Seminars in Developmental Biology最新文献

Sexual phenotype in gonochoristic species of hydra (species having separate males and females) is ultimately determined by cells committed to the sperm pathway. Little is known about the genetic basis for determining sex during embryogenesis, but the sex ratio of offspring is generally 1 : 1, implying a genetic component. Sexual phenotype of adult hydra is labile and sex-reversal in both directions can be induced by experimental manipulations involving addition or loss of the sperm lineage. Loss of the sperm lineage is induced by high temperature, causing males to switch to females. Introduction of the sperm lineage into females by grafting causes sex-reversal to male. These studies provide support for a model of sex determination based on cell-cell interactions.

Unique features of ciliates are reviewed in the framework of a speculative series of evolutionary transitions: a uninucleate protozoan gave rise to a multinucleate unicell and then a nuclear dimorphic unicell, with a germline micronucleus and a differentially amplified somatic macronucleus, possibly before the divergence of ciliates and heterokaryotic foraminiferans. Ciliates evolved to proliferate only in the diploid state. Macronuclear DNA fragmentation and amitotic karyokinesis arose multiple times. Variability arises in the macronucleus, a potential source of selectable diversity, and probably the cause of clonal senescence. Transposons may have played a role in micronuclear silencing, and proliferate rapidly in hypotrichous ciliate germlines due to their precise elimination from the developing macronucleus before its genes are expressed.

The production of mature spermatozoa requires a complex interaction between Sertoli cells and germ cells. Sertoli cells regulate aspects of germ cell division and differentiation while germ cells provide signals that modulate Sertoli cell functions. Germ cells can undergo some differentiation independent of Sertoli cells but at certain crucial points the interaction with Sertoli cells is required. There are several means by which this interaction may occur: (1) direct contact of components of the plasma membrane may act as a signal; (2) secondary messengers could be exchanged via gap junctions; (3) the secretion of paracrine factors may facilitate intercellular communication.

Interactions between germ cells and somatic cells are important at several stages of Drosophila development. The types of interactions that will be discussed include: (1) molecules physically transferred from one cell to another; (2) long range interactions by hormones; and (3) local interactions between germ cells and somatic cells when they are in close proximity. These interactions have been mostly characterized during oogenesis.

Germ cell development in Caenorhabditis elegans involves three processes: a shift from the mitotic to the meiotic cell cycle; the adoption of a male or female sexual identity; and differentiation into a functional gamete. All three aspects of germline development appear to be regulated, at least in part, by the soma. We discuss cell ablation, genetic and molecular studies that have shed light on the nature of the signal transduction systems mediating intercellular communication between germline and somatic tissues of the nematode.

Mouse germ cells during their migratory period depend for their survival, proliferation and guidance on growth factors and/or extracellular matrix produced by somatic tissue. The somatic development of the ovary appears more affected by the absence of germ cells than is the testis. Some factor emanating from testicular somatic tissue, probably from Sertoli cells, inhibits germ cells from entering meiosis before birth and thus induces them to embark on spermatogenesis rather than oogenesis. Genomic imprinting must also reflect some somatic influence.

This paper reviews the communication between the developing follicular germ cell, the oocyte, and its companion somatic cells, the granulosa cells. Both gap junctions and paracrine factors mediate this communication. Direct transfer of low molecular weight factors through the gap junctions is essential for oocyte growth and the regulation of meiosis. Paracrine factors secreted by granulosa cells, such as the c-kit ligand, also participate in these processes. Oocytes secrete paracrine factors that affect follicular organization, granulosa cell proliferation, and the ability of cumulus granulosa cells to produce hyaluronic acid. Thus the bidirectional communication between the germ cell and the somatic components of the ovarian follicle is essential for the development and function of both.