Cleavage Through Gastrulation

Sea Urchins


     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









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