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OSTEOBLASTS and OSTEOCYTES in developing INTRAMEMBRANOUS (or Membrane) BONE, INTRAMEMBRANOUS OSSIFICATION, 100X. Osteoblasts are bone-forming cells completely covering the surface of this U-shaped Bony Spicule in the image. When osteoblasts completely surround themselves with bony matrix that they secrete, they become isolated in spaces called Lacunae. Many Osteocytes are seen in the bony spicule and you can see evidence of a lacuna in several of the osteocytes. Intramembranous Bones are found in the broad, flat bones of the skull (e.g. frontal bone). Abundant undifferentiated Mesenchyme cells are also seen outside of the bony spicule. Mesenchymal cells are primitive cells that differentiate into Osteoblasts.