Study of the Lineage and Cell Cycle of Small Micromeres in Embryos of the Sea Urchin, Hemicentrotus pulcherrimus

Abstract

A study was made of 1 st cell cycle of small micromeres, segregated at the 5th cleavage cycle, in the sea urchin embryos of Hemicentrotus pulcherrimus. For identification of small micromeres, the embryos were pulse labeled with 5-bromodeoxyuridine (BrdU) at the 1 st cleavage. Using multiparametric microfluorometry equipped with a scanning stage (Tanaka, 1990), DNA content, extent of BrdU incorporation, protein content and the extent of 3H-thymidine labeling were measured on identical individual cells dissociated from an embryo. The findings of the present study are as follows. There is a short period of time between the telophase and onset of DNA replication. The period of DNA replication is 5 hr and after which, asynchronous mitosis takes place to produce 8 cells before hatching. The long S period is 83% the total 6 hr of the cell cycle. The rate of DNA accumulation is quite small during the initial one third of S but increases later in this phase. The degree of chromatin condensation remains high even during the S phase but it is low in large micromeres. The cell cycle may possibly be related causally to the development of small micromeres. The developmental significance of cell cycle duration, particularly that of DNA replication is discussed.

Role of the nanos homolog during sea urchin development

“…Given that the nanos gene has been previously detected in the germ line of animal species and is required for their development, it is thought that small micromere descendants are involved in germ cell development. This possibility was supported by the fact that other factors, which are reported to be expressed in the PGC of other animal species, were accumulated in small micromeres (Ogawa et al, 1999;Juliano et al, 2006) and that small micromeres exhibit a slow cell cycle like PGCs (Tanaka and Dan, 1990). However, it has been reported that functional germ cells can give rise to in adult sea urchins after depletion of the micromeres from the embryo (Ransick et al, 1996).…”

“…Labeling of small micromere descendants was performed according to the method reported by Tanaka and Dan (1990) with some modifications. Embryos were incubated in FSW containing 0.5 M BrdU just after MO injection for 1 hr.…”

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

“…5A), just after its formation. Its lineage was traced to the prism/pluteus stage, by which time the SMic descendants go through at least two cell divisions (Tanaka and Dan, 1990;Cameron et al, 1987). As a result, several labeled SMic descendants from a single SMic were found in the nascent coelomic pouches.…”

“…The developmental origin of the germ line and the specification mechanism of sea urchins have long been controversial. Sea urchin SMics have several PGC-like features, such as early segregation at the fifth division, a transcriptionally quiescent state (Pehrson and Cohen, 1986), a slower cell cycle (Tanaka and Dan, 1990), expression of germ line markers (Juliano et al, 2006), and a contribution to germ cell formation in the adult (Yajima and Wessel, 2011a). Yet, when the precursors of these cells, the Mics, are removed from the embryo, the resultant adults are still gravid and form eggs and sperm in the absence of the Mics (Ransick et al, 1996).…”

“…Recent studies, however, suggest that small micromeres (SMics) formed at the fifth cellular division have PGC features. The SMics are located at the vegetal tier in association with endomesodermal precursors, divide more slowly than their adjacent cells (Tanaka and Dan, 1990), express germ line-related genes such vasa, nanos and piwi (Juliano et al, 2006), and are involved in germ cell formation in the adult (Yajima and Wessel, 2011a). SMics are not necessary for larval development and contribute only to subregions of the coelomic pouches (Pehrson and Cohen, 1986), the precursor of the adult rudiment (see Fig.…”