How to Perform a Field Assessment of the Equine Placenta

Examination of the equine placenta can yield important information relating to both the reproductive health of the mare and the medical prognosis of the neonatal foal. A consistent, systematic, and thorough approach is essential to gain useful information.

1. Introduction

The fetus and the placenta are interdependent. Disturbances of placental development are reflected in compromise of the fetus. Similarly, disturbances in fetal development can adversely affect placental form and function. From a theriogenologist’s point of view, placental examination yields important information from the mare regarding the current state of the reproductive tract and prognosis for future reproductive performance. From an internist’s point of view, information gained by placental examination impacts on the immediate management of the neonatal foal, giving clues that may not be evident at initial examination as to the intrauterine environment and potential for neonatal compromise.

2. Normal Anatomy

Embryologic Origins

Chorion

The chorion forms from trophoblastic (ectodermal) cells of the early embryo [1]. Initially cuboidal, they develop into tall columnar cells with absorptive capabilities. Specialized trophoblastic cells situated in the chorionic girdle invade the maternal endometrium beginning at day 35 to form the endometrial cups. These represent the first physical attachment between the conceptus and endometrium. Placentation proper begins around day 70, characterized by widespread formation of microcotyledons [2]. The adjacent endometrium remodels and invaginates (forming microcaruncles) to receive these projections facilitating a marked increase in placental surface area for gaseous and nutrient exchange.

Allantois

The allantois develops on approximately day 21 as an outgrowth of the primitive hindgut, composed of an outer layer of highly vascular splanchnic mesoderm with an inner layer of non-vascular endoderm. Fusion of the allantois with the chorion results in the allantochorion. The highly vascular nature of the allantois is responsible for the vascularization of the amnion, umbilical cord, and chorion.

Amnion

The amnion forms from folds of ectoderm that envelop the developing fetus early in fetal development.

Fetal Membranes at Parturition

Allantochorion

Fusion of the allantois with the chorion results in the allatochorion. At term, this constitutes approximately two-thirds of the entire placental weight. The chorionic (villous) surface is intimately applied to the maternal endometrium and is responsible for the formation of microcotyledonary attachments. Grossly, it appears as a red velvet surface. Areas devoid of microcotyledons appear pale and fibrotic. The allantoic surface features prominent branching blood vessels that coalesce to form the placental arteries and vein.

Allantoamnion

The allantois surrounds the amnion by the fourth week of gestation, forming the allantoamnion. At term, this membrane is white and opaque, with large blood vessels coursing over the inner surface. At this time, it accounts for one-third of placental weight.

Umbilical cord

The umbilical cord results from the expansion of the amnion and allantois around the remnants of the yolk sac and vitelline duct [3]. It anchors the fetus to the original implantation site at the base of the gravid horn on the dorsal wall of the uterus. Cord length is directly correlated with the weight of the allantoamnion and the allantochorion.

Amniotic Portion.        
Plaques or hyperplastic nodules of glycogen-rich epithelium are often present close to the foal’s umbilicus. Along the majority of this portion of the umbilical cord, two arteries and one vein are present, the single vein resulting from anastomosis of two veins in the distal amniotic portion. Also present are the urachus and the vitelline duct. Urachal diverticula may be present; these are non-significant cystic structures. Deposits of amniotic detritus may be found; these tend to result from the attachment of epithelial cells, fragments of meconium, and fetal hair.

Allantoic Portion.        
In addition to the vascular structures noted above, yolk sac remnants can be noted. These may be ossified and attached by a vascular stalk; however, they are often found as a thin band of connective tissue parallel to the blood vessels. The two umbilical arteries perfuse separate areas of the allantochorion: one supplies the pregnant horn and cranial body, and the other supplies the non-pregnant horn, middle, and distal body.

Important Features of the Equine Placenta

There is no physical attachment between the allantoamnion and the allantochorion; therefore, the fetus encased by the allantoamnion floats freely in the allantoic fluid. This arrangement is unlike other large domesticated species such as the ruminants. The fetus is highly mobile and regularly traverses between the pregnant and non-pregnant horns in early pregnancy. Although rare, it is possible for the fetus to develop in the opposite horn from the umbilical cord attachment (the designated pregnant horn).

The umbilical cord is relatively long in the equine and will have a small number of twists evenly distributed along its length. In conjunction with the mobility noted above, torsion of the cord itself can occur, leading to suffocation of the fetus, urachal compromise, poor placental perfusion, and vascular compromise of the fetus.

Five avillous areas are regularly noted and have been assessed to be normal in the equine placenta [4]. These areas appear as flat, white, fibrous plaques and include the following: the area adjacent to the ostia of both oviducts; the site of invasion and subsequent sloughing of the endometrial cups; the cervical star; the area adjacent to the yolk sac attachment; and longitudinal folds centered over larger blood vessels (these result from traction on the allantochorion causing gathering of the membrane, and hence, loss of endometrial contact).

