通过检查灭绝恐龙留下的骨头来确定它们的性别的能力将彻底改变我们对恐龙古生物学的理解。然而，迄今为止，明确的性别特异性骨骼特征仍然难以捉摸或存在争议。尽管现存的恐龙（即现存的鸟类）表现出雌性所独有的称为髓质骨的性别特异性组织，但在非鸟类祖龙中自信地识别这种组织已被证明是一个挑战。由于活体物种中髓质骨组织的现有变异，追踪髓质骨的进化变得复杂。髓质骨结构或化学成分在进化过程中发生变化的假设；缺乏旨在将髓质骨与具有相同微观结构和发育特征的其他类型骨内膜组织（例如病理组织）区分开来的研究。最近的一项研究试图利用髓骨的分子特征，在活的鸟类中，髓骨含有特定的标记物，例如硫酸化糖胺聚糖硫酸角质素，以支持对非鸟类恐龙标本（霸王龙MOR）髓骨的鉴定。第1125章）据称，MOR 1125 的髓质骨样本对组织化学分析呈阳性反应，而测试的单一病理对照（禽类骨硬化症）却没有，这表明硫酸角质素的存在可能有助于在未来的研究中明确区分这些组织。为了进一步测试这些结果，我们对 20 种不同病因（18 个物种）的禽类骨病理样本和几个 MB 样本进行了采样。我们的新数据普遍支持硫酸角质素作为鸟类髓质骨的可靠标志物。 然而，我们还发现不同病理骨组织的反应性有所不同，某些病理中的反应性与 MB 无法区分。在当前样本中，一些由软骨样骨（通常是骨骼病理学的主要成分和脊椎动物中形成的骨折老茧）组成的病理学含有硫酸角质素。我们注意到，除了化学之外，软骨样骨与髓质骨具有许多特征（纤维基质、大量和大的细胞腔隙、潜在的骨内膜起源、小梁结构），并且髓质骨甚至被一些人认为是软骨样骨的一种。我们的结果表明，硫酸角质素的存在并不是 MB 的唯一证据，而是必须用作可用于识别非鸟类恐龙标本中的髓骨（以及怀孕雌性）的一套标准中的一个。未来的研究应该调查髓质骨和反应性软骨样骨之间是否存在明确的化学或微观结构差异，从而可以区分这些组织。
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Keratan sulfate as a marker for medullary bone in fossil vertebrates
The ability to determine the sex of extinct dinosaurs by examining the bones they leave behind would revolutionize our understanding of their paleobiology; however, to date, definitive sex‐specific skeletal traits remain elusive or controversial. Although living dinosaurs (i.e., extant birds) exhibit a sex‐specific tissue called medullary bone that is unique to females, the confident identification of this tissue in non‐avian archosaurs has proven a challenge. Tracing the evolution of medullary bone is complicated by existing variation of medullary bone tissues in living species; hypotheses that medullary bone structure or chemistry varied during its evolution; and a lack of studies aimed at distinguishing medullary bone from other types of endosteal tissues with which it shares microstructural and developmental characteristics, such as pathological tissues. A recent study attempted to capitalize on the molecular signature of medullary bone, which, in living birds, contains specific markers such as the sulfated glycosaminoglycan keratan sulfate, to support the proposed identification of medullary bone of a non‐avian dinosaur specimen (Tyrannosaurus rex MOR 1125). Purported medullary bone samples of MOR 1125 reacted positively to histochemical analyses and the single pathological control tested (avian osteopetrosis) did not, suggesting the presence of keratan sulfate might serve to definitively discriminate these tissues for future studies. To further test these results, we sampled 20 avian bone pathologies of various etiologies (18 species), and several MB samples. Our new data universally support keratan sulfate as a reliable marker of medullary bone in birds. However, we also find that reactivity varies among pathological bone tissues, with reactivity in some pathologies indistinguishable from MB. In the current sample, some pathologies comprised of chondroid bone (often a major constituent of skeletal pathologies and developing fracture calluses in vertebrates) contain keratan sulfate. We note that beyond chemistry, chondroid bone shares many characteristics with medullary bone (fibrous matrix, numerous and large cell lacunae, potential endosteal origin, trabecular architecture) and medullary bone has even been considered by some to be a type of chondroid bone. Our results suggest that the presence of keratan sulfate is not exclusive evidence for MB, but rather must be used as one in a suite of criteria available for identifying medullary bone (and thus gravid females) in non‐avian dinosaur specimens. Future studies should investigate whether there are definite chemical or microstructural differences between medullary bone and reactive chondroid bone that can discriminate these tissues.
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