Fetal villosity and microvasculature of the bovine placentome in the second half of gestation

Abstract

The architecture of the fetal villous tree and its vasculature in the bovine placentome were studied in the second half of gestation using both conventional histology and histology of ink-filled blood vessels. These were compared with corrosion casts of plastic fillings of the vasculature, prepared for scanning electron microscopy. This combination of morphological methods allows perception of the villous tree throughout gestation from broad-conical to tall-conical form where branch ramification occurs mainly at right angles to the stem. The stem villus typically contains a single central artery and several peripheral veins arranged in parallel. The proximal branches to the stem, the intermediate villi, contain a central arteriole and accompanying venules. The distal branches, the terminal villi, enclose capillary convolutions which consist of an afferent arterial capillary limb, capillary loops and efferent venous capillary limbs. Vascular interconnections exist within the terminal villi, as capillaries or venules between the capillary convolutions, serially bridging them in up to 5 places, and as capillary anastomoses between the capillary loops. Coiling and sinusoidal dilatations of these loops develop near the end of gestation. The intraplacentomal rearrangement of villous trees with progressive gestation and their morphological vascular adaptations are discussed in relation to placental function, including the ever increasing need for transplacental substance exchange. This adaptation allows the blood to traverse the shortest possible arterioarteriolar route to the periphery of the trees where exchange takes place. The need for an increasing blood flow stimulates capillary growth and at the same time optimises the blood flow reaching the placental barrier represented by the vessel cast surface. The capillaries also carry the blood back into the very voluminous system of venules and veins where back diffusion may occur. The total volume of terminal villi of bovine placentome, the ' working part ' of villous trees, hence distinctly increases with respect to the stem and intermediate villi, the ' supplying part ' of the villous tree. In morphological terms the efficiency of the bovine transplacental diffusional exchange is higher than in the closely related ' co-ruminants ' sheep and goats and distinctly higher when compared with the human placenta.

Histo-morphology of the Uterus and Early Placenta of the African Buffalo (Syncerus caffer) and Comparative Placentome Morphology of the African Buffalo and Cattle (Bos taurus)

“…Fetal villi of cattle, measured at the centre of pedunculated placentomes, were up to four times wider than in the African buffalo whereas the width of villous tips was similar in both genera. Villous trees in cattle therefore adopt a triangular shape which has been described as representing a broad "Thuja-tree" during early gestation [31] and a Christmas tree during midgestation [10] and [32]. In contrast, the villous trees of early S. caffer placentomes are best described as a "Tuscany-cypress" in appearance due to their slender form and rudimentary branching, rather resembling villi described in B. bubalis [8] and [27].…”

“…In contrast, the villous trees of early S. caffer placentomes are best described as a "Tuscany-cypress" in appearance due to their slender form and rudimentary branching, rather resembling villi described in B. bubalis [8] and [27]. It has been noted that during late gestation in cattle, tall-conical-shaped villous trees allow a denser arrangement of villi within a single placentome, which results in the maximal functional surface area of villous tissue in a given space [32]. In the African buffalo placenta, the simple yet dense villous arrangement during early gestation may compensate for the distinctly less developed villous branching when compared to that of cattle at similar fetal developmental stages.…”

Expression profile and protein levels of placental products as indirect measures of placental function in in vitro-derived bovine pregnancies

“…Under physiological conditions, the placenta grows faster than the foetus in early pregnancy but slower after mid-gestation, when foetal weight surpasses that of the placenta (Eley et al 1978, Prior & Laster 1979, Reynolds et al 1990. Morphologically, the bovine placenta is considered fully developed at mid-to late gestation, but foetal villous trees continue to develop (branching pattern) and rearrange (spatial relationship between endothelium and trophoblast) in placentomes until near term, increasing the villous SA, which will also accommodate foetal demands for growth towards the end of gestation (Leiser et al 1997). However, the placental tissue holds a great plastic capacity in facing unusual, adverse circumstances.…”

Iron transportation across the placenta

“…Leiser et al (1997) reported that the endocytosis of the fluid Anais da Academia Brasileira de Ciências (2012) 84(4): 1115-1120 (Annals of the Brazilian Academy of Sciences) Printed version ISSN 0001-3765 / Online version ISSN 1678-2690 www.scielo.br/aabc substance (pinocytosis) was structurally evident in the units of apical and basal membranes of all endothelial and epithelial placental barriers.…”