Lecture 11 - Bone Development and Calcium Homeostasis

Osteogenesis

Bone formation (fetal and early development)

Ossification

Process of replacing other tissue with bone

Calcification

Process of depositing calcium salts. Occurs during bone ossification

What are the two main forms of ossification?

1. Endochondral

2. Intramembranous

Define endochondral ossification and its process

Ossifies bone that originate as hyaline cartilage

1. Central chondrocytes enlarge and then DIE, leaving cavities within the cartilage

2. Blood vessels grow around the cartilage and perichondrium differentiates into osteoblasts

3. Blood vessels penetrate and fibroblasts differentiate into osteoblasts and produce spongy bone

4. Growth continues into medullary cavity. Epiphysis is replaced by bone shaft. Length and diameter increases

5. Capillaries and osteoblasts migrate to epiphysis and create secondary epiphysis centers

6. Epiphysis is filled with spongy bone. Epiphysis and diaphysis is split

7. Epiphyseal cartilage production slows, and the rate of osteoblast activity ACCELERATES

Epiphyseal closure

Epiphyseal cartilage gets narrower until it disappears

Appositional growth

Compact bone thickens and strengthens long bone with layers of circumferential lamellae

Epiphyseal growth

When long bone stops growing after puberty

1. Epiphyseal cartilage disappears

2. Epiphyseal line appears

Mature bones

Osteoblasts enlarge medullary cavity. Osteons form around the blood vessels in compact bone

Intramembranous ossification definition and process

Occurs in the dermis and produces the dermal bones

1. Mesenchymal cells cluster and form osteoblasts. They secrete organic components of matrix. The osteoid becomes mineralized with calcium salts, forming bone matrix

2. Some osteoblasts are trapped in bony pockets and differentiate into osteocytes, developing bone outward

3. Blood vessels branch within the region and bone growth accelerates with oxygen

4. Deposition of bone by osteoblasts located close to blood vessels results in a plate of spongy bone with blood vessels weaving throughout

5. Remodeling around blood vessels produces osteons. Osteoblasts on bone surface with connective tissue become periosteum

Process of remodeling

1. Maintains itself

2. Replaces mineral reserves

3. Recycles and renews bone matrix

4. Involves osteocytes, osteoblasts, and osteoclasts

Factors effective on bone remodeling

• Exercise

• Nutrition

• Hormone calcitriol

• Growth hormone, thyroxine

• Calcitonin and parathyroid hormone

• Estrogens and androgens

What are calcium ions vital to

• membranes

• Neurons

• muscle cells, especially heart cells

How is homeostasis maintained in the context of calcium?

Calcitonin and parathyroid hormone control storage, absorption, and excretion

Parathyroid hormone

Produced by parathyroid glands in the neck. INCREASES calcium ion levels by

• Stimulating osteoclasts to release stored calcium

• Increasing intestinal absorption of calcium

• Decreasing calcium excretion in the kidneys

Calcitonin

Secreted by parafollicular cells (C cells) in the thyroid. DECREASES calcium ion levels by:

• Inhibiting osteoclast activity while osteoblasts lock calcium in the bone matrix

• Intestinal absorption of calcium decreases

• Kidneys allow calcium loss

Perforating fibers

Collagen fibers of the periosteum connects with collagen fibers in the bone and with fibers of joint capsules. Attach tendons and ligaments to bone

Functions of the periosteum

1. Isolates bone from surrounding tissues

2. Provides a route for circulatory and nervous supply

3. participates in bone growth and repair

Endosteum

• Contains osteoblasts, osteoprogenitor cells, and osteoclasts

• Active in bone growth and repair

What is included in the blood supply of mature bones?

• Nutrient artery and vein

• Metaphyseal vessels

• Periosteal vessels

Nutrient artery and vein

A single pair of large blood vessels that enter the diaphysis through the nutrient foramen

Metaphyseal vessels

Supplies the epiphyseal cartilage. Where bone growth occurs

Periosteal vessels

Blood to superficial osteons. Secondary ossification

Fracture repair steps

1. Bleeding - Produces a clot (fracture hematoma). Establishes fibrous network. Bone cells in the area die

2. Cells of endosteum and periosteum divide and migrate into the fracture zone. External and internal calluses stabilize the break in bone

3. Osteoblasts replace central cartilage and external callus with spongy bone

4. Osteoblasts and osteocytes remodel the fracture for up to a year, reducing bone calluses

Effects of aging on bones

• Epiphysis, vertebrae, and jaw are the most affected

• Fragile limbs

• Height reduction

• Tooth loss

Osteoporosis

Severe bone loss that affects normal function

Hormones and bone loss

• Estrogens and androgens help maintain bone mass

• Bone loss in women accelerates after menopause

Cancer and bone loss

The osteoclast activating factor stimulates osteoclasts and produces severe osteoporosis. This is released by cancerous tissue