causes embryonic death but not embryonic infection. Stain (panels (e) and (f)): H&E.
Sources: Ward and Devor‐Henneman [44] with permission of Iowa State University Press, Bolon and Ward [39] with permission of CRC Press, and Szaba et al. [83] by permission of the Authors under a Creative Commons 4.0 International License.
Figure 5.18 A common cause of neonatal lethality is inability to suckle, which is shown by the inability to discern a “milk spot” (milk‐engorged stomach [arrow]) through the left abdominal wall. Relative to wild‐type (WT) littermates, knockout mice lacking the gene for cardiotrophin‐like cytokine (Clcf1tm1Zou) die by PND1 from dehydration and starvation resulting from deficient numbers of brainstem neurons needed to mediate nursing.
Source: Zou et al. [99] by courtesy of Sage Publications.
Figure 5.19 An outcome‐oriented decision tree for evaluating embryonic lethal phenotypes in developing mice. The initial analysis is performed near term (gestational day [GD] 17.5–18.5). If necessary, one or more follow‐up experiments may be needed earlier in gestation, usually either moving backward in two‐day intervals (GD16, GD14, GD12, GD10…) or selecting a new time point based on the external features of regressing implantation sites or dead embryos.
Pathologic Patterns in Mouse Placenta
The placenta is the first part of the conceptus that will be available for macroscopic evaluation, and it should be subjected to histopathological evaluation during investigations of embryonic lethality. The reasons for including this organ in the assessment are that many genes are expressed in both embryo and the embryonic portion of the placenta, and that abnormal placental development will lead soon thereafter to embryonic death even in the absence of lesions in the embryo proper [83]. About 650 abnormal placental phenotypes have been reported in mutant mouse embryos [93].
Since a properly conducted developmental pathologic evaluation should include an assessment of this organ, the placenta should be evaluated grossly before the embryo examination begins [94]. This placental review may be limited to rapid observations of size, shape, and color as many placental conditions present as alterations in these parameters; gross defects in placenta often are indicative of obvious embryonic defects as well (Figure 5.21). In some cases, placental weights may be obtained to discern more subtle variations in mass. For more extensive review and photo‐documentation, the placenta typically should be placed in a buffered saline solution to prevent drying and collapse. In general, however, the placenta should be assessed and then quickly fixed to prevent excessive autolysis of this highly vascular organ.
Figure 5.20 An outcome‐oriented decision tree for evaluating neonatal and juvenile lethal phenotypes. The initial analysis is performed at weaning (postnatal day [PND] 21–22). If necessary, one or more follow‐up studies may be needed at earlier postnatal time points, moving backward in three‐day intervals during the late juvenile period (PND18, PND15, PND12, PND9, PND6) and then at one‐ or two‐day intervals through the early juvenile and neonatal periods (PND4, PND, PND1). Source: Based on ref [22, 88, 89]
Figure 5.21 Placental dysfunction relative to wild‐type littermates (left column) may be observed macroscopically in knockout mice (right column) by reductions in vascularity of the yolk sac (top row) or size (i.e. hypoplasia, middle and bottom rows) of the definitive placenta. Findings may be visible from the side (upper two rows) or base (bottom row). Abbreviations: +/+ = wild‐type, −/− = knockout, C = chorion, D = decidua, L = labyrinth.
Sources:Mapk14tm1Mka (p38α knockout) mice from Tamura et al. [100], and Cited2tm1Jpmb (Cited2 knockout) mice from Withington et al. [101] with the permission of Elsevier.
Histopathological assessment of the placenta typically is performed on NBF‐fixed, paraffin‐embedded, H&E‐stained tissue sections, with serial sections used as needed to explore the expression of cell type‐specific molecular markers. Bouin's solution also may be used as a placental fixative (since embryos and placentas typically are fixed in a single container, and Bouin's penetrates older embryos better than NBF). However, Bouin's solution tends to rupture erythrocytes and destroys some fragile antigens, so this fixative should be employed with discretion for this organ. The key to successful histopathological evaluation of placenta is that the pathologist must be familiar with the laminar organization and cell populations within the various portions of the placenta (especially YS and the DP) during both early and late stages of gestation. In particular, the early placenta consists largely of a thick mass of maternal decidua encompassing a thin layer of trophectoderm (GD4.5–7.0); transitions to a thin undulating YS separated from a modest decidual margin by a thin layer of trophoblast giant cells in mid‐gestation (GD7.5–10.5); and then assumes a mature DP conformation with thick inner labyrinth, middle JZ with spongiotrophoblast nearest the labyrinth and trophoblast giant cells peripherally, and outer decidual rim (at GD12.5 and later).
Patterns of placental lesions leading to embryo mortality vary depending on the timing of the lethal event (Table 5.2 and Figure 5.22) [16, 17, 94, 95]. Early placental‐based lethality commonly involves either circulatory dysfunction in the YS with endothelial and/or hematopoietic disruption, leading to embryonic death between GD8.5–10.0, or failure of chorioallantoic fusion (i.e. umbilical cord formation) with embryonic death from GD9.5–11.0. Later placental‐based lethality typically results from malformed placental labyrinth due to faulty differentiation of one or more labyrinthine trophoblast lineages, leading to embryonic death occurring from GD13–16. In many cases, the precise cause and mechanism cannot be defined, so a generic diagnosis of “small labyrinth” is applied as a nonspecific end‐stage finding [17]. Disrupted formation of the placental labyrinth is by far the most frequent mechanism responsible for placental failure in mice. For example, lesions in the labyrinth due to gene inactivation or viral infection of endothelium [83] include trophoblast hyperplasia with mitotic figures (Figure 5.23a) and embryonic endothelial cell necrosis (Figure 5.23b); these changes may lead to placental failure and embryonic death. If an embryo dies during gestation and placental development is normal until the embryo dies, the placenta will regress as seen initially by necrotic debris within embryonic blood vessels in the labyrinth, which represents evidence of dead cells arriving from