Perivitelline threads in cleavage-stage human embryos: observations using time-lapse imaging

Area of Science:

• Human embryology
• In vitro fertilization (IVF)
• Preimplantation embryo development

Background:

• Time-lapse imaging reveals transient perivitelline threads in human preimplantation embryos.
• These threads appear at the two-cell stage, spanning the perivitelline space.
• Their formation is linked to cytoplasmic membrane contact with the zona pellucida.

Purpose of the Study:

• To describe the incidence and characteristics of perivitelline threads.
• To investigate the association of these threads with embryo development stages and cytoplasmic fragments.
• To determine if perivitelline threads affect early embryo development.

Main Methods:

• Retrospective analysis of 834 time-lapse videos from IVF cycles.
• Observation period: pronuclear formation to completion of the first cell cycle.
• Quantification of thread presence, formation timing, and association with cleavage and fragments.

Main Results:

• Perivitelline threads were observed in 56.4% of embryos.
• Threads predominantly formed after cytoplasmic membrane-zona pellucida contact (91.9%) and were visible at the first cleavage furrow (76.4%).
• 77% of threads were associated with cytoplasmic fragments at the two-cell stage; their presence did not affect embryo development.

Conclusions:

• Perivitelline threads are a common, transient phenomenon in human preimplantation embryos.
• Their occurrence is not linked to impaired embryo development.
• Further research is required to understand the functional significance of these structures.