Skeletal System Biol 105 - Lecture 8 Chapter 5

Outline Overview of the skeletal system Function of bones Bone structure Bone cells Cartilage Tendons and Ligaments Joints Bone development Hormonal regulation of bone growth Homeostasis Disorders of the skeletal system

Bones – rigid structure Cartilage – soft, cushions the joints Skeletal System The skeletal systems is composed of different types of connective tissues: Bones – rigid structure Cartilage – soft, cushions the joints Ligaments – attach bone to bone Tendons – attach muscle to bone. Tendons link the skeletal and the muscular systems

Figure: 25-08 Title: Elements of the Skeletal System Caption: Bones are the basic element of the skeletal system. Ligaments link one bone to another, and cartilage often serves as padding between bones. Tendons, while not part of the skeletal system, always link bone to muscle.

Supports and gives shape to the body. Protects soft body parts. Functions of Bone Supports and gives shape to the body. Protects soft body parts. Produce blood cells Stores minerals (calcium and phosphate) Stores fat. Along with the muscles, permits flexible body movement.

Calcified tissue = osseous tissue Blood vessels Nerves Bone Bones have different types of tissue Calcified tissue = osseous tissue Blood vessels Nerves Contain collagen fibers that give flexibility This combination of calcified tissue and collagen fibers make the bone strong but flexible

Compact bone – dense mineralized outer layer Bone Structure All bones have two layers of bone: Compact bone – dense mineralized outer layer Spongy bone – inner layer with open network filled with marrow

Typical Bone Structure Epiphysis – rounded end of the typical long bone, composed mainly of red bone marrow where blood cells are made Diaphysis – shaft of the typical long bone, has a cavity filled with yellow marrow where fat is stored.

Periosteum Periosteum – fibrous outer covering of the bone, contains nerves, blood vessels, lymphatic vessels. Functions in bone repair and growth

Typical Long Bone Structure Spongy bone (spaces contain red bone marrow) Compact bone on surface Yellow bone marrow Blood vessel Periosteum Central cavity (contains yellow bone marrow) (a) A long bone, such as the femur of the leg, consists of a shaft and two heads, or enlarged ends. Compact bone is located on the outer surface of the bone. Spongy bone is found in the heads. Figure 5.1a

Spongy Bone Figure 5.1e

Bone Is Living Tissue Figure 5.1b Osteon Periosteum Blood vessels and nerve in central canal Bone-forming cells (osteoblasts) Spongy bone Compact bone (b) The structural unit of compact bone is an osteon. Mature, living bone cells (osteocytes) are found in small spaces within the hard matrix. Figure 5.1b

Compact Bone Structure Osteon – structural unit of the compact bone

Osteon Structure Central Canal (Haversian) – contain blood vessels and nerves Osteocytes – Mature bone cells. Maintain bone structure and density and minerals Found in cavities called lacunae Canaliculi are canals that connect the lacunae to the central canal

Figure 5.1c The structure of bone

Figure 5.1d The structure of bone

Bone cells Osteoblasts Osteocytes Osteoclasts

Osteoblasts Osteoblasts = bone forming cells. Immature cells that secrete material called “bone ground substance” = non-cellular matrix that surrounds the cells. Osteoblasts mature into osteocytes after they have secreted enough material

Osteoclasts Osteoclasts – Bone absorbing cells. These are bone cells on the outer edge of bones. They release enzymes that eat away at the bone, releasing minerals

The arrow is pointing to: Diaphysis Epiphysis

Immature cells that secrete calcified material are: Osteoclasts Osteocytes Osteoblasts

Osteocytes are found in cavities called Central canals Collagen Lacunae Vellus

Cartilage in bones Two regions of cartilage remain at each end of the long bone The cap that covers the surfaces that rub against other bones A plate of cartilage called the epiphyseal plate, or growth plate

Bone development and growth The skeleton begins to form at about 6 weeks. Most bones grow through adolescence but some bones continue to grow through about age 25. Ossification – formation of bones 10-5

Bone development and growth The skeleton begins to form at about 6 weeks. Most bones grow through adolescence but some bones continue to grow through about age 25. Bone continue to change throughout life. Osteoclasts are removing the bone matrix while Osteoblasts are building the matrix up. Ossification – formation of bones 10-5

