. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. p58 AND ASCIDIAN MUSCLE CELL DETERMINANTS 221. Figure 6. Photomicrographs showing myosin heavy chain expression in a control larva, a cleavage- arrested eight-celled embryo, and a compressed and cleavage-arrested eight-celled embryo. (A) Section ol a control lar\a showing the expression of myosin heavy chain protein in tail muscle cells (arrow) situated along the notochord. (Bisection of a cleavage-arrested eight-celled embryo showing myosin in the periph- eral cytoplasm of blastomeres (long arrows). (C) Section of a co
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. p58 AND ASCIDIAN MUSCLE CELL DETERMINANTS 221. Figure 6. Photomicrographs showing myosin heavy chain expression in a control larva, a cleavage- arrested eight-celled embryo, and a compressed and cleavage-arrested eight-celled embryo. (A) Section ol a control lar\a showing the expression of myosin heavy chain protein in tail muscle cells (arrow) situated along the notochord. (Bisection of a cleavage-arrested eight-celled embryo showing myosin in the periph- eral cytoplasm of blastomeres (long arrows). (C) Section of a compressed and cleavage-arrested eight- celled embryo showing myosin in the peripheral cytoplasm of blastomeres (long arrows) and two additional blastomeres (short arrows). Scale bar in (A) equals 50 ^m/Same magnifications in (B and C) as in (A). within the FE. ME fragments of both size classes were either fixed for electron microscopy or immunocyto- chemistry, or they were transferred to tissue culture wells containing filtered seawater and cultured until the de- sired developmental stages were obtained. Microcompression of embryos Whittaker (1980) modified T. H. Morgan's micro- compression technique (1910) to reorient mitotic spin- dles of eight-celled Stye/a embryos. Compression re- sulted in the partitioning of the myoplasm of eight-celled Slvela embryos into four blastomeres instead of two blas- tomeres. A compression method similar to Whittaker's was used to alter the normal distributions of myoplasm in B. villasa embryos. Microcompression chambers were prepared by positioning strips of lens paper about 20 mm apart on a glass microscope slide. A drop of seawater containing four-celled embryos was positioned between the paper strips, and a coverslip was placed over the em- bryos. Seawater was gradually withdrawn from beneath the coverslip by capillary action to exert a gentle pressure on the embryos. The extent of compression was moni- tored under a microscope. Pressure
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