[MOD4LEC] Skeletal System And Joints

How many bones does the skeletal system consist of?

206± articulated bones and various ligaments and cartilages

At birth, there are 270 bones. They later fuse to 206, including examples like the os coxa

What are functions of the skeletal system?

• Provides a supporting framework for the body

• Protects softer tissues like the brain and lungs

• Provides movement in conjunction with muscles and nerves

• Serves as the primary mineral storage site, such as calcium and phosphate ions

• Works as a buffer against excessive changes in pH

• Produces blood cells

What the three types of cartilage?

• Hyaline cartilage - the most common type of cartilage

• Elastic cartilage - similar to hyaline cartilage, but with more elastin and less collagen

• Fibrocartilage - similar to hyaline cartilage, but with more collagen and less elastin

Hyaline cartilage [Types Of Cartilage]

Made up of chondrocytes, which are located within lacunae

• Lacunae are spaces in cartilage where chondrocytes live

• Surrounded by abundant matrix

• Abundant collagen, ground substance, and fewer elastin

Hyaline cartilage serves as a framework, protects tissues, and supports body parts

What are functions of hyaline cartilage?

Serves as a framework, and is the precursor to bone within the fetal skeleton and the growth of long-bones

• Protects tissues like costal cartilage and joints

• Supports body parts like the larynx

Hyaline cartilage heals slowly because it has a poor blood supply

What are some locations of hyaline cartilage?

• Tip of the nose

• Trachea and bronchi

• Ends of long bones at joints

Most of the fetal skeleton begins as hyaline cartilage

Hyaline cartilage is covered by a tissue sheath known as the periochondrium

Elastic cartilage [Types Of Cartilage]

Similar to hyaline cartilage but with more elastin and less collagen

What is the function of elastic cartilage?

Serves as a framework for body parts

What are some locations of elastic cartilage?

• External ears

• Larynx

Fibrocartilage [Types Of Cartilage]

Similar to hyaline cartilage but with more collagen and less elastin

What are functions of fibrocartilage?

• Shock absorption

• Acts as a protective cushion

What are some locations of fibrocartilage?

• Intervertebral disks

• Menisci

• Pubic symphysis

What is osseous tissue?

Compact bone and spongy bone

Compact bone provides bones with shape and structure. Spongy bone lessens the weight of the compact bone of the skeleton, and its red bone marrow produces blood cells

What structures is compact bone comprised of?

• Osteocytes, which reside within lacuna

• Lacuna are structured in concentric rings called lamellae

• Lamellae encircle a central canal

An osteon refers to an instance of lamellae encircling a central canal

What type of substance is abundant in the matrix of compact bone? What type of substance is abundant in the ground substance of compact bone?

Collagen is abundant in the matrix of compact bones

Calcium phosphate (CaPO₄) and calcium carbonate (CaCO₃) are abundant in the ground substance of compact bone

What structures is spongy bone comprised of?

• Trabeculae in sponge-like arrangement

• Red bone marrow, which produces blood cells

Spongy bone is located in compact bone, within the epiphysis of long bones

Synovial membranes

Some joints of the skeletal system are lined by fibrous synovial membranes, made only of connective tissue

• Span the gap from one bone to the next and secret synovial fluid into the joint

Name some cancers and where they originate

• Carcinomas - originate in epithelial tissue, and account for approximately 90% of all cancers

• Sarcomas - originate in connective tissue like bone and muscle, and account for approximately 10% of all cancers

• Lymphomas - originate in the lymph nodes

• Melanomas - originate in pigment cells of the epidermis

• Leukemias - originate in blood-forming tissues such as bone marrow

Osteogenic cells [Types Of Bone Cells]

Stems cells that develop from embryonic mesenchymal cells

• Give rise to most other bone cell types

• Found in the endosteum and inner layer of the periosteum

• Multiply continually to become osteoblasts

Osteoblasts [Types Of Bone Cells]

Bone-forming cells that form a single layer on the bone surface under the endosteum and periosteum

• Resemble a cuboidal epithelium

• Nonmitotic; new ones can only be generated by mitosis and differentiation of osteogenic cells

• Synthesize the soft organic matter of the bone matrix, and further hardens through mineral deposition

