An important consequence of fertilization: Quiescent (resting) Egg’s metabolism is transformed into a highly active metabolic state > CLEAVAGE

Start with first Division ->

Cleavage is a stage of VERY fast cell divisions. It starts immediately after fertilization. But, how do we mark its end? Cell cycling slows, indicating end of cleavage stage

Typical cell cycle

Typical accelerated cell cycle during cleavage stage

occurs for most phyla

Cell division includes cytokinesis and karyokinesis

Table 8.1. Karyokinesis and cytokinesis Major disruptive drug LocationMajor protein composition Mechanical agentProcess Colchicine nocodazole a Central cytoplasmTubulin microtubules Mitotic spindleKaryokinesis CytochalasinBCortical cytoplasmActin microfilaments Contractile ringCytokinesis

In MOST cases, cleavage reduces a large egg volume into many smaller cells The relation of nuclear volume to cytoplasmic volume changes during cleavage stage The ratio of: nuclear volume/cytoplasmic volume in each cell, and in the whole organism, increases

Two Important Cleavage Geometry Factors -Positioning (& timing) of spindle *maternal factors *here-also sperm influence Here, physical positioning of spindle according to microtubules contributed by sperm cell

Two Important Cleavage Geometry Factors -1 Positioning (& timing) of spindle -2 Retardation of traversal of yolk during cytokinesis

Retardation of traversal of yolk in cytokinesis

Up to Here: Cleavage, generalized. note also: 3. strategies of nutrition 4. when are axes specified? NOW, the specific cases

Sea Urchin Cleavage

Vade Mecum

Blastocoel

-Basal lamina chondroitin sulf. collagen laminin fibronectin Peri-vitelline space -Hyaline

Spiral holoblastic Cleavage:e.g. Molluscs

Factors that regulate spindle placement: Genetic proof for maternal factors.

Vade Mecum

Cleavage in Zebrafish

Chicken – our first amniote Amniotes Lay eggs on land: Reptiles Birds...Mammals END

Chicken –AND Zebrafish Discoidal

Tunicate: Sea Squirt - Urochordate

Cleavage in Zebrafish -Restart 0.5 mm

Large to huge amount of yolk Moderate amount of yolk Very little yolk Tetrapods: Even monotremes, but smaller

‘Our’ cleavage (as placental mammals) is very different from birds and reptiles We’ll look at non-placental mammals to bridge the gap, in order to understand ‘our’ cleavage

Monotremes – egg laying mammals: Figure 1. Photomicrograph of a uterine egg (4mm diameter) of O. anatinus (Platypus) showing an ellipsoidal blastodisc with eight blastomeres (BL) exhibiting meroblastic cleavage. Scale bar, 2mm. אכידנה Echidna ברוזן Platypus

Monotreme vs. Frog 11 mm

1 mm Urchin, human

Monotreme vs. Frog, Urchin, Human eggs (and human thumb)

Marsupial Phylogenetic relationship of tetrapods, and of Amniotes

Chicken – well studied amniote Amniotes Lay eggs on land: Reptiles Birds...Mammals Center of disc – embryo. Edges - extraembryonic

Monotremes – egg laying mammals: Figure 1. Photomicrograph of a uterine egg (4mm diameter) of O. anatinus (Platypus) showing an ellipsoidal blastodisc with eight blastomeres (BL) exhibiting meroblastic cleavage. Scale bar, 2mm. Echidna Platypus

Diagram of a Monotreme Egg. 1) Shell; 2) Yolk; 3) Yolk Sac; 4) Allantois; 5) Embryo; 6) Amniotic Fluid; 7) Amniotic Membrane; 8) Membrane CHICK

NOW: Eutherians – placental mammals:

Figure 1 Amniote key adaptation: the amniotic egg Hard-shelled egg or internal gestation. Removed need for moist environment for reproduction.