ABSTRACT
Comparative pathologists who engage in mouse developmental pathology studies must possess a thorough understanding of normal anatomic features in order to effectively nd and characterize lesions in developing mice. Most investigators will possess a fair degree of assurance in their ability to assess structural traits in near-term fetuses (gestational day [GD] 17.5 or older), neonates, and juveniles (up to postnatal day [PND] 35). Condence in this regard is founded in the close structural resemblance of most cells, organs, and tissues during these late developmental stages to the appearance of the corresponding components in adult mice. However, developmental anatomy is not conned merely to consideration of the normal three dimensions of length, width, and height that constitute the usual focus of clinicians, pathologists, and other biomedical scientists. Instead, prociency in the art and science of developmental anatomy necessarily includes comprehension of structural changes in the fourth dimension of time as well (see Figure 1.4). The transient, sometimes weird, constantly shifting morphology of the embryo and the extraembryonic membranes early during gestation affords a particular challenge to comparative pathologists tasked with unraveling the cause of embryonic lethal phenotypes.
