Histology Mentor

1. Lamella is a layer consisting of an organic (35%) and an inorganic (65%) component.

2. Osteocytes

1. Circumferential - inner and outer, are on inner and outer surfaces of bone; usually incomplete.

2. Concentric - 8 to 15 lamellae surrounding neurovascular (haversian) canals. canal + lamellae = haversian system or osteon.

3. Interstitial lamellae are fragments of prior osteons or circumferential lamella - indicative of the constant turnover of bone.

4. Trabeculae or spicules have small segments of lamellae but usually no osteons - spongy (cancellous) bone.

1. Periosteum - dense irregular fibrous connective tissue sheath surrounding the bone. The layer next to the bone is highly vascular, contains many nerves, and is the location of osteoprogenitor cells or preosteoblasts, which can be activated in the event repair or remodeling is needed. Thick collagen fibers from ligaments and tendons penetrate the periosteum and become embedded in the underlying bone, where they are known as Sharpey's fibers.

2. Endosteum - a very thin layer of vessels and connective tissue cells, including osteoprogenitor cells, lining the marrow cavity and osteonic canals. These are largely separated from the bone surfaces during slide preparation. Look for bone trabeculae and fat cells mingled with developing blood cells in a section of rib; this is the picture of red marrow.

3. Osteonic (Haversian) canals consist of loose connective tissue, blood vessels, some lymph vessels and vasomotor nerves. Osteoprogenitor cells are found along the surface of the bone surrounding the canal. Bone is organized so that few osteocytes are more than 200 microns from a blood vessel. Blood vessels enter the osteonic canals from the marrow as well as from the very vascular periosteum via transversely oriented canals, which interconnect osteonic canals and are known as Volkmann's or interosteonic canals.

1. Produce organic matrix - osteoid. In young bone, collagen is irregularly arranged (woven bone). In mature bone, collagen is regularly oriented in layers with differing orientations (lamellae).

2. Facilitate mineralization - lags behind the formation of osteoid. These are overlapping sequential processes. In bone formation, osteoblasts differentiate on a surface and begin forming osteoid. Then mineral is gradually deposited in the matrix at the expense of water. The alkaline phosphatase of osteoblasts is thought to participate in this mineralization process. The two classes of bone formation are simply based on two kinds of surfaces on which osteoblasts differentiate. Osteoblasts differentiate from preosteoblasts (osteoprogenitor cells), which are associated with the periosteum and endosteum.

Osteoblasts differentiate from osteoprogenitor cells and begin to function on collagen bundles in a connective tissue membrane (e.g. skull). The collagen is irregularly arranged at first, and this primary bone is called woven bone; it becomes lamellar bone when remodeling takes place.

Osteoblasts differentiate on longitudinal remnants of a cartilage model as its horizontal components disintegrate. The process begins first in the diaphysis or shaft, with the formation of a bone collar along with hypertrophy and fusion of chondrocyte lacunae in the cartilaginous diaphysis. A blood vessel penetrates this site bringing with it cells which facilitate the cartilage erosion and the deposition of bone matrix on the cartilage remnants. This is the primary ossification center. Then the epiphyses (secondary ossification center) begin to be replaced.

1. Zone of resting cartilage

2. Zone of proliferation

3. Zone of maturation and hypertrophy

4. Zone of hypertrophy and calcification

5. Zone of ossification (the metaphysis)

The epiphyseal plate "closes" at puberty.

The calcium ion concentration of the body fluids is normally controlled within narrow limits. Bone acts as a buffer for calcium so that the moment to moment control of calcium concentration is determined by the rate at which calcium enters and leaves bone. These rates are greatest and the most susceptible to control by parathormone, 1, 25 dihydroxy vitamin D, and calcitonin, in remodeling units and young osteons. To make room for the formation of new osteons, old osteons that have the highest concentration of mineral and the lowest of water are constantly being replaced. The process by which osteoclasts form an erosion tunnel followed by the formation of a new osteon is called remodeling. Remodeling occurs both inside compact bone as well as at bone surfaces. Wherever new bone is laid down on an eroded surface, a thin layer of cementing substance is first produced. This thin layer is called a cementing or reversal line. It is highly mineralized and contains little collagen. Since both erosion and formation proceed simultaneously but are separated in space, a three-dimensional structure can be visualized, called the remodeling unit. Erosion proceeds at one end of the unit, and bone formation at the other end. About 10% of bone turns over each year.

1. Source - myeloid tissue having a common ancestor with monocytes. Osteoclasts are large, multinucleated cells with numerous mitochondria. When active they are attached to the bone surface in an annular region called the clear zone that surrounds the ruffled border.

2. Factors that regulate recruitment

3. Attachment to bone in regions of clear zone by membranous integrin binding to bone matrix osteopontin & osteonectin

4. Matrix digestion - achieved by secreting protons through the ruffled border using the lysosomal proton pump and lysosomal enzymes by exocytosis.

5. Calcitonin receptors - activation of these causes the osteoclast to release its attachment to bone and cease to function. This leads to a decrease in blood calcium levels.

1. Source - stromal cells

2. Secrete organic matrix - collagen Type I, calcium binding proteins osteocalcin, osteopontin, osteonectin), growth factors (transforming growth factor, insulin-like growth factor. They have a well developed rough endoplasmic reticulum and Golgi apparatus.

3. Promote mineralization