Their similarities derive from their common origin within a tetrapod ancestor that possessed, among the other bones, a humerus. Our goal today and in this week's lab is first to learn some fundamentals of the arrangements of cells in tissues. The mineralized tissues, more than any other tissue, connect us with our past because they readily fossilize. I like to think that our dermal cranium derives from the bony armor of the ostracoderms, ancient jawless fishes. Fossil head skeleton in a Cretaceous teleost ca 100 Myr ago.
Teeth fossilize even better than bone, because the enamel is harder than bone.
See PJH for tooth structure and development. The hard tissues derive from soft tissues, present in an early embryo as shown here. In embryonic epithelia the cells have sheet-like arrangement. To the inside is mesenchyme , the second basic kind of embryonic tissue, with looser cell arrangement, and ECM within the extracellular spaces. Also shown here are mesenchymal condensations - here from embryonic sources that will be important for us later in the course, termed neural crest and placode.