Ooplasmic Determinants and Somatic Cell Determination
Eggs of tunicates go through typically mosaic development in which the various blastomeres become determined to follow their particular paths of differentiation very early after fertilization. The determination is signaled through determinants present in the egg cytoplasm which become segregated in particular blastomeres during the first three cleavage divisions. Intensive descriptive, experimental and biochemical studies on some type of tunicate species like Styela and Ciona have confirmed that determination is brought about under the affect of intrinsic cytoplasmic (ooplasmic) determinants and not by interaction among the blastomeres or alteration in the genome in the nuclei of different blastomeres. This is particularly true for the determination of blastomeres which give rise to muscle cells of the tadpole that hatches from the egg. You have studied determination of germ cells in the mosaic eggs of Ascaris through cytoplasmic determinants. Now muscle cell determination in tunicates based upon the studies on Styela and Ciona.
This is a good instance of determination of a somatic cell type (muscle) by cytoplasmic determinants in mosaic eggs. Eggs of tunicates consist of colored cytoplasmic regions. For instance, Styela eggs contain a yellow colored region, the myoplasm. In another tunicate Boltenia the myoplasm region is orange colored. Besides, the egg contains ectoplasm and endoplasm regions as well. Cytoplasmic movements known as ooplasmic segregation localise these regions in eggs after fertilisation. In Styela and Boltenia the ooplasmic segregation is obvious due to the colored regions of the cytoplasm. In the unfertilised egg of Styela the clear ectoplasm is present in the type of germinal vesicle near the animal pole. The grey yolky endoplasm occupies the entire vegetal half and the yellow myoplasm is confined to the peripheral conical region. Instantaneously after fertilisation there occurs displacement of the three materials (plasmas) in the zygote. The yellow myoplasm flows down to the vegetal pole.
The germinal vesicle breaks and the clear ectoplasm moves into the vegetal hemisphere. At similar time the endoplasm is displaced into the animal half of the egg. This arrangement of cytoplasmic contents of the zygote does not last for much time and soon by a second phase of segregation the three cytoplasmic materials are rearranged again. The myoplasm moves to a position below the equator of the egg and forms a crescentic region on the future posterior side of the egg. It is known as the yellow crescent. Most of the grey yolky endoplasm moves to the future anterior half and the ectoplasm to the animal half of the egg simultaneously. Opposite to the yellow crescent. a fourth cytoplasmic region, the chordoplasm is formed. This is also a crescent located beneath the equator on the anterior side of the vegetal half of the egg. This arrangement of various cytoplasmic regions clearly shows the bilateral organisation of zygote contents.