# Ossification --- **Ossification** or **osteogenesis** is how [[bone|bones]] are initially formed in the embryo. There are two different pathways for making bone. **Intramembranous ossification**. which builds up bone from scratch which is how most of the bones in the face and head, as well as the collar bones is created. The rest of the bones are formed from a process called **endochondral ossification** that first builds up a cartilage "template" and then gradually replaces the cartilage with bone. ## Intramembranous Ossification **Intramembranous ossification** is the process of ossification that builds up bone directly from sheets of [[connective tissue|mesenchymal tissue]]. This pathway makes a relatively few number of bones in the face and head, as well as the collarbones. The process begins *in utero* and does not finish until well after birth. This is why babies have soft skulls. ![[intramembranous ossification.png]] ### Step 1: The Ossification Center Intramembranous ossification starts when some cells within the [[connective tissue|mesenchyme]] differentiate into [[bone|osteogenic cells]] (which quickly become [[bone|osteoblasts]]) and cells for forming [[capillaries]]. These cells start out in a cluster called the **ossification center**. ### Step 2: Osteocytes The osteoblasts secrete a protein mixture called **osteoid** which calcifies with (I'm pretty sure anyway) [[bone#Hydroxyapatites|hydroxyapatites]] in it, trapping the osteoblasts in a case of bone. The osteoblasts then transform into [[bone|osteocytes]] and the osteogenic cells turn into new osteoblasts. ### Step 3: Trabecular Matrix & Periosteum The osteoid hardens around the [[capillaries]] which forms the **trabecular matrix** of the spongy bone. Osteoblasts on the outside surface turn into the [[bone#Endosteum Periosteium|periosteum]]. (I'm not sure on this next but, but what it sounds like is:) The osteoblasts of the periosteum then make a casing of dense bone sandwiching the spongy bone. ### Step 4: Bone Marrow The trabecular bone constrains the capillaries so that they eventually smoosh together to form [[bone marrow|red bone marrow]]. ## Endochondral Ossification **Endochondral ossification** is one way of ossification where the bone tissue gradually replaces a "template" bone made out of [[cartilage]]. [[bone#Classifications|Long bones]] are made this way. It takes a lot longer than [[ossification]], and is used to form the majority of the bones in our body, as well as during [[bone repair]]. ![[endochondral ossification.png]] This is how most bones are formed. Some others are made though a process called [[ossification]]. ### Step 0: The Cartilage Template The bone template is made when [[connective tissue|mesenchymal cells]] differentiate into [[cartilage|chondrocytes]] that form bone-shaped pieces of [[cartilage#Types|hyaline cartilage]]. The [[cartilage|perichondrium]] forms soon after. [[stem cell|Mesenchymal]] cells live under the perichondrium. ### Step 1: Angiogenesis Angiogenesis occurs and blood vessels invade the perichondrium. ### Step 2: Osteoblasts Differentiate Mesenchyme converts to [[bone cells|osteoblasts]]. ### Step 3: Periosteal Collar The osteoblasts secrete [[bone cells|osteoid]] to convert the perichondrium to the periosteum. It initially forms as a circle of compact bone around the mostly-still-cartilage diaphysis. This is called the **periosteal collar**. ### Step 4: The Primary Ossification Center Before the matrix can ossify, chondrocytes enlarge and then calcify. As the matrix calcifies the chondrocytes are cut off from a blood supply and die off. This leaves space for more [[capillaries]] to enter the space (which will eventually become the [[bone#Structure|medullary cavity]]) and bring with them [[bone|osteogenic cells]]. The **primary ossification center** forms in that growing central cavity of bone tissue. Between the primary ossification center and the periosteal collar there is, for some time, still cartilage remaining. ### Step 5: Secondary Ossification Centers After birth new **secondary ossification centers** develop at the epiphyses of the long bones in the same way as the primary ossification center does. ### Step 6: Epiphyseal Seals Remain The primary and secondary ossification centers continue to grow and expand until they take up most of the bone, with the exception of the [[long bone|epiphyseal plate]]. This is the site of [[interstitial growth]] during childhood and adolescence. ___