Courtesy - Patrick M. McCue DVM, PhD,
Diplomate American College of Theriogenologists
The equine placenta consists of
an outer membrane called the
chorioallantois that is attached to the
uterus and an inner membrane called
the amnion, which surrounds the
foal. The chorioallantois is connected
to the uterine wall by millions of
microscopic ‘velcro-like’ attachments
called microcotyledons, except for the
area of the placenta that is in contact
with the cervix. Absence of uterine
glands on the cervix leads to a failure
of microcotyledon development on the
adjacent part of the placenta.
Consequently, whereas a majority
of the outer chorionic surface of the
placenta has a brick-red velvety
appearance due to the presence of
the microcotyledons, the region of
placenta abutting the cervix lacks
microcotyledons and has a smooth
white appearance. This area is known
as the cervical star. Attachment of
the placenta to the uterus via the
microcotyledons is critical for the
transfer of nutrients and oxygen from
the mare across the placenta to the
foetus.
1.
The mare breaks water.
Page 64 • The Australian Quarter Horse Magazine • July • August 2015
With the onset of labour, the cervix
starts to relax and the uterus begins
to contract. Increased pressure from
the uterus causes the chorioallantois
to bulge into the open cervix. In the
normal progression of foaling, the
chorioallantois ruptures at the cervical
star and results in the release of a
large quantity of allantoic fluid (i.e. the
“water breaks”). A thin, transparent,
greyish-white membrane, the amnion,
should begin protruding through the
vulva within 5 to 10 minutes after the
mare breaks her water. As the mare
2.
The amnion, should begin protruding through the vulva within 5 to 10
minutes after the mare breaks her water.