📝 STANDARD DESCRIPTION:
Cleavage = rapid mitotic divisions of zygote without cell growth between divisions.
Result: Cells (blastomeres) get smaller with each division; total embryo size unchanged.
Sequence (mammalian):
• 1-cell zygote → 2-cell → 4-cell → 8-cell → 16-cell → 32-cell stage.
• At 16-32 cells = MORULA ('mulberry stage') — solid ball of cells inside zona pellucida.
• Compaction occurs at 8-16 cell stage (cells flatten + form tight junctions).
Type: Holoblastic (complete) + nearly equal cleavage in mammals.
Location: Cleavage occurs as embryo travels down uterine tube → reaches the uterus at a species-specific time (cow morula about day 4–5; bitch morula about day 8–10).
📚 DETAILED DESCRIPTION:
CLEAVAGE & THE MORULA
Cleavage is a series of rapid mitotic divisions following fertilization. Unlike normal cell cycles, there is no cell growth (cell growth is minimal and early cell-cycle phases are abbreviated, although mammalian cycles do not universally omit G1 and G2) — daughter cells simply become smaller with each division.
FEATURES of cleavage:
• Total embryo volume stays nearly constant.
• Surface-to-volume ratio of each blastomere increases.
• Nuclear-to-cytoplasmic ratio increases.
• Cell cycle is short (mostly S + M phases).
• Eventually new genes 'switch on' (zygotic genome activation, around 4-8 cell stage in cow).
SEQUENCE (mammalian):
• Day 1: Fertilization in ampulla → 1-cell zygote.
• Day 1-2: First cleavage → 2-cell stage. (Slow — ~24 h).
• Day 2: 4-cell stage.
• Day 3: 8-cell stage. Compaction: cells flatten and form tight junctions — outer cells become a tight epithelium.
• Day 3-4: 16-cell MORULA (Latin morum = mulberry). Has outer + inner cell populations.
• Blastocyst timing is species-specific: in cattle cavitation and blastocyst formation usually occur around day 6–7; canine development and uterine entry are later.
CELL FATE BY POSITION:
From the 16-cell stage:
• Outer cells → Trophectoderm/Trophoblast → contributes to chorion and the fetal placenta; other fetal membranes also include epiblast-derived tissues (NOT the embryo proper).
• Inner cells → Inner Cell Mass (ICM) / Embryoblast → forms the embryo proper + a few extra-embryonic structures (yolk sac, amnion).
POTENCY at this stage:
• Up to 8-cell stage in cow/sheep, blastomeres are totipotent — each can form a complete embryo (basis of identical twinning by embryo splitting in cattle).
• After compaction, cells become committed.
TIMING (Embryo arrival in uterus):
• Cow: morula → uterus on day 4-5.
• Mare: late morula/early blastocyst → uterus day 5-6.
• Bitch: morula → uterus on day 8-10.
• Sow: morula → uterus on day 4.
CLINICAL USE:
• In Vitro Embryo Production (IVEP): Standard in dairy cattle — fertilize oocytes in lab, culture to morula/blastocyst, transfer to recipient. Allows rapid genetic improvement.
• Embryo Splitting: Cleave a morula in 2 → identical twins (cattle).
// COMPARATIVE ANALYSIS
| COW |
First cleavage ~24 h post-fert. Reaches uterus at morula stage (~day 5). Hatching from zona ~day 9. |
| MARE |
Reaches uterus as blastocyst (~day 6). Mobile blastocyst phase — embryo moves throughout uterus to signal pregnancy. |
| BITCH |
Long uterine tube transit — morula reaches uterus at 8-10 days. Late implantation (~day 17). |
CLINICAL RELEVANCE
Embryo Transfer (ET): Routine in cattle for elite genetics. IVEP: Lab-made embryos. Embryo Splitting: Identical twin production. Early Embryonic Death (EED): Major cause of fertility loss; ~30-40% of conceptions in dairy cows are lost at this stage.
