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Embryonic liver has a unique external morphology and quantitative morphometry, based on magnetic resonance imaging data of human embryos from the Kyoto Collection of Human Embryos. Liver morphogenesis is strongly affected by the adjacent organs and tissues. The left ventricle develops to the left medial-caudal side, which results in the formation of a depression at left medial region and a prominence bilaterally at the cranial surface of the liver between Carnegie Stage (CS)17 and CS19. An imprint of the stomach that formed at the dorsal left-medial region of the liver became more marked with development until CS23. A depression induced by the umbilicus formed at the ventral region of the liver between CS16 and CS19. An indentation caused by the right adrenal gland formed at the dorsal-caudal region of the liver surface from CS20. Morphometric analysis revealed that the volume of the liver increased exponentially from CS14 through CS23. The liver developed preferentially along the dorsoventral axis and right/left axis until CS17, along the craniocaudal axis between CS17 and CS19, and then in all directions after CS19. Several important developmental phenomena, such as differentiation of the diaphragm, the extension of the body axis of the embryo, and the physiologic herniation of the intestine into the umbilical cord, may affect morphometric data. These data contribute to a better understanding of liver development as well as the morphogenesis of adjacent organs, both temporally and spatially, and serve as a useful reference for fetal medicine and prenatal diagnosis. Anat Rec, 2012. © 2011 Wiley Periodicals, Inc.
The liver occupies a large space in the abdominal cavity during most of the prenatal period and plays an important role in the development of functional organs (Lemaigre, 2009; Sadler and Langman, 2010). The liver becomes a hematopoietic organ after 6 weeks (Drews, 1995) and begins to metabolize important biochemical materials for development, such as albumin, bile, glycogen, and fetal-specific proteins, at around 8 weeks (Carlson, 2009).
The development of the liver proceeds in a unique manner. The liver develops at Carnegie Stage (CS) 11 (30 days after fertilization) as an outgrowth of the endodermal epithelium, the liver bud, from the caudal part of the foregut. The liver originates from two different tissues: angioblastic tissue from the coelomic surface cells and epithelial columns sprouting from the hepatic evagination of the gut epithelium (O'Rahilly and Müller, 1987). The liver lies at an active center of angiogenesis in the early embryonic period. The asymmetricity of the afferent venous vessels of the liver derives from two specific circulation systems: the vitelline and umbilical veins, which are acquired between CS13 and CS16 (Mall, 1906; Dickson, 1957; Collardeau-Frachon and Scoazec, 2008). Efferent venous vessels, including the right, left, and middle hepatic veins (HVs) and the inferior vena cava (IVC), form at similar stages. The developmental process of the efferent venous vessels is not as well studied as that of the afferent venous vessels (Mall, 1906; Dickson, 1957; Couinaud, 1996; Collardeau-Frachon and Scoazec, 2008).
Among recent three-dimensional (3D) imaging techniques, magnetic resonance (MR) microscopy is a powerful tool for 3D measurements. It is a noninvasive and nondestructive imaging method, and has been applied to analyze embryonic development in different animal models (Bone et al., 1986; Smith et al., 1992, 1994, 1996). MR imaging of embryos is highly advantageous (Effmann et al., 1988; Smith et al., 1992; Haishi et al., 2001), providing a resolution of 40 μm/pixel or better with long scan times. Kyoto and Tsukuba Universities began a project in 1999 to acquire 3D MR microscopic images of thousands of human embryos using a super-parallel MR microscope operated at 2.34T (Shiota 2007; Matsuda et al., 2003, 2007; Yamada et al., 2006).
In the present study, the precise external morphology and morphometry of the embryonic liver was studied using MR imaging data of human embryos from the Kyoto Collection of Human Embryos (http://bird.cac.med.kyoto-u.ac.jp). These data will serve as a useful reference for evaluating the development of the embryonic liver and adjacent organs and how they morphologically affect each other.
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- MATERIALS AND METHODS
- LITERATURE CITED
- Supporting Information
The liver bud grows rapidly, and the embryonic liver occupies most of the abdominal cavity after the end of the 6th gestational week (ca. CS16; Hutchins and Moore, 1988; Lemaigre, 2009). The details of the morphologic and morphometric features of the liver during the early embryonic period, however, have remained unknown. Mall (1906) made wax models of the liver exterior from serial histologic sections of the human embryo to study the positional relationship of the gall bladder and vascular system from an outside view. He only described the morphologic changes of the liver in embryos between 17.5 and 24 mm in size; “By comparing the livers of three embryos it is seen that only their upper surfaces are regular in form from stage to stage; the processes extending into the abdominal cavity are irregular, to fit into the spaces that there are for them to grow into.” Severn (1971) examined serial histologic sections of 38 human embryos from CS9 through CS11. For his detailed histologic observation, 3D drawings of the developing foregut and hepatic diverticulum were made, showing the change in the external appearance. Hutchins and Moore (1988) reported that the liver appeared at CS11 and grew to over 90 mm3 in volume by CS23. They calculated the difference in the volume between right- and left-halves of the liver, divided by the median sagittal plane; the right half was large with an average proportion of 57.8%, and the ratio was almost constant in all embryos from CS11 through CS23. In the present study, external morphologic and morphometric analysis of the liver during embryonic periods was performed using MR imaging data acquired from embryos obtained from the Kyoto Collection. The present data revealed a unique external morphology as well as the quantitative morphometry of the embryonic liver.
