Blood Clots in Dinosaur Bones
will be reviewed shortly. Te second complementary preservation process is based on the natural degradation that takes place when reducing sugars decay to advanced glycation end products over time. Tis mechanism cannot guarantee preservation of labile tissues, but it may add resistance to degradation and microbial decomposition of endogenous proteins over time [3,10]. With respect to metal-catalyzed crosslinking of vessels, tran-
Figure 4: Intact veins from HCTH-02 (DIC microscopy). Scale bar, 40 µm. scanning electron microscopy (SEM) (Figure 7, leſt side,
arrows) especially when imaged as uncoated specimens. We predict that with more deliberate methods, bone cells with lon- ger dendrites will be liberated from dinosaur bone.
Soft Tissue Preservation Mechanisms It was once thought that bacterial biofilms mimicked the
morphology of dST, however those claims have been elimi- nated [11]. Armitage [26] cryo-thin sectioned sheets of fibrillar bone aſter mechanically peeling them from exposed horncore (T. horridus HCTH-02). Frozen sections
(9 µm thickness)
revealed rows of embedded osteocytes stacked three-dimen- sionally on each other. It is impossible for biofilms to replicate three-dimensionally stacked (and filipodia-linked) cells. Presently two complimentary systems of preservation are
proposed to account for the survival of endogenous blood vessels through deep time. Te first of these has been described as tran- sition metal-catalyzed intermolecular crosslinking [6,11], which
sition metals are necessary for protein stabilization. Free iron (heme) is normally bound up by intracellular proteins in large quantities within tissues and cells. Cells mediate cytotoxicity by binding heme with specialized proteins [27]. Heme is also very highly concentrated within erythrocytes [28]. It is proposed by some workers that heme (or hemoglobin from red blood cells) is liberated post-mortem and acts to generate hydroxyl radicals (Fenton reactions), which “fix” tissues similar to the way aldehydes fix tissue for microscopy [10,11]. Hydroxyl (HO) and peroxyl (HOO) radicals are said to stabilize collagen and elastin structural proteins via crosslink formation, particularly within vessel wall amino acids [10]. Te added stability might confer stiffness and harden resistance to degradation of vessels over deep time. Hemoglobin is clearly the stated source for iron mediated
stabilization of dST [11] as there is an “intimate association” between iron particles and soſt tissues, particularly bone cells (see section Ultraviolet Fluorescence and Autofluorescence of Bone). “Blood cells rich in iron-containing HB flow through vessels and have access to bone osteocytes through the lacuna- canalicular network,” [11]. What is not explained is how hemo- globin trapped in erythrocytes avoids the inescapable blood cascade reaction known as thrombosis, which initiates clot pro- duction immediately upon tissue injury or body trauma [29]. In response to tissue damage, prothrombin in the blood becomes activated to thrombin. Trombin rapidly cleaves fibrinogen into fibrin fibers. Deposition of fibrin fibers (along with the expression of a host of co-factors) into the blood vessels acti- vates the clotting cascade. Fibrin segments form a meshwork of fibers upon which the rest of the clot is assembled, particularly aſter platelets are activated to assist in clot formation. Erythro- cytes are captured and sequestered within clots adding rigid- ity. Trombin production continues and peaks within about 6 minutes aſter injury in a “burst” eruption [30,31]. Tis lasts as long as platelets and co-factors are plenti- ful in plasma and on cells to maintain the reaction, but in less than 30 minutes clot formation enters a stability phase [31]. Intriguingly, it has been shown that
Figure 5: Intact neural filament from HCTV-22 showing birefringence under polarized light with a ¼ wave plate. Scale bar, 40 µm.
2020 September •
www.microscopy-today.com
when whole blood is exposed to metal-cata- lyzed oxidizing reagents, fibrinogen can be modified by hydroxyl radicals thus inhibit- ing thrombin-induced clot formation [31]. However, it has not been determined how thrombosis, once activated, might influ- ence the production of hydroxyl radicals in dinosaur tissues or inhibit the passage of free water through vessels, which is required for Fenton reaction longevity. For Fenton hydroxyls to reach, envelop, and preserve tissues deeply sequestered in bone, several requirements must be met; RBCs
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