A rigid form of connective tissue which forms the skeleton of higher vertebrates. As with all connective tissues, there are three primary components of osseous tissue:
- Associated with bone formation, these cells are located where new bone is forming, e.g., in the periosteum.
- The cells vary in shape from cuboidal to pyramidal.
- Osteoblasts often appear stratified as in an epithelium.
- The nucleus is large with a single prominent nucleolus.
- The cytoplasm is marked by basophilia due to the presence of abundant ribosomes. These organelles are responsible for the synthesis of the proteins of the bony matrix.
- Osteoblasts contain the enzyme alkaline phosphatase used to calcify the osseous matrix.
- Basically, an osteoblast that has been enclosed within the bony matrix in a space called the lacuna.
- The single nucleus is darkly staining.
- The cytoplasm of the osteocyte is faintly basophilic containing fat droplets and granules of glycogen.
- The space occupied by the osteocyte, the lacuna, has an irregularly oval shape. Fine cytoplasmic processes of osteocytes travel some distance into the thread-like tunnels called canaliculi. The canaliculi extend between lacunae of osteocytes and the Haversian canals.
- In developing bone, the cytoplasmic processes from one osteocyte make contact with the processes from adjoining osteocytes. In mature bone, the processes are withdrawn almost completely.
- In mature bone the empty canaliculi remain as passageways for the diffusion of nutrients and wastes between bone and blood.
- Giant, multinuclear cells which vary greatly in shape.
- They are found on the surfaces of osseous tissue usually in shallow depressions called Howship’s lacunae.
- The cytoplasm is slightly basophilic and contains lysosomal vacuoles.
- Under E.M. the cell surface facing the osseous matrix shows numerous cytoplasmic projections and microvilli described as a ruffled border.
- It is thought that osteoclasts arise by fusion of uninucleated osteoprogenitor cells or from fused monocytes which emigrate from the blood.
- The osseous matrix facing the osteoclast appears demineralized. This and other studies have led to the assumption that the osteoclast is involved in bone resorption.
- When bone resorption activity ceases in a region of bone (e.g., following the healing of a fracture) osteoclasts are not apparent
Osseous Matrix - The osseous matrix is composed of two components:
- Organic Component – Chiefly osteocollagenous fibers
- These fibers are difficult to see in ground bone preparations.
- Fibers are held together with a special glue consisting of glycosaminoglycans (a glycoprotein). This organic material tends to be acidophilic due to the small amount of chondroitin sulfate present.
- The Inorganic Component – The inorganic component is principally calcium phosphate crystals.
- These crystals are deposited within the cement between the osteocollagenous fibers.
- Typically, bony matrix is deposited in layers or lamellae 3 to 7 um thick.
- The presence of discrete layers around the central canal is due to the fact that collagen fiber is laid down spirally in layers that are at 90 degrees of orientation to each other.
Architecture of Bone – There are two primary forms that bones take. In one case, the forms an elongated tube, e.g., a long bone like the femur. The second basic bone form is a flat plate as in the frontal bone. Each of these forms shows histological and developmental characteristics that are unique.
III. Typical Long Bone Architecture
Periosteum – a fibrous sheath covering a long bone’s shaft (diaphysis) but not the articulating surfaces. The periosteum consists of two layers:
- A dense fibrous, vascular, outer layer
- An inner more loosely arranged connective tissue containing:
- elastic fibers
- osteoblasts and osteoprogenitor cells for growth and repair
- coarse Sharpey’s fibers which anchor the periosteum to the underlying bone
- 3. Functions of the periosteum include:
a. Anchors tendons and ligaments to bone
- b. Allows passage of blood vessels, lymphatics and nerves into and out of the bone
- c. Participates in growth (appositional) and repair through the activities of the osteoprogenitor
Compact Bone – located around the diaphysis.
- Osseous tissue found immediately below the periosteum.
- The most significant feature of compact bone is the Haversian system.
- Each Haversian system consists of:
- Concentrically arranged lamellae of calcified matrix (each lamella is about 3 to 7 um thick).
- Hair - like canaliculi cross lamellae and connect the lacunae containing living osteocytes with a central Haversian canal.
- The longitudinal Haversian canals branch or anastomose freely with each other.
- Volkmann’s canals enter compact bone from the endosteal and periosteal surfaces and join the Haversian canals at right angles.