Morphogenesis of the liver was strongly affected by the adjacent organs and tissues. The characteristic effects of stage- and organ-specific changes are summarized in Fig. 10. The left ventricle developed to the left medial-caudal side, which resulted in the formation of a clear depression in the left medial region and prominence bilaterally on the cranial surface of the liver between CS17 and CS19 (Fig. 3A-c,d). An imprint of the stomach formed at the dorsal left-medial region of the liver, and became more marked with development until CS23 (Fig. 4A-a,c,e). A depression caused by the umbilicus formed in the ventral region of the liver between CS16 and CS19 (Fig. 5A-b). An indentation created by the right adrenal gland was formed at the dorsal-caudal region of the liver surface from CS20 (Fig. 6-a,b). Therefore, the morphology of the embryonic liver reflects the development of the adjacent organs during organogenesis.
Figure 10. Development of adjacent organs and tissues that may affect liver morphogenesis. Temporal (stage-specific) and organ-specific effects are indicated and named according to the Carnegie stage.
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Morphometric analysis in the present study revealed that the volume of the liver increased exponentially from CS14 through CS23, and the ratio of LTR, LDV, and LTH to LCC presented here indicated that the direction of growth changed at around CS17 and CS19 (Figs. 8B, 9B). That is, the liver developed preferentially along the dorso/ventral axis and right/left axis until CS17, along the cranio/caudal axis between CS17 and CS19, and then in all three directions. The occurrence of several important developmental phenomena around CS17 may affect the morphometric data (O'Rahilly and Müller, 1987; Moore, 2008; Schoenwolf and Larsen, 2009). When the septum transversum begins to differentiate into the diaphragm, development in the cranial direction is limited, while development towards the abdominal cavity is accelerated likely due to extension of the body axis of the embryo and physiologic herniation of the intestine into the umbilical cord, which creates space and transform the inner structures of the abdominal cavity (O'Rahilly and Müller, 1987).
Original and first-hand data regarding the stages of development of the vascular architecture of the liver are scarce (Collardeau-Frachon and Scoazec, 2008). Though the asymmetry of the hepatic vascular structure may be acquired between CS13 and CS16, the precise stages of development could not be determined. The right umbilical vein, which is an important indicator of the symmetrical stage, was clearly detected at CS13 (O'Rahilly and Müller, 1987). In the present study, only one case showed the right umbilical vein at CS14 and other 61 of 62 cases had already lost the right umbilical vein by CS14; that is, the hepatic vascular structure was already asymmetrical. The present data suggest that the fundamental architecture of the asymmetrical stage is acquired between CS13 and CS14 in almost embryos.
The terminal HVs formed at a similar stage as the afferent circulation system, as mentioned earlier. In the present study, three HVs were observed in 68.4% of the cases after CS15, indicating that the three HVs are acquired around CS15 in most cases. These data are consistent with those of a previous study reporting profound remodeling of the efferent venous system during the 5th gestational week (Dickson, 1957; Collardeau-Frachon and Scoazec, 2008). Three HVs were not identified until after CS17 in 19.6% of cases, suggesting that there are several individual variations in the number and arrangement of the terminal HVs, in contrast to the afferent venous circulation systems. It is so far impossible to distinguish an anomaly from a variation in individual embryos, mainly because only terminal HVs were detected on the MR image. Detailed identification of such a small branch of the vessels depends on the resolution of the imaging technique. Further improvements in imaging modalities are expected that will allow for more precise detection of the intrahepatic vascular system and application to analyses at CS13 or earlier.
Recent advances in medical imaging allow for earlier assessment of human development and prenatal diagnosis in the first trimester. Data about normal development during the embryonic stages, however, remain inadequate for guiding such clinical evaluations. Insights into the dynamic and complex processes during organogenesis will require accurate morphologic data with dynamic modeling of embryonic structures. Furthermore, 3D reconstructions are necessary to elucidate the complex anatomic remodeling that occurs during these early embryonic stages. From this point of view, the present data will be useful for evaluating the appropriate development of the embryonic liver based on the external morphology, and for evaluating adjacent organs that affect the morphology of the liver stage-specifically. This information will be an indispensable reference for clinical evaluation with obstetrical ultrasonography in the early gestational weeks, which will be useful for fetal medicine and prenatal diagnosis.