocytes – mature bone cells

• Osteoclasts – large cells that resorb or break down bone matrix

• Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen


Chemical Composition of Bone: Inorganic

• Hydroxyapatites, or mineral salts

• Sixty-five percent of bone by mass

• Mainly calcium phosphates

• Responsible for bone hardness and its resistance to compression


Bone Development

• Osteogenesis and ossification – the process of bone tissue formation, which leads to:

• The formation of the bony skeleton in embryos

• Bone growth until early adulthood

• Bone thickness, remodeling, and repair


Formation of the Bony Skeleton

• Begins at week 8 of embryo development

• Intramembranous ossification – bone develops from a fibrous membrane

• Endochondral ossification – bone forms by replacing hyaline cartilage


Postnatal Bone Growth

• Growth in length of long bones

• Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive

• Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth


Long Bone Growth and Remodeling

• Growth in length – cartilage continually grows and is replaced by bone as shown

• Remodeling – bone is resorbed and added by appositional growth as shown

Figure 6.10


• During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone

• During puberty, by testosterone and estrogens

• Initially promote adolescent growth spurts

• Cause masculinization and feminization of specific parts of the skeleton

• Later induce epiphyseal plate closure, ending longitudinal bone growth

Hormonal Regulation of Bone Growth During Youth


Bone Remodeling

• Remodeling units – adjacent osteoblasts (deposit) and osteoclasts (resorb) bone at periosteal and endosteal surfaces


Bone Deposition

• Occurs where bone is injured or added strength is needed

• Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese

• Alkaline phosphatase is essential for mineralization of bone

• Sites of new matrix deposition are revealed by:

• Osteoid seam – unmineralized band of bone matrix

• Calcification front – abrupt transition zone between the osteoid seam and the older mineralized bone


Bone Resorption

• Accomplished by osteoclasts

• Resorption bays – grooves formed by osteoclasts as they break down bone matrix

• Resorption involves osteoclast secretion of:

• Lysosomal enzymes that digest organic matrix

• Acids that convert calcium salts into soluble forms

• Dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood


Importance of Ionic Calcium in the Body

• Calcium is necessary for:

• Transmission of nerve impulses

• Muscle contraction

• Blood coagulation

• Secretion by glands and nerve cells

• Cell division


Control of Remodeling

• Two control loops regulate bone remodeling

• Hormonal mechanism that maintains calcium homeostasis in the blood

• Mechanical and gravitational forces acting to the skeleton


Hormonal Mechanism

• Rising blood Ca2+ levels trigger the thyroid to release calcitonin

• Calcitonin stimulates calcium salt deposit in bone

Figure 6.11


Hormonal Mechanism

• Falling blood Ca2+ levels signal the parathyroid glands to release PTH

• PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood

Figure 6.11


Response to Mechanical Stress

• Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it

Figure 6.12



• Observations supporting Wolff’s law include:

• Long bones are thickest midway along the shaft (where bending stress is greatest)

• Curved bones are thickest where they are most likely to buckle

Figure 6.12



• Large, bony projections occur where heavy, active muscles attach

Figure 6.12


Bone Fractures (Breaks)

• Bone fractures are classified by:

• The position of the bone ends after fracture

• Completeness of the break

• The orientation of the bone to the long axis

• Whether or not the bones ends penetrate the skin


Types of Bone Fractures

•Learn table with fracture types 6.2?


Stages in the Healing of a Bone Fracture

• Hematoma formation

• Torn blood vessels hemorrhage

• A mass of clotted blood (hematoma) forms at the fracture site

• Site becomes swollen, painful, and inflamed

Figure 6.13.1



• Fibrocartilaginous callus forms

• Granulation tissue (soft callus) forms a few days after the fracture

• Capillaries grow into the tissue and phagocytic cells begin cleaning debris

Figure 6.13.2



• Bony callus formation

• New bone trabeculae appear in the fibrocartilaginous callus

• Fibrocartilaginous callus converts into a bony (hard) callus

• Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later

Figure 6.13.3



• Bone remodeling

• Excess material on the bone shaft exterior and in the medullary canal is removed

• Compact bone is laid down to reconstruct shaft walls

Figure 6.13.4


Homeostatic Imbalances

• Osteomalacia

• Bones are inadequately mineralized causing softened, weakened bones

• Main symptom is pain when weight is put on the affected bone

• Caused by insufficient calcium in the diet, or by vitamin D deficiency



• Rickets

• Bones of children are inadequately mineralized causing softened, weakened bones

• Bowed legs and deformities of the pelvis, skull, and rib cage are common

• Caused by insufficient calcium in the diet, or by vitamin D deficiency



• Osteoporosis

• Group of diseases in which bone reabsorption outpaces bone deposit

• Spongy bone of the spine is most vulnerable

• Occurs most often in postmenopausal women

• Treatment

• Calcium and vitamin D supplements

• Increased weight bearing exercise

• Hormone (estrogen) replacement therapy (HRT)

