Glucocorticoids

 The human body has 206 bones   
            There are 26 bones in the foot
​                        The hand and wrist contains 54 bones
                                  The femur is the longest and strongest bone
                                           The stapes, in the middle ear, is the smallest and lightest bone
                                                     Only the hyoid bone isn't connected to another bone
                                                              Only 10% of the world's animals have an internal skeletal system

Dermal bone forms directly in the skin. Osteoblasts deposit a calcium phosphate/salt matrix then they differentiate into osteocytes. So the difference between dermal ossification and endochondral ossification is that dermal ossification is the development of bone from fibrous tissue while the bone in endochondral ossification starts with cartilage.
Dermal scales are exhibited by fishes and a few reptiles. Dermal scales are bony plates that overlap or fit tightly together forming a dermal skeleton. In turtles, the bony plates form a "shell" which is really a dermal skeleton attached to their endoskeleton.

GH

          The Skeletal System

 Hormone Control:
Calcitonin and Parathyroid Hormone (PTH)

Evolution of Bone

​ Just an FYI re actual complexity of process

Rickets the softening  of bones in children. Rickets occurs often because of extreme
vitamin D deficiency.

Thyroid Hormones/ T3 T4

 Calcitonin acts to reduce blood Ca+ levels, opposing the action of parathyroid hormone (PTH). Calcitonin inhibits the activity of osteoclasts and decreases the resorption of calcium in the kidneys. 
PTH acts to resorb calcium from the bone, increase calcium resorption by the kidneys.

​ Osteoblasts synthesize and secrete a matrix of collagen and calcium salts. When the area around the osteoblast calcifies, the osteoblast becomes trapped and transforms into an osteocyte. 

 Bone marrow
 is the spongy tissue found   in the medullary cavities of long bones. Red bone marrow (myeloid tissue) and yellow bone marrow (fatty tissue). Red marrow houses blood stem cells which are able to differentiate into WBCs used to fight infections, RBCs which carry O2 and platelets which participate in clot formation. Red bone marrow exhibits a highly vascular fibrous tissue containing hematopoietic stem cells/blood-forming stem cells. Yellow bone marrow exhibits mesenchymal (marrow stromal) stem cells which give rise to fat, cartilage, and bone.

Early vertebrate skeletons were cartilaginous endoskeleton.
Some chondrichthyans then formed skeletal parts though the process of endochondral ossification which occurs when mineral skeleton are created by replacing cartilage templates. This process now occurs during the growth/development of  every mammalian embryo.  Skeletal function expanded from support/protection to also as a reservoir for calcium and phosphorus and as a site of blood cell production.

          bone remodeling

Excessive production of GH in children (before the epiphyseal plates fuse) results in  gigantism and continued growth of long bones.

Estrogens/Androgens