Histological Properties of the Chorionic and Basal Plates and Maintenance of the Mechanical Environment in the Human Placenta

Abstract

The human placenta is covered with the chorionic and basal plates, which face the fetal and maternal sides, respectively. Each plate shows its own characteristics in tissue structure so that these plates would have quite different mechanical properties: The mechanical environment of the placenta would be dependent on its position, a fetal side or maternal side. In the meantime, considering that the blood circulations in the placenta, the fetal blood flows in the blood vessels, which pass through the umbilical cord, chorionic plate, and villous trees, and the maternal blood flows in the blood vessels in the basal plate and intervillous space. The chorionic and basal plates would be necessary for the fetal and maternal blood circulations, respectively. In this chapter, the influence of the chorionic and basal plates on the mechanical environment of the placenta, and the fetal and maternal blood circulations, is explained.

1. Introduction

The substances in the fetal and maternal blood, such as nutrition and gas, are successfully exchanged so that a fetus grows well. The fetal blood flows into the umbilical artery, circulates the blood vessels in the villous trees of the placenta, and returns to the fetus through the umbilical vein, while the maternal blood in the endometrial spiral artery flows into the intervillous space, corresponding to the space around the villous tree, and after circulating the space, returns to the endometrial vein. The fetal and maternal blood circulations do not share the space, but their substances are exchanged through the villous trees in the placenta. The condition of the fetal and maternal circulations would directly influence the substance exchange. Considering that the mechanical environment around these circulations could change the flow pattern, the environment would be modulated in order to keep the proper conditions of the circulations.

The umbilical cord shows the histological characteristics [1], which would be helpful to keep the mechanical environment suitable for the blood flow [2]. Also, the active contraction of the villous tree [3, 4], the contractile cells around the fetal blood vessels [5, 6, 7, 8], expected to cause the contraction, have been reported. The computational models based on these previous reports [3, 4, 5, 6, 7, 8] have been developed and indicated that the contractile system would be helpful for the fetal and maternal blood circulations [9, 10]. Moreover, it has been reported that the dilation of the spiral artery, caused by the loss of smooth muscle and elastic lamina, could modulate the maternal blood flow properly [11]. In the meantime, the deformation of the placenta and blood flow caused by the uterus contraction, evaluated by MRI images, has been reported [12, 13]. According to the previous report [14, 15, 16], the thickness and diameter of the placenta at the third trimester were about 40 mm and 200 mm, respectively. Comparing the placental size with the resolution of the MRI images (2.4 × 2.4 × 5 or 2.5× 2.5 × 6 [mm³]) [12], these quantitative values would be helpful in the evaluation along the circumferential direction of the uterus.

The placenta at term is covered with the chorionic plate, marginal zone, and basal plate [17, 18]. The chorionic plate connects to the umbilical cord while the basal plate is next to the placental bed. The marginal zone is placed between these two plates. These regions, which are continuously connected with each other, show the two types of fibrinoid, the fibrin-type and matrix-type fibrinoid [17, 18, 19, 20]. Also, smooth muscle was observed at the chorionic plate [5] and marginal zone [18]. The distribution was expanded to the basal plate [18]. The mechanical properties of the chorionic plate were evaluated by the elastic moduli and thickness in the amniotic and chorionic layers [21]: the amniotic layer, 1.90 MPa and 47 μm; the chorionic layer, 2.2 MPa and 185 μm; the entire layer, 4.7 MPa and 243 μm. The elastic moduli of the placenta were varied by the methods [22]: The elastic moduli at tension and shear were more than twice of those at compression. Considering that the amniotic and chorionic layers could influence the elastic moduli at tension and shear more than that at compression, the elastic modulus of the region surrounded by the layers would be much lower than those in the layers.

In this study, a computational model of the human placenta was developed in order to evaluate changes in the mechanical environment of the human placenta, caused by the elongation and contraction of the uterus. Based on the results, how the histological characteristics of the placenta assist the fetal and maternal blood circulations will be indicated.

