Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

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Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

Oocyte apoptosis: like sand through an hourglass.

To obtain a reliable molecular probe to trace the origin of germ cell lineages in birds, we isolated a chicken homolog (Cvh) to vasa gene (vas), which plays an essential role in germline formation in Drosophila. We demonstrate the germline-specific expression of CVH protein throughout all stages of development. Immunohistochemical analyses using specific antibody raised against CVH protein indicated that CVH protein was localized in cytoplasm of germ cells ranging from presumptive primordial germ cells (PGCs) in uterine-stage embryos to spermatids and oocytes in adult gonads. During the early cleavages, CVH protein was restrictively localized in the basal portion of the cleavage furrow. About 30 CVH-expressing cells were scattered in the central zone of the area pellucida at stage X, later 45–60 cells were found in the hypoblast layer and subsequently 200–250 positive cells were found anteriorly in the germinal crescent due to morphogenetic movement. Furthermore, in the oocytes, CVH protein was predominantly localized in granulofibrillar structures surrounding the mitochondrial cloud and spectrin protein-enriched structure, indicating that the CVH-containing cytoplasmic structure is the precursory germ plasm in the chicken. These results strongly suggest that the chicken germline is determined by maternally inherited factors in the germ plasm.

Isolation of chicken vasa homolog gene and tracing the origin of primordial germ cells.

: This synopsis compiles and reviews the available information on the identity, distribution, life history, populations, exploitation, protection, and management of the loggerhead sea turtle Varetta caretta (Linnaeus 1758) a species threatened by exploitation and the alteration and destruction of its habitat.

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Synopsis of the Biological Data on the Loggerhead Sea Turtle Caretta Caretta (Linnaeus 1758).

Maternally inherited mitochondria and other cytoplasmic organelles play essential roles supporting the development of early embryos and their germ cells. Using methods that resolve individual organelles, we studied the origin of oocyte and germ plasm-associated mitochondria during Drosophila oogenesis. Mitochondria partition equally on the spindle during germline stem cell and cystocyte divisions. Subsequently, a fraction of cyst mitochondria and Golgi vesicles associates with the fusome, moves through the ring canals, and enters the oocyte in a large mass that resembles the Balbiani bodies of Xenopus, humans and diverse other species. Some mRNAs, including oskar RNA, specifically associate with the oocyte fusome and a region of the Balbiani body prior to becoming localized. Balbiani body development requires an intact fusome and microtubule cytoskeleton as it is blocked by mutations in hu-li tai shao, while egalitarian mutant follicles accumulate a large mitochondrial aggregate in all 16 cyst cells. Initially, the Balbiani body supplies virtually all the mitochondria of the oocyte, including those used to form germ plasm, because the oocyte ring canals specifically block inward mitochondrial transport until the time of nurse cell dumping. Our findings reveal new similarities between oogenesis in Drosophila and vertebrates, and support our hypothesis that developing oocytes contain specific mechanisms to ensure that germ plasm is endowed with highly functional organelles.

A Balbiani body and the fusome mediate mitochondrial inheritance during Drosophila oogenesis.

Chick primordial germ cells (PGCs) which separated from the "germinal crescent" entoderm in the period from stages 4 to 8 circulated mostly through the developing blood vessels from stage 10 onward and finally migrated into the gonad. The PGCs making their appearance up to this stage were generally spherical in profile, about 14 mum in diameter. Some of the PGCs in contrast, did not enter the blood vessels but remained in the tissue (mesenchyme) of the embryo proper (tissue PGCs) and possessed pseudopodial processes, suggesting their migration by means of amoeboid movements. The circulating PGCs emerged from blood vessels in the vicinity of developing gonads by three days (gonadal PGCs). The principal mechanism responsible for the subsequent migration of gonadal PGCs is assumed to be amoeboid movements as in the case of tissue PGCs. Notable amounts of PAS-positive glycogen were demonstrated in the cytoplasm of PGCs in all stages obsreved. They also contained yolk and lipids intracytoplasmically, the former dissipating in relatively early stages of development. Electron microscopic observation revealed the electron-opaque, "fragmented nucleolus" in the large nucleus (8 mum in diameter), which represented another prominent feature of chick PGCs. PGCs contained a well-developed Golgi complex and endoplasmic reticulum.