Cervical Star

The cervical star is the portion of the allantochorion overlying the internal os of the cervix. Because there are no microcotyledonary attachments, it has the appearance of raised white bands radiating from a central area. Usually the cervical star is not seen intact; instead, it appears as a white edge on the allantochorion at the site of exit of the foal.

Allantochorionic Pouches

Maternal rejection of the endometrial cups results in their sloughing into adjacent invaginations of the overlying allantochorion. These appear as pedunculated structures forming a ring around the site of umbilical cord attachment at the dorsal wall of the pregnant horn. When opened, they contain brown viscous material. The lumen of the pouch can be demonstrated to communicate with the endometrial surface.

Allantoic Calculus (Hippomane)

The allantoic calculus is composed of concentric layers of amorphous proteinaceous material deposited on a nucleus of epithelial debris. Minerals, especially calcium phosphate, are also deposited. These are first apparent around 90 days gestation as small white objects, enlarging to 5 - 10 cm and becoming tan in color by term.

3. Recording Information

To avoid the loss of potentially useful information and permit maximal use of diagnostic services, the recording of examination findings on a standardized form is recommended. This form should be clearly laid out and have sections for the following information: mare - signalment, reproductive history, assistance at parturition; foal - presentation, outcome of delivery, signalment, weight.

Gross examination findings of the placenta including weight of constituent parts, length of umbilical cord, location, and appearance of gross lesions, missing pieces, and sample collection areas. A standardized placental diagram should be available to draw important findings. Record ancillary diagnostics if performed.

4. Procedure for Gross Examination

Meaningful examination of the placenta relies on a systematic and repeatable approach. After removal of gross debris, the constituent parts of the placenta should be weighed (this can be left until the end of the exam if it is more expedient to do so). Recording the weight is especially important with the grossly edematous placenta, where fluid leakage during storage may falsely decrease placental weight. Therefore, appropriate storage of the placenta before examination includes containers that promote the retention of this fluid, low ambient temperature, and avoidance of destruction by domestic animals.

Allantochorion

The equine allantochorion is usually expelled with the allantoic surface outermost. This is the result of traction on the allantochorion by the foal during delivery. It is vitally important to examine both chorionic and allantoic surfaces. Depending on individual preference, the membrane can be examined either side first. It is the author’s preference to expose the chorionic (maternal) surface at the beginning of the examination. The placenta is laid out on a clean, level surface with good lighting. The examiner proceeds to identify the body and both uterine horns. Once this is done, arrange the membranes in a capital "F" position, with the pregnant horn uppermost and the body forming the vertical bar of the "F" (Fig. 1). The body characteristically displays a bulge corresponding to the ventral deviation of the pregnant horn.

The allantochorion should be examined for signs of completeness and any signs of rupture identified. The tip of the non-pregnant horn is the most likely part of the placenta to be retained and must be identified in all examinations (Fig. 2). Abnormal structures and lesions should be noted. The site of fetal exit and remnants of the cervical star require special attention, with any allantochorionic thickening or exudate sampled. The placenta is inverted to expose the other side. It is the author’s preference to examine the allantoic (fetal) side after the maternal side. Identification of tissue suspected lost after examination of the maternal side is aided by reconstruction of the placental vessels of the allantoic side.

Figure 1. Placenta positioned for systematic examination.

Figure 2. Placenta positioned as in Fig. 1. Note incomplete non-pregnant horn and meconium-stained allantoamnion.

Allantoamnion

This membrane should be assessed for uniform thickness and color. If present, edema should be noted.

Umbilical cord

Assessment of length, degree of twisting, and the presence of any vascular compromise are important. Normal findings include the presence of a blood clot at the torn end, with some hemorrhage possible down the cord. The umbilical arteries can project up to 6 cm from the end of the cord as they rupture inside the foal proximal to the site of venous separation. As noted previously, in some cases, the site of attachment of the umbilical cord lies at the base of the horn contralateral to where fetal development occurred.

5. Interpretation

Deviations in the appearance of the placenta from that considered normal provides information of importance to both the mare and the foal.

Placental Weight

In healthy thoroughbred mares, the placenta weighs ~11% of the foal’s birth weight (Table 1) [5]. In addition, a linear relationship between foal and allantochorionic weight has been shown to exist [6]. Increased weight may indicate the presence of edema such as from intoxication with Neotyphodium coenophialum (fescue toxicosis). It is likely that fetal nutrition and gas exchange has been compromised, and neonatal maladjustment is possible.

Allantochorion

Color

The chorion usually is a rich red or maroon color with the gross appearance of a velvet-like surface. Pale or fibrotic areas indicate sites of placental detachment or lack of microcotyledon formation (Fig. 3). Sharply demarcated deeply red engorged areas often occur: these are of unknown significance and have not been associated with any pathology. The non-pregnant horn is generally paler than the rest of the chorion.

Figure 3. Avillous areas of chorion in association with allantoic vasculature (non-pathologic).

