BONE TISSUE: CHARACTERISTICS, STRUCTURE, FORMATION AND GROWTH - BIOLOGY - 2025

The bone tissue is one that consists bones. Bone, along with enamel and dentin, are the hardest substances in the body of animals. Bones make up the structures that protect vital organs: the brain is protected by the skull, the spinal cord by the spinal column, and the heart and lungs are protected by the rib cage.

The bones also serve as “levers” for the muscles that are inserted in them, multiplying the force that those muscles generate during the execution of the movements. The rigidity provided by the bone allows locomotion and the support of loads against gravity.

Bone tissue cells (Source: OpenStax College via Wikimedia Commons)

Bone is a dynamic living tissue that is constantly changing and these changes are stimulated by the pressure and stresses to which this tissue is subjected. For example, pressure stimulates resorption (destruction) and stress can stimulate new bone formation.

Bones are the main deposit of calcium and phosphorus in the body: almost 99% of the total calcium in the human body is stored in bone tissue. Total bone mass varies throughout an animal's life. During the growth phase, bone formation overcomes resorption (destruction) and the skeleton grows and develops.

Initially it increases its length and then its thickness, reaching its maximum between 20 and 30 years in humans. In adults (up to about 50 years) there is a balance between bone formation and resorption.

This balance is given by a replacement process known as "bone remodeling" and that affects, per year, 10% to 12% of total bone mass. Subsequently, a degenerative process begins in which resorption exceeds formation and bone mass slowly decreases.

Characteristics and structure

The bone has a central cavity called the medullary cavity, which houses the bone marrow, a hematopoietic tissue, that is, a tissue that forms blood cells. These structures are covered by periosteum, except for the areas corresponding to the synovial joints.

The periosteum has an outer layer of dense fibrous connective tissue and an inner layer with osteogenic cells, which are bone-forming cells or osteoprogenitor cells.

The central part of the bone is lined by a monolayer of cells of thin, specialized connective tissue called the endosteum. The endosteum has osteoprogenitor cells and osteoblasts. The bone thus covered, has its cells integrated in a calcified extracellular matrix.

Osteoprogenitor cells differentiate into osteoblasts and are in charge of the secretion of the bone matrix. When surrounded by matrix, these cells are inactivated and are called osteocytes.

The spaces that osteocytes occupy in the matrix are called gaps.

90% of the organic matrix is ​​made up of type I collagen fibers, a structural protein also present in tendons and skin, and the rest is a homogeneous gelatinous substance called the fundamental substance.

Compact bone and cancellous bone

The collagen fibers of the matrix are arranged in large bundles and, in compact bone, these fibers form concentric layers around the channels through which the blood vessels and nerve fibers run (Haversian channels). These layers form cylinders known as "osteons".

Each osteon is delimited by a cementation line formed by calcified fundamental substance with few collagen fibers and is nourished by the vessels that are in the Haversian canals.

Large surface plaques or spicules form in cancellous bone and cells are nourished by diffusion of extracellular fluid from the bone into the trabeculae.

The inorganic components of the matrix constitute around 65% of the dry weight of bone and are mainly made up of calcium and phosphorus, in addition to some elements such as sodium, potassium, magnesium, citrate and bicarbonate, among others.

Calcium and phosphorus are found to form hydroxyapatite crystals. Calcium phosphate is also found in amorphous form.

The hydroxyapatite crystals are arranged in an orderly way along the type I collagen fibers, which are overlapping concentrically, which also makes the crystals overlap like bricks of a wall.

Bone formation and growth

The bones of the skull are formed by a process known as "intramembranous ossification." In contrast, long bones are first shaped into cartilage and then transformed into bone by ossification, which begins at the shaft of the bone and is called "endochondral ossification."

Most flat bones develop and grow by intramembranous bone formation and ossification. This process occurs in highly vascularized mesenchymal tissue, in which mesenchymal cells differentiate into osteoblasts that begin to produce bone matrix.

This is how a network of spicules and trabeculae is formed, whose surfaces are populated by osteoblasts. These regions of initial osteogenesis are called the primary center of ossification. This is how the primary bone is formed with randomly oriented collagen fibers.

