Embriologia equina

have been solved. It is the local smooth muscle-relaxing
properties of the stage-dependent secretion of PGE2 by the
day 5 morula that is the key to its onward passage into the
uterus. However, thequestion of whether this unusual
control of oviductal transport in equids is nothing more than
an evolutionary quirk, or is a necessary developmental
mechanism to delay entry of the embryo intothe uterus
until such time as the uterus is hormonally prepared to
nurture the embryo, remains of interest for future
investigation.
Maternal recognition of pregnancy
Short (1969) coined the phrase‘maternal recognition of
pregnancy’ when comparing the different strategies used
by the common domestic animal species to ensure
continuation of the life and secretory function of the corpusluteum beyond its normal cyclical lifespan, and so maintain
the uterus in the correct progestational state to support the
pregnancy. A series of elegant experiments in sheep, cows
and pigs carried outby R. M. Moor, L. E. A. Rowson, E. J. C.
Polge and others at The Animal Research Station in
Cambridge earlier in the 1960s (for review, see Moor, 1968)
had demonstrated that: (i) the luteolytichormone that
induces cyclical destruction of the corpus luteum is
secreted by the uterus; (ii) this uterine luteolysin reaches the
ovary via a local vascular pathway instead of the peripheralcirculation; and (iii) one or more embryos must be present
in the ipsilateral uterine horn between day 12 and day 14
after ovulation to suppress uterine release of luteolysin and
so achieve the necessaryluteostasis of pregnancy. A further
series of experiments in the early 1970s established that: (i)
prostaglandin F2a (PGF2a) is the essential component of the
uterine luteolysin in mammals; (ii)PGF2a is released from
the endometrium in spike-like bursts late in dioestrus; and
(iii) PGF2a reaches the corpus luteum via direct local
countercurrent transfer between the uterine vein and the...