Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

IV. Clinical Aspects of Reproductive Endocrinology
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( Linzell and Heap, 1968 ). After this time period, the fetoplacental
unit secretes significant amounts of progesterone.
The dog requires the presence of CL for all of gestation.
Sokolowski (1971) found that ovariectomy even as late as
day 56 postbreeding resulted in premature delivery.
The necessity of secondary CL, which are formed in the
mare between days 40 and 60 of gestation, has been discussed
over the years. The secondary CL are the result of
eCG secretion by the endometrial cups, which are formed
from a circular band of cells of placental origin (chorionic
girdle cells) that invade the endometrium and form isolated
endocrine organs of temporary function ( Allen, 1969 ).
Although secondary CL are not essential for the maintenance
of pregnancy in the mare in that the primary CL
continues to function for up to 150 days, they do add extra
progestational support for the pregnancy during the time
placental production of progesterone is being established.
Progesterone support of pregnancy in the mare begins to be
taken over by the placenta as early as day 50 of gestation but
is not complete for all mares until approximately 100 days
or later, a time that coincides with the beginning demise of
both primary and secondary CL ( Holtan et al. , 1979 ).
It has been demonstrated that eCG has a close immunological
relationship with equine LH ( Farmer and Papkoff,
1979 ); further, incubation studies of CL have shown a
luteotropic effect of eCG ( Squires et al. , 1979 ). If fetal loss
occurs after the formation of endometrial cups, continuing
eCG production supports luteal activity, even to the point
of making lysis of these CL difficult in conjunction with
the pharmacological administration of prostaglandin F 2 α .
The endocrine activity of the fetoplacental unit can
be monitored through the measurement of conjugated
estrogens, especially estrone sulfate in peripheral blood
plasma or urine ( Fig. 21-6 ). In the pregnant sow, concentrations
of estrone sulfate become detectable at day 17 of
pregnancy, increase until about day 28, then decline to
low or undetectable values and then increase again beginning
around day 75 to 80 of pregnancy and remain high
until parturition ( Robertson and King, 1974 ). In the mare,
estrone sulfate values begin to increase around day 35 to
40 of pregnancy with accentuated production at 80 to 90
days and with highest values obtained from day 150 to parturition
( Kindahl et al. , 1982 ; Terqui and Palmer, 1979 ).
In the ewe ( Tsang, 1978 ), estrone sulfate levels can be
detected from around day 70 to 80 after conception and
in the goat concentrations start to increase around day
50 of pregnancy ( Chaplin and Holdsworth, 1982 ). In the
pregnant cow, estrone sulfate concentrations in milk start
to increase between days 100 to 120 ( Hatzidakis et al. ,
1993 ; Henderson et al. , 1994 ). In all species, high values
of estrone sulfate in the peripheral blood, milk, or urine are
strong evidence for the presence of a viable fetus.
The mare produces two estrogens during pregnancy
that are unique to equids, equilin, and equilenin, both of
which have unsaturated B rings. Pashen and Allen (1979)
have shown through fetal gonadectomy in the horse the
importance of fetal gonads in the production of estrogen in
cooperation with the placenta. Little evidence of estrogen
production during pregnancy has been reported for the dog
( Hadley, 1975 ). Estrogen production by the fetoplacental
unit in the cat also appears to be minimal during gestation
( Verhage et al. , 1976 ).
An important endocrine change, which occurs before
parturition in the cow, ewe, and sow, involves an increase
in the synthesis and release of unconjugated estrogens by
the fetoplacental unit. This increased estrogen synthesis is
reflected in elevated plasma estrone concentrations in the
pregnant cow beginning between 30 and 20 days prepartum
( Edqvist et al. , 1973 ), in the ewe about 2 days before
parturition ( Challis et al. , 1971 ), and in the pig about
1 week before delivery ( Robertson and King, 1974 ). In the
cow ( Edqvist et al. , 1973 ; Stabenfeldt et al. , 1970 ) and
bitch ( Smith and McDonald, 1974 ), parturition is preceded
by an abrupt fall in progesterone concentrations between
48 and 24 h before delivery. In the ewe ( Stabenfeldt et al. ,
1972 ), mare ( Noden et al. , 1978 ), and sow (Baldwin and
Stabenfeldt, 1975 ), partial withdrawal of progesterone
occurs before delivery. In the cow ( Edqvist et al. , 1978 ;
Fairclough et al. , 1975 ), ewe ( Liggins et al. , 1972 ), and
bitch ( Concannon et al. , 1988 ), it has been demonstrated
that prostaglandin release initiates regression of the CL and
thus is responsible for the withdrawal of progesterone.
High estrogen and prostaglandin concentrations combined
with low progesterone concentrations increase the
contractile state of the uterus. Prostaglandins may also initiate
cellular changes within the cervix, in addition to the
effect of relaxin, which result in cervical softening and
dilation. Cervical stimulation, the result of the initial entry
of the fetus into the pelvic canal, causes the reflex release
of oxytocin from the posterior pituitary. This increases
the intensity of uterine contractions and thus aids the final
delivery process.
IV . CLINICAL ASPECTS OF REPRODUCTIVE
ENDOCRINOLOGY
Important differences exist between humans and animals
as concerns endocrine analysis. For example, steroid hormone
concentrations are much lower (10-fold less as concerns
estrogens) in animals, which produces a requirement
for more rigorous assay systems for animals. In animals,
the most useful information comes from assessing gonadal
or fetoplacental activity (versus pituitary activity) and
thus emphasizes the determination of steroid hormones.
Besides the fact that gonadotropin values are less useful
for the assessment of clinical situations, variations in
amino acid composition among species as concern specific
hormones mean that multiple systems have to be developed
to determine the content of one protein hormone