Figure 5.3 Steps of bone formation in long and short bones, from an embryo into childhood Fetal Development Cartilaginous surface Birth Spongy bone formation Spongy bone 9 months 3 months 2 months Calcified cartilage Cartilage model Cartilaginous (epiphyseal) growth plate Blood vessel Bone collar Compact bone Deteriorating cartilage matrix Step 1: A cartilaginous model of the future bone forms. Step 2: Osteoblasts form a collar of bone around the shaft of the model. Step 3: The shaft of the cartilage model begins to hollow out, and spongy bone fills the space. Blood vessels continue to penetrate the area, and the region of bone formation expands. Step 4: Secondary centers of bone formation develop in the ends of the bone. Step 5: Cartilage remains only on the surfaces that rub against other bones and in the cartilage growth plates.

Bone Growth Bone growth is stimulated by growth hormone during childhood Thyroid hormones ensure that the skeleton grows with the proper proportions At puberty, increasing levels of male or female sex hormones initially stimulate cartilage cells to divide, but eventually allow for the growth plates to fuse, and bone can no longer increase in length

Hormonal Regulation of Bone Growth Growth hormone (GH) - directly stimulates growth of the epiphyseal plate and bone growth in general. Vitamin D - converted to a hormone causing the intestines to absorb calcium. Sex hormones - adolescents experience a growth spurt due to an increased level of hormones. 10-7

Hormonal regulation of Bone Calcium Levels Parathyroid hormone (PTH) – produced by the parathyroid gland - accelerates bone recycling and increases blood calcium. Calcitonin – produced by the thyroid gland - a hormone that decreases blood calcium levels, deposits calcium into the bone. Homeostasis – maintaining a balance of calcium levels in blood and in bone.

Figure: 25-01 Title: Calcium Levels: An Example of Negative Feedback Caption: Calcium levels in the bloodstream are regulated in part by the parathyroid glands. These glands can sense when calcium levels are too low (the stimulus in the drawing at left). In response, the parathyroids secrete a hormone that causes the bones to release calcium. This bring about high calcium levels, which the glands also can sense (the stimulus in the drawing at right). High calcium levels cause the parathyroids to halt their production of the calcium-liberating hormone.

Healing broken bones Broken bones and fractures are the same thing Bone fractures are healed by fibroblasts and osteoblasts. When a break occurs, there is bleeding followed by a clot. Fibroblasts secrete collagen fibers that form a callus linking the two parts of the bone. This is later replaced by bone.

Bone Fractures are Healed by Fibroblasts and Osteoblasts Formation of blood clot Formation of cartilaginous callus Cartilaginous callus Spongy bone struts Blood clot (hematoma) Step 1: Within hours after the fracture, a blood clot forms. Step 2: A cartilaginous callus is formed by invading fibroblasts. Figure 5.4 (1 of 2)

Bone Fractures are Healed by Fibroblasts and Osteoblasts Formation of bony callus Bone remodeling New blood vessels Bony callus Healed fracture Step 3: Osteoblasts form new bone, converting the cartilaginous callus to a bony callus. Step 4: The fracture is healed and bone is remodeled, restoring bone to original shape. Figure 5.4 (2 of 2)

This hormones accelerates bone recycling and increases blood calcium Growth hormone Calcitonin Parathyroid hormone

Calcitonin is produced by the ______ gland Parathyroid Thyroid Anterior pituitary Hypothalamus

Cartilage Cartilage - flexible connective tissue not as strong as bone tissue. Function: Cushion joints, flexiblilty Found at the ends of long bones, nose, ends of ribs, larynx and trachea, disks between vertebrae and knee, ear flaps and epiglottis. Cartilage cells are chondrocytes Cartilage lacks blood vessels, so slow to heal

10-4

Tendons and Ligaments Ligaments – connective tissue that connects bone to bone Tendons – connective tissue that connects muscle to bone

Synovial Joints Freely movable joints held together by ligaments. Synovial membrane - produces synovial fluid, an excellent lubricant for the joints. Bursae – fluid filled sacs, reduce friction Menisci – cartilage between the bones, acts as a cushion Cartilage is also found at the ends of the bones 10-18

