Periodic patterns in Rodentia: Development and evolution

Mammalian periodic pigment patterns, such as spots and stripes, have long interested mathematicians and biologists because they arise from non-random developmental processes that are programmed to be spatially constrained, and can therefore be used as a model to understand how organized morphological structures develop. Despite such interest, the developmental and molecular processes underlying their formation remain poorly understood. Here, we argue that Arvicanthines, a clade of African rodents that naturally evolved a remarkable array of coat patterns, represent a tractable model system in which to dissect the mechanistic basis of pigment pattern formation. Indeed, we review recent insights into the process of stripe formation that were obtained using an Arvicanthine species, the African striped mouse (Rhabdomys pumilio), and discuss how these rodents can be used to probe deeply into our understanding of the factors that specify and implement positional information in the skin. By combining naturally evolved pigment pattern variation in rodents with classic and novel experimental approaches, we can substantially advance our understanding of the processes by which spatial patterns of cell differentiation are established during embryogenesis, a fundamental question in developmental biology.

Assessment of sex specific endocrine disrupting effects in the prenatal and pre-pubertal rodent brain

BACKGROUND

Brain sex differences are found in nearly every region of the brain and fundamental to sexually dimorphic behaviors as well as disorders of the brain and behavior. These differences are organized during gestation and early adolescence and detectable prior to puberty. Endocrine disrupting compounds (EDCs) interfere with hormone action and are thus prenatal exposure is hypothesized to disrupt the formation of sex differences, and contribute to the increased prevalence of pediatric neuropsychiatric disorders that present with a sex bias.

OBJECTIVE

Available evidence for the ability of EDCs to impact the emergence of brain sex differences in the rodent brain was reviewed here, with a focus on effects detected at or before puberty.

METHODS

The peer-reviewed literature was searched using PubMed, and all relevant papers published by January 31, 2015 were incorporated. Endpoints of interest included molecular cellular and neuroanatomical effects. Studies on behavioral endpoints were not included because numerous reviews of that literature are available.

RESULTS

The hypothalamus was found to be particularly affected by estrogenic EDCs in a sex, time, and exposure dependent manner. The hippocampus also appears vulnerable to endocrine disruption by BPA and PCBs although there is little evidence from the pre-pubertal literature to make any conclusions about sex-specific effects. Gestational EDC exposure can alter fetal neurogenesis and gene expression throughout the brain including the cortex and cerebellum. The available literature primarily focuses on a few, well characterized EDCs, but little data is available for emerging contaminants.

CONCLUSION

The developmental EDC exposure literature demonstrates evidence of altered neurodevelopment as early as fetal life, with sex specific effects observed throughout the brain even before puberty.

Investigating gradients of gene expression involved in early human cortical development

The division of the neocortex into functional areas (the cortical map) differs little between individuals, although brain lesions in development can lead to substantial re-organization of regional identity. We are studying how the cortical map is established in the human brain as a first step towards understanding this plasticity. Previous work on rodent development has identified certain transcription factors (e.g. Pax6, Emx2) expressed in gradients across the neocortex that appear to control regional expression of cell adhesion molecules and organization of area-specific thalamocortical afferent projections. Although mechanisms may be shared, the human neocortex is composed of different and more complex local area identities. Using Affymetrix gene chips of human foetal brain tissue from 8 to 12.5 post-conceptional weeks [PCW, equivalent to Carnegie stage (CS) 23, to Foetal stage (F) 4], human material obtained from the MRC-Wellcome Trust Human Developmental Biology Resource (http://www.hdbr.org), we have identified a number of genes that exhibit gradients along the anterior-posterior axis of the neocortex. Gene probe sets that were found to be upregulated posteriorally compared to anteriorally, included EMX2, COUPTFI and FGF receptor 3, and those upregulated anteriorally included cell adhesion molecules such as cadherins and protocadherins, as well as potential motor cortex markers and frontal markers (e.g. CNTNAP2, PCDH17, ROBO1, and CTIP2). Confirmation of graded expression for a subset of these genes was carried out using real-time PCR. Furthermore, we have established a dissociation cell culture model utilizing tissue dissected from anteriorally or posteriorally derived developing human neocortex that exhibits similar gradients of expression of these genes for at least 72 h in culture.

