ReviewBovine placenta: A review on morphology, components, and defects from terminology and clinical perspectives
Introduction
An excellent review published in 2000 [1] focused on the ontogeny of the most important cellular populations of the bovine placenta from the utero-fetal defense perspective and stressed the importance of placentology and placental pathology. The review identified the etymological origin of the word “placenta.” It is derived from Latin, meaning “flat cake,” due to the apparent gross similarity of the human discoid placenta to round, flat loaves of unleavened bread common in ancient times. Although differentiation of the early conceptus is very similar in humans and cattle in size [2], the initial establishment with the uterine epithelium and subsequent growth and differentiation of the placenta are very different between these two species [1].
Defining and adopting terminology used in bovine reproduction that is clear, precise, understandable, and available in a single source would make the exchange of clinical and research information and outcomes more efficient. Furthermore, diagnoses and inference of results between and among studies can be correctly interpreted and substantiated or negated, and therapy and hypotheses can be formulated without unnecessary confusion and redundancy in treatments and experiments. On the basis of these objectives, a series of articles on ovarian dynamics [3], biology of the preattachment embryo [4], and production and manipulation of preattachment embryos [5] have been published. This article will review the development and clinically relevant features associated with the bovine placenta. As in the previous reviews [3], [4], [5], the focus is on terminology and its application in clinical and research settings. Clinical management of specific placental conditions is beyond the scope of this review; however, this information is readily available in standard Theriogenology textbooks.
The term placenta points to the structure that has both maternal (endometrium) and fetal components [6], [7]. These are identified as maternal placenta and fetal placenta, respectively [6], [7]. The term fetal placenta refers to the chorioallantois [7], whereas the term fetal membranes includes the amnion and chorioallantois [8]. These distinctions will be applied to the clinical conditions that are discussed in this review. This review will attempt to define the terms applicable to the morphology, components, and defects of the bovine placenta, whereas endometrial defects are beyond the scope of this review.
Section snippets
Morphology
Currently, the bovine placenta is described as “cotyledonary synepitheliochorial,” on the basis of its morphology that is established at approximately Days 40 to 50 of pregnancy. Earlier classification as syndesmochorial was on the basis of a misunderstanding of the number and form of the layers intervening between the fetal and maternal circulation and needed correction. The earlier assumption that the uterine epithelium is lost in the process of placentation resulting in direct apposition of
Fetal membranes
An arrangement of transporting epithelia between maternal and fetal circulations more vividly encapsulates the functional aspects of the placenta. It is primarily made up of fetal membranes, which include the amnion, allantois, and chorion (collectively termed extraembryonic membranes). The trophectoderm and the nonvascular mesoderm form the chorion, which is also known as “somatopleur.” The chorion produces folds to enclose the embryo to form a fluid-filled sac, the amnion. A Day-35 embryo
Clinical relevance
In reproductive management, early pregnancy diagnosis is critically important. Rapid and accurate detection of nonpregnant cattle as soon as possible after breeding is essential, because it facilitates prompt rebreeding or perhaps timely culling. An additional benefit includes the potential to identify reasons for pregnancy failures. Many methods have been developed over the years. The focus here will be to document the methods wherein pregnancy diagnosis is determined by identifying the
Developmental defects
During the early part of pregnancy, there is maximal placental growth, differentiation, and vascularization. These are critical for subsequent fetal development. Major factors that can impede this process are nutrition, and environmental factors such as stress including hypothermia and infection. Among these factors, the influence of nutrition has been studied [99], [100], [101]. Both nutrient restriction and overnourishment during pregnancy suppressed placental cell proliferation and
Summary
The primary objective of this review, similar to the previous reviews [3], [4], [5], was to provide a lexicon of terms in bovine reproduction that can be used by veterinarians and scientists involved in applied and basic research, so that diagnoses and inference of results between and among studies can be correctly interpreted and substantiated or negated, and therapy and hypotheses can be formulated without unnecessary confusion and redundancy in treatments and experiments. This review is
Acknowledgments
The author gratefully acknowledges the valuable assistance provided by Don Schlafer and Peter Wooding in critiquing the review. The author expresses his sincere appreciation to Peter Wooding for the monographs and Marco Colazo for ultrasonograms.
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