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The biology system of the penguin is very complex, no less complex than our own. In fact, there are systems present in penguins that do not exist in mammals. There are several ways to analyze the biology of penguins, and the first place to start is at the cellular level. So that's where we'll begin. Bone
What are all these features of penguin bone? Hydroxyapatite is the amalgamation of calcium in bone. You've heard the expression that calcium builds bone. Well, calcium is the primary component of hydroxyapatite crystals, 40 um length depositions throughout calcified bone. Hydroxyapatite crystals constitute the bulk of the inorganic matrix of bone. The organic matrix of bone is composed primarily of collagen (Type I) fibers. Both these matrices are elaborated by osteoblasts and osteocytes. Osteoblasts begin the elaboration, and when osteoblasts become surrounded by what they've elaborated, the become known as osteocytes. Osteocytes secrete much less collagen than do osteoblasts. Osteocytes are the permanent living cells of the bone matrix. The final cell in bone is the osteoclast. Osteoclasts are what remodel bone. They are stimulated by Vitamin D on the surfaces of osteocytes. When so stimulated, osteoclasts break down bone and allow growth and remodeling to take place in penguins.
So, where do penguins get calcium to build their bones? And where do they get the Vitamin D they need? Penguins eat a lot of seafood, and fish liver oil contains a significant source of Vitamin D. Those same fish contain soft bones, and bones, as you know, are rich in calcium. To build their own bones, the small fish and crustaceans that penguins consume provide these necessary nutrients. Typical foods penguins eat to build their calcium and Vitamin D stores:
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The cardinal lesson that bone teaches is that the foundation of all endoskeleton creatures
is that bone is living tissue. Bone consists of the following structures:
The image to the left is a typical cross-section of bone stained with the standard
hematoxylin and eosin stain. H&E provides the characteristic pink and red stains you might find in histological
sections such as this. The dark pink web-link branching throughout the image you see on the left is
decalcified bone. That is, the calcium/hydroxyapatite has been removed from this section through
staining techniques. Punctuated along the decalcified bone are the osteoblasts. You can also see blood
vessels (BV) running through the marrow cavity (MC). The presence of blood vessels in bone implies that
bone is indeed a living tissue and that the cells require nutrients, especially oxygen. The marrow cavity is
the site of blood cell proliferation. In the adult penguin, blood cells in the body stem from the marrow cavities. Those
of you who would like to see a labeled structure can click 
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