Bones of the Appendicular Skeleton & Bone Homeostasis
Appendicular Skeleton Overview
The appendicular skeleton comprises the bones within the pectoral and pelvic girdles and their attached limbs.
Pectoral (Shoulder) Girdle and Upper Limb: Specialized for flexibility and range of motion.
Pelvic (Hip) Girdle and Lower Limbs: Specialized for strength and weight-bearing.
Pectoral (Shoulder) Girdle and Upper Limb
The body has two pectoral girdles (left and right).
Each pectoral girdle consists of:
Scapula: Also known as the shoulder blade.
Clavicle: Also known as the collarbone.
Clavicle:
Extends across the top of the thorax.
Articulates with the sternum (breastbone) and the scapula.
Scapula:
A visible bone in the back.
Muscles of the arm and chest attach to it.
The glenoid cavity of the scapula articulates with the head of the humerus.
The glenoid cavity is much smaller than the humeral head, allowing for extensive arm movement in almost any direction.
This flexibility, however, reduces joint stability, making it the joint most prone to dislocation.
Stabilized by ligaments and tendons.
Rotator Cuff: Formed by tendons extending to the humerus from small muscles originating on the scapula. Vigorous circular arm movements can lead to rotator cuff injuries.
The components of the pectoral girdle freely follow the movements of the upper limb.
Upper Limb Bones:
Humerus: The bone of the upper arm.
Has a smoothly rounded head that fits into the glenoid cavity of the scapula.
The shaft features a deltoid tuberosity (protuberance) where the deltoid shoulder muscle attaches.
The size of the deltoid tuberosity can indicate if a person performed significant heavy lifting during their lifetime.
The distal end has two protuberances that articulate with the radius and ulna at the elbow.
Radius and Ulna: Bones of the forearm.
When the palm faces forward, the radius and ulna are approximately parallel.
When the palm faces backward, the radius crosses in front of the ulna, facilitating the twisting motion of the forearm.
Hand Bones: The hand's many bones contribute to its flexibility.
Carpals: Eight small, pebble-like bones forming the wrist.
Metacarpals: Five bones fanning out from the carpals, forming the framework of the palm.
The metacarpal leading to the thumb is opposable to the other digits.
An opposable thumb can touch each finger separately or cross the palm to grasp objects.
Knuckles: The enlarged distal ends of the metacarpals.
Phalanges: Bones of the fingers and thumb (digits).
In the hand, phalanges are long, slender, and lightweight.
Pelvic (Hip) Girdle and Lower Limb
Pelvic Girdle (Hip Girdle): Consists of two heavy, large hip bones, also known as os coxae (or innominate bones).
Pelvis: A basin-shaped structure composed of the pelvic girdle, sacrum, and coccyx.
Bears the weight of the body.
Protects organs within the pelvic cavity.
Serves as an attachment point for the legs.
Each Hip Bone (Os Coxa): Composed of three parts fused in adults:
Ilium: The largest part of the hip bone.
Ischium: The part we sit on, featuring a posterior spine for muscle attachment.
Pubis: The anterior part of the hip bone.
These three bones meet at the acetabulum, the hip socket.
The two pubic bones are joined by a fibrocartilaginous joint called the pubic symphysis.
Male vs. Female Pelvis:
Female Pelvis: Iliac bones are more flared, the pelvic cavity is shallower, and the outlet is wider.
These adaptations facilitate the birthing process during vaginal delivery.
Lower Limb Bones:
Femur: The thigh bone, the longest and strongest bone in the body.
Its head articulates with the hip bone in the acetabulum.
A short neck positions the legs for walking efficiently.
Has two large processes for the attachment of thigh and buttocks muscles.
At its distal end, it articulates with the tibia of the leg at the knee region, involving the patella.
Patella: The kneecap.
Held in place by the quadriceps tendon, which continues as a ligament attaching to the tibial tuberosity.
Tibia: The larger, weight-bearing bone of the lower leg.
Fibula: The more slender bone in the lower leg.
Its head articulates with the tibia.
Foot Bones: The many bones of the foot provide flexibility, especially on uneven surfaces.
Ankle: Contains several tarsal bones.
Talus: One tarsal bone that moves freely where it joins the tibia and fibula.
Calcaneus: The heel bone, also part of the ankle.
The talus and calcaneus support the body's weight.
Instep: Formed by five elongated metatarsal bones.
The distal ends of the metatarsals form the ball of the foot.
Weakened ligaments binding the metatarsals can lead to flat feet.
Phalanges: Bones of the toes, similar to finger bones.
In the foot, phalanges are stout and extremely sturdy.
Articulations (Joints)
Bones are joined at joints (articulations).
Classification of Joints:
Fibrous Joints: Immovable, e.g., sutures between cranial bones.
Cartilaginous Joints: Slightly movable.
Connected by hyaline cartilage, e.g., costal cartilages joining ribs to the sternum.
Formed by fibrocartilage, e.g., intervertebral discs.
Synovial Joints: Freely movable; have several general classes.
Ligaments: Connect bone to bone and strengthen the joint.
Fibrous Joint Capsule: Formed by ligaments, surrounds the bones at the joint.
Lined with a synovial membrane that secretes synovial fluid to lubricate the joint.
Bursae: Fluid-filled sacs that ease friction between bone areas, overlapping muscles, or skin and tendons.
Menisci: C-shaped pieces of fibrocartilage between bones, providing added stability and acting as shock absorbers.