- Blood vessels, lymphatics and nerves pass through Volksmann’s canals, Haversian canals and all of their branches (anastomoses). As a result, there is a very extensive transport system within compact bone.
Spongy Bone – is commonly seen within the epiphyses and immediately beneath the endosteum of the diaphyses of long bones. It is also found sandwiched between two layers of compact bone in a flat bone.
- Consists of trabeculae or plates forming an interconnected network.
- The trabeculae are comprised of a varying number of lamellae with lacunae containing osteocytes.
- In prenatal spongy bone and in regions of regenerating bone, the lamellae cannot be distinguished since the osteocollagenous fibers are arranged in an irregular fashion. This is called woven bone.
- In older spongy bone, the fibers between the lamellae are arranged at 90 degrees to each other. This arrangement allows the lamellae to be distinguished from one another.
- Hematopoietic (blood-forming) tissue can be found within the spaces of spongy bone especially within the epiphyses.
Endosteum – a delicate layer lining the marrow cavities, as well as, Haversian and Volksmann’s canals.
1. It is a condensed reticular tissue in the adult.
2. It is believed to contain both osteoprogenitor and hematoprogenitor cells
IV Development and Growth of Bone – Bone has special characteristics which will influence how bone grows and develops.
- The solid, mineralized matrix of bone is permeated with an extensive system of canaliculi which
extend from each lacuna to the surfaces of bone and the canal system. The fluid in these spaces and
surfaces permit an exchange of metabolites and gases between the blood system and the bone cells.
This allows the survival of osteocytes even though they are encased in a bony capsule.
- Bone is a richly vascular tissue. At a distance of 0.5 mm, the canalicular system becomes highly inefficient in supplying osteocytes with food and oxygen. The extensive network of Haversian and Volkmann’s canals bring the blood close enough to the osteocytes so that the diffusion distance through the canaliculi does not exceed 0.5 mm for any osteocyte.
- Bone can only grow appositionally, i.e., a process in which new bone is added to one surface. Bone cannot grow interstitially, from within by the division of bone cells because of the presence of rigid calcium salts within the matrix.
- The architecture of bone is not static. Bone is constantly being destroyed and reformed in different regions simultaneously. It is a constantly reforming tissue.
Intramembranous Bone Formation – This is a process of osteogenesis which leads to the formation of flat bones.
- This process begins within flat sheets of mesenchyme. Mesenchyme may be considered to be the progenitor of all connective tissues.
- The mesenchyme cells within the sheet are connected to each other by their cytoplasmic processes.
- In the middle of the sheet, mesenchymal cells begin to different into osteoblasts.
- A dense fibrous substance begins to surround the osteoblasts. This material is called osteoid.
- The osteoblasts begin to secrete alkaline phosphatase which causes the deposition of calcium salts on the collagenous fibers in the osteoid.
- As calcified matrix is deposited around the osteoblasts and their processes, lacunae and canaliculi are formed. Since the cytoplasmic processes of the adjoining osteoblasts were contiguous (touching), the canaliculi of these osteoblasts become connected.
- The bone forming in the middle of the sheet is woven spongy bone. Vascular tissue fills the spaces between the trabeculae and becomes hematopoietic.
- On the outer surfaces of the sheet, a fibrous periosteum forms over a layer of osteoblasts. As new osteoblasts form from the underside of the periosteum, appositional growth occurs. The older osteoblasts become surrounded by matrix and transform into osteocytes.
- The bone forming on both outer surfaces of the sheet becomes compact. Between these compact plates, spongy bone (diploe) remains. Within it blood forming tissue can be found.
Intracartilagenous Bone Formation – This type of ossification uses the hyaline cartilage "skeleton" of the fetus as the model upon which the bones will form. This process begins during the third month of gestation and typically in the long bones.
In the center of the shaft, chondrocytes enlarge and secrete alkaline phosphatase. This causes the deposition of calcium salts in the matrix. The calcified matrix is intensely basophilic in stained sections.
The calcified matrix around these chondrocytes prevents the cells from receiving nutrients and oxygen. As a result they die.
As the chondrocytes die, their lacunae begin to coalesce forming a cavity in the middle of the diaphysis. This is the early primary marrow cavity.
