Bone Lab

Bone Introduction

The skeletal system, which is made up of bone and cartilage, serves three primary functions:

• Mechanical support for sites of muscle attachment
• Protection of vital organs
• Ion reserve of Ca and PO for metabolic functions

Macroscopic Bone Structure

Long bones are composed of both cortical and cancellous bone tissue. They consist of several areas:

• The epiphyses are at the ends of the long bone and are the parts of the bone that participate in joint surfaces.

• The diaphysis is the shaft of the bone and has walls of cortical bone and an underlying network of trabecular bone.

  

  Types of Bone

  Bone is divided into two types that are different structurally and functionally. Most bones of the body consist of both types of bone tissue:
• Compact bone, or cortical bone, mainly serves a mechanical function. This is the area of bone to which ligaments and tendons attach. It is thick and dense.
• Trabecular bone, also known as cancellous bone or spongy bone, mainly serves a metabolic function. This type of bone is located between layers of compact bone and is thin and porous. Located within the trabeculae is the bone marrow.

Bone Cells and Matrix

Bone is a tissue in which the extracellular matrix has been hardened to accommodate a supporting function. The fundamental components of bone, like all connective tissues, are cells and matrix. There are three key cells of bone tissue: osteoblasts, osteocytes, and osteoclasts. They each have unique functions and are derived from two different cell lines.

Osteoblasts

Osteoblasts synthesize the bone matrix and are responsible for its mineralization. They are derived from osteoprogenitor cells, a mesenchymal stem cell line. Osteoblasts produce osteoid which is a matrix of type I collagen and glycosaminoglycans (GAGs). Eventually, the collagen fibers in osteoid become mineralized with calcium hydroxyapatite to produce bone.

Osteocytes

Osteocytes are inactive osteoblasts that have become trapped within the bone they have formed.

Osteoclasts

Osteoclasts break down bone matrix through phagocytosis. They are derived from the monocyte (macrophage) cell line. Osteoclasts produce acid to dissolve mineralized bone and enzymes, such as collagenase, that digest type I collagen. They are large, multinucleated cells.

Bone Development

Bone development begins with the replacement of collagenous mesenchymal tissue by bone. This results in the formation of woven bone, a primitive form of bone with randomly organized collagen fibers that is further remodeled into mature lamellar bone, which possesses regular parallel rings of collagen. Lamellar bone is then constantly remodeled by osteoclasts and osteoblasts. Endochondrial ossification and intramembraneous ossification are two different methods by which bone is produced from mesenchymal tissue:

Endochondrial Ossification

Endochondral ossification is the process by which cartilage is progressively replaced by bone at the epiphyseal growth plates. This occurs in long bones, the vertebrae, and the pelvis.

Intramembraneous Ossification

Intramembranous ossification is the process by which mesenchymal tissue is directly replaced by bone without an intermediate cartilage step. It occurs most notably in the bones of the skull.

Bone Remodeling

In adults, after growth has ceased, bone is formed by the osteoblasts only where it was previously resorbed by the osteoclasts. This follows a specific sequence of events, and takes about three months in total to complete.

Activation

In the process of activation, osteoblasts induce osteoclasts to break down bone matrix. This occurs via the Receptor Activator for NFkB-Ligand (RANK-L) signaling pathway, in which RANK-L on the surface of osteoblasts binds to RANK on osteoclasts to turn them on. This process lasts for approximately 3 days.

Resorption

In resorption, the ruffled border of the osteoclast forms a sealing zone which isolates the area of bone erosion. Organic acids and lysosomal enzymes dissolve the mineral component and break down the organic matrix, respectively. This process occurs at approximately 14 days.

Reversal

Over time, osteoblasts begin to replace osteoclasts at the site of bone turnover.

Formation

Osteoblasts begin to lay down new lamellar bone on top of old bone. In doing so, cement lines are created to mark the borders between old and new bone matrix. This can take up to 70 days to complete.

Virtual Microscope Slides

1. Microscopic Organization of Bone
Observe this slide at low magnification. The calcified bone is purple. The non-calcified elements are light blue. Find areas of compact bone and trabecular bone.
2. Endochondrial Ossification
This slide shows endochondral ossification, the process by which cartilage is calcified to form bone. Begin by distinguishing bone from the surrounding muscle tissue and from the cartilage epiphyses. Indicate the regions in which cartilage is being concerted into bone. Identify the specific areas where chondrocytes are resting, proliferating, maturing, undergoing hypertrophy, and calcifying.

Lab Quiz

1. Name the precursor to this cell.

Answer: This is an osteocyte, which is an osteoblast that has become encased in bone. The embryonic precursor to both of these is the osteoprogenitor cell, a mesenchymal stem cell.
2. Identify the labeled cells.

Answer: A = osteoclast; note multiple nuclei. B = osteoblast
3. What will these cells secrete?

Answer: Lysosomal hydrolases, collagenase, acid pH
4. What is the pale, blue-staining region?

Answer: Epiphyseal Plate
5. What type of bone formation is taking place?

Answer: Endochondral Ossification
6. What is the function of this cell and what is its embryological precursor?

Answer: This is an osteoclast, which resorbs bone. It's embryological precursor is a monocyte. In contrast, the precursors of osteoblasts are mesencyhmal stem cells.
7. This slide shows a resorption canal, where new bone is being laid down. Identify A, B, C, D and E

Answer: A = osteoblasts, B = osteocytes, C = osteoid, D = cement line E = bone