Page 456 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 431

VetBooks.ir 2.31 P

E
eCG
Amount of hormone FG

Fig. 2.31 Summary of hormonal events
in the pregnant mare. P = progesterone;
E = oestrogen; eCG = equine chorionic
gonadotropin; FG = fetal gonad weight.
(Adapted from McKinnon AO, Voss JL (1993) Ov 30 60 90 120 180 240 300 Par
(eds) Equine Reproduction. Lea and Febiger, Days of gestation
Philadelphia, pp. 27–175.)

without ovulation. Secondary CL secrete progester- cups produce eCG and in the mare this has mainly
one. Primary and secondary CL persist until after LH-like effects. It causes resurgence of the primary
day 160 and have virtually disappeared by day 210 of CL and luteinisation of the secondary CL, result-
pregnancy. The fetoplacental unit begins to secrete ing in progestagen levels remaining elevated. It is
progesterone from day 60 and by day 100 is pro- also involved in the maternal immunotolerance of
ducing enough progestins to maintain pregnancy. foreign antigens produced by the fetus. eCG is used
Pregnancy in the mare, therefore, is maintained as the basis for a pregnancy test in mares from days
predominately by the placenta. Progesterone assays 40–42. Since cEG is synthesised by cells that origi-
can help in pregnancy diagnosis and assessment of nate from the embryo, it is a good test for pregnancy;
the quality of the primary CL, but it is important to however, there can be false positives, with the preg-
realise that elevated progesterone levels are an indi- nancy having been lost after the endometrial cups
cation of luteal tissue, and not of pregnancy. If blood have formed. The lifespan for the endometrial cups
is taken from a mare 18–22 days after the last ovula- is up to 100–120 days of pregnancy but they may
tion and shows elevated progesterone levels, it sug- occasionally persist beyond this, causing abnormal
gests that an active CL is still present and therefore cycling in the next breeding season.
the mare may be pregnant; however, false positives
do occur, and a high progesterone level at 18–22 days Oestrogen
post ovulation may also be due to: Maternal serum oestrogen levels rise from about day
35 of pregnancy. It is interesting to note that the rise
• Dioestrus ovulation. in oestrogen concentration at around day 35 occurs
• Embryonic loss after maternal recognition of with a functional CL being present. eCG from endo-
pregnancy (see above). metrial cups apparently stimulates luteal steroido-
• Failure of luteolysis. genesis, resulting in increased oestrogen synthesis
• Pyometra. and secretion from CL tissue. After day 45, addi-
tional oestrogens are produced by the fetoplacen-
Equine chorionic gonadatropin tal unit and released into the maternal circulation.
Cells from the embryo attach to the endometrium at Oestrogen levels peak at around days 210–250 and
about day 35, forming small ‘islands’ of tissue called then slowly decline. Oestrogens are thought to play
‘endometrial cups’, usually at the base of what will a role in the development of the vascular supply and
become the pregnancy uterine horn. Endometrial endometrial hypertrophy during pregnancy. Assays