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GonadMorphogenesis in Vertebrates: Divergent Means to a Convergent End
Tony DeFalco and Blanche Capel
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710; email: t.defalco@cellbio.duke.edu, b.capel@cellbio.duke.edu

Annu. Rev. Cell Dev. Biol. 2009. 25:457–82 First published online as a Review in Advance on July 14, 2009 The Annual Review of Cell and Developmental Biologyis online at cellbio.annualreviews.org This article’s doi: 10.1146/annurev.cellbio.042308.13350 Copyright c 2009 by Annual Reviews. All rights reserved 1081-0706/09/1110-0457$20.00

Key Words
sex determination, germ cell, testis, ovary, sex reversal

Abstract
A critical element of successful sexual reproduction is the generation of sexually dimorphic adult reproductive organs, the testisand ovary, which produce functional gametes. Examination of different vertebrate species shows that the adult gonad is remarkably similar in its morphology across different phylogenetic classes. Surprisingly, however, the cellular and molecular programs employed to create similar organs are not evolutionarily conserved. We highlight the mechanisms used by different vertebrate model systems togenerate the somatic architecture necessary to support gametogenesis. In addition, we examine the different vertebrate patterns of germ cell migration from their site of origin to colonize the gonad and highlight their roles in sex-specific morphogenesis. We also discuss the plasticity of the adult gonad and consider how different genetic and environmental conditions can induce transitions between testisand ovary morphology.

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Contents
INTRODUCTION . . . . . . . . . . . . . . . . . . MECHANISMS OF GONAD MORPHOGENESIS IN VARIOUS SPECIES . . . . . . . . . . . . . . Mouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Turtle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fish . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . GERM CELLS IN GONAD FORMATION . . . . . . . . . . . . . . . . . . . . Germ Cell Migration . . . . . . . . . . . . . . Requirement for Germ Cells in Gonad Differentiation . . . . . . . . PLASTICITY OF GONAD PHENOTYPES: THE TRANSITION BETWEEN TESTIS AND OVARY . . . . . . . . . . . . Seasonal Breeders . . . . . . . . . . . . . . . . . . Masculinized Females . . . . .. . . . . . . . . Bidirectional Sex Change . . . . . . . . . . . Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . CONCLUSION . . . . . . . . . . . . . . . . . . . . . 458

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Granulosa cell: somatic support cell of the ovary that forms part of the follicle Bipotential (indifferent): undifferentiated state of the gonadalprimordium that can develop into testis or ovary Primary sex determination: development of sexual phenotype in the somatic cells of the gonad Genetic sex determination (GSD): when chromosomal elements direct sexual phenotype regardless of environment

INTRODUCTION
The propagation of all vertebrate species depends on the development of reproductive organs that support the differentiation of the germcell lineage into two types of functional gametes: sperm and eggs. The morphological structures of the adult testis and ovary show a remarkable level of similarity across vertebrates (Figure 1). The testis is organized into testis cord structures in which somatic cells surround cohorts of germ cells that differentiate asynchronously to produce a continual supply of mature sperm. Spermatogonial...