2. Computational model of the human placenta and its analysis

The model development and computations were carried out by the finite element analysis software (COMSOL Multiphysics®, 6.1). The computational model of the human placenta, developed in this study, is shown by Figure 1. The morphological characteristics and mechanical properties of the components in this model are indicated by Tables 1 and 2, respectively. As Figure 1 shows, the placenta is set as the ellipse (A), whose major and minor axes were based on the radius and thickness of the placenta at the gestational age (40 weeks), respectively [15]. The placenta was covered with the thin area (B₂) entirely, composed of the chorion (part of the chorionic plate), marginal zone, and basal plate. The thickness was corresponding to that of the chorion [21]: Because the thickness of the chorionic plate was larger than the sum of those at the amniotic and chorionic layers [21], the difference was evenly added to each layer’s thickness. Also, the region B₂ was surrounded by the two thin layers (B₁ and B₃): B₁, amnion (part of the chorionic plate); B₃, the boundary region between the placenta and myometrium. The thickness in B₁ and B₃ was the same as that in amnion [21]. B₁ and B₂, corresponding to the chorionic plate, were extended to the point, 24 mm from the ellipse A (about 10% of the major axis), keeping parallel with its major axis. Both ends of B₁ and B₂, F₁ and F₂ in Figure 1 were fixed at all the computation. The region C, the myometrium, was put the thin regions (B₂ and B₃) with the thickness before labor [23]. Every region in the region C, including the side of the region A, and the region under the region A, was connected smoothly, with the radius of curvature, the same as its thickness. Considering that the uterus is constrained in the various ways, including cardinal ligament, broad ligament of the uterus ligament of ovary, and round ligament of uterus [26, 27], the region D (surroundings) under the region C (myometrium) was set. The bottom of the region D, F₃ in Figure 1, was fixed at all the computation. Assuming that the uterus could move to some degree, the elastic modulus of the region D was set much lower than that of the others. The elastic modulus of the placenta was corresponding to that at compression [22] in order to weaken the influence of the chorionic plate. The elastic modulus at chorion [21] was used for B₂. Because the region B₃ faces the maternal side, which is composed of the loose fibers and a complex vascular plexus [17], its elastic modulus was set at the same as B₁. While Poisson’s ratio at myometrium was 0.499 for incompressibility as muscular tissue, that in other parts was 0.3, corresponding to gel [24], because incompressibility was not clear. Table 3 shows the elongation and contraction of the uterus (Mode 1, Mode 2, and Mode 3). Figure 2 shows the model with the triangular elements (305,336 elements).

Figure 1.

A computational model for the human placenta and its surroundings, developed in this study. A, placenta; B1, amnion (chorionic plate); B2, chorion (chorionic plate), marginal zone, and basal plate; B3, boundary region between the placenta and myometrium; C, myometrium; D, surroundings. The details of the region R1 (broken lines, upper) and R2, part of R1 (broken lines, lower, left), are shown at the lower images (left and right), respectively. F1, F2, and F3 were fixed in all the cases.

Region	Component	Type	Size
A	Placenta	Ellipse	Major axis = 104 mm [15]
			Minor axis = 22.5 mm [15]
B1	Amnion (chorionic plate)	Layer	Thickness = 52.5 μm [21]
			Side = 24 mm
B2	Chorion (chorionic plate), Marginal zone, Basal plate	Layer	Thickness = 190.5 μm [21]
			Side = 24 mm
B3	Boundary region between the placenta and myometrium	Layer	Thickness = 52.5 μm
C	Myometrium	Layer	Thickness = 8.48 mm [23]
			Side = 24 mm
D	Surroundings	Layer	Thickness = 92.8–115.5 mm
			Side = 24 mm

Table 1.

Morphological characteristics in the computational model.

Region	Mechanical properties
	Elastic modulus (Pa)	Poisson’s ratio
A	5.0 × 10^4 [22]	0.3 [24]
B1	1.9 × 10^7 [21]	0.3 [24]
B2	2.2 × 10^6 [21]	0.3 [24]
B3	1.9 × 10^7	0.3 [24]
C	1 × 10^7 [25]	0.499
D	1	0.3 [24]

Table 2.

Mechanical properties in the computational model.

Category	Mode	Fixation	Direction	Force/m (N)
Mode 1	Contraction	F1, F2, and F3	From F1 to F2	8.48 [28]
Mode 2	Contraction		From F1 and F2 to B3C
Mode 3	Elongation		From B3C to F1 and F2

Table 3.

Elongation and contraction of the myometrium.

B_3C, the center of B3.

Figure 2.

Mesh in the computational model (305,336 elements).

The mechanical environment was evaluated by von Mises stress, the first principal stress, displacement, and the first principal strain. The distribution in each parameter was visualized. Also, the fluctuation of each parameter was evaluated along the lines, depicted by Figure 3.

Figure 3.