New insights into the role of androgens in wolffian duct stabilization in male and female rodents

Androgen-mediated wolffian duct (WD) development is programmed between embryonic d 15.5 (e15.5) and 17.5 in male rats, and WD differentiation has been shown to be more susceptible to reduced androgen action than is its initial stabilization. We investigated regulation of these events by comparing fetal WD development at e15.5-postnatal d0 in male and female androgen receptor knockout mice, and in rats treated from e14.5 with flutamide (100 mg/kg/d) plus di-n(butyl) phthalate (500 mg/kg/d) to block both androgen action and production, testosterone propionate (20 mg/kg/d) to masculinize females, or vehicle control. In normal females, WD regression occurred by e15.5 in mice and e18.5 in rats, associated with a lack of epithelial cell proliferation and increased apoptosis, disintegration of the basement membrane, and reduced epithelial cell height. Exposure to testosterone masculinized female rats including stabilization and partial differentiation of WDs. Genetic or chemical ablation of androgen action in males prevented masculinization and induced WD regression via similar processes to those in normal females, except this occurred 2-3 d later than in females. These findings provide the first evidence that androgens may not be the only factor involved in determining WD fate. Other factors may promote survival of the WD in males or actively promote WD regression in females, suggesting sexually dimorphic differences in the preprogrammed setup of the WD.

Chorioallantoic and Yolk Sac Placentation in the Dassie Rat Petromus typicus and its Significance for the Evolution of Hystricognath Rodents

Placental characters are most important in understanding the evolutionary history of hystricognath rodents of which some act as animal models for human pregnancy. The data available deal mostly with species native to South America, but the current paper presents novel findings on chorioallantoic and yolk sac placentation in an Old World hystricognath and discusses its significance for the evolution of the group. Several hystricognath stem species characters are verified for Petromus, such as the unique trophoblast growth pattern within the chorioallantoic placenta. Subsequently, a novel set of characters belonging to the visceral yolk sac is added to the stem species pattern of the group. The nourishment of the embryo is facilitated by an inverted visceral yolk sac placenta from early pregnancy onward, later complemented by the chorioallantoic placenta. About mid term, the visceral yolk sac becomes partly folded and attached to the parietal yolk sac cover of the chorioallantoic placenta, suggesting a functional shift to the transfer of substances between the two placental types. Thus, the chorioallantoic and yolk sac placenta collaborate in nurturing the embryo. This apparently represents an evolutionary transformation along the stem lineage of hystricognaths.

The development of mastication in rodents: from neurons to behaviors

The development of suckling behavior is a fundamental characteristic of mammalian development. The occurrence of this behavior across mammals allows us to extrapolate information from animal models to better understand normal and abnormal masticatory development in infants. This review focuses on prenatal cell, molecular, and morphological changes in rat and/or mouse masticatory muscles, trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) that accompany the development of suckling behavior. A special emphasis is placed on N-methyl-d-aspartate (NMDA) receptor subunit expression because of the important role that NMDA receptors play in the production of rhythmical jaw movements and neuronal development. Prenatally the timing of NMDA subunit changes follows neuromuscular junction formation in masticatory muscles, and is coincident with the emergence of rhythmical jaw movements and in vitro rhythmical trigeminal activity. Our data suggest that NMDA receptor subunit changes in Mo5 and Me5 are synchronized with the emergence of rhythmical jaw movements and trigeminal motor activity.

Prenatal development of hypothalamic neuropeptide systems in the nonhuman primate

In the rodent, arcuate nucleus of the hypothalamus (ARH)-derived neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons have efferent projections throughout the hypothalamus that do not fully mature until the second and third postnatal weeks. Since this process is likely completed by birth in primates we characterized the ontogeny of NPY and melanocortin systems in the fetal Japanese macaque during the late second (G100), early third (G130) and late third trimesters (G170). NPY mRNA was expressed in the ARH, paraventricular nucleus (PVH), and dorsomedial nucleus of the hypothalamus (DMH) as early as G100. ARH-derived NPY projections to the PVH were initiated at G100 but were limited and variable; however, there was a modest increase in density and number by G130. ARH-NPY/agouti-related peptide (AgRP) fiber projections to efferent target sites were completely developed by G170, but the density continued to increase in the postnatal period. In contrast to NPY/AgRP projections, alphaMSH fibers were minimal at G100 and G130 but were moderate at G170. This study also revealed several significant species differences between rodent and the nonhuman primate (NHP). There were few NPY/catecholamine projections to the PVH and ARH prior to birth, while projections were increased in the adult. A substantial proportion of the catecholamine fibers did not coexpress NPY. In addition, cocaine and amphetamine-related transcript (CART) and alpha-melanocyte stimulating hormone (alphaMSH) were not colocalized in fibers or cell bodies. As a consequence of the prenatal development of these neuropeptide systems in the NHP, the maternal environment may critically influence these circuits. Additionally, because differences exist in the neuroanatomy of NPY and melanocortin circuitry the regulation of these systems may be different in primates than in rodents.