Types of Synovial Joints:
Ball and Socket Joints: Found at the hips and shoulders, allowing movement in all planes, including rotation.
Hinge Joints: Found at the elbow and knee, primarily permitting movement in one direction, similar to a door hinge.
Movements Permitted by Synovial Joints:
Skeletal muscles, attached to bones by tendons spanning joints, cause movement when they contract, moving one bone relative to another.
Bone Growth and Homeostasis
Bone Development Timeline:
Skeleton begins forming at about of embryonic development, when the embryo is approximately long.
Most bones grow in length and width through adolescence.
Some bones continue enlarging until about age .
Bone Remodeling:
Bones are living tissues capable of responding to stress by changing size, shape, and strength throughout a lifetime.
This continuous process of bone renewal is called remodeling.
Bone Repair: If a bone fractures, it can heal through bone repair.
Bone Cells: Involved in bone growth, remodeling, and repair:
Osteoblasts: Bone-forming cells.
Secrete the organic matrix of bone.
Promote the deposition of calcium salts into the matrix.
Osteocytes: Mature bone cells derived from osteoblasts.
Maintain the structure of the bone.
Reside within lacunae after being surrounded by calcified matrix.
Osteoclasts: Bone-absorbing cells.
Break down bone.
Assist in returning calcium and phosphate to the blood.
Throughout life, osteoclasts remove bone matrix while osteoblasts build it up; this dynamic balance constitutes bone remodeling.
Bone Development and Growth (Ossification)
Ossification: The process of bone formation.
Bones of the skeleton form in two distinct ways during embryonic development:
Intramembranous Ossification:
Forms flat bones, such as cranial bones of the skull.
Bones develop between sheets of fibrous connective tissues.
Connective tissue cells differentiate into osteoblasts in ossification centers.
Osteoblasts secrete the organic matrix (mucopolysaccharides and collagen fibrils), adding calcium salts to promote ossification.
Results in soft sheets or trabeculae of spongy bone.
Spongy bone remains inside the flat bone.
A periosteum forms outside the spongy bone, with osteoblasts from the periosteum carrying out further ossification.
Trabeculae fuse to form a compact bone bone collar surrounding the internal spongy bone.
Endochondral Ossification:
Forms most bones of the human skeleton.
Bone forms within a cartilage model (hyaline cartilage).
Cartilage is gradually replaced by calcified bone matrix, making bones capable of bearing weight.
Bone formation spreads from the center to the ends.
Steps of Endochondral Ossification:
Cartilage Model: In the embryo, chondrocytes lay down hyaline cartilage shaped like the future bones (cartilage models). As models calcify, chondrocytes die.
Bone Collar: Osteoblasts (derived from the newly formed periosteum) secrete organic bone matrix, which calcifies. This compact bone collar covers the diaphysis and thickens over time.
Primary Ossification Center: Blood vessels bring osteoblasts to the interior, initiating spongy bone formation in the diaphysis (bone shaft). This is the first center for bone formation.
Medullary Cavity and Secondary Ossification Centers: Osteoclasts absorb the spongy bone in the diaphysis, creating the medullary cavity. Shortly after birth, secondary ossification centers form in the epiphysis (ends of the bone), where spongy bone persists along with red bone marrow.
Epiphyseal Growth Plate (Growth Plate): A band of un-ossified cartilage remains between the primary and each secondary ossification center. This plate allows limbs to increase in length as long as it is present.
Layers of the Epiphyseal Plate:
Resting Zone: Nearest the epiphysis, cartilage remains.
Proliferating Zone: Chondrocytes produce new cartilage cells.
Degenerating Zone: Cartilage cells die off.
Ossification Zone: Bone is formed, causing the length of the bone to increase.
Bone Diameter Enlargement: As bones lengthen, their diameter enlarges.
Osteoblasts from the periosteum deposit new compact bone on the external surface.
Osteoclasts enlarge the medullary cavity from the inside, preventing bones from becoming too heavy and thick.
Final Size of Bones:
When epiphyseal plates close (ossify completely), bone lengthening ceases.
Epiphyseal plates typically close in females at about of age.
Epiphyseal plates typically close in males at about age .
Portions of other bone types may continue growing until age .
Hormonal Control of Bone Growth
Hormones (chemical messengers from endocrine glands) control the activity of the epiphyseal plate and bone growth.
Vitamin D:
Can be formed in the skin via sunlight exposure or consumed in the diet (e.g., fortified milk).
Converted to a hormone in the kidneys that acts on the intestinal tract.
Chief function: Intestinal absorption of calcium.
Deficiency in children can cause rickets, leading to bone deformities (e.g., bowed long bones).
Growth Hormone (GH):
Directly stimulates growth of the epiphyseal plate and bone growth in general.
Ineffective if cellular metabolic activity is not promoted (e.g., by thyroid hormone).
Too little GH in childhood: Dwarfism.
Too much GH in childhood: Excessive growth, leading to gigantism.
Excess GH in adults (after epiphyseal fusion): Acromegaly, causing excessive growth of bones in hands and face.
Thyroid Hormone: Promotes the metabolic activities of cells, necessary for effective growth hormone action.
Sex Hormones (e.g., Estrogen, Testosterone):
Increased levels during adolescence cause a dramatic growth spurt.
Stimulate osteoblast activity.
Rapid growth causes epiphyseal plates to be