What tissue makes up anterior and posterior fontanelles? ?

The anterior and the posterior fontanelles is made up of the MESENCHYMAL CONNECTIVE TISSUES.
The anterior and the posterior fontanelles are two areas that are normally found on infant heads; at these areas, the skull bones did not cover the brain completely. The anterior fontanelles is located towards the front of the head while the posterior fontanelles is located at the back of the head. As the baby grow older, the mesenchmal tissues of the fontanelles turns into bone tissues.

The answer to this question would be mesenchymal connective tissue.

The fontanelle is a soft spot in the skull that can be found in neonates or baby. But the spot will undergo ossification and become a hard bone latter in their life. This process is important because a neonates/baby head is still growing, so the bone should not close before their growth is finished.