The subplacenta of the red-rumped agouti (Dasyprocta leporina L)

BACKGROUND

Hystricognath rodents have a lobed placenta, comprising labyrinthine exchange areas and interlobular trophoblast. These correspond to the labyrinthine and spongy zones of other rodent placentae. Beneath them, however, is a structure unique to hystricognath rodents called the subplacenta. We here describe the subplacenta of the red-rumped agouti and examine the possible functional correlates of this structure.

METHODS

Placentae were collected from early in midgestation to near term of pregnancy and examined by standard histological techniques, immunohistochemistry and transmission electron microscopy. In addition, to study the microvasculature of the subplacenta, vessel casts were inspected by scanning electron microscopy.

RESULTS

In the subplacenta, lamellae of connective tissue support a layer of mononuclear cytotrophoblast cells. Beneath this is found syncytiotrophoblast. Clusters of multinuclear giant cells occur in the transition zone between the subplacenta and decidua. There are prominent intercellular spaces between the cytotrophoblast cells. The basal membrane of these cells is often close to fetal blood vessels. The syncytiotrophoblast surrounds an extensive system of lacunae. Microvilli project into these lacunae from the plasma membrane of the syncytiotrophoblast. The syncytial cytoplasm contains electron-dense granules. This is probably the amylase-resistant PAS-positive material identified by histochemistry. The subplacenta is supplied entirely from the fetal circulation. Within it the vessels pursue a tortuous course with sinusoidal dilatations and constrictions.

CONCLUSION

The functions that have been attributed to the subplacenta include hormone production. Our findings are consistent with this interpretation, but suggest that hormone secretion is directed towards the fetal circulation rather than the maternal tissues.

The eternal tooth germ is formed at the apical end of continuously growing teeth

Rodent incisors are known to be continuously growing teeth that are maintained by both the cell-proliferation at the apical end and the attrition of the incisal edge. This type of tooth had a special epithelial structure for the maintenance of stem cells, showing the bulbous epithelial protrusion at the apical end. The morphological transition of the epithelial-mesenchymal compartment by serial transverse sections of the apical end toward the incisal direction is likely to reflect the development of the tooth germ in the prenatal stage. Based on the present histological and previous molecular biological studies, the special structure at the apical end is obviously different from the cervical loop giving rise to Hertwig's epithelial root sheath (HERS), in human, mouse and rat molar tooth germs. Hence, we propose a new concept that the eternal tooth bud producing various dental progeny is formed at the apical end of continuously growing teeth, and a new term "apical bud" for indicating this specialized epithelial structure. Furthermore, BrdU labelling analysis suggested that the guinea-pig molars, which were continuously growing teeth, also possessed plural specific proliferative regions and "apical bud" at the apical end.

Molecular mechanisms for morphogenesis of the central nervous system in mammals*

The mammalian central nervous system (CNS) is a highly organized structure. In the beginning of CNS development, neural precursor/stem cells are dividing in the neuroepithelium. After a while, these precursors gradually start to differentiate into neurons and glial cells. Various factors are involved in the proliferation and differentiation of neural precursors. Recent studies have demonstrated that the basic helix-loop-helix (bHLH) transcription factors play important roles in differentiation processes. Hairy and Enhancer of split homolog (HES) 1 and HES5 are bHLH-type repressors and inhibit neural differentiation. Mammalian achaete-scute complex homolog (MASH) 1 and mammalian atonal homolog (MATH) 1 are positive bHLH regulators expressed in neural precursors. A balance between positive and negative regulators may determine whether differentiation proceeds or not. The data suggest that this balance is controlled by Notch signaling. Other extracellular signals also govern CNS morphogenesis. To elaborate the primary shape of the CNS, proliferation of neural precursors should be strictly regulated in a spatial and temporal manner. A recent study suggests that a Sonic hedgehog-dependent signaling relay controls growth of the diencephalon and midbrain. Nutrition is another critical factor for development. Expression analysis of Folate binding protein 1 implied the close association between folate uptake and anterior neural tube closure.