Thickness

Compared with the pregnant horn, the non-pregnant horn is thinner in section, puckered, and smaller in size. Both horn tips are edematous, the pregnant horn being more prominently so.

Cervical Star Thickening

The majority of intrauterine infections ascend through an incompetent cervix. Gross thickening, exudate, and a line of demarcation between this area and the adjacent placental body are characteristic findings of placentitis (Fig. 4). Detachment and deviation of the cervical star from the internal os may occur, preventing the foal from exiting the allantochorion at the weakest point, which can result in premature placental separation.

Figure 4. Ascending placentitis caused by cervical incompetence.

Avillous Areas

The chorion can be considered to develop as a mirror of the endometrial surface. In addition to the previously noted normally occurring small avillous areas, large areas without villi can result from twinning and a devitalized endometrial surface. Apposition between the allantochorion of multiple fetuses leads to large areas devoid of villi and hence decreased absorptive capacity. Development of microcotyledons only occurs in areas of normal endometrial architecture; therefore, their absence indicates endometrial pathology. Gross appearance varies from decreased density of villi to complete absence. These changes are not diagnostic; however, histologic changes are present in the allantochorion of fetuses aborted because of placental insufficiency.

Body Pregnancy

Although usually resident in one uterine horn with extension into the body of the uterus, the occasional fetus will develop largely within the body of the uterus. The characteristic placental finding is short, symmetrical placental horns. Because of the limited placental contact area afforded by this arrangement, these pregnancies usually spontaneously terminate because of functional insufficiency.

Exudate

The presence of exudate may be considered as presumptive evidence of microbial intrauterine activity and should be collected in appropriate storage media. Common organisms include Streptococcus zooepidemicus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Leptospira sp., Aspergillus fumigatus, and Crossiella equi [7,8].

Allantoamnion

The allantoamnion should be a smooth, white, opaque, uniformly thin membrane. Proliferative areas may occur as previously noted; however, these should be focal in nature. Widespread edema and thickening may indicate fetal compromise because of decreased nutrient and gaseous exchange. Gross lesions are uncommon: hemorrhage and necrosis adjacent to the umbilical cord indicates occurrence of umbilical cord torsion. Thickening may result from diffuse mineralization centered on the blood vessels. Amnion nodosum is the result of proliferative changes caused by embedded hair and other fetal debris.

Umbilical Cord

Pathology of the umbilical cord often leads to death of the fetus. Abnormal findings include aneurysms, intramural hematomas, thrombosis, edema, and intimal tears (Fig. 5). Elongation is associated with strangulation of the fetus or torsion predisposing to umbilical and urachal compromise. In 95% of normal equine pregnancies, the umbilical cord measures between 36 and 83 cm, with an average length of 55 cm [3] (Table 1). The most common abnormality is increased length, which leads to three possible complications: fetal strangulation, torsion of the cord, and hypoperfusion of the allantochorion [9]. Hypoperfusion may result in congestion, thrombosis, and mineralization of the allantochorion. During delivery, an elongated cord may prevent the fetus from rupturing the allantoamnion inside the mare, allowing the fetus to exit the mare completely enveloped and at risk of suffocation.

Decreased cord length (<30 cm) predisposes to tearing and premature placental separation causing intrapartum death.

Torsion of the cord results in hemorrhage and urachal compression. This can lead to bladder distension and rupture in utero, and urachal dilatation, patent urachus, and elongated navel stump after delivery. Excessive torsion can be suspected when the cord is unwound but maintains an obvious sharp kink at one or more points along its length (Fig. 6).

Figure 5. Intimal damage to umbilical vasculature secondary to torsion.

Figure 6. Umbilical cord with non-reducible kink at allantoic end, indicating previous torsion.

6. Further Examination

Sample Submission

Sections of the following should be routinely collected and clearly identified for histological examination in those cases requiring further diagnostic information.

• Allantochorion: pregnant horn, non-pregnant horn, body, cervix
• Allantoamnion
• Umbilical cord
• Focal lesions considered not characteristic of the equine placenta. It is important to include both the lesion and adjacent normal tissue.

The presence of histological abnormalities is more indicative of compromise to the foal than the proportion of the placenta involved in the pathology [6].

Collection of exudates is also recommended for microbial culture where fetal sepsis or infectious abortion is suspected.

7. Conclusions

Examination of the equine placenta is a vitally important part of reproductive and neonatal practice. Detection of placental pathology may be the earliest indication of the potential for neonatal compromise. Placental weight and umbilical cord length can be easily measured in field situations and deviations from normal values are noteworthy. Significant gross visual examination findings include meconium staining of the amnion (Fig. 7), excessive hemorrhage associated with umbilical cord separation, excessive rotation and kinking of the umbilical cord, avillous chorion other than areas noted as normal, and any placental thickening, edema, or exudate. Additionally, information pertinent to the reproductive prognosis of the mare can be gained from close inspection of the chorionic surface and cervical area.

Figure 7. Gross meconium staining and thickening of allantoamnion.