• Prevented or delayed by sufficient calcium intake and weight-bearing exercise


Paget’s Disease

• Characterized by excessive bone formation and breakdown

• Pagetic bone with an excessively high ratio of woven to compact bone is formed

• Pagetic bone, along with reduced mineralization, causes spotty weakening of bone

• Osteoclast activity wanes, but osteoblast activity continues to work


Developmental Aspects of Bones

• Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton

• The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms

• At birth, most long bones are well ossified (except for their epiphyses)


Developmental Aspects of Bones

• By age 25, nearly all bones are completely ossified

• In old age, bone resorption predominates

• A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life


Joints (Articulations)

• Weakest parts of the skeleton

• Articulation – site where two or more bones meet

• Functions

• Give the skeleton mobility

• Hold the skeleton together


Classification of Joints: Structural

• Structural classification focuses on the material binding bones together and whether or not a joint cavity is present

• The three structural classifications are:

• Fibrous

• Cartilaginous

• Synovial


Classification of Joints: Functional

• Functional classification is based on the amount of movement allowed by the joint

• The three functional class of joints are:

• Synarthroses – immovable

• Amphiarthroses – slightly movable

• Diarthroses – freely movable


Figure 8.11c

Example - Synovial Joints: Major Ligaments and Tendons (Anterior View)


Inflammatory and Degenerative Conditions

• Bursitis

• An inflammation of a bursa, usually caused by a blow or friction

• Symptoms are pain and swelling

• Treated with anti-inflammatory drugs; excessive fluid may be aspirated

• Tendonitis

• Inflammation of tendon sheaths typically caused by overuse

• Symptoms and treatment are similar to bursitis


Arthritis

• More than 100 different types of inflammatory or degenerative diseases that damage the joints

• Most widespread crippling disease in the U.S.

• Symptoms – pain, stiffness, and swelling of a joint

• Acute forms are caused by bacteria and are treated with antibiotics

• Chronic forms include osteoarthritis, rheumatoid arthritis, and gouty arthritis


Osteoarthritis (OA)

• Most common chronic arthritis; often called “wear-and-tear” arthritis

• Affects women more than men

• 85% of all Americans develop OA

• More prevalent in the aged, and is probably related to the normal aging process


Osteoarthritis: Treatments

• OA is slow and irreversible

• Treatments include:

• Mild pain relievers, along with moderate activity

• Magnetic therapy

• Glucosamine sulfate decreases pain and inflammation

• SAM-e (s-adenosylmethionine) builds up cartilage matrix and regenerates tissue


Rheumatoid Arthritis (RA)

• Chronic, inflammatory, autoimmune disease of unknown cause, with an insidious onset

• Usually arises between the ages of 40 to 50, but may occur at any age

• Signs and symptoms include joint tenderness, anemia, osteoporosis, muscle atrophy, and cardiovascular problems

• The course of RA is marked with exacerbations and remissions


Rheumatoid Arthritis: Course

• RA begins with synovitis of the affected joint

• Inflammatory blood cells migrate to the joint, causing swelling

• Inflamed synovial membrane thickens into a pannus

• Pannus erodes cartilage, scar tissue forms, articulating bone ends connect

• The end result, ankylosis, produces bent, deformed fingers


Rheumatoid Arthritis: Treatment

• Conservative therapy – aspirin, long-term use of antibiotics, and physical therapy

• Progressive treatment – anti-inflammatory drugs or immunosuppressants

• The drug Embrel, a biological response modifier, removes cells that promote inflammation


Gouty Arthritis

• Deposition of uric acid crystals in joints and soft tissues, followed by an inflammation response

• Typically, gouty arthritis affects the joint at the base of the great toe

• In untreated gouty arthritis, the bone ends fuse and immobilize the joint

• Treatment – colchicine, nonsteroidal anti-inflammatory drugs, and glucocorticoids


Developmental Aspects of Joints

• By embryonic week 8, synovial joints resemble adult joints

• Few problems occur until late middle age

• Advancing years take their toll on joints including:

• Ligaments and tendons shorten and weaken

• Intervertebral discs become more likely to herniate

• OA is inevitable, and all people of 70 have some degree of OA

• Prudent exercise (especially swimming) that coaxes joints through their full range of motion is key to postponing joint problems


Sprains

• The ligaments (bone to bone) reinforcing a joint are stretched or torn

• Partially torn ligaments slowly repair themselves

• Completely torn ligaments require prompt surgical repair


Cartilage Injuries

• The snap and pop of overstressed cartilage

• Common aerobics injury

• Repaired with arthroscopic surgery


Dislocations

• Occur when bones are forced out of alignment

• Usually accompanied by sprains, inflammation, and joint immobilization

• Caused by serious falls and are common sports injuries

• Subluxation – partial dislocation of a joint

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Developmental Aspects of Tissue Primary germ layers: ectoderm, mesoderm, and endoderm.

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