• Stress and fractures stimulate osteogenic cells to multiply more rapidly, thus generating increased numbers of osteoblasts that reinforce or rebuild the bone

Osteoblasts have an endocrine function

Osteocytes [Types Of Bone Cells]

Former osteoblasts that have become trapped in the matrix as a result of being deposited

• Reside in tiny cavities called lacunae

• Distributed in lamellae (concentric rings) around a central canal

• Reach into the canaliculi to contact the processes from neighboring osteocytes

• Neighboring osteocytes can pass nutrients and chemical signals to one another

• Pass their metabolic wastes to the nearest blood vessel for disposal

Name functions of osteocytes

• Resorption of the bone matrix

• Deposition of materials into the bone matrix

• Work as strain sensors, that sense adjustments in bone shape and density to adapt to stress

Osteocytes contribute to homeostasis of bone density and blood concentrations of calcium and phosphate ions

Osteoclasts [Types Of Bone Cells]

Bone-dissolving cells on the surfaces of bones

• Develop from the same bone marrow stem cells that give rise to the blood cells

• Each osteoclast is formed by the fusion of several stem cells

• Bone remodeling results from the combined action of bone-dissolving osteoclasts and bone-depositing osteoblasts

Osteogenic cells, osteoblasts, and osteocytes all belong to one cell lineage. However, osteoclasts have an independent origin

What structures does bone matrix consist of?

• Proteins - mostly collagen fibers, synthesized by osteoblasts

• Ground substance - 85% crystalized calcium phosphate (CaPO₄) and 10% calcium carbonate (CaCO₃)

• In lamellar arrangement around central canal

What structures do central canals consist of?

• Blood vessels

• Nerves - serve cells and matrix

Perforating canals connect juxtaposed central canals

What structures does spongy bone consist of?

• Spicules - thin rods or spines

• Trabeculae (thin bony plates)

• Red bone marrow

Red bone marrow produces blood cells

Where is yellow bone marrow located?

The medullary cavity of the diaphysis (shaft) of long bones

What categories of bones are there?

• Long bones - humerus, femur, metacarpals, metatarsals

• Short bones - carpals, tarsals

• Flat bones - sternum, scapulae, ribs

• Irregular bones - vertebrae, sphenoid bone

• Miscellaneous bones - sesamoid bones

• Wormian or sutural bones - occur between sutures of cranial bones

Tori are extra bones in the mouth, which are elevations that create a hard visible mass in the mouth

Describe the process of osteogenesis (bone development)

Begins at the first few weeks of prenatal development, concluding around 25 to 30 years old

1. Intramembranous ossification - mesenchyme (undifferentiated connective tissue) is transformed into spongy and compact bone by osteoblasts

2. Endochondral ossification - hyaline cartilage, such as in a newborn baby, is transformed into compact or spongy bone

Primary ossification center; longitudinal growth at center of diaphysis

Secondary ossification center; growth center at epiphyseal plate

Some bones consist of both intramembranous and endochondral ossification; clavicles, mandibles, facial bones

What factors affect bone growth, development, and maintenance?

Vitamin A, C, and D are necessary for bone growth, development, and maintenance

• Vitamin A - normal bone degradation and resorption

• Vitamin C - normal synthesis of collagen for matrix

• Vitamin D (calcitriol) - is actually a hormone, but is referred to as vitamin D; principal function is to raise the blood calcium concentration

In what ways does vitamin D raise the blood calcium concentration?

1. It increases calcium absorption by the small intestine

2. It increases calcium resorption from the skeleton. Vitamin D binds to osteoblasts, which release a chemical messenger that causes osteoclast-producing stem cells to differentiate or transform into osteoclasts. The new osteoclasts then liberate calcium and phosphate ions from bone

3. It weakly promotes the reabsorption of calcium ions by the kidney, so less calcium is lost in the urine

How is vitamin D produced?

1. Epidermal keratinocytes use ultraviolet radiation from sunlight to convert the steroid 7-dehydrocholesterol into previtamin D_3, which is carried to the bloodstream by a transport protein

2. The liver adds a hydroxyl group to the molecule and converts it into calcidiol

3. The kidney adds another hydroxyl group, converting calcidiol into calcitriol, the most active form of vitamin D