Hello everybody, this is Rob Swatsky from the York Campus of HACC, and in this podcast we'll be continuing on with chapter 6 on bone tissue with a focus on bone formation. Bone forms through a process called ossification, also called osteogenesis, which means the making of bone. Bones form during embryological and fetal development. They grow throughout our early childhood until the adult bone sizes are achieved in our late teens or early 20s. Bones form during the remodeling process, as old bone is continuously replaced with new bone, and growth occurs during fracture repair. Bone first forms during embryological and fetal development out of mezzan-kime tissue, which is embryonic connective tissue, that is in the rough shape of the bones. The formation of cartilage and the first bone tissue through ossification begins during the sixth week of embryological development. Bones can develop in two different ways as the original connective tissues are replaced with bone tissue. The simpler method of bone formation is called intra-membranous ossification. Intra-membranous means between the membranes, and involves the formation of bone directly within the thin, flat membrane-like layers of mezzan-kime. Endocondral ossification, endocondral meaning within the cartilage, involves the formation of bone within the high-elun cartilage that originates from the mezzan-kime. Intra-membranous ossification is the process that forms the flat and irregular bones of the skull, face, and mandible, or lower jaw, as well as hardens the fontanelles or soft spots of the fetal skull. The steps of intra-membranous ossification include development of the ossification center, calcification, formation of trabeculi, and finally development of the periosteum. In the first step, mezzan-kime cells gather together at the locations of where the bone will develop and begin to differentiate into osteogenic cells, which will then become osteoblasts that start to secrete the extracellular matrix. This grouping of cells is called an ossification center. Here we see the ossification center in the illustration, made up of the osteoblasts, and surrounded by the mezzan-kime cells, and a skeleton of collagen fibers. Here we see a cluster of osteoblasts that are in the process of building up the bone tissue, secreting that matrix. In step 2, ultimately the production of the matrix stops, and the osteoblasts are now osteocytes within lakuni, which are the chambers within that hardening extracellular matrix. Canneliculi connect the lakuni and the osteocytes to each other, and during this calcification process calcium and more mineral salts are continuously deposited in the matrix as it hardens. In the third step, as the matrix hardens, it forms the trabeculi that merge and fuse with each other to form spongy bone. The spaces within spongy bone contain blood vessels and red bone marrow forms. Here we see the trabeculi fused together of the spongy bone and plenty of spaces to allow blood vessels and marrow to form. In the fourth step, the development of the periosteum, as the trabeculi form, the outer mezzan-kime cells become more concentrated and eventually develop into the periosteum. The central region of the new bone remains a spongy bone, while the surface layers of the spongy bone are transformed into compact bone. This new bone will undergo lots of remodeling over time as it is molded into its specific adult shape and size. During endocondrolocification, cartilage is replaced by bone. This process forms most of the bones of the body, but can be best seen in a long bone. It is a little more complex than intramembranous ossification and consists of six major steps. Development of the cartilage model, growth of the cartilage model, development of the primary ossification center, development of the medullary or marrow cavity, development of the secondary ossification center, and finally formation of the articular cartilage and epivisial plate. Endocondrolocification begins with the formation of the cartilage model. At the location of the new bone, mezzan-kime cells cluster together and take on the basic shape of the bone that will develop. These cells then become condroblasts. The prefix condrow refers to cartilage, and we know the suffix blast means to build. These cells start secreting cartilage extra cellular matrix and develop into the cartilage model made of the hyaline cartilage. The paracondrium develops around the cartilage model and this acts as a covering to contain the cell cluster and maintain the overall shape and length of the model. In the second stage, growth of the cartilage model, the conjuroblasts surround themselves in the extra cellular matrix they're producing and become conjurous sites or cartilage cells. The conjuroblasts divide and grow the cartilage model lengthwise in a process called interstitial growth. It's also known as endogenous growth, which means growth from within. As new extra cellular matrix is deposited on its surface, the cartilage model can grow in thickness in a process called oppositional growth or exogenous growth. As the model grows, conjurocytes in the middle of the model undergo hypertrophy, which is an increase in size, and the nearby matrix begins to harden through calcification. Conjurocytes within this region die as they are cut off from nutrients as the matrix hardens around them. The space is remaining as the conjurocytes die fused together and form lacuny. The third step, development of the primary ossification center, starts when a nutrient artery moves through the paracondrium and the cartilage model via a nutrient foramen. Osteogenic cells in the paracondrium differentiate into osteoblasts and bone begins to form. The paracondrium is now called the periosteum. In the middle of the model, periosteal capillaries grow into the calcifying cartilage and trigger the growth of the primary ossification center. This area is where most of the cartilage will be replaced by bone tissue. Osteoblasts secrete bone matrix over the calcified cartilage and form trebecule of spongy bone. Primary ossification now spreads from the middle of the model outward towards the ends. In the fourth step, development of the medullary or marrow cavity, osteoclasts start to carve away and break down some of the spongy bone trebecule as the primary ossification center grows outward. The osteoclasts ultimately carve out the medullary cavity within the diaphysis. With the remaining diaphysis wall, ultimately transformed into compact bone. Step 5. Development of the secondary ossification centers occur when branches of the epiphysial artery enter the proximal and distal epiphyses. The process of bone formation at the ends of the developing bone is similar to the primary ossification centers. The big difference is that spongy bone remains in the middle of the epiphyses and no medullary cavities are formed. In the final step, step 6, formation of articular cartilage and the epiphysial plates, most of the highland cartilage in the epiphyses becomes spongy bone except for the outermost coverings, which remain as highland cartilage and function as articular cartilage, which is found at the joints or articulations where bone meets bone. We also have cartilage left at the epiphysial or growth plates, which function in growing the length of the bone.