Effects of restraint stress in gestation: implications for rodent developmental toxicology studies

Restraint has been used as a procedure to study the effects of stress on gestation outcome in rodents. The effects of restraint could potentially be used as a model for the impact of general stress produced by high doses of toxicants and other interventions. In mice, restraint in the peri-implantation period leads to implantation failure, and restraint at appropriate times in organogenesis produces cleft palate, supernumerary ribs, and resorption. In rats, there is some evidence for an association with restraint for implantation failure, but not for the morphological anomalies. Restraint in late gestation alters adult sexual behavior of male rat offspring, but consequences for their fertility are not known. Intrauterine growth retardation is not commonly associated with gestational restraint. In the few studies where they have been directly compared, different restraint procedures produced graded, qualitatively different, or no effects. Adrenocortical hormones have been implicated as mediating the effect of restraint on cleft palate, but not on supernumerary ribs, implantation failure, or sexual differentiation. Given the variety of restraint procedures and the varying species-dependent consequences, it is not possible to infer a generalizable pattern of developmental effects due to gestational stress from the restraint literature. As an alternative approach, contemporary methods in gene expression and developmental biology could profitably be applied to understanding different patterns of stress-mediated effects of toxicant exposures on intrauterine development.

Morphological aspects of in vivo cleavage in Myocastor coypus (coypu)

The objective of the present work was to characterize the in vivo cleavage stage of Myocastor coypus embryos. For this purpose a colpocytological follow-up and controlled mating of 18 females were performed. Specimens from the beginning of the first cleavage to the acquisition of a morula appearance were considered to be in cleavage stage. Embryos in cleavage were collected between days 3 and 6 post-coitus. Of the collected embryos, 80% presented an even number of blastomeres and the remaining 20% an odd number. Embryos from 3 to 7 cells were blastomere associations in a spherical disposition within the zona pellucida. Blastomeres were spherical or ovoid, presenting slight flattening in areas contacting with other blastomeres. Embryos of 8 and 9 cells were as a group of blastomeres slightly elongated, surrounded by a spherical zona pellucida. The percentage of peri-vitelline space occupied by the embryonic mass ranged from 74.1 to 95.8% for all the substages. The cleavage pattern, developed in the oviduct, was of a rotational holoblastic type and asynchronic.

Pak1 and its T212 phosphorylated form accumulate in neurones and epithelial cells of the developing rodent

The serine/threonine kinase Pak1 is a target of the RhoGTPases Rac and Cdc42 and an important regulator of cell morphology and migration. Recent work from several laboratories has indicated that Pak1 controls microtubule dynamics as well as the organisation of F-actin microfilaments. Pak1 is phosphorylated on T212 by the p35/Cdk5 or cyclin B1/Cdc2 kinase in postmitotic neurones and mitotic cells, respectively. To understand its function during development, we have carried out a detailed temporal and spatial analysis of Pak1 expression and phosphorylation on T212. In the embryonic forebrain, Pak1 and Pak1T212(PO4) were seen to accumulate in the corpus callosum, intermediate zone, lateral olfactory tracts, and anterior commissures. Epithelial cells of the mouse embryo lung, kidney, intestine, and skin also exhibited high levels of Pak1 and Pak1T212(PO4), suggesting a previously unsuspected role in epithelial differentiation. Pak1T212(PO4) was undetectable in all adult tissues. Together, these data indicate a specific, developmentally regulated role of the Pak1 kinase.

