Embryology of the Horse

Learning Outcomes

  • Describe embryology of the horse.
  • Explain its role in healthy development and genetic diversity.

DNA Overview

  • Chromosomal Structure:
    • Horse has 64 chromosomes (2n).

Genes and Alleles

  • Genes code for proteins that determine physical traits.
  • Alleles are variations of the same gene (e.g., the MDR1 gene in collies).

Genetic Variation

  • Occurs during meiosis (sex cell formation).
  • Causes of genetic variation include:
    • Mutations can be inherited.
    • Gene flow influenced by human activities in horse populations.

Genotype vs Phenotype

  • Genotype: Combination of alleles an individual possesses.
  • Phenotype: Physical expression or traits of an individual's genotype.
  • Genotypes affected by:
    • Genetic variation
    • Meiosis
    • Fertilization
  • Phenotypes are influenced by inheritance patterns.

Punnett Squares

  • Tool for predicting genotype and phenotype probabilities:
    • BB, Bb, bb for genotypes, resulting in color probabilities such as blue and white.

Coat Colours

  • Extension Gene (E or e): Controls production of black or red hair.
  • Agouti Gene (A or a): Controls distribution of black hair.
    • Genotypes affecting coat colors:
    • ee (recessive) leads to red hair.
    • AA/Aa/aa can lead to black hair.
    • EE/Ee can be homozygous for black.

Stages of Embryo Development

  1. Zygote (2-8 cells)
  2. Morula (16 cells, ICM)
  3. Blastocyst (early, expanded, late)
  4. Embryo to Fetus (>60 days).

Equine Embryo Development

  • Key features:
    • Morula:
    • Zona Pellucida
    • Formation of blastocoele and inner cell mass (ICM) and trophoblasts.
    • Gap Junctions: Form between inner cells, allowing cell polarization.
    • As fluid accumulates, trophoblast cells create a cavity (blastocoele).

Early Days of Development

  • 5.5 days: Embryo enters the uterus.
  • 16 days: Embryo attaches to the uterine wall.
  • 20-30 days: Formation of cardiovascular and nervous systems.
  • 30-60 days: Development of other organs.
  • >60 days: Embryo recognizable as a horse (fetus).

Sex Determination

  • SRY gene on the Y chromosome determines male sex.
  • Mullerian duct: Develops into female reproductive structures.
  • Wolffian duct: Develops into male reproductive structures.
  • Presence of anti-mullerian hormone during male development.

Extra-Embryonic Membranes Development

  • Day 14: Mesometrial and Myometrial contraction; attachment to the endometrium.
  • Day 18: Initial formation of extra-embryonic membranes.
  • Day 24: Development of the chorion.
  • Day 36: Formation of the allanto-chorion, critical for nutrient transfer.