cleavage

• The blastocyst is the stage at which an embryo hatches from its zona pellucida.
• Embryonic cleavage is the division of cells in the early embryo
• Cleavage begins during the transport of the embryo through the oviduct
• In normal-developing embryos, cell division occurs every 18-20 hours
• Cleavages occur almost without cellular growth, blastomeres become smaller as the original cytoplasm of the zygote
• Once there are approximately 16-32 blastomeres, the zygote becomes a morula
• Morula formation marks the end of cleavage
• Compaction is a process where the outermost cells of the morula differentiate into an epithelium, attach firmly to each other, and give the embryo a smoother surface
• Blastulation is the formation of a blastula from a morula
• Blastula is a hollow ball of cells that forms between 5-8 days after fertilization
• Blastomeres move to the edge of the cell, leaving a fluid-filled space in the center called the blastocoel
• In mammals, the blastula is referred to as a blastocyst
• The mammalian blastula undergoes differentiation of cells into the inner cell mass (ICM) and trophectoderm
• The inner blastomeres become positioned at one pole of the embryo forming the inner cell mass (ICM) (embryoblast)
• The ICM will give rise to the definitive structures of the fetus
• The trophectoderm cells will give rise to the embryonic part of the placenta
• Around the time of hatching, the inner cell mass (ICM) reorganizes into two cell populations: hypoblast and epiblast
• The hypoblast cells contribute to extraembryonic structures (yolk sac)
• The epiblast cells will form the embryo and give rise to the three primary germ layers (ectoderm, endoderm, and mesoderm)
• The Rauber’s layer is subsequently lost, and the epiblast merges with the trophectoderm to establish the bilaminar embryonic disc
• After hatching, the blastocyst retains its spherical shape at first
• In ruminants and pig, blastocyst elongation continues, and the embryo becomes tubular and later filamentous
• During elongation, the blastocyst secretes estrogens for corpus luteum maintenance and progesterone production for uterus preparation for pregnancy