The origin, migration and morphology of the primordial germ cells in the chick embryo

Germ cells of the chick ovary which develop asymmetrically on the right and left sides of the embryo were observed by electron microscopy from 7 to 17 days of incubation. Germ cells showed different patterns in the right and left ovaries, and also differences in the cortex and in the medulla. At 7 to 9 days, germ cells were individually present in both the right and left ovaries. They proliferated from 10 days onward and formed many cell aggregates. In each aggregate, germ cells were joined by intercellular bridges, and their cell cycles were synchronized. Although meiotic features of the germ cells were frequently found in the cortex of the left ovaries at 16 days, some germ cells exhibited these features at earlier days. In the meiotic germ cells, the "Balbiani body," which was composed of a concentration of cell organelles shifted to one pole of the cell, was always seen. The appearance of a meiotic nucleus and Balbiani body was characteristic of germ cells located in the left ovarian cortex, whereas those were never seen in the left medulla and in the right ovary. At 16 and 17 days, some germ cells were released into the lacunae of the medulla of both the right and left ovaries. The meaning of this phenomenon is discussed. Degenerating germ cells were frequently observed in the right ovary, but rarely in the left.

A fine morphological study of germ cells in asymmetrically developing right and left ovaries of the chick

Chick primordial germ cells (PGCs) in the final course of their migration were observed by scanning (SEM) and transmission (TEM) electron microscopy, with an emphasis on their extravasation. The profile of the PGCs leaving blood vessels were first revealed under SEM. Chick embryos at stages 16 and 17 (about 2.5 days of incubation) were employed for the present study, since the PGCs emerging out of the vessels in teh gonadal area could be observed in high frequency at this developmental period. PGCs in the vessels showed a round profile, possessing many microvilli, while extravasating PGCs were rather oligovillous except the one side of the cell, where long filopodia extended toward the wall of the vessels. These filopodia seem to adhere to the wall of the vessels prior to emerging out. After extravasation, PGCs moved toward adjacent prospective gonadal epithelium and invaded it by amoeboidism. Following the settling down in the epithelium, PGCs showed rugged surface with few microvilli.

Scanning and Transmission Electron Microscopic Observations of Chick Primordial Germ Cells with Special Reference to the Extravasation in Their Migration Course

The right ovary of chick embryos is known to degenerate during embryogenesis, while the left one advanced its normal growth. The present study demonstrates germ cell death in the degenerating right ovary. In the process of germ cell death, chromatin condensation, reduction of cell size, and formation of apoptotic bodies were observed. The cell organelles were almost normal in the early stages of the process. Fragments of broken germ cells were phagocytosed by neighboring cells or macrophages. From these observations, it may be assumed that germ cell death in the right ovary of chicks is related to apoptosis. In the normal left ovary, similar cell death was only observed in the germ cells in the medulla, but not in the cortex. Therefore, germ cell death seems to be a phenomenon common to the medullae of both the right and left ovaries.

https://bioone.org/journals/zoological-science/volume-13/issue-4/zsj.13.559/Germ-Cell-Death-in-the-Degenerating-Right-Ovary-of-the/10.2108/zsj.13.559.pdf

Germ cell death in the degenerating right ovary of the chick embryo.

Primordial germ cells (PGCs) in the turtle embryo (Caretta caretta) were observed with light and transmission electron microscopes. Identification of the PGCs for light microscopy was made by the periodic acid-Schiff (PAS) technique. PGCs were first found in the yolk-sac endoderm through the 5th to 6th day of development. PGCs freed from the endoderm then migrated to the root area of the dorsal mesentery and the coelomic angle between the 7th and the 11th day of development, and finally settled down in the gonadal anlage by the 14th day. Turtle PGCs were characterized by a large size (16 μm in diameter) and large nuclei with distinct nucleoli, and by the presence of large numbers of lipid droplets, yolk platelets and glycogen particles in the cytoplasm. Cell organelles were well-developed in PGCs at later stages. Amoeboid features of the PGCs were observed in the mesenchyme, indicating active locomotion. PGCs were usually surrounded or encircled by neighboring somatic cells. No intravascular PGCs were detected at any stage of development examined.

Atsumi Ukeshima

Papers

The origin, migration and morphology of the primordial germ cells in the chick embryo

A fine morphological study of germ cells in asymmetrically developing right and left ovaries of the chick

Scanning and Transmission Electron Microscopic Observations of Chick Primordial Germ Cells with Special Reference to the Extravasation in Their Migration Course

Germ cell death in the degenerating right ovary of the chick embryo.

Observations of primordial germ cells in the turtle embryo (caretta caretta): light and electron microscopic studies