Animal models for brain and pituitary gonadal disturbances

Ovarian autografts in sheep as a model for studying folliculogenesis

The studies outlined in this review indicate that the cortical autograft, in addition to its clinical application as a means to restore fertility, represents a valuable experimental model that can be exploited to examine aspects of both early and terminal follicle development. The autograft procedure is a means to experimentally deplete the follicle population in an individual and this procedure results in similar endocrine changes and reproductive cycle perturbances as those observed in aged sheep and women with incipient ovarian failure. This methodology therefore represents a non-primate large animal model to study the consequences of, and possible interventions to overcome, reproductive problems associated with depleted ovarian follicular reserves. Without the necessity of keeping animals for large periods of time so that this depletion can occur naturally. In terms of early follicle development, the fact that the ischaemia that occurs during revascularisation of the autograft effectively synchronises follicle development at the primordial stages of development means that the autograft can be used as a model to study the control of early follicle development. This model has been used to examine the role of FSH (follicle stimulating hormone) in the control of early follicle development and the preliminary data presented provides strong evidence that FSH does indeed modulate early folliculogenesis, confirming the value of this model as a means of performing experimental investigations in this area. Further work using this model will concentrate on the role of other endocrine and local factors in the control of early folliculogenesis and the identification of the key developmental checkpoints during this process, with a view to designing physiological culture systems to support early follicle and oocyte development.

Hypogonadism and obesity in mice with a targeted deletion of the Nhlh2 gene

The family of basic helix-loop-helix (bHLH) genes comprises transcription factors involved in many aspects of growth and development. We have previously described two bHLH transcription factors, Nhlh1 and Nhlh2 (originally named NSCL1 and NSCL2). The nucleotide and predicted protein sequences of Nhlh1 and Nhlh2 are homologous within their bHLH domain where there are only three conservative amino acid differences. During murine embryogenesis, Nhlh1 and Nhlh2 share an overlapping but distinct pattern of expression in the developing nervous system. To improve our understanding of the role of these genes during neurogenesis, we have generated mice containing targeted deletions of both genes and here describe our results for Nhlh2. Loss of Nhlh2 results in a disruption of the hypothalamic-pituitary axis in mice. Male Nhlh2-/- mice are microphallic, hypogonadal and infertile with alterations in circulating gonadotropins, a defect in spermatogenesis and a loss of instinctual male sexual behaviour. Female Nhlh2-/- mice reared alone are hypogonadal, but when reared in the presence of males, their ovaries and uteri develop normally and they are fertile. Both male and female homozygotes exhibit progressive adult-onset obesity. Nhlh2 is expressed in the ventral-medial and lateral hypothalamus, Rathke's pouch and in the anterior lobe of the adult pituitary. Our results support a role for Nhlh2 in the onset of puberty and the regulation of body weight metabolism.