Ossification is started by the formation of layers of undifferentiated
connective tissue that hold the area where the flat bone is to come. On a baby, those spots are known as
fontanelles. The fontanelles contain connective tissue
stem cells, which form into
osteoblasts, which secrete
calcium phosphate into a matrix of canals. They form a ring in between the membranes, and begin to expand outwards. As they expand they make a bony matrix. This hardened matrix forms the body of the bone. Since flat bones are usually thinner than the
long bones, they only have
red bone marrow, rather than both red and
yellow bone marrow (yellow bone marrow being made up of mostly fat). The bone marrow fills the space in the ring of osteoblasts, and eventually fills the bony matrix. After the bone is completely ossified, the osteoblasts retract their calcium phosphate secreting tendrils, leaving tiny canals in the bony matrix, known as
canaliculi. These canaliculi provide the nutrients needed for the newly transformed osteoblasts, which are now called
osteocytes. These cells are responsible for the general maintenance of the bone. A third type of bone cell found in flat bones is called an
osteoclast, which destroys the bone using enzymes. There are three reasons that osteoclasts are normally used: the first is for the reparation of bones after a break. They destroy sections of bone that protrude or make reformation difficult. They are also used to obtain necessary calcium that osteoclasts are used is for growing. As the bone grows, its shape changes. The osteoclasts dissolve the part of the bone that must change. ==Additional images==