Look! A scrap of soft tissue extracted from dinosaur bone:

Demineralized fragments of endosteally derived tissues lining the marrow cavity of the T. rex femur. The demineralized fragment is flexible and resilient and, when stretched (arrow), returns to its original shape.

Dr GH has already promised to describe this new find in more detail on The Panda's Thumb, so I'm going to be very brief—keep an eye on The Thumb for more.

Anyway, it has been reported in Science this week that well-preserved soft tissues have been found deep within the bones of a T. rex, and also within some hadrosaur fossils. This is amazing stuff; fine structure has been known to be preserved to this level of detail before, but these specimens also show signs of retaining at least some of their organic composition. What the authors have done is to carefully dissolve away the mineral matrix of the bone, exposing delicate and still flexible scraps of tissue inside.

Here, for example, is a piece of endothelial tissue, or the tubelike epithelia that line blood vessels and form capillaries. It is compared to a similarly prepared piece from fresh ostrich bone; you can tell the T. rex fragment has undergone some changes, but it's comparable in size and organization to the bird sample.

(I) T. rex vessel fragment showing detail of branching pattern and structures morphologically consistent with endothelial cell nuclei (arrows) in vessel wall. (J) Ostrich blood vessel liberated from demineralized bone after treatment with collagenase shows branching pattern and clearly visible endothelial nuclei.

Looking more closely with a scanning electron microscope, here's a similar piece of T. rex blood vessel that has ruptured, spilling out its contents. Maybe those cells don't look perfectly preserved, but they're darned close.

Exploded T. rex vessel showing small round microstructures partially embedded in internal vessel walls.

And lastly, here's a closeup of the surface of that epithelia, compared with an ostrich epithelium. The cells here are very, very flat, and the nuclei are the thickest part, bulging up and giving the surface a pebbled appearance. The T. rex epithelium has a similar pebbly look, suggesting that just maybe there is even some subcellular structure preserved.

(E) Higher magnification of a portion of T. rex vessel wall, showing hypothesized endothelial nuclei (EN). (F) Similar structures visible on fixed ostrich vessel. Striations are seen in both (E) and (F) that may represent endothelial cell junctions or alternatively may be artifacts of the fixation/dehydration process.

How could this be? Here's the authors' explanation.

…we demonstrate the retention of pliable soft-tissue blood vessels with contents that are capable of being liberated from the bone matrix, while still retaining their flexibility, resilience, original hollow nature, and three-dimensionality. Additionally, we can isolate three-dimensional osteocytes with internal cellular contents and intact, supple filipodia that float freely in solution. This T. rex also contains flexible and fibrillar bone matrices that retain elasticity. The unusual preservation of the originally organic matrix may be due in part to the dense mineralization of dinosaur bone, because a certain portion of the organic matrix within extant bone is intracrystalline and therefore extremely resistant to degradation. These factors, combined with as yet undetermined geochemical and environmental factors, presumably also contribute to the preservation of soft-tissue vessels. Because they have not been embedded or subjected to other chemical treatments, the cells and vessels are capable of being analyzed further for the persistence of molecular or other chemical information.

So, basically, these cells were entombed in a thick mineral sarcophagus, protected from bacteria and other external insults. There have to have been other factors at play—cells are full of enzymes that trigger a very thorough self-destruct sequence at death—so I'm definitely looking forward to the molecular analysis. Even if their form was preserved, I expect these cells to be denatured monomer soup on the inside.

Schweitzer MH, Wittmeyer JL, Horner JR, Toporski JK (2005) Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science 307(5717):1952-1955.