Testis determination and the H-Y hypothesis

A neuroscience-based approach to the assessment of sexual behavior in animals

Sexual behavior in animals is important in ensuring the continuity of the generation. These behaviors differ in animal species. Sexual behaviors are shaped under the control of the reproductive system. Physiological stimuli produced by the reproductive system find their counterparts in the organism as reproductive activity. Reproductive activity display a critical role by transferring on the genetic heritage of organisms to the next generations. This activity, which is built on delicate balances, is associated with many systems in the organism. Nervous system, hormonal system, and circulatory system are the main ones. The regular formation of the reproductive activity in species is due to the effect of various factors. In domestic mammals, the reproductive activity is regulated by hormones secreted from brain and endocrine glands. Many hormones have duties in terms of the sustainability of reproductive activity. GnRH is the main hormone responsible for initiating this reproductive activity. Gonadotropin-releasing hormone (GnRH), which is a small molecule peptide from certain nerve cells in the nucleus infundibularis region of the hypothalamus and consists of different amino acids, is secreted under the influence of smell, temperature, light, and physical stimulation. Besides, GnRH release is controlled by various neurotransmitters (adrenaline, noradrenaline, dopamine, acetylcholine, serotonin). On the other hand, various genetic factors in secretory glands, gonadal cells, reproductive tissues can lead to significant changes on reproductive activity through specific molecular pathways and mechanisms.

Genes involved in testicular development and function

The development of the testis requires the highly regulated expression of a series of genes. Many of the genes involved are transcription factors, such as steroid hormone receptors and growth factors. Investigators have used gene cloning, mutation analysis, transgenic mice, and gene-deletion studies to define the role of specific genes in testicular development and function. In the past 5 years, investigators have defined a gene on the Y chromosome, SRY, thought to be required for testis determination. This protein is a member of a larger family of related transcription factors. Expression of this gene triggers a cascade of events that leads to the development of the Sertoli cell, Leydig cells, and the testis. The development of the male phenotype is dependent on the presence and action of androgens, which exert their effect after combining with a receptor in the nucleus of the target cell that stimulates gene transcription. Defects in the androgen receptor gene lead to a full spectrum of morphological defects in the male. Interestingly, mutations in other members of the steroid receptor superfamily, such as the estrogen receptor gene, also affect male fertility. A number of "orphan" receptors (i.e., receptors whose ligans have not been identified) are also required for normal testicular development and function, as are several genes normally thought to be tumor-suppressor genes (e.g., Wilms' tumor-suppressor gene). In contrast, alpha-inhibin has been thought to be an endocrine hormone, yet it functions as a tumor-suppressor gene in the testis. Testicular development and normal spermatogenesis require the proper function and coordination of a large number of transcription factors, steroid hormone and orphan receptors, and growth factors. There are likely to be a large number of other, as yet unidentified genes that are necessary for male gonadal development.

Demonstration of a stage-specific expression of the ZFY protein in fetal mouse testis using anti-peptide antibodies

The zinc finger Y (Zfy) gene is located on the Y chromosome of all placental mammals. Although it is phylogenetically conserved and is expressed in mouse fetal testis, it is not the sex determining Y (Tdy) gene. To address the possible function of the Zfy gene in mice, the distribution of Zfy protein in fetal mice was investigated by immunocytochemical staining using several specific antisera against synthetic peptides of the mouse Zfy protein. Analysis of various fetal tissues at different embryonic stages demonstrated a specific staining only in fetal testis. In particular, reactive protein was initially observed in male fetal gonads at day 11.5 postcoitum (p.c.). The immuno-staining intensified in fetal testes at day 12 and 12.5 p.c., decreased drastically in those at day 13 and 14 p.c. and became undetectable in those at day 15 p.c. and beyond. The reactive molecules were distributed mostly within the seminiferous tubules of the embryonic testis. The present observations confirm the previous findings with RT-PCR analysis and indicate that Zfy or Zfy-like protein is expressed in stage-specific manner during early testis differentiation. Its location in the seminiferous tubules suggests a possible role in early germ cell development.

The role of cell-cell interactions in postembryonic development of the Caenorhabditis elegans germ line

This review addresses the role of cell-cell interactions in the development of the Caenorhabditis elegans germ line: specifically, the relative contributions of germ-line-soma interactions versus autonomous processes are considered. Current knowledge of the interacting cell types and the genes essential for various aspects of germ-line development is discussed.