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Keywords:

  • Hyrtl's anastomosis pregnancy-induced hypertension;
  • normotensive;
  • umbilical arteries;
  • umbilical cord;
  • placenta

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

Umbilical arteries carry the blood from the fetus to the placenta and are typically connected by Hyrtl's anastomosis, a connection that is located near where the umbilical cord meets the placenta. The investigation of the anastomosis in pathological conditions, including pregnancy-induced hypertension is limited. Hence, 200 placenta and umbilical cords, 100 from normotensive and 100 from pregnancy-induced hypertensive subjects, were dissected and measurements were recorded. A single anastomosis between the umbilical arteries was observed in167 specimens. In 16 cases, the two umbilical arteries were fused, in 15 cases there was no anastomosis, and in two cases there was a single umbilical artery. In one specimen from a normotensive case, a double anastomosis was observed. To our knowledge this is only the second report of this rare anatomical variant. When an anastomosis is present, the connecting vessel can be transverse to or form an oblique angle with the umbilical arteries. We observed a striking increase in the number of artery pairs connected by a transverse vessel in specimens from hypertensive subjects relative to those from normotensive subjects. Moreover, placentas from hypertensive donors were small if the umbilical arteries were connected by an oblique anastomosis. In addition, the length of the anastomosis and its distance from the cord insertion was shorter in specimens from hypertensive compared to normotensive subjects. We conclude that pregnancy-induced hypertension alters the anatomy of Hyrtl's anastomosis, and in some circumstances, the placenta. Anat Rec, 297:819–825, 2014. © 2014 Wiley Periodicals, Inc.

Abbreviations used
ANOVA

analysis of variance

FVW

flow velocity waveform

SUA

single umbilical artery.

The placenta is essential for proper nutrient and gaseous exchange and removal of unwanted products during the growth of the fetus, serving as a vital link between fetus and mother. The umbilical vein and arteries link the fetus and the placenta. The later arise from the internal iliac arteries and usually anastomose near the umbilical cord insertion into the placenta. This is known as Hyrtl's anastomosis. It was discovered by Hebenstreit in 1737, drawn for the first time in 1748 by Albin, and investigated extensively in 1870 by Hyrtl (Hyrtl, 1870). Over the next 100 years, a number of studies helped to further characterize the anastomosis, but they mainly were performed in poorly characterized specimens, limiting their value (Bacsich et al., 1938; Priman, 1959; Szpakowski, 1974).

Stieve and Strube (1933) and Patten (1946) reported that the placental circulation is established during the 3rd and 4th week of the embryonic period. They observed networks with multiple anastomosis in the umbilical arteries. Hyrtl reported only a single anastomosis (Hyrtl, 1870). Consistent with this, Stutz and Shordania (1929) noted that the networks observed early in development diminished gradually to a single connection as development progressed, though they observed rare instances of double anastomosis (Shordania, 1929). Further, in a study that used corrosion casting, no anastomosis between the arteries was observed except at the level of Hyrtl's anastomosis (Bacsich and Smout, 1938).

Studies of this anastomosis mainly have been performed post-delivery by examining the cord and the placenta (Benirschke and Kaufmann, 1995). One well-studied morphological feature is the angle of the anastomosis with the umbilical arteries. It appears that in most cases, this is an acute angle leading from the higher flow artery to the weaker flow artery (Priman, 1959). In those cases where transverse anastomosis are found, such that, if required, blood can flow in either direction between the arteries, the area of the placenta bathed by the two umbilical arteries is almost equal (Priman, 1959). In contrast, a large difference in the placental areas perfused by the two vessels was reported in oblique anastomoses, though quantitative data were not presented (Szpakowski, 1974). In the same study, the author indicated that in oblique anastomoses, the artery into which the anastomosis empties supplies a larger area of placenta (Szpakowski, 1974).

Many have speculated on the function of Hyrtl's anastomosis. It has been suggested that in conditions of compression and blockage of the arteries, the anastomosis functions similarly to the circle of Willis for the brain and behaves like a safety valve (Priman, 1959; Benirschke and Kaufmann, 1995). Bacsich and Smout (1938) proposed that when the pressure in the intervillous space and cotyledon is increased, the anastomosis acts like a buffering system. While the anastomosis may serve these functions, studies over the last 20 years suggest that a key function is that originally proposed by Hyrtl (1870). That is, the anastomosis appears to equalize the blood pressure in the umbilical arteries, helping to equalize the area of placental coverage between the two vessels.

