Figure 5

Histology and fine structure of the early kink stage of Saccoglossus kowalevskii . (A) Semithin sagittal section showing the inner organization of the embryo. The lateral evagination of the endodermal wall constitutes the rudiment of the earlier connection to the mesocoela. Fine structure of the tubular connection is shown in C. (B) The longitudinal epitheliomuscle cells (lec) of the protocoel (pc) are rich in myofilaments (myo) and mitochondria (mi). Neighbouring cells are interconnected by zonulae adherentes (za). (C) The cells comprising the former connection to the mesocoela are monociliated and resemble those of the endodermal lining. Inset: Each cilium (ci) is anchored to the cytoplasm by a short horizontal rootlet and a second vertical rootlet. (D) The endodermal cells (edc) are columnar and monociliated. They are covered by numerous bulbus microvilli (mv) and several yolk granules (yo) are present within the cytoplasm. (E) Cross section of the mesocoelic mesoderm composed of two layers. The proximal mesothelium (pms) is made up of flattened cells, while the distal mesothelium (dms) contains more cuboid and voluminous cells. Note the course of the ecm (arrowheads). (F) The proximal flattened cells contain basal myofilaments in a circular arrangement. There is a virtual lumen present between both cell layers as indicated by interspersed cilia (see S2D for high mag). (G) The distal cells contain basal myofilaments in a longitudinal arrangement. (H) SEM of a dissected embryo showing the same area as seen in E. The mesodermal is composed of a distal and proximal layer of cells. ac accessory centriole, cr ciliary rootlet, ec ecdoderm, ecc ectodermal cell, ed endoderm, lu lumen, ms mesocoel, msc mesodermal cell, mt metacoel, nu nucleus, vs vesicles.