Pre-implantational timetable of embryonal development of Myocastor coypus (Coypu)

The purpose of the present study was to determine the chronology of the pre-implantation embryonic development in Myocastor coypus (coypu). It was carried out by daily colpocytological examination and controlled mating of 33 females. Oocytes and embryos were obtained by flushing from day 0 to day 10 post-coitus (p.c.). On day 1 p.c., oocytes predominated whereas on day 2 p.c. zygotes were predominant. The cleavage period was from day 3 to day 6 p.c.. Morulae were collected from day 6 to day 9 p.c., whereas blastocysts were collected on days 8 and 9. From oviduct flushing, the embryos in the zygote stage and up to the morula stage with less than a 30-cell stage were recovered. Embryos in the morula stage with 30 or more cells and up to the growing blastocyst stage were collected from the flushing of hemiuteri.

[Rapid determination of sex in Myocastor coypus embryos in the first stage of gestation]

The early knowledge of the sex may be crucial for the understanding of many features of ecological and evolutive biology, including offspring sex-ratio adjustment and evolution of breeding systems. In coypu (Myocastor coypus), significant variation in birth sex-ratios can be observed and selective abortion of entire litters is one of the cited mechanisms. In order to determine the sex of coypu embryos in the earlier stages of gestation (second week), we developed a molecular technique based on PCR amplification of a region of the Sry gene. These method used the combination of two sets of primers: one specific of the Y-chromosome; the other one, autosomal, is a positive control for amplification. Because of the direct amplification of embryo lysate without DNA extraction, the present sexing technique is rapid, relatively simple and inexpensive, and presents numerous advantages for the study at population scale.

Early development of the neural plate, neural crest and facial region of marsupials

Marsupial mammals have a distinctive reproductive strategy. The young are born after an exceptionally short period of organogenesis and are consequently extremely altricial. Yet because they must be functionally independent in an essentially embryonic condition, the marsupial neonate exhibits a unique suite of adaptations. In particular, certain bones of the facial region, most cranial musculature and a few additional structures are accelerated in their development. In contrast, central nervous system structures, especially the forebrain, are markedly premature at birth, resembling an embryonic d 11 or 12 mouse. This review examines the developmental processes that are modified to produce these evolutionary changes. The focus is on the early development of the neural plate, neural crest and facial region in the marsupial, Monodelphis domestica, compared with patterns reported for rodents. Neural crest begins differentiation and migration at the neural plate stage, which results in large accumulations of neural crest in the facial region at an early stage of development. The early accumulation of neural crest provides the material for the accelerated development of oral and facial structures. The first arch region is massive in the early embryo, and the development of the olfactory placode and frontonasal region is advanced relative to the forebrain region. The development of the forebrain is delayed in marsupials relative to the hindbrain or facial region. These observations illustrate how development may be modified to produce evolutionary changes that distinguish taxa. Further, they suggest that development is not necessarily highly conserved, but instead may be quite plastic.

Identification of rCop-1, a new member of the CCN protein family, as a negative regulator for cell transformation

By using a model system for cell transformation mediated by the cooperation of the activated H-ras oncogene and the inactivated p53 tumor suppressor gene, rCop-1 was identified by mRNA differential display as a gene whose expression became lost after cell transformation. Homology analysis indicates that rCop-1 belongs to an emerging cysteine-rich growth regulator family called CCN, which includes connective-tissue growth factor, CYR61, CEF10 (v-src inducible), and the product of the nov proto-oncogene. Unlike the other members of the CCN gene family, rCop-1 is not an immediate-early gene, it lacks the conserved C-terminal domain which was shown to confer both growth-stimulating and heparin-binding activities, and its expression is lost in cells transformed by a variety of mechanisms. Ectopic expression of rCop-1 by retroviral gene transfers led to cell death in a transformation-specific manner. These results suggest that rCop-1 represents a new class of CCN family proteins that have functions opposing those of the previously identified members.

