4 Different Stages in Embryonic Development:

1-

2-

3-

4-

In early development, we will discuss.. , ... and..

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Fertilisation

• - Formation of a diploid.. from haploid egg and sperm. Molecules involved:

• --> Sperm penetrate protective layer around egg

• --> R.. on egg surface bind to molecules on.. surface

• --> Change at egg surface prevents p.. (entry of multiple sperm nuclei)

Acrosomal (tip of sperm) and cortical (granules) reactions during sea urchin fertilisation

• 1- ... reactions trigger when sperm meets egg

• 2- Acrosome at tip of sperm released.. enzymes that digest material around egg

• 3- ... bridge forms between sperm and egg

Acrosomal (tip of sperm) and cortical (granules) reactions during sea urchin fertilisation

• 4- Membrane f.. between sperm and egg

• 5- The sperm.. enters egg

• WAYS EGG STOPS POLYSPERMY: Fast Block to Polyspermy (not in mammals)

• - Seconds after sperm binds, ... channels open in egg plasma membrane

• - ... ion influx causes.. of egg membrane (as NA+2 are +ive)

• WAYS EGG STOPS POLYSPERMY: Cortical Reaction

• - Reaction initiated by sperm..

• - Seconds after, ... in egg open and release calcium ions and form a fertilisation..

• - This acts as a s.. block to polyspermy

• --> IMPORTANT as it forms a calcium ions wave that.. from one side to the other, forming impermeable layer

Cleavage

• - Fertilisation is followed by cleavage, a period of rapid cell.. without growth

• - Cleavage partitions embryo into smaller cells called b..

• - b.. is a ball of cells (1) with fluid-filled cavity called b.. (2)

Cleavage

• Overlapping Process: Cell.. (mitosis): from one cell, division produces many cells

Gastrulation: Rearranges cells of the.. into layered embryo called the..

--> Blastocoel: the.. in the blastula

--> Invagination: On one side, pushing cells inwards/outwards

--> First initial stage of cell.. as inner layer will be different from outer layer (outer becomes skin, inner becomes.. system)

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Gastrulation: Rearranges cells of the Blastula into layers embryo called the Gastrula

• Overlapping Process: Cell.. (cells become specialised)

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Embryonic Layers

• - Germ Layer: Embryonic tissue (collection of specialised cells)

• - Ectoderm: Germ layer covering embryo's.. (skin)

• - Endoderm: Innermost germ layer and lines developing.. tube, called the a.. (gut)

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Embryonic Layers

• Diploblastic: Animals have an e.. and e.. , and includes non-bilaterians (no bilateral)

• Triploblastic: Animals have intervening 3rd layer, a m.. , this includes all bilaterians

Endoderm: d.. tract and organs derived from it as well as s.. organs

Ectoderm: e.. and n.. system, as well as hair, cornea, etc

Mesoderm: Coelom, notochord, (in chordates), m.. , bones, c.. system, spleen, kidneys, reproductive system, etc

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Overlapping Process

• - M.. : Tissue formation, Differentiated cells form generalised tissues and organs

Overlapping Process

• - Morphogenesis: Tissue formation, Differentiated cells form generalised tissues and organs

• Blastopore fate

• - In Protostomes, Blastopore becomes mouth/anus

• - In Deuterostomes, Blastopore becomes mouth/anus

Summary:

• 1- Fertilisation: Formation of a diploid.. from haploid egg and sperm.

• 2- Cleavage: period of rapid cell.. without growth

• 3- Gastrulation: Rearranges cells of the.. into layered embryo called the.. . (First stage of cell.. )