Emergency Services Information

The Equine Embryo: A Gallery

The equine embryo is special in many ways:

Capsule: The embryonic capsule is unique among domestic mammals. This is a glycoprotein protective coat formed by the trophoblast layer beneath the zona pellucida. It forms only after the embryo enters the uterus, so embryos grown in a laboratory do not develop the capsule until after they are transferred. If not protected by either the zona pellucida or the embryo capsule, an embryo cannot survive in the uterus.
Selective transport: Fertilization occurs and the embryo is formed within the oviduct. It remains in the oviduct for the first five and a half days after fertilization, until it signals the oviduct to move it into the uterus with the production of a specific form of prostaglandin. Without this signal, the oviduct tends to retain ova that are not fertilized or embryos that do not reach this stage of development. This means, unlike most domestic animals, unfertilized equine oocytes will generally not be recovered on a uterine flush unless they have accompanied an embryo into the uterus.
Growth rate: Once the embryo becomes a blastocyst and starts expanding around day seven, the embryo will nearly double in size each day until day 12. This allows early ultrasound determination of pregnancy status, as an embryo around 300-400 microns (0.3-0.4mm) in size when recovered at day seven will grow to 3-4mm, large enough to be visualized on an ultrasound scan, by day 11.
Mobility phase: The early equine embryo does not sit quietly in the uterus, but moves rapidly along the entire endometrial surface until approximately day 17, when it becomes too large to move. The embryo needs to have close contact with the endometrial surface to stimulate maternal recognition of the pregnancy, so the mare will maintain a good environment for the embryo and not return to heat.

: Grade 2 early blastocyst with a fractured zona pellucida. The embryo capsule is protruding through the breech in the zona. Extruded blastomeres are trapped between the zona pellucida and the capsule.

: Grade 2 early blastocyst with prominent internally extruded blastomeres.

: Grade 2 compact morula just starting to form a blastocoele.

: Grade 2 compact morula.

: Grade 2 expanded blastocyst with zona partially flaked off.

: Unfertilized oocyte (UFO).

: Grade 2 early blastocyst with abnormal cumulus cells remaining adhered to the zona pellucida.

: Grade 3 expanded blastocyst showing significant capsule damage.

: Grade 2.5 to 3 expanded blastocyst. There has been significant leakage of fluid from the blastocoele but the capsule remains relatively intact.

: Unfertilized oocyte (commonly called UFO) with a sperm cell adhered to the zona pellucida. UFOs are commonly flattened, but this one was still spherical but degenerated ooplasm marks it as a UFO.

: Grade 4 morula.

: Triplet embryos, grades 3, 2, and 1 from a single flush. All three resulted in pregnancies.

: Grade 3 early blastocyst (note capsule formation evident in zona pellucida deficit and under extruded blastomere.

: Grade 2 expanded blastocyst exhibiting large extruded blastomeres between and the capsule and zona pellucida.

: Grade 3 compact morula.

: Grade 1 expanded blastocyst.

: Twin grade 3 expanded blastocysts.

: Grade 3 expanded blastocyst recovered from a cloudy flush. Shrunken blastoceole, capsule damage, and darkened cells in and beneath the trophoblast layer.

: Grade 3 expanded blastocyst, shrunken with damaged capsule and leakage from the blastocoele.

: Grade 3 large expanded blastocyst that has had significant fluid leakage issues. The shrunken blastocoele is significant but the loss of integrity of the capsule is of greater concern.

: Grade 2 early blastocyst with a large amount of extruded blastomere debris beneath the zona pellucida.

: Unfertilized oocyte (UFO) showing the common flattened characteristic shape and degenerated ooplasm.

: Grade 1 expanded blastocyst.

: Grade 2 early blastocyst .

: Grade 3 expanded blastocyst with fluid loss from blastocoele and capsule.

: Grade 2 expanded blastocyst with supranumery sperm embedded in the damaged zona pellucida.

: Grade 1 expanded blastocyst twins both exhibiting capsule formation under the thinning zona pellucida, prominent inner cell masses, and migrating hypoblast cells.

: Grade 1 expanded blastoycst.

: Grade 1 early blastocyst with a large raft of cumulus cells still adhered to the zona pellucida.

: Grade 1 expanded blastocyst showing a prominent inner cell mass and hypoblast cells originating from the inner cell mass spreading inside the trophoblast layer.

: Unfertilized oocyte with adherent sperm.

: Grade 1 expanded blastocyst with UFO adhered to the remnant of the zona pellucida.

: ICSI early blatocyst.

: Grade 2 compact morula.

: Grade 2.5 large expanded blastocyst with the hypoblast layer partially avulsed from the trophoblast layer during shipment.

: Grade 2 early blastocyst.

: Grade 2 compact morula. The extruded blastomeres here have not been compressed yet and are still discernable as cells.

: Grade 4 morula.

: Day 4 ICSI embryos of varying quality.

: Grade 2 large expanded blastocyst with an internally extruded blastomere. Most blastomeres are extruded out of the embryo but they can occasionally be trapped in the blastocoele.

: Grade 1 expanded blastocyst with a prominent capsule beneath a thinning zona highlighted by the extruded blastomeres trapped between the layers.

: Closeup of a grade 1 early blastocyst with a supranumery sperm embedded in the zona pellucida.

: Grade 1 expanded blastocyst.

: Grade 1 expanded blastocyst with some prominent hypoblast cells.

: Two unfertilized oocytes (UFOs) adhered to one another.

: Unfertilized oocyte (UFO) and debris. Photo taken to show the flattened aspect of the typical UFO.

: Grade 2 compact morula just starting to form a blastocoele.

: Grade 3 expanded blastocyst that has debris adhered externally, is somewhat shrunken with fluid loss from both the blastocoele under the capsule and from under the capsule as well.

: Grade 1 expanded blastocsyt showing the hypoblast cell layer migrating inside the trophoblast layer.

: Grade 1 expanded blastocyst with the microscope focused to highlight the inner cell mass.

: Grade 2 early blastsocyst showing a beginning blastocoele.

: Grade 1 expanded blastocyst with prominent hypoblast cell migration from the inner cell mass.

: Three ICSI early blastocysts showing good trophoblast layer and the dark coloring typical of in vitro produced equine blastocysts.

: One 2 cell embryo and 1 fertilized oocyte 24 hours following fertilization. Both exhibit extruded debris commonly expelled from the oocyte a few hours prior to the first cell division.

: Grade 1 early blastocyst with an unfertilized oocyte (UFO) adhered to the zona pellucida.