Post-meiotic mechanism of facultative parthenogenesis in gonochoristic whiptail lizard species.

Facultative parthenogenesis (FP) has historically been regarded as rare in vertebrates, but in recent years incidences have been reported in a growing list of fish, reptile, and bird species. Despite the increasing interest in the phenomenon, the underlying mechanism and evolutionary implications have remained unclear. A common finding across many incidences of FP is either a high degree of homozygosity at microsatellite loci or low levels of heterozygosity detected in next-generation sequencing data.

The status of the human embryo in various religions.

Research into human development involves the use of human embryos and their derivative cells and tissues. How religions view the human embryo depends on beliefs about ensoulment and the inception of personhood, and science can neither prove nor refute the teaching of those religions that consider the zygote to be a human person with an immortal soul. This Spotlight article discusses some of the dominant themes that have emerged with regard to how different religions view the human embryo, with a focus on the Christian faith as well as Buddhist, Hindu, Jewish and Islamic perspectives.

Widespread failure to complete meiosis does not impair fecundity in parthenogenetic whiptail lizards.

Parthenogenetic species of whiptail lizards in the genus Aspidoscelis constitute a striking example of speciation by hybridization, in which first-generation hybrids instantly attain reproductive isolation and procreate as clonal all-female lineages. Production of eggs containing a full complement of chromosomes in the absence of fertilization involves genome duplication prior to the meiotic divisions. In these pseudo-tetraploid oocytes, pairing and recombination occur exclusively between identical chromosomes instead of homologs; a deviation from the normal meiotic program that maintains heterozygosity.

Laboratory synthesis of an independently reproducing vertebrate species.

Speciation in animals commonly involves an extrinsic barrier to genetic exchange followed by the accumulation of sufficient genetic variation to impede subsequent productive interbreeding. All-female species of whiptail lizards, which originated by interspecific hybridization between sexual progenitors, are an exception to this rule. Here, the arising species instantaneously acquires a novel genotype combining distinctive alleles from two different species, and reproduction by parthenogenesis constitutes an effective intrinsic barrier to genetic exchange.

Unisexual reproduction among vertebrates.

The past decade has seen a remarkable revision of perspectives on unisexual reproduction in vertebrates. One can no longer view it as a rare curiosity far outside the mainstream of evolution. More than 80 taxa of fish, amphibians, and reptiles are now known to reproduce by parthenogenesis (Greek for 'virgin birth') or its variants, and they persist in nature as all-female lineages.

Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards.

Although bisexual reproduction has proven to be highly successful, parthenogenetic all-female populations occur frequently in certain taxa, including the whiptail lizards of the genus Aspidoscelis. Allozyme analysis revealed a high degree of fixed heterozygosity in these parthenogenetic species, supporting the view that they originated from hybridization events between related sexual species. It has remained unclear how the meiotic program is altered to produce diploid eggs while maintaining heterozygosity.

Ontological and ethical implications of direct nuclear reprogramming.

Scientific breakthroughs rarely yield the potential to engage a foundational ethical question. Recent studies on direct reprogramming of human skin cells reported by the Yamanaka lab in Japan and the Thomson lab in Wisconsin suggest that scientists may have crossed both a scientific and an ethical threshold. The fascinating science of direct nuclear reprogramming highlights empirical data that may clarify the ontological status of cellular activity in the early stages of what could become a human fetus and justify ethical options for research in this controversial field.

Normal stem cells and cancer stem cells: the niche matters.

Scientists have tried for decades to understand cancer development in the context of therapeutic strategies. The realization that cancers may rely on "cancer stem cells" that share the self-renewal feature of normal stem cells has changed the perspective with regard to new approaches for treating the disease. In this review, we propose that one of the differences between normal stem cells and cancer stem cells is their degree of dependence on the stem cell niche, a specialized microenvironment in which stem cells reside.

Live human germ cells in the context of their spermatogenic stages.

Background: Various types of live, dispersed, human testicular cells in vitro were previously compared with the morphologic characteristics of human spermatogenic germ cells in situ within seminiferous tubules. The current study extends those observations by placing live human germ cells in the context of their developmental steps and stages of the spermatogenic cycle.

Methods: Live human testicular tissue was obtained from an organ-donating, brain-dead person.

A comparative morphological study of human germ cells in vitro or in situ within seminiferous tubules.

For many infertile couples, intracytoplasmic germ cell/spermatozoon injection into unfertilized eggs may be their only hope for producing their own biological children. Thus far, success with injection of pre-spermatozoan germ cells such as round spermatids has not been as great as that of spermatozoon injection. This could be due in part to the difficulty of identifying younger (less mature) male germ cells in testicular biopsy dispersions.

Effect of age on the composition of seminiferous tubular boundary tissue and on the volume of each component in humans.

Seminiferous tubular boundary tissue thickens with age. The objective was to characterize the composition of boundary tissue in 16 young adult (20 to 29 years) and 18 older adult (51 to 84 years) men. Testes were perfused with glutaraldehyde, placed in osmium, and embedded in Epon 812 (Ladd Research Industries, Burlington, VT).

Quantification of human spermatogenesis: germ cell degeneration during spermatocytogenesis and meiosis in testes from younger and older adult men.

Germ cell degeneration during spermatocytogenesis and meiosis was investigated to explain the age-related decline in daily sperm production (DSP). Numbers of Types A-dark, A-pale, and B-spermatogonia, potential daily sperm production per g parenchyma (PDSP) based on type B-spermatogonia, early primary spermatocytes, and late primary spermatocytes, and DSP per g based on early spermatids were determined in 15 men aged 20 to 48 yr (mean +/- SEM, 33 +/- 2 yr) and 15 men aged 52 to 90 yr (65 +/- 3 yr). Testes obtained within 15 h of death (largely due to trauma or heart failure) were perfused vascularly with glutaraldehyde.

Seminiferous tubules and daily sperm production in older adult men with varied numbers of Leydig cells.

Previous studies of adult men have failed to reveal a relationship between numbers of Leydig cells in the testes and rates of sperm production, perhaps because of a functional excess of these cells in younger men. Hence, a possible relationship between Leydig cell numbers and sperm production was sought in 50 older men, aged 50-90 years, in whom the Leydig cell population had been depleted by age-related attrition. When these men were sorted by increasing numbers of Leydig cells per man into two, three, or five groups, no difference could be found between or within these groups when daily sperm production per man (DSP); seminiferous tubular volume, diameter, or length; or seminiferous epithelial volume was examined.

Characterization of the testicular abnormality in 5 alpha-reductase deficiency.

The testes of five phenotypic women (from four families) with 5 alpha-reductase deficiency were studied. In one of the patients, the enzyme deficiency was similar in the testis and epididymis and in fibroblasts cultured from the labia majora. In testes from four of the patients, the concentrations of the 5 alpha-reduced steroids dihydrotestosterone and 3 alpha-androstanediol were less than 10% of those in normal subjects.

Age-related variation in seminiferous tubules in men. A stereologic evaluation.

Tubular boundary tissue and seminiferous epithelia were evaluated stereologically in testes from 28 men aged 20 to 48 years and 28 men aged 50 to 90 years. Testes obtained at autopsy within 15 hours of death were perfused with glutaraldehyde, embedded in Epon (Ladd Research Industries, Inc., Burlington, VT), sectioned at 0.