Corpus: Bone

Bone, or bone tissue, refers to a very dense and rigid type of connective and supportive tissue that makes up the human skeleton. Bones are highly specialized structures. The branch of medicine that focuses on the study of bones is called osteology.

Teeth are considered a distinct type of tissue.

All vertebrates have a skeleton composed of many bones that support their bodies. Each bone has a unique shape based on its location and is constantly shaped by functional demands. The size of bones ranges from the tiny ossicles in the middle ear, only a few millimeters long, to the large and sturdy thigh-bone, known as the femur.

The human body consists of about 210 bones, with most sources citing a range of 206 to 212. The exact number can vary between individuals due to the presence of accessory bones, which occur in some people, and the ossification of cartilage over a person's lifetime.

4.1. ...according to origin

Bones are classified based on their embryonic origin:

• Membranous (or connective tissue) bones: These bones form directly from connective tissue and are primarily found in the cranial region. They develop by merging smaller bone segments into more compact structures.
• Endochondral (or replacement) bones: These bones develop through a process called chondral ossification, where they are formed from pre-existing cartilage. Endochondral bones make up the deeper parts of the skeleton, such as the spine and the bones of the arms and legs.

4.2. ...by morphology

Bones can also be classified based on the arrangement or morphology of their tissue:

• Spongy bone (substantia spongiosa): This type of bone tissue is porous and resembles a sponge. It is typically found in the center of bones and contains bone marrow.
• Compact bone (substantia compacta): This is a dense and solid bone tissue that forms the outer layer, or cortex, of bones, providing strength and stability.

4.3. ...according to shape

Bones are also categorized based on their shape or unique structure:

• Long bones (ossa longa): These are tubular bones, such as the femur, humerus, ulna, radius, tibia, fibula, as well as the metatarsal and metacarpal bones. They consist of two ends (epiphyses) and a central shaft (diaphysis).
• Flat bones (ossa plana): These bones have a flattened structure, such as the scapula, sternum, and ribs.
• Short bones (ossa brevia): These are short, compact bones that are cube- or cylinder-shaped, like the bones of the wrist (carpals) and ankle (tarsals).
• Pneumatized bones (ossa pneumatica): These are hollow bones found in the skull, such as the maxilla, frontal bone, sphenoid bone, and ethmoid bone, containing air spaces.
• Sesamoid bones (ossa sesamoidea): These bones develop within tendons and can vary in number. The largest sesamoid bone is the patella (kneecap).

Bones that do not fit into any of the above categories are called irregular bones (ossa irregularia).

The outer surface of a bone is covered by a dense connective tissue membrane called the periosteum. Beneath the periosteum lies the cortical layer (corticalis), which is made up of compact bone tissue (substantia compacta). Deeper within, the compact bone transitions into a spongy network of trabeculae known as the spongy bone (substantia spongiosa). The spaces within this spongy framework form the medullary cavity (cavum medullare), which is lined by a thin membrane called the endosteum. The medullary cavity contains bone marrow, which, in most bones, is gradually replaced by yellow fatty marrow as a person ages.

Bone tissue is made up of a network of living bone cells, called osteocytes, which are embedded in a hard extracellular substance known as the bone matrix. These osteocytes are interconnected through tiny channels called canaliculi. A dedicated blood vessel system provides nutrients and oxygen to the bone cells. The fundamental structural unit of bone, organized around a central blood vessel, is known as an osteon.

6.1. Bone cells

Three different types of cells are found in bone tissue:

• Osteoblasts: These cells originate from less differentiated precursor cells and are responsible for producing the bone’s organic matrix, called osteoid. They also produce alkaline phosphatase, an enzyme that regulates bone mineralization.
• Osteocytes: Osteocytes are mature bone cells that develop from osteoblasts once they become embedded in the bone matrix they secrete. They maintain the bone matrix and play a crucial role in regulating calcium levels. Osteocytes communicate with each other through extensions called canaliculi.
• Osteoclasts: These are large, multinucleated cells derived from monocytic stem cell lines. Osteoclasts are responsible for bone resorption, breaking down bone tissue. They are typically located in resorption areas of the bone, known as Howship’s lacunae.

6.2. Extracellular substance

Regarding the spatial organization of the extracellular matrix between bone cells, there are two forms of bone:

• Woven bone: In woven bone, the collagen fibers within the bone matrix are arranged in a random, criss-cross pattern, giving it a less organized structure.
• Lamellar bone: In lamellar bone, the collagen fibers are arranged in parallel layers, resulting in a more organized and structured appearance.

6.3. Bone formation

The process of bone formation is known as ossification or osteogenesis. The human body can form bone in different ways, and these methods are distinguished histologically:

• Chondral ossification
• Desmal ossification

Bone is composed of approximately 60 to 70 % inorganic minerals, 10 to 15 % water, and 20 to 25 % organic matter. The minerals are mainly calcium salts, present in the form of hydroxyapatite crystals. The organic matrix of bone primarily consists of collagen type I, along with proteoglycans and other proteins such as osteonectin, osteocalcin, and sialoprotein, which are crucial for bone mineralization.

8.1. Shaping

Bones provide the structural framework and support for the human body. To effectively perform this essential function, bone tissue undergoes continuous remodeling, responding to mechanical loading and unloading. This dynamic process allows bones to adapt their strength according to the demands placed upon them.

8.2. Movement

Bones, along with muscles, tendons, ligaments, and joints, contribute to the biomechanical properties of the human body. They play a crucial role in transmitting and transferring forces throughout the body.

8.3. Protection

Another important function of bones is to protect delicate internal organs. The skull bones protect the brain, while the ribcage shields the heart and lungs. Additionally, due to its high mineral density, bone offers some degree of radiation protection for the sensitive bone marrow housed within it.

8.4. Mineral metabolism

Bone tissue serves to store mineral salts (primarily calcium), which can be released again when required, and is able to provide buffer substances to regulate the pH value in the blood. The bone substance can also bind harmful heavy metal ions to a certain extent and eliminate them from the bloodstream.

8.5. Blood formation

From around the end of the fourth embryonic month, during the beginning of the medullary phase, the bone marrow becomes the primary hematopoietic organ in humans. Hematopoiesis occurs almost exclusively in the red bone marrow. In infants, red bone marrow is present in all bones, but in adults, it is primarily located in flat and short bones.

Diseases and injuries of the skeletal system and bones are primarily managed by the fields of osteology, orthopedics, and trauma surgery, but bone disorders are also relevant in many other medical specialties. Some of the most significant bone-related conditions include:

• Trauma: fractures
• Degenerative processes: Osteoporosis
• Metabolic disorders:
  • Osteomalacia (adults)
  • Rickets (children)