Bone structure and cellular composition
Anatomically, a bone consists of two layers. The outer compact layer is formed by osteons—structural units made of concentric lamellae containing blood vessels and nerves. The inner spongy layer is a network of interwoven trabeculae forming cavities for red bone marrow.
The dry weight of the bone matrix is distributed equally: 50% are inorganic minerals (hydroxyapatite crystals) and another 50% are organic proteins and water. The non-mineralized framework secreted by the cells is called osteoid.
Three types of cells are responsible for tissue remodeling:
- Osteoblasts: synthesize collagen and matrix proteins (e.g., osteonectin), regulate mineralization, and control the activity of bone-resorbing cells.
- Osteocytes: mature mononuclear cells entombed within lacunae. Their task is to maintain the matrix structure.
- Osteoclasts: multinucleated macrophages engaged in bone resorption (breakdown).
Bone dysplasias
Dysplasias are genetically determined cartilage and bone pathologies leading to growth defects and skeletal malformations.
Osteopetrosis (marble bone disease) is associated with impaired osteoclast function. Since the resorption function is lost, bone synthesis becomes excessive. Medullary canals narrow, squeezing out the bone marrow, which leads to severe anemia. A characteristic radiological sign is the "bone-in-bone" phenomenon.
Achondroplasia is a form of congenital dwarfism. A mutation in the FGFR3 gene causes the receptor to constantly suppress chondrocyte proliferation. Cartilage cells are not replaced by bone tissue, which drastically halts endochondral ossification (longitudinal growth). Meanwhile, patients' intelligence is fully preserved.
Osteogenesis imperfecta (" brittle bone disease") occurs due to mutations in type 1 collagen genes. The main problem is bone mass deficiency and extreme fragility. There are several types of the disease, ranging from lethal prenatal forms to milder ones accompanied by multiple fractures, blue sclerae, and hearing loss.
Paget's disease and fibrous dysplasia
Paget's disease (osteitis deformans) most commonly manifests after age 40. The leading theory is viral: a paramyxovirus infects osteoblasts, forcing them to release interleukin-6, which hyperactivates osteoclasts. The disease goes through three stages:
- Osteolytic: rapid destruction of the cortical plate.
- Osteoblastic: chaotic construction of bone trabeculae, formation of the characteristic coarse-fibered mosaic structure.
- Osteosclerotic: decrease in the number of osteoclasts, replacement of bone marrow spaces with adipose tissue.
The result is thickening, deformation, and roughness of the affected bones. The process can be complicated by fractures, kidney stones due to calcium release into the blood, and, in rare cases, sarcoma.
In fibrous dysplasia, healthy bone tissue is locally replaced by fibrous connective tissue containing primitive bone trabeculae inclusions, which also leads to deformations.
Metabolic disorders: Osteoporosis
These disorders are associated with metabolic disruptions. The primary nosology in this group is osteoporosis, characterized by a decrease in tissue mass per unit volume of bone. The pathogenesis comes down to resorption exceeding synthesis.
Primary generalized osteoporosis makes bones fragile throughout the body. The most common variant is postmenopausal (type I). When estrogen levels drop in women, osteoblasts (which have receptors for these hormones) begin to produce cytokines. This powerfully stimulates osteoclast activity, accelerating skeletal destruction.
Briefly: answers to topical questions
What functions do osteoblasts perform during bone remodeling?
Osteoblasts perform synthesis, formation, and mineralization functions of bone tissue. During remodeling, they carry out the following tasks:
- Synthesis of procollagen and other matrix proteins.
- Secretion of alkaline phosphatase involved in calcium and electrolyte metabolism.
- Matrix calcification by chemical means (secretion of phosphoproteins) and physical means (release of matrix vesicles).
- Regulation of osteoclast activity (bone-resorbing cells).
What components make up the organic matrix of bone tissue?
The organic component of the bone matrix includes collagens, predominantly type I collagen (90–95%) and type V collagen, non-collagenous proteins (fibronectin, osteonectin, etc.), as well as glycosaminoglycans.
What clinical and morphological manifestations are characteristic of osteopetrosis?
Osteopetrosis is characterized by excessive bone formation and mineralization due to a loss of osteoclast function. Clinical and morphological manifestations include:
- Short stature, bone fragility, and brittleness with a tendency to fractures.
- Progressive narrowing of the medullary cavities, leading to anemia and decreased resistance to infections.
- Extramedullary hematopoiesis causing hepatosplenomegaly.
- Cranial nerve palsies and blindness.
- Osteomyelitis.
- Generalized increased bone density and the radiological "bone-within-a-bone" sign.
What complications can accompany Paget's disease?
Paget's disease can be accompanied by a number of severe complications affecting the skeletal, nervous, and cardiovascular systems. These include:
- Pathological fractures due to bone weakening.
- Malignant transformation (in 1% of cases) with the development of osteosarcoma, fibrosarcoma, or chondrosarcoma.
- Nephrolithiasis (kidney stone formation) resulting from hypercalcemia and hypercalciuria.
- Cranial nerve compression leading to hearing loss and visual disturbances.
- Tooth loosening.
- High-output heart failure.
What microscopic changes are observed in fibrous dysplasia of bone?
In fibrous dysplasia of bone, microscopic examination reveals the replacement of normal bone tissue with cellular-fibrous osteogenic tissue. The following changes are characteristic:
- Proliferation of connective tissue with foci of myxomatous changes.
- Formation of primitive bone trabeculae, reflecting the phenomenon of arrested osteogenesis.
- Specific sign: immature bone trabeculae have irregular outlines, visually resembling "Chinese characters" (on hematoxylin and eosin staining).
- Presence of isolated primitive bone inclusions within the tissue.
Which primary benign bone tumors are distinguished in pathological anatomy?
In pathological anatomy, primary benign tumors of bone and cartilage tissue include:
- Osteoma is a dense bone formation with a lobulated surface.
- Osteoid osteoma is a tumor presenting as a granular mass with distinct borders.
- Osteoblastoma is a variant of osteoid osteoma.
- Chondroma is a tumor structured similarly to hyaline cartilage.
- Osteochondroma, chondroblastoma, and chondromyxoid fibroma.
To remember cell roles, use this association: osteoBlasts = Builders, osteoclasts = Crashing (Destroyers), osteocytes = Keepers (Preservers).
What else to study on this topic
- Calcium and phosphorus metabolism in bone tissue
- Types of osteogenesis imperfecta (I–IV)
- Hyperostoses: melorheostosis and pycnodysostosis
- Role of 1α,25-dihydroxycholecalciferol in osteoid mineralization
- Parathyroid osteodystrophy and rickets
Master the entire topic in the "Pathological Anatomy" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Bone Pathology" and 4,414 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 153 video lessons on the subject
Other topics in the section "Chapter 22. Diseases of the Musculoskeletal System"
Free video lessons: pathological anatomyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.