Osteogenesis and Bone Growth Quiz

Osteogenesis and Bone Growth

• Bone formation is called osteogenesis or ossification.
• Two types of ossification occur: intramembranous and endochondral.
• Intramembranous ossification forms flat bones like those in the face.
• Endochondral ossification forms long bones, e.g., at the base of the skull.
• Osteoprogenitor (osteogenic) cells are undifferentiated cells that can become osteoblasts, found in the inner layer of the periosteum and endosteum.
• Osteoblasts synthesize bone matrix and collagen fibers but cannot divide.
• Osteocytes are mature bone cells that maintain the bone matrix.
• Osteoclasts are huge cells that break down bone tissue.

Four Types of Bone Cells

• Osteoprogenitor (osteogenic) cells: undifferentiated cells which divide to replace themselves and become osteoblasts, found in inner layer of periosteum and endosteum.
• Osteoblasts: form matrix and collagen fibers but cannot divide.
• Osteocytes: principal cells of bone tissue; mature cells that no longer secrete matrix
• Osteoclasts: huge cells from fused monocytes (WBCs); serve to break down bone tissue, function in bone resorption at surfaces such as endosteum.

Learning Objectives

• Describe the process of ossification and ossification centers
• Describe the processes involved in the two types of ossification
• Describe the process of bone growth and remodeling
• Describe fracture healing

What is Ossification?

• All embryonic connective tissue begins as mesenchyme.
• Intramembranous ossification forms bone directly from fibrous connective tissue membranes.
• Endochondral ossification forms bone from hyaline cartilage models.

Intramembranous Ossification

• Begins in utero, during fetal development.
• Continues into adolescence.
• The skull and clavicles are not fully ossified at birth.
• Sutures of the skull are not closed, which allows for deformation during passage through the birth canal.
• The last bones to ossify are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.
• Compact and spongy bone develops directly from sheets of mesenchymal connective tissue.
• Mesenchymal cells gather and differentiate into capillaries and osteogenic cells.
• Early osteoblasts appear in a cluster called an ossification center, spreading out to form bone tissue.

Endochondral Ossification

• Bone develops by replacing hyaline cartilage.
• Cartilage serves as a template, which is completely replaced by new bone.
• Takes longer than intramembranous ossification.
• Mesenchymal cells differentiate into chondrocytes, forming cartilaginous skeletal precursors.
• Perichondrium (a membrane covering the cartilage) appears.
• Chondrocytes in the center of the cartilage model grow in size; matrix calcifies and nutrients cannot reach chondrocytes.
• Chondrocytes die and disintegrate.
• Blood vessels invade the resulting spaces and osteogenic cells (osteoblasts) differentiate.
• Bone cell development occurs in a region deep in the periosteal collar—the primary ossification center.
• Chondrocytes and cartilage continue to grow at the ends of the bone (future epiphyses).
• Length of bone increases as bone replaces cartilage in diaphyses.
• Cartilage remains at joint surfaces (articular cartilage) and between diaphysis and epiphysis (epiphyseal plate) is responsible for longitudinal bone growth.
• Secondary ossification centers appear in the epiphyses, with cartilage replacement by bone continuing after birth.

Bone Growth in Length

• The epiphyseal plate is the area of growth in a long bone.
• Is a layer of hyaline cartilage where ossification occurs in immature bones.
• Cartilage is formed on the epiphyseal side of the epiphyseal plate.
• Cartilage is ossified on the diaphyseal side, with diaphysis growth in length.
• Epiphyseal plate is composed of four zones of cells and activity. - Bone growth ceases in adults (ages 18-25).

Zones of the Epiphyseal Plate

• Proliferative zone- makes new chondrocytes (via mitosis.) to replace those that die at the diaphyseal end of the plate.
• Zone of maturation and hypertrophy- older and larger chondrocytes, situated closer to the diaphyseal end of the plate.
• Zone of calcified matrix- most chondrocytes are dead due to calcified matrix. Capillaries and osteoblasts penetrate this zone, with osteoblasts forming bone tissue on remaining calcified cartilage.
• Bone grows when osteoblasts add tissue to the diaphysis.

Bone Growth in Thickness (Appositional Growth)

• Bones increase in diameter while increasing in length.
• Osteoclasts resorb old bone in the medullary cavity.
• Osteoblasts form new bone beneath the periosteum, increasing the diameter of the diaphysis and medullary cavity.
• This process is called modeling.

Bone Remodeling

• Continual redistribution of bone matrix along lines of mechanical stress.
• Osteoclasts destroy old bone and osteoblasts rebuild bone.
• The distal femur is fully remodeled every 4 months.
• Several hormones and calcitriol control bone growth and bone remodeling.
• Hormones include calcitonin and parathyroid hormone, regulating calcium and phosphate levels.

Factors Affecting Bone Growth

• Nutrition- adequate levels of minerals (calcium and phosphorus) and vitamins (C, K, and B12) are necessary for bone growth and protein synthesis.
• Sufficient levels of specific hormones during childhood (insulin-like growth factor, growth hormone, thyroid hormones, estrogen, and testosterone) are needed to promote cell division.

Growth Hormone (GH)

• GH secretion is regulated by hypothalamus and various mediators (GHRH, somatostatin, ghrelin, IGF-1).
• Principal physiological regulations include neural endogenous rhythm, sleep, stress, exercise, and nutritional and metabolic signals.
• GH deficiency can be hereditary or acquired.
• GH deficiency can be treated with recombinant human GH in children and in adults.
• GH hypersecretion can cause acromegaly or gigantism.

Hormonal Abnormalities Affecting Bone Growth

• GH oversecretion during childhood leads to gigantism.
• GH undersecretion during childhood leads to dwarfism.
• In both men and women, lack of estrogen receptors results in taller growth than normal before closure of growth plate.

Fracture Repair

• Formation of fracture hematoma-blood clots in 6-8 hours after injury, bringing in phagocytes for cleanup.
• Formation of fibrocartilaginous callus- fibroblasts invade and lay down collagen fibers and chondroblasts create fibrocartilage to span broken ends of bone.
• Formation of bony callus-osteoblasts secrete spongy bone- lasts about 3-4 months- joins broken bone ends.
• Bone remodeling- compact bone replaces spongy bone in the bony callus, which is then remodeled back to normal shape.

Test your knowledge on osteogenesis, bone cell types, and their roles in bone formation. This quiz covers key concepts such as intramembranous and endochondral ossification, and the various types of bone cells including osteoblasts, osteocytes, and osteoclasts.