Endoplasm and Ectoplasm

Under the light microscope, we can find two regions in the-cytoplasm .of the amoeba;

1.  The central region, the endoplasm which is fluid like sol and
2. The ectoplasm, the region of the cytoplasm just beneath the plasma membrane, which is gel-like.

In the phase contrast microscope, we can see that the endoplasm contains abundant particles and membranous organellae, found in constant random motion, indicating their freedom of movement in the sol region of the cytoplasm. Ectoplasm contains a three-dimensional network of cross-linked actin fibres, and all other organelle are excluded from the region. This gel region apparently decides the shape of the pseudopodium and may transmit tension from the regions of cellular contraction to the sites of contact with the substratum. It is believed that the ectoplasm contains non-cross-linked actin filaments and probably myosin filaments also.

As a pseudopodium elongates and the sol-like endoplasm streams into it, the region of the endoplasm near the tip of the pseudopodium apparently transforms into - gel-like ectoplasm. Simultaneously, the ectoplasm elsewhere in the cell transforms into sol-like endoplasm, probably by an uncrossing of linking actin fibres. Proteins such as actin, fimbrin and fodrin are involved in the sol-to-gel transition. They cross-link actin filaments and bundle them to each other. Cross linking of actin filaments produces a network confining the movement of individual actin molecules and results in the semisolid gel state.