Page 509 - Anatomy and Physiology of Farm Animals, 8th Edition

494 / Anatomy and Physiology of Farm Animals

Amniotic cavity Allantoic cavity Uterine Fetal
Endometrium epithelium epithelium
VetBooks.ir Digestive Chorioallantois
Allantois
tract
Amnion
Urinary
bladder
Urachus Endothelial cell
Yolk sac
Red blood cell

Figure 28-2. Equine fetus in the placenta. The cho-
rioallantois is the outer allantois plus the chorion. The
chorion is associated intimately with the endome- Maternal tissue Fetal tissue
trium. The inner allantois is fused with the amnion.
Source: adapted from Reece, 1997. Reproduced with Figure 28-3. Epitheliochorial type of placenta.
permission of John Wiley & Sons, Inc.

The fluid‐filled amniotic cavity or sac is direct contact with the epithelium of the
sometimes called the second water bag. uterus of the dam (Fig. 28‐3). (The placenta
The terms first and second water bag refer of ruminants may also be termed a synepi-
to fetal membranes at the time of parturi- theliochorial placenta because a number
tion, when the allantoic sac is expelled first of fetal trophoblast cells fuse with endo-
and the amniotic sac, second. metrial cells to form binucleate cells.)
Branches of the paired umbilical arter- A hemochorial type of placenta, in which
ies and the umbilical vein run through fetal vessels and chorion are invaginated
the connective tissue between the allantois into pools of maternal blood, is found in
and chorion. These vessels are an impor- humans and some rodents. An endotheli-
tant part of the fetal circulation. The ochorial type, in which the chorion is in
umbilical arteries carry unoxygenated direct contact with the endothelium of
blood from the fetus to the placenta, and blood vessels of the dam, is found in carni-
the umbilical vein carries oxygenated vores. Mammalian placenta can also be
blood from the placenta to the fetus. As a classified at the gross anatomic level based
general principle, blood from the fetus on the distribution of the microscopic sites
never mixes with blood from the dam. of exchange. In the mare and sow, exten-
However, the two circulations are close sions of the chorion (chorionic villi) pro-
enough at the junction of chorion and ject into crypts scattered over the entire
endometrium to permit oxygen and nutri- endometrium, and this placental type is
ents to pass from the maternal blood to termed diffuse (Fig. 28‐4). Ruminants have
the blood of the fetus and waste products a cotyledonary type of placental attach-
to pass from the fetal blood into the blood- ment (Fig. 28‐4) in which exchange takes
stream of the dam. place at structures termed placentomes.
The relationship between the fetal and Placentomes are formed by the invagina-
maternal tissues at the histologic (micro- tion of a specific region of fetal chorionic
scopic) site of exchange is a basis for tissue, cotyledons, into a mushroom‐like
classification of mammalian placenta. The projection from the surface of the endome-
tissue of the maternal side is usually named trium, caruncles (Figs. 28‐5 and 28‐6).
first, then the fetal side. The placenta of These caruncles project out from the
most domestic mammals (sow, mare, ewe, surface of the uterus approximately 1.25 cm
and cow) is classified as epitheliochorial. and vary in diameter from 1.25 cm to more
In this type, the chorion of the fetus is in than 10 cm.