For example, in the absence of an anastomosis, discordant flow accompanied by asymmetric placental coverage was reported (Hitschold, 1992) .Also, although there is a large significant difference in the resistance to blood flow in the umbilical arteries early in pregnancy, as pregnancy advances and the anastomosis matures, the difference decreases (Predanci et al., 1998). In discordant umbilical arteries, the equalizing function in resistance to blood flow of the anastomosis has been demonstrated (Predanci, 1998). Finally, using flow velocity waveform (FVW) analysis, Raio et al (1999) assessed FVW in the anastomosis and in the umbilical arteries both up and downstream of the anastomosis, and confirmed the equalizing function.

Surprisingly, despite this seemingly important function, the presence of an intermediate vessel in the umbilical cord seems to be recent evolutionary development. Lower primates (e.g. lemurs) lack the anastomosis and 80% of the old world primates and only 30% of new world primates (Platyrrhine) have it (Young, 1972).

Variations in the types of anastomoses have been described, including fusion of the arteries, anastomosis via branches, double anastomosis, and no anastomosis, but the most frequently occurring anastomosis is an intermediate vessel between the umbilical arteries that is present in >80% of cases (Priman, 1959; Szpakowski, 1974). FVW data to date have yet to provide convincing links between anatomical variations and placental abnormalities or pregnancy outcomes, and Young (1972) suggested that linking any factor to the presence or absence of the anastomosis was impossible. Further, in a study where differences in the area of the placenta served by the arteries were present, the arteries were found to be of equal size in the presence of the anastomosis (Benirschke and Kaufmann, 1995). Still, there is some evidence that placental anomalies (e.g., velamentous or -marginal cord insertion and infarcts) maybe related to the absence of an anastomosis (Dolkart et al., 1992; Raio et al., 1999).

In the study carried out by Ullberg (2003) of 64 placentas, 60 had an anastomosis in the umbilical arteries. It was found that no placentas had more than one anastomosis. A true vessel represented the anastomosis in 56 placentas and only two cases of fenestration and two of fusion were reported. Verification by dissection confirmed the absence of anastomosis in one case and a single umbilical artery in two cases (SUA).

The variability in the anatomic structure of Hyrtl's anastomosis is large, but studies on possible physiological correlates between structural variations and function are notably lacking .Because of the apparent importance of the anastomosis in distributing flow between the umbilical arteries and in the placenta, we investigated potential hypertension related anatomical changes. Here we report a relationship between anatomical variations in Hyrtl's anastomosis in normotensive and pregnancy- induced hypertension.

MATERIALS AND METHODS

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

All procedures were approved by the Institutional Ethics Committee (IEC No: SMIMS/IEC/2012-Phd.2). Placentas from 100 normotensive mothers and from 100 pregnancy-induced hypertensive mothers were collected and fixed in 10% formalin in phosphate buffered saline. Women were considered hypertensive if they had a systolic arterial blood pressure >140 mmHg and a diastolic pressure >90 mm Hg measured on two or more occasions at least four hour apart after the 20th week of gestation .Women with or without oedema and/or proteinuria were included. Exclusion criteria include essential hypertension, diabetes mellitus, platelet disorders, chronic renal disease, and epilepsy.

Umbilical cord dissections extended to the point of cord insertion into the placenta. The umbilical arteries, vein and Hyrtl's anastomosis were examined visually and by palpation and then photographed. Anatomical features were recorded. These included the (1) presence or absence of Hyrtl's anastomosis; (2) presence of one or two umbilical arteries; (3) angle of Hyrtl's anastomosis with umbilical arteries (transverse or oblique); and (4) types of anastomosis (normal, fused, branched, or fenestrated).

Measurement of the anastomosis of the umbilical arteries was done with digital vernier calliper (resolution: 0.01mm, speed ≤ 1.5m/s, working temperature: 0∼ + 40°C), photographs were taken, and other abnormalities were examined. The length of the Hyrtl's anastomosis was measured from the stem of one umbilical artery to the other, and the distance of the anastomosis to the placenta (A to P distance) was measured from the junction of the umbilical artery and Hyrtl's anastomosis to the point of the cord insertion to the placenta. The distance was measured from the midpoint of the anastomosis to the cord insertion. All the observations and measurements were recorded in a tabular form.

STATISTICAL ANALYSIS

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

Assuming a 20% difference in the morphology between the two groups a sample size of at least 71 was required in each group to detect a significant difference at α(P) ≤ 0.05 and Power (1 – β = 80%) for a two sided test. Mean values ± SD was considered for continuous variables. Non continuous variables were expressed as percentages. Data was analysed by one way analysis of variance (ANOVA) and the difference was considered significant when P ≤ 0.05. SPSS 16.0 was used for data analysis.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

Upon examination of 200 umbilical cords, we observed all five previously described types of anastomoses: transverse (Fig. 1a), oblique (Fig. 1b), fenestrated (Fig. 1c), fused (Fig. 1d), and branched (Fig. 1f). In addition, we noted two specimens had only a SUA (Fig. 1e) and several pairs of arteries lacked an anastomosis (Fig. 1g).

image

Figure 1. Different types of Hyrtl's anastomosis. a-transverse, b-oblique, c-fenestrated, d-fused, e-no anastomosis, f-anatomosis by branch, g-single umbilical artery.