At the about the same time, osteoblasts differentiate within the fibrous perichondrium. These cells begin to form a thin collar of bone around the circumference of the shaft called the periosteal collar. As soon as bone begins to form, the perichondrium becomes a periosteum. The periosteal collar compensates, structurally for the hollowing of the shaft.
The periosteal bud, a conglomerate of blood vessels, nerves, lymphatics, osteoblasts and osteoclasts breaks through the periosteum and enters the primary cavity within the diaphysis.
This incoming osteogenic tissue forms osteoid and bone on the calcified cartilage trabeculae remaining within the primary cavity. This region is now referred to as the primary ossification center.
Within this center, the woven spongy bone with a calcified cartilage core is reabsorbed by the osteoclasts forming the primary medullary cavity which rapidly fills with hematopoietic tissue.
The fibrous, nonmineralized lining of the medullary cavity is the endosteum. Osteoblasts form in the endosteum and begin the formation of endosteal bone. The appositional growth of endosteal bone is closely regulated to prevent closure of the primary marrow cavities and destruction of bone marrow.
Bi-directional Ossification – Following the formation of the primary ossification center, bone formation extends towards both ends of the bone from the center of the shaft.
- Cartilage cells on the leading edges of ossification are dying.
- Osteoblasts cover the cartilagenous trabeculae with woven, spongy bone.
- Behind the advancing front of ossification, osteoclasts are absorbing the spongy bone and enlarging the primary marrow cavity.
- The periosteal collar thickens and extends toward the epiphyses to compensate for the continued hollowing of the primary cavity.
Increase in Length of the Cartilage Model – The dramatic increase in the length of the cartilage model during the remainder of gestation depends on growth processes taking place in the epiphyses of the developing bone.
- The hyaline cartilage within the epiphyses shows little or no indication of growth, i.e., mitosis. This is referred to as the Quiescent or Reserve Zone.
- Moving a little closer to the diaphysis you begin to see chondrocytes that have undergone numerous mitoses. In this Zone of Proliferation, the dividing chondrocytes have aligned themselves in distinct rows or columns parallel with the long axis of the bone. The cells within the row are crowded, flattened and separated by very little matrix. Each row grows by the addition of new cells at the distal end of the row.
- Moving further from the epiphysis toward the diaphysis, the chondrocytes in the row enlarge becoming cuboidal in shape. The cytoplasm of these cells accumulates glycogen. This region is referred to as the Maturation Zone. The zones of proliferation and maturation account for most of the increase in length seen during this period.
- At the diaphyseal end of the maturation zone, the chondrocytes begin to release alkaline phosphatase. This produces a deposition of calcium salts around the chondrocytes. The matrix in this Zone of Calcification is intensely basophilic.
- The calcification of the cartilage leads to the death of the chondrocytes and the breakdown of the the delicarte strip of matrix between the lacunae within the row. This produces the characteristic "honeycomb" appearance of this Zone of Retrogression. Marrow tissue from the primary marrow cavity moves into these spaces.
- In the Zone of Ossification, osteoblasts differentiate from marrow mesenchyme. They begin the process of bone formation on the framework of the remnants of calcified cartilage in the extremities of the diaphysis.
- The woven spongy bone which has formed in the zone of ossification will be reabsorbed by the progression of osteogenic tissue from the diaphysis. The diaphyseal marrow cavity extends into this Zone of Reabsorption. This completes the formation of the diaphyseal marrow cavity, now called the secondary marrow cavity.
Formation of Secondary Centers of Ossification (Epiphyseal centers) – At about the time of birth, secondary centers of ossification appear in both ends of most long bones.
Epiphyseal chondrocytes proliferate and hypertrophy.
Mature chondrocytes secrete alkaline phosphatase. The resulting deposition of calcium salts leads to the death of the chondrocytes.
Vascular osteogenic buds enter the epiphysis from the exterior of the bone or from the diaphysis.
The processes of osteogenesis and reabsorption occur in all directions replacing most of the hyaline cartilage in the epiphysis with spongy bone. The spaces between the trabeculae become filled with marrow tissue.
This red marrow remains an active center of hematopoiesis throughout life. The red marrow of the secondary marrow cavity in the diaphysis loses its hematopoietic function as it becomes replaced with adipose tissue (yellow marrow).
Hyaline cartilage remains intact in two regions of the epiphysis:
- Over the free ends of the bone as articular cartilage.
- Within the epiphyseal plate which forms between the epiphysis and the diaphysis.