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 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 showing a prominent inner cell mass and hypoblast cells originating from the inner cell mass spreading inside the trophoblast layer.
Grade 2 compact morula.
Grade 2 early blastocyst .
Grade 3 expanded blastocyst recovered from a cloudy flush. Shrunken blastoceole, capsule damage, and darkened cells in and beneath the trophoblast layer.
Grade 2.5 to 3 expanded blastocyst. There has been significant leakage of fluid from the blastocoele but the capsule remains relatively intact.
Grade 1 expanded blastoycst.
Grade 2 early blastocyst.
Grade 2 early blastsocyst showing a beginning blastocoele.
Grade 3 expanded blastocyst with fluid loss from blastocoele and capsule.
Grade 2 expanded blastocyst with zona partially flaked off.
Grade 1 expanded blastocsyt showing the hypoblast cell layer migrating inside the trophoblast layer.
Twin grade 3 expanded blastocysts.
Closeup of a grade 1 early blastocyst with a supranumery sperm embedded in the zona pellucida.
Day 4 ICSI embryos of varying quality.
Grade 1 expanded blastocyst with the microscope focused to highlight the inner cell mass.
Grade 3 compact morula.
Triplet embryos, grades 3, 2, and 1 from a single flush. All three resulted in pregnancies.
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.
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 2 compact morula. The extruded blastomeres here have not been compressed yet and are still discernable as cells.