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As shown in Table 1, although all types of anastomosis were found in umbilical cords from both normotensive and hypertensive mothers, the distribution of these varied. The transverse and oblique types were essentially equally common in normotensive cases (24% and 26%. respectively). In contrast, in hypertensive cases, transverse and oblique anastomosis were observed in 43% and 19% of cords, respectively, a 2.3-fold difference. Also, transverse anastomoses were found 1.8-fold more times in cords from hypertensive than in normotensive mothers. The fraction of cords with a fenestrated anastomosis was more than double in specimens from normotensive compared to hypertensive mothers (18 vs. 8), but the fractions of fused, branched and no anastomosis were similar in the two groups. A single case of a single artery was observed in each group.

Table 1. Types of Hyrtl's anastomoses in normotensive and pregnancy-induced hypertensive pregnancies
TypeNormotensivePregnancy-induced hypertensionTotal
Transverse
Number % of total in subtype24 (35.8%)43 (64.2%)67
Oblique
Number % of total in subtype26 (57.8%)19 (42.2%)45
Fenestrated
Number % of total in subtype18 (69.2%)8 (30.8%)26
Fused
Number % of total in subtype8 (50.0%)8 (50.0%)16
Branched
Number % of total in subtype14 (48.3%)15 (51.7%)29
No anastomosis
Number % of total in subtype9 (60.0%)6 (40.0%)15
Single umbilical artery
Number % of total in subtype1 (50.0%)1 (50.0%)2
All types
Number100100200
Double anastomosis number010001

In addition, a double anastomosis was found in one of the normotensive cases (Fig. 2). The umbilical arteries were connected by a transverse 4.2-mm long anastomosis lying 4.27 mm from the site of the cord insertion. A second 26.0-mm long anastomosis branched off the right umbilical artery and connected to the stem of the left umbilical artery near its insertion into the placenta.

image

Figure 2. Double anatomosis. 1-anterior view, 2-posterior view.

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The overall distance of the anastomosis from the placental insertion was significantly longer (P < 0.001) in the normotensive group than hypertensive group (Table 2). Among anastomosis types, however, only in transverse anastomoses was the distance of the anastomosis from the insertion significantly greater (P = 0.002) in the normotensive group (Table 2).

Table 2. Distance from Hyrtl's anastomosis to the umbilical artery insertion into the placenta in normotensive and pregnancy-induced hypertensive pregnancies
TypeDistance of hyrtl's anastomosis from insertion into placenta (mm)Significant difference
NormotensivePregnancy-induced hypertension
Transverse mean6.133.63 
N24430.002
STD deviation±4.77±1.39 
SEM0.970.214 
Oblique mean4.646.28 
N2619 
STD deviation±3.41±5.020.196
SEM0.671.15 
Fenestrated mean5.874.96 
N188 
STD deviation±2.85±1.670.4111
SEM0.670.59 
Fused mean14.659.71 
N88 
STD deviation±9.81±9.670.328
SEM3.473.42 
Branched mean3.733.60 
N1415 
STD deviation±3.11±1.560.888
SEM0.830.403 
All types mean6.034.81 
N9093 
STD deviation±5.27±4.110.000
SEM0.560.43 

Similar finding were observed with respect to the length of the Hyrtl's anastomosis (Table 3). The overall average length of the anastomosis was longer in the normotensive than in hypertensive group (P < 0.001), but among the types, only in the transverse types was the length significantly longer (P = 0.044) in the normotensive group.

Table 3. Length of Hyrtl's anastomosis in normotensive and pregnancy-induced hypertensive pregnancies
TypeLength of hyrtl's anastomosis (mm)Significant difference
NormotensivePregnancy-induced hypertension
Transverse mean5.113.67 
N24430.044
STD deviation±3.71±2.04 
SEM0.760.31 
Oblique mean6.225.24 
N2619 
STD deviation±3.17±2.810.290
SEM0.620.65 
Fenestrated mean4.133.66 
N188 
STD deviation±1.59±2.050.527
SEM0.380.73 
Fused mean3.682.91 
N88 
STD deviation±0.96±0.680.083
SEM0.340.24 
Branched mean3.533.26 
N1415 
STD deviation±2.24±1.740.718
SEM0.590.45 
All types mean4.863.84 
N9093 
STD deviation±2.95±2.220.000
SEM0.3110.23 

In contrast, as shown in Table 4, overall placental surface areas did not differ between the two groups (P = 0.340), although surface areas were significantly larger (P=0.005) in the normotensive than hypertensive group when the anastomosis was of the oblique type.

