Cleavage Through Gastrulation

Sea Urchins

Cleavage

• Series of mitotic divisions - a large egg is divided into many smaller nucleated cells - called blastomeres

– Cytoplasmic volume decreases - no growth occurs between divisions

• In most species these early cell divisions are under the control of proteins and mRNAs stored in the oocyte (zygote genome does not participate)

– Mammals are an exception

Cleavage Rates

From Fertilization to Cleavage

• Blastomeres have a biphasic cell cycle(No -G₁ and G₂ phases)

• Transition to cleavage accomplished by mitosis promoting factor (MPF)

• MPF is first involve d in the resumption of the meiotic division following ovulation in the frog

• Regulates the biphasic cell divisions of early blastomeres through cyclic activity

• Blastomere cell cycles consist of M (mitosis) and S (DNA synthesis) only

Cyclic MPF Activity

• MPF - two subunits - cyclin B (large subunit) - shows periodic behavior

– Accumulates during S and then degraded after the cells reached M

• Cyclin b regulates the small subunit of MPF - cyclin-dependent kinase

– The small subunit phosphorylates several proteins - this brings about mitosis

Cell Cycles in Early Blastomeres and Somatic Cells

Cytoskeletal Mechanism of Mitosis

• Cleavage - two processes

– First - karyokinesis - mitosis

– Mechanical agent is the mitotic spindle - made of microtubules made of tubulin

• Second - cytokinesis - division of cytoplasm

– Mechanical agent is a contractile ring of microfilaments made of actin

Creates the cleavage furrow - bisects the plane of mitosis - results in two genetically equivalent blastomeres

Microtubules and Microfilaments - Cell Division

Holoblastic Cleavage

Spiral Cleavage

Meroblastic Cleavage

Overview of Gastrulation

• The cells of the blastula stage are dramatically rearranged

• Cells get new positions and new neighbors

• The multilayered body plan of the organism is established

Overview of Gastrulation

• Cells that form the endodermal organs and mesodermal organs are brought into the interior

• Cells that form the skin and nervous system are spread of the surface of the embryo

• Thus the three germ layers are formed - and the stage is set for new interactions of the newly positioned cells

Gastrulation Movements

• Invagination - infolding of a region of cells

• Involution - inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells

• Ingression - migration of individual cells from the surface into the interior of the embryo

• Delamination - splitting of one cellular sheet into two more or less parallel sheets

• Epiboly - movement of epithelial sheets - that spread as a unit, rather than individually, to enclose the deeper layers of the embryo

Types of Cell Movements

Gastrulation

Axis Formation

• Three axes must be established that are the foundations of the body

1. anterior-posterior axis - head to tail

2. dorsal-ventral axis - back to belly

3. right-left axis - line between the two lateral sides of the body

Axes of a Bilaterally Symmetric Animal

Cleavage in Sea Urchins

• Radial holoblastic cleavage

• 1st & 2nd are meridional & perpendicular to each other

• Third cleavage is equatorial - perpendicular to the first two

Cleavage in Sea Urchins

• Fourth cleavage is different from the first three - top four cells divide meridionally into 8 blastomeres each with the same volume

• Called mesomeres

• The vegetal tier undergoes an unequal equatorial cleavage - get four large cells - macromeres & four small cells the micromeres

Cleavage in Sea Urchins

Micromere formation

Blastula Formation in Sea Urchins

• Begins at the 128 cell stage - after the 7th division

• Hollow sphere of cells around the blastocoel - at this point all cells are of the same size

• Tight junctions connect the cells into a seamless epithelial sheet

Blastula Formation in Sea Urchins

• After 10th cleavage - synchrony of cell division ends - mid-blastula transition

• Cells on the outside form cilia - embryo rotates

• Cells of the vegetal pole now begin to thicken - vegetal plate

• Cells of the animal pole- secrete enzyme that digest the fertilization envelope

Sea Urchin Blastula

Fate Map in Sea Urchin

Fate Map in Sea Urchin

• The veg₂ cells are specified by the micromeres

• The veg₂ layer help specify the veg₁ layer

• Without the veg₂ layer - the veg₁ cells are able to produce endoderm but the endoderm is not specified for foregut, midgut or hindgut.

Fate Map in Sea Urchin

• Thus there seems to be a cascade wherein the vegetal pole micromeres induce the cells above to be come veg₂ cells

• The veg₂ cells induce the cells above to be veg₁ - Thus micromeres undergo autonomous specification to be come skeletogenic mesenchyme & these go on to induce cell tiers above

b- catenin - Transcription Factor

• Molecule involved in specifying micromeres

• Activated Via the Wnt pathway

• Accumulates in the nuclei of those cells fated to become endoderm and mesoderm - this is autonomous specification

• Responsible for specifying the vegetal half of the embryo

• Finally b- catenin is essential for giving the micromeres their inductive ability

Sea Urchin Gastrulation

• After blastula hatches from the fertilization envelope - vegetal side thickens

• A small cluster of cells begin to extend and retract thin processes(filipodia)

• Then they dissociate from the surface - ingress into the blastocoel - these micromere derived cells are called - primary mesenchyme - will form larval skeleton - skeletogenic mesenchyme

• The ingression occurs because the cells lose affinity for their neighbors and for the hyaline membrane

• Also acquire a strong affinity for a protein lining the blastocoel

Ingression of Primary Mesenchyme

Archenteron Invagination

• As the primary mesenchyme leave other remaining cells still attached to one another and the hyaline layer move to fill in the gaps caused by the ingression of primary mesenchyme

• The vegetal plates also bends inward and invaginates about 1/4 to 1/2 the way into the blastocoel

• Then invagination stops - called the archenteron

• Opening is called the blastopore

Mechanism of Invagination

• Fibropellins from the cortical layer of the oocyte form a meshlike network over the embryo by the blastula stage

• At invagination - vegetal pole cells secrete chondroitin sulfate proteoglycan into the inner lamina of the hyaline layer - directly beneath them

Mechanism of Invagination

• This proteoglycan secretion is very hygroscopic - swells the inner lamina but not the outer

• Now the vegetal region of the hyaline layer buckles

• Then a second force arises from movements of the epithelial cells adjacent to the vegetal plate - draws the buckled layer inward

Invagination of the Vegetal Plate

2nd Stage of Archenteron Invagination

• Following initial invagination - the archenteron extends dramatically - may triple its length

• Cells migrate over one another and flatten out - called convergent extension

• Cell division also continues - more endodermal cells and new secondary mesenchyme cells

Third Stage of Archenteron Invagination

• In some sea urchins - third stage of archenteron elongation - tension from secondary mesenchyme cells

• Then cells form at the tip of the archenteron

• Filipodia extend from these to the inner surface of the blastocoel wall - then shorten and pull on the archenteron

• Ablating these cells results in a shorter archenteron

Mid-Gastrula - Sea Urchin