Calcification then occurs and the osteoblasts trapped in the matrix become osteocytes, the processes of which give rise to canaliculi. As the trabecular networks form like a sponge, the vascular connective tissue gives rise to the bone marrow.

The addition of peripheral trabeculae increases the size of the bone. In the occipital bone (a cranial bone in the posterior area) there are several ossification centers that fuse together to form a single bone.

In newborns, the fontanelles between the frontal and parietal bones are areas of ossification that have not yet fused.

Compact bone formation

The regions of mesenchymal tissue that remain uncalcified in the internal and external portions will form the periosteum and endosteum. The areas of cancellous bone near the periosteum and dura will become compact bone and form the inner and outer table of flat bone.

During growth, in long bones, specialized areas in the epiphyses are separated from the diaphysis by a highly active plate of cartilage called the epiphyseal plate.

The length of the bone increases as this plate deposits new bone at each end of the shaft. The size of the epiphyseal plate is proportional to the rate of growth and is affected by various hormones.

Regulation

Among the hormones that modulate this plaque is growth hormone (GH) released by the anterior pituitary and regulated by growth hormone-releasing hormone (GRH), produced by the hypothalamus, and by a somatomedin, which is a factor of type I insulin-like growth (IGF-I) produced by the liver.

As long as the rate of mitotic activity in the proliferation zone is similar to the rate of bone resorption in the zone, the size of the epiphyseal plate remains constant and the bone continues to grow.

After 20 years of age, mitotic activity decreases and the ossification zone reaches the cartilage zone, joining the medullary cavities of the diaphysis and the epiphyses.

Longitudinal bone growth ends when epiphyseal closure occurs, that is, when the diaphysis joins the epiphysis. Epiphyseal closure follows an orderly time sequence ending with the last closure after puberty.

The growth in width of the long bone is produced by appositional growth, which is the product of the differentiation of the osteoprogenitor cells of the inner layer of the periosteum into osteoblasts that secrete bone matrix towards the subperiosteal areas of the diaphysis.

Bone remodeling

Throughout the life of a human being, the bone is in constant change through the processes of formation and resorption, that is, of destruction of old bone and formation of new bone.

In infants, calcium undergoes a 100% annual turnover, while in adults it is only 18% annually. These processes of resorption and formation or replacement are called bone remodeling.

The remodeling begins with the action of the osteoclasts that destroy the bone and leaving cracks that are then invaded by the osteoblasts. These osteoblasts secrete the matrix that will later ossify and give rise to new bone. This cycle requires, on average, more than 100 days.

At any given time, about 5% of all skeletal bone mass is in the process of remodeling. This implies the participation of some two million remodeling units.

Differences in the remodeling of compact and cancellous bone

The annual remodeling rate of compact bone is 4% and that of cancellous bone is 20%.

The difference between the remodeling rates of the two types of bone is most likely due to the fact that the cancellous bone is in contact with the bone marrow and is directly influenced by the cells with paracrine activity in the bone marrow.

The osteoprogenitor cells of compact bones, on the other hand, are found in the haversian canals and in the inner layers of the periosteum, far away from the bone marrow cells and depend, for the initiation of remodeling, on the hormones that arrive by blood.

Many are the hormonal and protein factors involved in the activity of osteoblasts and osteoclasts in bone remodeling, however, the function of each has not been clearly elucidated.

Bone cells

-Types of bone cells and their characteristics

Bone cells are osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts. Each of these cells has particular functions in bone physiology and has well-differentiated histological characteristics.

Osteoblasts, osteocytes, and osteoclasts, together, form the bone shaping unit.

Osteoprogenitor or osteogenic cells

These cells are found in the inner layer of the periosteum and in the endosteum. They derive from the embryonic mesenchyme and can give rise, by differentiation, to osteoblasts. Under certain stress conditions they can also differentiate into chondrogenic cells.

They are spindle-shaped cells with an oval nucleus, scant cytoplasm, little rough endoplasmic reticulum (RER) and a poorly developed Golgi apparatus. They have abundant ribosomes and are very active during the period of bone growth.

