Transmission electron microscopy (TEM) of equine conceptuses at 14 and 16 days of gestation.

The research article provides a comprehensive examination of developing horse embryos (conceptuses) at 14 and 16 days after ovulation. Using Transmission Electron Microscopy (TEM), the study identifies major structural differences and various cell activity in the embryos at these stages.

Methodology

• The researchers used Transmission Electron Microscopy (TEM) to study horse embryos 14 and 16 days post-ovulation. TEM is a powerful imaging technique that can display minute details of the studied object.
• Two embryos were examined at Day 14, while three were observed at Day 16 to note the cellular and structural development into an early equine conceptus.

Day 14 and Day 16 Observations

• On Day 14, the embryos only exhibited primitive structures. Notably, the ectoderm of the embryo and the surrounding trophoblast ectodermal cells displayed specific cell surface differentiations.
• By Day 16, the embryos had progressed to neurulation (the first step in the formation of the brain and spinal cord) and mesodermal somites formation (blocks of mesoderm along the back of the embryo that will develop into the vertebral column, musculature, and dermis).

Detailed Observations Using TEM

• Significant protrusions and fusion of cytoplasmic projections were observed on both Day 14 and Day 16 at the embryonic ectodermal cell surface. These are typically associated with macropinocytotic events, mechanisms involved in fluid and large protein uptake.
• Absorptive features on the apical trophoblast membrane were emphasized by the presence of numerous microvilli (small extensions of the plasma membrane that increase surface area) and coated endocytotic pits.
• The endodermal cells were loosely arranged with only apically located cellular junctions. This configuration led to large intercellular compartments.
• At the border of the embryonic disc, regular detection of apoptotic cells (cells undergoing programmed death) indicated high remodeling activities.

Findings and Future Suggestions

• Interestingly, blister-like structures were observed between the ectoderm and mesoderm in the trilaminar part of both Day-14 and Day-16 conceptuses. Despite being strictly circumscribed, these blisters were not sealed by cellular junctions between germinal layers.
• The researchers suggest that these blisters might play a role in embryo positioning, although further studies are necessary to determine their actual purpose.