PRACTICAL 2 . THE FOETAL MEMBRANES AND PLACENTA
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LEARNING OBJECTIVES.
To understand the external and internal gross anatomy of the pregnant
uterus of the ewe.
Understanding of the arrangement of the foetal membranes-->chorion,
amnion allantois and the fused membranes, chorioamnion and
chorioallantois.
To identify the amniotic and allantoic cavities and fluids, observe the
volume of the fluids and understand the functions.
Understand the different placental shapes and sites of attachment to
the uterus: diffuse (e.g. pig,horse), cotyledonary (e.g. ruminant),
zonary (e.g. carnivore).
Understanding the gross anatomy and histology of the placenta and its
foetal and maternal components.
Understand the principal blood supply to, and drainage of blood and
lymph from the placenta.
II. DISSECTION OF THE PREGNANT UTERUS AND FOETAL
MEMBRANES OF THE SHEEP.
Examination of the external features
(i).Examine the ovary and identify ovarian follicles.
(ii). Note the immature and graafian follicles and corpora lutea. How many
corpora lutea are present in each ovary? Have both ovaries been ovulated?
Make free-hand sections of the ovary.
(iii).Examine the horns of the uterus for pregnancy. Is there a foetus in both
horns? Is there sign of foetus absorption?.
(iv). Identify the ligaments
 The utero-ovarian proper ligament
 The suspensory ligament (if present)
 The mesovarium
 The mesometrium
 Intercornual ligament
(v)Note size of the pregnant horn, the thickness of the uterine wall. The
placentomes can be seen through it.
(vi). Locate the body of the uterus, cervix, vagina, the external genital organs
and the bladder if these are still present.
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Dissection
Divide the uterine horns by cutting through the intercornual ligament. Divide
the horns but leave the body of the uterus intact.
Open up the uterine horn very carefully, cutting through the myometrium
between the placentomes. Be careful not to damage the underlying
endometrium and foetal membranes. The endometrium maybe fused with the
outermost foetal membrane, the chorion.
Observe the arrangement of the foetal membranes-->chorion, amnion allantois
and the fused membranes, chorioamnion and chorioallantois.
Open the chorion to expose the inner membranes (allantois and amnion). Try
to isolate the allantois and trace it to the umbilical cord. Note the amnion
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extends completely round the dorso-lateral aspects of the embryo but the
allantoic cavity is restricted to the ventral part.
5. Open the amniotic cavity and expose the embryo.
6. Observe the amnion covered with small, white, oval ectodermal projections.
These are amniotic plaques containing large amounts of glycogen. The
allantois has white or brown allantoic calculi/hippomanes. Allantoic calculi
are cellular debris derived from endodermal epithelium of the foetal gut.
7. Observe the structure of the umbilical cord.
III. PROSECTED SPECIMENS OF FOETAL MEMBRANES AND
PLACENTA.
(i). Observe the gross anatomy of the placenta in different species-->cat, dog; sheep,
pig, cow and horse
(ii). Identify the foetal and maternal membranes and the structure of the foetalmaternal interface.
(iii). Identify the different placental shapes and sites of attachment to the uterus:
diffuse (e.g. pig,horse), cotyledonary (e.g. ruminant), zonary (e.g. carnivore).
HISTOLOGY OF PLACENTA IN HISTOLOGY PRESENTATIONS ON
DESK TOP
(i).Images of endothelial placenta of cat and dog
Observe:
 Low magnificationdifferentiate the placental from the paraplacental
zone.
 The foetal membraneschorion and allantois and the foetal blood
vessels.
 Structure of the placentathe foetal villi penetrating the maternal
endometrium.
 Extensive blood lacunae in the placenta.
Makai-kai
June 2012
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