Osteoblasts

Osteoclasts are cells derived from osteogenic cells. They are responsible for synthesizing the organic matrix of bone, that is, collagen, proteoglycans and glycoproteins. They are arranged in overlapping layers on the surface of the bone.

Its nucleus is on the opposite side to the secretory portion rich in vesicles. They have abundant RER and a well-developed Golgi apparatus. They have short projections or extensions that make contact with other neighboring osteoblasts. Other long processes connect them with the osteocytes.

As the osteoblasts secrete matrix, it surrounds them, and when the osteoblasts are completely included in the matrix, that is, surrounded by it, they become inactivated and become osteocytes.

Despite the fact that most of the bone matrix is ​​calcified, around each osteoblast and even each osteocyte, a thin layer of non-calcified bone matrix remains, which is called the osteoid, which separates these cells from the calcified matrix.

There are different types of receptors in the cell membrane of osteoblasts. Of these receptors, the most important is the receptor for parathyroid hormone (PTH), which stimulates the secretion of an osteoclast-stimulating factor that promotes bone resorption.

Osteoblasts can also secrete enzymes capable of removing osteoid and thus bringing osteoclasts into contact with the calcified bone surface to initiate resorption.

Osteocytes

These are cells derived from inactive osteoblasts and are called mature bone cells. They are housed in the aforementioned lagoons of the calcified bone matrix. There are between 20,000 to 30,000 osteocytes for every cubic millimeter of bone.

From the lagoons, the osteocytes radiate cytoplasmic processes that link them together, forming interstitial junctions through which ions and small molecules can be exchanged between cells.

Osteocytes are flattened cells, with flat nuclei and few cytoplasmic organelles. They are capable of secreting substances under mechanical stimuli that cause tension in the bone (mechano transduction).

The space that surrounds the osteocytes in the lacunae is called the periosteocitic space and is filled with extracellular fluid in the non-calcified matrix. The surface area of ​​the periosteal walls is estimated to be about 5000m2 and that it houses a volume of about 1.3 liters of extracellular fluid.

This fluid is exposed to about 20 g of exchangeable calcium that can be reabsorbed into the bloodstream from the walls of these spaces, which contributes to the maintenance of blood calcium levels.

Osteoclasts

These cells are derived from the same progenitor cells as tissue macrophages and circulating monocytes; These are found in the bone marrow and are the progenitor cells of granulocytes and macrophages (GM-CFU).

Mitosis of these progenitor cells is stimulated by macrophage colony-stimulating factors and in the presence of bone, these progenitors fuse to form multinucleated cells.

An osteoclast is a large, multinucleated, mobile cell. It measures about 150μm in diameter and can have up to 50 cores. It has a basal area where the nuclei and organelles are found, a brush border in contact with the calcified bone, clear areas peripheral to the brush border and a vesicular area.

The main function of these cells is that of bone resorption. Once they work, they undergo apoptosis (programmed cell death) and die. To start the process of bone resorption, the osteoclast attaches itself to the bone through proteins called integrins.

Next, proton pumps that are H + dependent ATPases move from the endosomes into the brush border membrane and acidify the medium until the pH drops to approximately 4.

Hydroxyapatite dissolves at such a pH and collagen fibers are degraded by acid proteases also secreted by these cells. The end products of the digestion of hydroxyapatite and collagen are endocyted within the osteoclast and are then released into the interstitial fluid to be subsequently eliminated in the urine.

Types of bone tissue (types of bone)

As may have been noted in the text, there are two types of bone tissue, namely: compact or cortical bone and trabecular or cancellous bone.

The first constitutes 80% of the total bone mass and is found in the diaphyses of long bones, which are the tubular portions arranged between the two ends (epiphyses) of these bones.

The second type of bone is typical of the bones of the axial skeleton, such as the vertebrae, the bones of the skull and pelvis, and the ribs. It is also found in the center of the long bones. It makes up 20% of the total bone mass and is of vital importance for the regulation of calcium metabolism.

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