Synovial Joints Femur Fat pad Tibia (a) A bursa is a fluid-filled sac that cushions certain joints and reduces friction between tendons and ligaments. Femur A layer of cartilage on the articulating surfaces of the bones reduces friction as the bones move. The joint cavity is filled with synovial fluid, which serves as a shock absorber and lubricant. Fat pad The synovial membrane forms the inner surface of the joint cavity and secretes synovial fluid. Tibia (a) Synovial joints, such as the knee shown here, permit a great range of movement. Figure 5.13a

Synovial Joints Quadriceps tendon Femur Posterior cruciate ligament Lateral collateral ligament Anterior cruciate ligament Medial collateral ligament Patellar ligament Fibula Patella (kneecap) Tibia (b) Ligaments hold bones together, support the joint, and direct the movement of the bones. Figure 5.13b

10-19

Cartilage cells are fibrocytes osteocytes chondrocytes

This type of connective tissue connects bone to bone Ligaments Tendons

fluid filled sacs are called: Synovia Menisci Bursae

Disorders of the Skeletal System Osteoarthritis - deterioration of cartilage at joints. Rheumatoid Arthritis – chronic inflammatory disorder of the joints. Sprains – When ligaments are torn or stretched Bursitis – inflammation of bursa Tendonitis – inflammation of tendon sheath Fracture – broken bone simple = bone breaks but does not penetrate the skin compound = bone breaks and protrudes through the skin 10-22

Osteoporosis When more bone is reabsorbed than is formed, this is when the homeostasis is not maintained and more calcium is taken out of the bones than is replaced. Estrogen and Testosterone help maintain the bone density. After menopause women produce less estrogen and this can lead to less bone density

Risk Factors Post menopausal Not enough exercise Poor diet, not enough calcium in the diet Smoking Low vitamin D levels Certain hormonal disorders

Treatment Exercise Increased Calcium in the diet, calcium supplements if needed, the type of calcium supplement is important Estrogen replacement therapy Prescription drugs

Ages Amount mg/day Birth - 6 months 210 6 months - 1 year 270 1-3 500 Recommended Calcium Intakes* Ages Amount mg/day Birth - 6 months 210 6 months - 1 year 270 1-3 500 4-8 800 9-13 1300 14-18 19-30 1000 31-50 51-70 1200 70 or older

Food Calcium (mg) % DV* Yogurt, plain, low fat, 8 oz. 415 42% Yogurt, fruit, low fat, 8 oz. 245-384 25%-38% Sardines, canned in oil, with bones, 3 oz. 324 32% Cheddar cheese, 1 ½ oz shredded 306 31% Milk, non-fat, 8 fl oz. 302 30% Tofu, firm, ½ cup 204 20% Mozzarella, part skim 1 ½ oz. 275 28% Orange juice, calcium fortified, 6 fl oz. 200-260 20-26% Turnip greens, boiled, ½ cup 99 10% Kale, cooked, 1 cup 94 9% Ready to eat cereal, calcium fortified, 1 cup 100-1000 10%-100% Cottage cheese, 1% milk fat, 1 cup unpacked 138 14%

Decreased Calcium Absorption Oxalic acid – potent inhibitor of calcium absorption, and is found in high concentrations in spinach and rhubarb and in somewhat lower concentrations in sweet potato and dried beans. Sodium – Increased sodium intake results in increased loss of calcium in the urine. Protein – As dietary protein intake increases, the urinary excretion of calcium also increases. Caffeine – Caffeine in large amounts increases urinary calcium content for a short time.

Fractured bones are the same thing as broken bones True False

Important Concepts Read Ch 7 What are the four components of the skeletal system, and their functions What are the functions of bone What are the two layers of bone What is the periosteum and what is its function What is the structure of the osteon, what are the features found in the osteon

Important Concepts What are the three types of bone cells and their functions What are the functions of cartilage, tendons, and ligaments. What are the parts of synovial joints and their functions What are the hormones involved in bone growth and homeostasis, what glands produce calcitonin and parathyroid hormones

Important Concepts What are the disorders of the skeletal system How do bones heal What is osteoporosis, what are the risk factors, what can you do to prevent it.

Definitions Compact bone, osteon, central canal (haversian canal), Spongy bone, Epiphysis, Diaphysis, Periosteum, lacunae, callus, fibroblasts, Osteoblasts, Osteocytes, Osteoclasts, Chondrocytes, growth hormone, Parathyroid hormone (PTH), Calcitonin, Vitamin D, epiphyseal plate, growth plate