A tough, elastic glycoprotein capsule envelops the equine blastocyst between
Days 6 and 23 after ovulation. It maintains the spherical configuration of,
and provides physical support for, the embryo as it traverses the entire
uterine lumen during Days 6–17, propelled by myometrial contractions
that are stimulated by pulsatile release of prostaglandin
F2α and prostaglandin
E2. The capsule also accumulates constituents of the
exocrine secretions of the endometrial glands (‘uterine milk’) as
nutrients for the mobile embryo as it releases its antiluteolytic maternal
recognition-of-pregnancy signal to the whole of the surface of the
endometrium. Mobility ceases abruptly on Day 17 with a sudden increase in
uterine tonicity that ‘fixes’ the conceptus at the base of one of
the uterine horns. At Day 35, the trophoblast of the spherical conceptus has
separated into its invasive and non-invasive components. The former,
distinguished as the thickened, annulate chorionic girdle, invades the
maternal endometrium to form the unique endometrial cups. These secrete a
chorionic gonadotrophin that synergizes with pituitary follicle-stimulating
hormone to induce secondary luteal development in the maternal ovaries. The
cup cells express foreign fetal antigens that stimulate strong maternal
humoral and cell-mediated immune responses, which curtail their lifespan. The
non-invasive trophoblast of the allantochorion establishes a stable
microvillous contact with the endometrial epithelium around Day 40 and, over
the next 100 days, develops a complex multibranched interdigitation with the
endometrium to form the microcotyledonary haemotrophic exchange units that
cover the entire surface of the diffuse epitheliochorial placenta. Reduction
in the effective total area of fetomaternal contact at this placental
interface, by competition between twin conceptuses for the limited area of
available endometrium, by attachment of the allantochorion to an imperfect
endometrium in a mare with endometrosis, or following cross-breeding or embryo
transfer between a sire and dam of dissimilar size, will all induce
intrauterine growth retardation of the fetus and runting of the foal, which
persists into adult life. Over 40 years ago, Professor Roger Short and his
colleagues determined that the high concentrations of conventional and unique
ring B unsaturated oestrogens in the blood and urine of mares during the
second half of pregnancy stem from placental aromatization of large quantities
of C-19 precursor molecules secreted by the temporarily hypertrophic fetal
gonads. Placental production of progesterone and 5α-reduced
progestagens, on the other hand, depends on both maternal and fetal adrenal
sources of pregnenelone.