Skeletal System

Skeletal System Biology 105 - Lecture 8

Chapter 5

Outline

I. Overview of the skeletal system II. Function of bones III. Bone structure IV. Bone cells V. Cartilage VI. Tendons and ligaments VII.Joints VIII.Bone development IX. Hormonal regulation of bone growth X. Homeostasis XI. Disorders of the skeletal system

Skeletal System

The skeletal system 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.

1. Supports and gives shape to the body. 2. Protects soft body parts. 3. Produces blood cells. 4. Stores minerals (calcium and phosphate). 5. Stores fat. 6. Along with the muscles, permits flexible

body movement.

Functions of Bone

Bone Structure

Bones contain 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 makes the bone strong but flexible.

Bone Structure

All bones have two layers: Compact bone – dense mineralized outer

layer Spongy bone – inner layer with open

network filled with marrow

Spongy Bone

Figure 5.1e

Typical Long Bone Architecture

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

Typical Long Bone Architecture

Figure 5.1a

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.

Bone Is Living Tissue!

Figure 5.1b

Blood vessels and nerve in central canal

Spongy bone Compact bone

Osteon

Bone-forming cells (osteoblasts)

(b) The structural unit of compact bone is an osteon. Mature, living bone cells (osteocytes) are found in small spaces within the hard matrix.

Periosteum

Compact Bone Structure

Osteon – structural unit of the compact bone.

Osteon structure: Central canal (Haversian) – contains blood

vessels and nerves.

Osteocytes – mature bone cells that maintain bone structure, 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”: This is 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:

Epiphysis

50%50%1. Diaphysis 2. Epiphysis

Immature cells that secrete calcified material are:

1. Osteoclasts 2. Osteocytes 3. Osteoblasts

Osteocytes are found in cavities called:

tral c

Lacuna

Vellus

25% 25%25%25%

1. Central canals 2. Collagen 3. Lacunae 4. Vellus

The skeleton begins to form at about 6 weeks after conception.

Most bones grow through adolescence, but some bones continue to grow through about age 25.

Bone continue to change throughout life: Osteoclasts remove the bone matrix, while

osteoblasts build up the matrix.

Ossification – formation of bones.

Bone Development and Growth

Figure 5.3 Steps of bone formation in long and short bones, from an embryo into childhood

2 months

Fetal Development

Cartilage model

3 months 9 months

Bone collar Deteriorating

cartilage matrix

Calcified cartilage

Blood vessel

Birth Spongy

bone formation

Childhood

Spongy bone

Compact bone

Step 2: Osteoblasts form a collar of bone around the shaft of the model.

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.

Cartilaginous surface

Step 1: A cartilaginous model of the future bone forms.

Cartilaginous (epiphyseal) growth plate

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.

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 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.

Growth hormone (GH) – directly stimulates growth of the epiphyseal plate and stimulates bone growth in general.

Vitamin D – converted to a hormone that causes the intestines to absorb calcium.

Sex hormones – adolescents experience a growth spurt due to an increased level of hormones.

Hormonal Regulation of Bone Growth

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,

and deposits calcium into the bone.

Homeostasis – maintaining a balance of calcium levels in blood and in bone.

Hormonal Regulation of Bone Calcium Levels Hormonal Regulation of Bone Calcium Levels

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

Figure 5.4 (1 of 2)

Blood clot (hematoma)

Step 1: Within hours after the fracture, a blood clot forms.

Step 2: A cartilaginous callus is formed by invading fibroblasts.

Formation of blood clot

Spongy bone struts

Formation of cartilaginous callus

Cartilaginous callus

Bone Fractures are Healed by Fibroblasts and Osteoblasts

Figure 5.4 (2 of 2)

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.

New blood

vessels

Formation of bony callus Bone remodeling

Bony callus Healed

fracture

This hormones accelerates bone recycling and increases blood calcium:

wth hor

itonin

Parathyro

33% 33%33%1. Growth hormone 2. Calcitonin 3. Parathyroid hormone

Calcitonin is produced by the ______ gland.

Parathyro

Thyroid

ior pitu

othala

25% 25%25%25%1. Parathyroid 2. Thyroid 3. Anterior pituitary 4. Hypothalamus

Cartilage

Cartilage – flexible connective tissue that is not as strong as bone tissue.

Function: cushions joints, provides flexibility Where found: Ends of long bones, nose, ends of ribs, larynx and trachea, disks between vertebrae, knee joint, ear flaps, and epiglottis.

Cartilage cells are called chondrocytes. Cartilage lacks blood vessels, so it is very slow

to heal!

Tendons and Ligaments

Ligaments – connective tissue that connects bone to bone.

Tendons – connective tissue that connects muscle to bone.

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.

Synovial Joints

Figure 5.13a

A layer of cartilage on the articulating surfaces of the bones reduces friction as the bones move.

(a) Synovial joints, such as the knee shown here, permit a great range of movement.

A bursa is a fluid-filled sac that cushions certain joints and reduces friction between tendons and ligaments.

The joint cavity is filled with synovial fluid, which serves as a shock absorber and lubricant.

The synovial membrane forms the inner surface of the joint cavity and secretes synovial fluid.

Fat pad

Synovial Joints

Figure 5.13b

Quadriceps tendon

Fibula

Posterior cruciate ligament

Anterior cruciate ligament

Medial collateral ligament

Patellar ligament

Patella (kneecap)

(b) Ligaments hold bones together, support the joint, and direct the movement of the bones.

Lateral collateral ligament

Cartilage cells are called:

osteocy

chondr

33% 33%33%

1. Fibrocytes 2. Osteocytes 3. Chondrocytes

This type of connective tissue connects bone to bone:

Ligamen

Tendon

50%50%1. Ligaments 2. Tendons

Fluid-filled sacs are called:

Synovia

33% 33%33%1. Synovia 2. Menisci 3. Bursae

Osteoarthritis – deterioration of cartilage at joints. Rheumatoid Arthritis – chronic inflammatory

disorder of the joints. Sprain – 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

through the skin. Compound = bone breaks and protrudes through

the skin.

Disorders of the Skeletal System

Osteoporosis

Occurs when more bone is reabsorbed than is formed: Homeostasis is not maintained, and more

calcium is taken out of the bones than is replaced.

Estrogen and testosterone help maintain bone

density.

After menopause, women produce less estrogen and this can lead to reduced bone density.

Risk Factors for Osteoporosis

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 may be needed. The type of calcium supplement is important.

Estrogen replacement therapy Prescription drugs

Recommended Calcium Intakes*

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

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% Turnip greens, boiled, ½ cup 99 10% 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. 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.

50%50%1. True 2. False

Important Concepts Read chapter 5

What are the four components of the skeletal system, and what are 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, and 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? Which glands produce calcitonin and parathyroid hormones?

Important Concepts

What are the disorders of the skeletal system we discussed in this lecture? How do bones heal?

What is osteoporosis, what are the risk factors

for osteoporosis, and 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

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