Skeletal System Histology & Ossification

What are the three main cartilages?

1. hyaline cartilage

2. elastic cartilage

3. fibrocartilage

Which cartilage is the most prominent/abundant?

hyaline cartilage

What are lacunae (cartilage)?

cavities/space in the cartilage where the chondrocytes are located

What are some general characteristics of cartilage?

• avascular

• nutrients come from diffusion from the matrix

• matrix is predominantly water and proteoglycans

What are chondrocytes?

mature cells in the lacunae

What are chondroblasts?

Immature cartilage cells that become chondrocytes

What does hyaline cartilage form?

• primitive vertebrate skeleton (lampreys, sharks)

• fetal skeletons

• articular surfaces of bones

• tracheal rings

• costal cartilages

• larynx

What does elastic cartilage make up?

external ear, epiglottis

Where is fibrocartilage found?

• intervertebral discs

• ligament and tendon insertions

Describe the intervertebral discs

• vertebral joint - no space between the vertebrae

• composed of the:

  • annulus fibrosus: fibrocartilage ring

  • nucleus pulposus: center

Describe endochondral ossification

• encompasses the development of long bones

• occurs at epiphyseal growth plates/epiphyseal lines

• bone replaces hyaline cartilage

What are the steps of endochondral ossification?

1. matrix mineralization

2. chondrocyte death

3. blood vessel penetration with osteogenic cells

4. osteoblasts lay down bony matrix

What is ossification regulated by?

hormones

What determines the length of the long bones?

the zone of ossification

Describe the general process of intramembranous ossification.

• occurs in the skull and other flat bones

• no hyaline cartilage is present

• happens via sheets of undifferentiated connective tissue

• osteoclasts clump together in trabeculae and become osteocytes

• connective tissue is mineralized into lamellar bone

When comparing histology of endochondral and intramembranous ossification, how can you tell the difference?

Endochondral ossification appears more organized, with zones of ossification occurring at epiphyseal plates. Intramembranous ossification is not as organized.

Diaphysis

the shaft/compact bone of a long bone

Epiphysis

the enlarged ends of the long bones; spongy bone

Periosteum

the fibrous membrane covering lone bones

Articular cartilage

thin cartilage that covers epiphyseal surfaces

Medullary cavity

marrow cavity inside the diaphysis

Endosteum

the membrane lining the medullary cavity

Degradation of what cartilage of the long bones leads to arthritis?

articular cartilage

Haversian canal

longitudinal canal in the center of a Haversian system; disperses nutrients

Haversian system

a circular region of the bone

Lamellae

concentric rings of calcified matrix around a Harversian canal, connected by canaliculi

Lacunae (bone)

cavities between lamellae that contain bone cells

Osteocytes

mature bone cells in the lacunae

Canaliculi

small canals that radiate from the Haversian canals to connect lacunae; helps with nutrient dispersion

Osteoclasts

remove bone matrix during bone remodelling

Osteoblasts

lay down new lamaellae for minerals (Ca) to bind

Describe bone remodelling

Osteoclasts remove bone matrix while osteoblasts lay down new lamella for minerals to bind to. This leaves a central Haversian canal in the center of the area of remodeling.

What regulates bone remodeling?

balance regulated by parathyroid hormone from the thyroid