Development & Growth
Bones form by replacing connective tissue. There are two types of bone growths: intramembranous and endochondral.
Intramembranous bone growth forms within sheet-like layers of connective tissue. It is most common in the skull. Osteoblasts form bone around the cells. They continue to lay down the bone in large clusters. Osteoblasts are then called osteocytes when they reside in the lacunae, within the bony matrix they have created. Cells on the outside of the bone matrix create the periosteum.
Intramembranous bone growth is limited in the size and length that it can create. It is not well adapted to create the length required for long bones.
Endochondral bone growth is responsible for most bone growths, especially long bone growth. Endochondral bone growth replaces cartilage with the bone. It goes through stages where it lays down cartilage to increase length and then breaks the cartilage down, replacing it with bone. This is the type of bone growth which utilizes growth plates, or epiphysis plates.
The first step in endochondral bone growth is hyaline cartilage formation in the diaphysis. Blood vessels migrate into the diaphsyis and osteoblasts activity increases. The osteoclasts begin breaking down the cartilage and replace it with spongy bone in the diaphysis. The first area of bone production in the diaphysis is called the primary ossification center. Periosteum begins to develop on the outside of the diaphysis.
Osteoblasts underneath the periosteum begin laying down compact bone between the periosteum and spongy bone. Secondary ossification centers begin to develop in the epiphysis of long bones. They go through similar development to produce growth at the ends of long bones. In between the epiphysis and diaphysis, a region of highly active cartilage growth develops. Layers of cartilage cells are undergoing mitosis (cell division). The replicating cells increase layers of cartilage between the epiphysis and diaphysis. This region is called the epiphyseal plate, also known as the growth plate.
Osteoclasts then break down a layer of the cartilage and osteoblasts lay down additional bone in its place. This process of breaking down cartilage and laying down bone increases length of a bone.
Epiphyseal plates increase length of a bone. Intramembranous ossification underneath the periosteum results in increased thickness of long bones. Osteoclasts also break down the center of the diaphysis, creating the medullary cavity.
Questions:
Which bone growth is most common in the skull? ________________________________________
Which bone growth allows for lengthening of bones? _____________________________________
What forms the periosteum? _________________________________________________________
Which bone growth allows for increase in thickness of flat and long bones? ___________________
What is the first area of calcified bone called? ___________________________________________
Where are secondary ossification centers formed?________________________________________
Does the primary ossification center form compact or spongy bone?__________________________
Where is compact bone produced?____________________________________________________
An area of highly replicating hyaline cartilage is called? ___________________________________
This area of cartilage allows for increased length of long bones T/F? __________________________