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.