First cleavage in mammalian embryos occurs in:
In mammals fertilization occurs in ampulla of fallopian tube and first mitotic cleavage initiates during embryonic transit toward uterus. Zygote undergoes cleavage divisions within oviductal lumen bathed in fluid secreted by epithelial cells providing pyruvate, lactate and growth factors, while still enclosed by zona pellucida. No contact with uterine epithelium occurs yet. Morula reaches uterus after three to four days. Thus oviduct supplies protected, nutritive microenvironment sustaining genome activation, compaction, cavitation preparation before implantation becomes possible inside uterin
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Early cleavage and embryonic transit within oviductal environment.