Lines for evaluation in the mechanical environment of the placenta and surroundings. 7 lines (a–g), parallel with each other, were set 40 mm each along the horizontal direction.

3. Results

3.1 Von Mises stress and the first principal stress

Figure 4 shows the results of von Mises stress in each computation. While the shape of the placenta was largely deformed because of the myometrium contraction and elongation, von Mises stress in the placenta was kept much lower than that in the chorionic and basal plates, marginal region, and myometrium. Also, von Mises stress at these layers fluctuated largely. Figure 5 shows the results of the first principal stress. As the results of von Mises stress, the first principal stress in the placenta was kept lower. The chorionic and basal plates, and marginal region, whose elastic moduli were much higher than the placenta, would prevent changes in the mechanical environment of the placenta. The stable condition in von Mises stress and the first principal stress would promote the effective usage of the perivascular contractile cells in the villous tree and chorionic plate for the fetal and maternal blood circulations.

Figure 4.

von Mises stress in the computational model of the human placenta and surroundings, caused by the myometrium elongation and contraction (Mode 1, Mode 2, and Mode 3 (Table 3)). The stress distribution is shown by the entire image (right), and the values along each line, indicated by Figure 3 (left).

Figure 5.

The first principal stress in the computational model of the human placenta and surroundings, caused by the myometrium elongation and contraction (Mode 1, Mode 2, and Mode 3 (Table 3)). The distribution of the stress is shown by the entire image (right), and the values along each line, indicated by Figure 4 (left).

3.2 Displacement and the first principal strain

Figure 5 shows the result of displacement in each computation. The deformation in the placenta at Mode 1 was much larger than that in other cases. The elongation and contraction the myometrium would cause various deformation patterns in the placenta so that the fetal and maternal blood could be stirred. As Figure 6 shows, the first principal strain showed the same tendency (Figure 7).

Figure 6.

Displacement in the computational model of the human placenta and surroundings, caused by the myometrium elongation and contraction (Mode 1, Mode 2, and Mode 3 (Table 3)). The distribution of the displacement is shown by the entire image (right), and the values along each line, indicated by Figure 3 (left).

Figure 7.

The first principal strain in the computational model of the human placenta and surroundings, caused by the myometrium elongation and contraction (Mode 1, Mode 2, and Mode 3 (Table 3)). The distribution of the strain is shown by the entire image (right), and the values along each line, indicated by Figure 4 (left).

4. Discussion

In this study, the computational model of the human placenta and its surroundings has been developed, and the influence of the uterus contraction and elongation on the mechanical environment in the placenta was evaluated by the model. The mechanical load, such as force and stress, in the villous trees and intervillous space, which is corresponding to the placenta region in this model, would be stable although the large and various deformation could be caused by the uterus contraction. As the previous reports have indicated, the characteristic uterus contraction and elongation could deform the placenta [12, 13]. While the structural characteristics of the chorionic and basal plates and its change along gestational age have been investigated [11], how these plates could influence the mechanical environment on the placenta has been barely examined. It has been reported that the smooth muscle in the marginal zone would actively influence on the uteroplacental vein [18], but its function for the blood circulation in the placenta has been barely examined.

While the thickness of the chorionic plate was just about 1% of the placental thickness [14, 15, 16, 21], the chorionic plate is composed of chorion and amnion, whose mechanical characteristics are quite different [11, 21]. Although the thickness and mechanical properties of the basal plate and marginal zone have been barely examined, these parameters in this model were the same as those of the chorionic plate in this model because they connect to the chorionic plate continuously. The results of this computational model have shown the importance of these thin plates for maintaining the mechanical environment of the intervillous space and villous tree.

The structural characteristics of the chorionic and basal plates and marginal zone has been simplified in this model. For example, amnion could be easily separated at “spongy layer” in the chorionic plate. This behavior would be mechanically important so that this layer will be taken into the computational model in the future. Also, fibrinoid, contained a lot in the placenta, including the chorionic and basal plates, is considered in the model development. Although the mechanical properties have been barely investigated, the estimation based on the previous reports [11, 19, 20] will be helpful.

5. Conclusion

The computational model of the human placenta and its surroundings, including the chorionic and basal plates and marginal zone, has been developed. The computation based on this model has indicated that the uterus contraction could cause the large deformation of the placenta, and mechanical load would be maintained so that the fetal and maternal blood circulation could be assisted by the contractile cells around the blood vessels.

Conflict of interest

The author declares no conflict of interest.