Equus caballus, horse: embryonic & post-natal development

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• prenatal Average gestation period is 11 months (320-365 days). Fertilization rates during natural mating are high - up to 90%. However, an estimated 30-40% of pregnancies are lost within 2 weeks.
  • embryo MeSH
    • pre-implantation embryo
      • zygote Fertilized egg; one-cell embryo. At ovulation mature oocytes enter oviduct where one or two of them become fertilized. After ovulation, equine embryo spends first 6 days in the oviduct, near the ampullary-ischmic junction of that organ. The early embryo is surrounded by tough transparent acellular membrane called Zona pellucida. One unusual feature of early equine embryogenesis is the polarity and opacity of the oocyte and zygote. Fertilized egg starts its journey toward uterus, while unfertilized eggs are left behind.
      • cleavage By day 2, the embryo reaches 4-cell stage.
      • morula MeSH By day 4, the embryo reaches the morula stage, a compact mass of 16-32-cells (called blastomeres); it resembles a cluster of mulberries with two types of cells, outer cells and inner cells. At this stage the embryo is elongated (in contrast with cow and pig embryos, which are spherical).
      • blastocyst MeSH
        • early blastocyst 6-7 day post-ovulation. Embryo is about 0.14 mm in diameter. The horse embryo reaches the uterus as an early spherical blastocyst, which represents a hollow ball, which has two distinctive tissues: the outer layer of trophoblasts (trophectoderm) that gives rise to extra-embryonic tissues, and the inner cell mass (ICM), which gives rise to the embryonic disc and eventual embryo proper. In horses, the ICM remain much more dispersed than in ruminants and pigs and fill much more of the space within the distinct palisade of trophoblast cells. This makes it more difficult to distinguish between morulae and early blastocysts in horses. At this stage one of the most prominent features of equine embryonic development becomes noticeable: an unusual acellular glycoprotein capsule that forms between trophectoderm and Zona pellucida. The function of the capsule is not well understood, however, the position of the capsule at the maternal-fetal interface suggests that it is likely to play role in the maternal-fetal signaling to establish and maintain pregnancy (maternal recognition of pregnancy). The capsule is essential to embryonic survival. The development of the capsule is of practical significance: it makes cryopreservation and micromanipulation of the embryo very difficult.
        • expanded blastocyst 7-8 days post-ovulation. At this stage, the embryo shed its Zona pellucida (hatches) and is enveloped only by the capsule. The capsule maintains the spherical shape of the conceptus throughout the first 6-8 weeks (in contrast with cow and pigs, whose embryos elongate dramatically at this stage) and is essential for continuation of early equine embryogenesis until at least day 21^st. Until implantation, the encapsulated conceptus exchanges essential signals with the mare by moving throughout the uterus in response to myometrial contractions.
        • yolk sac The yolk sac is formed by endodermal tissue which emanates from the inner cell mass and expands spherically, lying just under the outer trophectoderm layer. The resultant two-layered structure can be first observed by day 10 to 11 and persists, in ever diminishing size throughout pregnancy. Thus, during the first 3 to 4 weeks of pregnancy, equine placentation can be described as choriovitteline. Growth of the embryo is especially rapid between days 11 and 16 (from about 4 mm in diameter at day 11 to about 20-30 mm at day 16).
        • mesoderm formation Mesoderm begins its development from embryonic disc between the endodermal and ectodermal layers at about day 14. The resultant three-layered structure develops spherically from the region of the inner cell mass toward the embryonic pole. The process is accompanied by vascular development. At this period, fully distended equine conceptus differs from the somewhat flaccid, wrinkled elongating conceptuses of pigs and ruminants.
        • "fixation" The conceptus is about 20-30 mm in diameter. At about 16 to 17 day post-ovulation, the mobility of the conseptus is brought to an end, and the conceptus becomes "fixed" at the site of eventual placentation. The glycoprotein capsule becomes flaccid, which reflects the loss of sialic acid. At around day 18 the capsule ceases to grow and begins to attenuate, raptures at about day 21 and completely disappears by day 23. The fixation coincides with the proper orientation of the embryo. It is still relatively easy to recover the intact conceptus transcervically from the standing mare at least until about day 35 (in no other domestic species can entire conceptuses be obtained from the uterus so easily and non-traumatically).