Table 4. Placental surface area in in normotensive and pregnancy-induced hypertensive pregnancies
TypePlacental surface area (cm)Significant difference
NormotensivePregnancy-induced hypertension
Transverse mean208.47194.79 
N24430.253
STD deviation±53.04±42.58 
SEM10.836.49 
Oblique mean217.26168.96 
N2619 
STD deviation±54.74±53.530.005
SEM10.7412.28 
Fenestrated mean214.18191.52 
N187 
STD deviation±40.78±42.780.231
SEM9.6116.17 
Fused mean205.83209.98 
N88 
STD deviation±35.17±57.550.864
SEM12.4320.35 
Branched mean219.27228.90 
N1414 
STD deviation±50.04±31.270.547
SEM13.48.36 
All types mean211.98196.75 
N100100 
STD deviation±48.22±47.850.340
SEM4.824.76 

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

A literature search conducted on Hyrtl's anastomosis determined that the anastomosis between the umbilical arteries had been investigated in the context of pregnancy- induced hypertension. Hence, we examined specimens from normotensive mothers and those with pregnancy-induced hypertension.

We first studied historic data, regarding Hyrtl's anastomotic pattern and noted that the anastomosis were found to be present in the umbilical arteries near the cord insertion (Hebenstriet, 1737; Hyrtl, 1870). These features were identifiable in the present study from the specimens that we examined in both normotensive and pregnancy-induced hypertension.

Moreover, these previous studies demonstrated that the umbilical arteries are connected by a single connection and were essentially never connected by two or more anastomosis (Shordania, 1929; Hyrtl, 1870). A single report in the literature mentions a specimen in which there were two anastomoses (Priman, 1959). This is supported by the one case of double anastomosis in this study.

The important functional role of the Hyrtl's anastomosis appears to equalize the blood pressure between the placental territories supplied by the umbilical arteries (Hyrtl, 1870). This probable role in regulating the distribution of the blood flow to the placenta has been supported by computer modelling studies (Gordon et al., 2006).

Although numerous authors have described the anastomosis in the umbilical arteries, they have done so without differentiating whether they were associated only with normal subjects or with mothers with certain conditions like essential hypertension, pregnancy-induced hypertension, or diabetes.

This study focuses on the pattern of the anastomoses in pregnancy-induced hypertension. Since in pregnancy-induced hypertension there is an increase in the vascular resistance (Mitra, 2000),we hypothesized that there might be structural differences in umbilical cords and placentas derived from normotensive versus pregnancy-induced hypertensive mothers.

In earlier investigations, different types of Hyrtl's anastomoses (transverse, oblique, fused) have been characterized using the injection corrosion technique, Doppler method, and angiography (Arts, 1961; Priman, 1959; Ullberg, 2001). In the transverse type, in which the blood can flow in either direction, the area of the placenta perfused by the two arteries was found to be nearly equal (Priman, 1959). In the present study, the finding that a greater percentage of transverse anastomoses occurred in pregnancy-induced hypertension, supports the idea that the anastomosis aids in equalizing the pressure gradients in the umbilical arteries.

Hemodynamic analysis of inclined (oblique) anastomoses, suggest that there is a drop in pressure between the two arteries which affects the flow pattern (Gordon et al., 2006).Further, a significant difference in the placental area served by the two vessels was observed in oblique type (Szpakowski, 1974).

We observed a significant difference in the placental surface area in the oblique-type anastomosis. In placenta from normotensive subjects the average placental surface area was found to be 217.26 cm2 while those from pregnancy-induced hypertensive subjects averaged 168.96 cm2. According to our current understanding, there is a disturbance in the flow of blood in the oblique anastomosis, suggesting a drop in the pressure. This may account for the observed change in the surface area of the placenta.

In conclusion, occurrence of a greater percentage of transverse anastomoses associated with pregnancy-induced hypertension suggests an important functional role for Hyrtl's anastomosis in regulating the blood flow in the umbilical arteries. This study also clearly demonstrates that the size of the placental surface area differs considerably in relation to the type of anastomosis. This study reveals the novel finding that the characteristics of Hyrtl's anastomosis differ in the umbilical cord of fetuses from normotensive mothers and those with pregnancy-induced hypertension.

ACKNOWLEDGEMENTS

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

The authors thank Dr. Ratnabali Sengupta, for her guidance and encouragement, Dr. Benoy Upreti for constant support. My sincere gratitude to Dr. Gina Schatteman, for her valuable time and comments to have this manuscript improvised for which I shall always remain in debt.

LITERATURE CITED

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. ABSTRACT
  3. MATERIALS AND METHODS
  4. STATISTICAL ANALYSIS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. LITERATURE CITED
  9. Supporting Information

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