  • post-implantation embryo
    • choriollantoic
      placenta Developing embryo is surrounded and protected by 2 membranous sacs: innermost is called amnion, and outermost is called chorion. The chorion evolves into the fetal contribution of the placenta. Allantois, an extra-embryonic membranous sac, derived from the yolk sac, arises from the hindgut of the embryo and expands toward abembryonic pole (opposite to the embryonic pole where the primordial embryo is located). An exocoelom develops at the area of advancing allantois and the retreating yolk sac. By about day 30, the area of this exocoelom, which encircles the embryo, takes on a characteristic appearance. This area is very avascular and varies in width from one to several millimeters. It is known as the chorionic girdle. The cell of the girdle are highly proliferative and begin to invade the maternal endometrium between days 36 and 38 where they phagocytize maternal epithelial cells and continue to migrate, penetrating the basement membrane of the maternal epithelial cells to the stroma, where they locate between the uterine glands. Here they differentiate and hypertrophy to become mature endometrial cups, large raised structures that protrude above the uterine endometrial surface. At about this time the heart beat is detectable by ultrasound.
    • organogenesis development of the horse
      • day 40 The 3/4 inch embryo within embryonic vesicle has 4 legs, head, eyelids, rudimentary ears, ridges where the nostrils will be, and functional elbows an stifle joints.
      • days 50-55 The embryo is now slightly over an inch long, nesting within the 3-inch vesicle. Tiny ribs under skin are recognizable; domed head has developed a distinct skull; ears are visible; the hock and fetlock joints have developed. Umbilical cord is developing. At this stage, the embryo becomes a fetus.
      • day 60 The fetus is about 2 1/2 inches long (the size of a hamster). Because of the development of hooves it starts to resemble a horse. Its head is still tucked, but less so than before. The fetus is hairless.
      • day 100 7-inch fetus is about the size of a 6-week old kitten. It has a little hair on its lips; its now 1/2 inch ears are unfurling from its head.
      • days 120-150 Gaining more than a pound every 10 days, the fetus now is about the size of a rabbit. It has hair on the chin, muzzle, and eyelids with eyelashes. Umbilical cord is fully developed.
      • day 180 The fetus has quadrupled its weight in just 30 days. Mane and tail hairs have appeared; it's about the size of a Beagle. Hair grows over the body. External sex organs form.
      • day 240 Now about the size of a small lamb, the fetus has whisker-like hairs on its chin, throat and muzzle. Mane grows; hair grows along the spine.
      • day 270 Fetus now looks like a foal: fine hair covers its body, and it now has a swatch of hair on its tail. It's about the size of a German Shepherd. It lies on back, ready to turn over into position for birth.
      • day 320 In the last week or so, the fetus's lungs have developed to the point that they can function; its legs have strengthened so that they can support is weight; and its hair has coarsened. Teeth show through gums. Head rests between forelegs. The fetus is in final position and is ready to be born. Forelegs and head of foal emerge first.
• post natal
  • neonate First 1-2 weeks. Foal is born with its eyes open. The newborn is characterized by large head, short face, small mouth, short body, soft wooly coat (milk hair); long legs with soft hooves are wide apart for stability. Foals are precocious developers and, unlike calves or fawns, which tend to lie in undergrowth, can gallop with their dams within a few hours of birth. Foals begin to engage in exploratory trips away from the dam with other foals between one and two months of age.
  • foal Under 1 years old. By 4 months of age smooth adult coat replaces milk hair. Hooves grow and harden. Cannon bone is longer. Juveniles are weaned at about 7 months. Body develops adult proportion and withers become higher than croup.
  • juvenile Less than 4 years old. A female horse is called filly and ungelded male horse is called colt. Sexual maturity is attained at about 1-2 years; successful breeding usually does not occur until 2-3 years of age.
  • adult The horse older than 4 years old. A female horse is called mare and ungelded male horse is called stallion. Castrated male horse is called gelding. The domestic horse today has a life expectancy of 25 to 30 years.

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