Prenatal development of the mammalian vomeronasal organ

The vomeronasal organ (VNO) originates from the medial wall of the olfactory pit shortly after the middle of the embryonic period in mammals. The Anlage stage consists of a cellular bud that grows dorsally, caudally, and towards the midline leaving a groove. The following stage, Early Morphogenesis, includes the closure of the vomeronasal groove to form a parasagittal blind-ended tube in the nasal septum, which opens into the nasal and/or oral cavities. The lumen adopts a crescent shape while the epithelial lining differentiates into an increasingly wider epithelium on the concave side and a gradually thinner epithelium on the convex side. The former goes on to occupy a medial position and develops neuroblasts among supporting and undifferentiated cells, with supporting cell nuclei tending to align in the upper rows. The lateral "non-sensory" epithelium furrows, giving a kidney-shaped appearance to the VNO cross section. The next stage, Late Morphogenesis is extended up to a difference in thickness between both epithelia becomes similar to the adult, generally by birth. An increasing number of ciliary generation complexes, larger and more abundant microvilli, and an evident glycocalyx are observed in the neuroepithelium at the luminal surface, while enzymatic activities become more intense. The non-sensory epithelium appears quite mature save for its luminal surface, which is still devoid of cilia. Blood capillaries penetrate the most basal region of the neuroepithelium and vomeronasal glands are very few and immature. At birth, some neurons appear well developed to support certain functionality; however, persistence of architectural, histochemical, and ultrastructural signs of immaturity, suggests that full performance of the VNO does not occur in newborn mammals, but in prepubertal ages.

Non-photic signalling in the suprachiasmatic nucleus

Scheduled arousal by handling and sub-cutaneous saline injection entrains the free-running clock of the adult Syrian hamster and outbred (ID(ICR)) but not inbred (C57B16) mice. Syrian hamsters bearing lesions of the intergeniculate leaflet of the thalamus remain able to entrain and phase-shift to light, but the lesions block completely entrainment by serial arousal, even though lesioned animals continue to respond acutely to the arousing cue. This suggests that the innervation from the IGL to the SCN is a necessary component of the pathways which signal an aroused state to the clock. Siberian hamsters do not entrain to serial arousal but they do entrain to serial injections of melatonin, whereas in adult Syrian hamster, systemic treatment with melatonin has no effect above that of arousal. In contrast to the adult, the foetal and neonatal Syrian hamster can be entrained by melatonin. These variations in sensitivity correlate with inter-specific and developmental differences in the pattern and level of expression of melatonin receptors in the SCN. The perinatal hamster can also be entrained by dopaminergic agonists. SCN tissue from neonatal Syrian hamsters was used to characterise the biochemical actions of dopamine and melatonin. In primary culture and tissue explants, forskolin, dopamine and glutamatergic agonists all stimulated the phosphorylation of the transcription factor CREB. This probably occurred via convergent actions through Ca2+ (glutamate) and cyclic AMP-dependent (forskolin, dopamine) signalling pathways. Dopamine induced phospho-CREB-ir exclusively in GABA-ir neurons and melatonin reversed this effect of dopamine, indicative of an inhibitory Gi protein linking via the Mel1a receptor to adenylyl cyclase. The regulation of phospho-CREB by multiple entraining cues in the SCN highlights its position as a point of convergence for regulators of the clock, and indicates a possible role in entrainment.

The ultrastructure of the trophoblastic layer of the degu (Octodon degus) placenta: a re-evaluation of the 'channel problem'

Previous studies on immersion-fixed specimens of the haemomonochorial labyrinthine chorioallantoic placenta of the degu have made visible channels or pores which completely crossed the trophoblastic layer; for the first time hints for an open connection between maternal blood spaces and fetal interstitium were demonstrated in a placenta. This important finding was re-evaluated by means of transmission electron microscopy paying particular attention to the influence of the mode of fixation and of ischaemic periods prior to fixation. Thirty placentae from seven near-term degu (estimated gestational age 80-90 days) were fixed at various times (5-45 min) after maternal death. Placentae were perfused in situ via the aorta/uterine arteries or the umbilical vein with 2.2 per cent phosphate-buffered glutaraldehyde (n = 7, ischaemic periods < 5-45 min) or lanthanum hydroxide/osmium tetroxide (n = 7, ischaemic periods < 5 min), or were immersion-fixed in 2.2 per cent phosphate-buffered glutaraldehyde (n = 16, ischaemic periods 10-45 min). In material with brief ischaemic periods (n = 3, 5-10 min) no open connections between the maternal blood lacunae and fetal interstitium could be detected. Instead, occasional foci of trophoblast reduced to a thickness of 0.1 micron were found. After ischaemic periods exceeding 10 min, the thin trophoblastic diaphragms had partly disappeared resulting in complete transtrophoblastic pores. It is likely, therefore, that visible trophoblastic pores or channels in the degu placenta are ischaemic artefacts. Application of lanthanum hydroxide from the fetal circulation showed that a branching system of membrane-lined tubules (15-50 nm wide) intruded the trophoblast from its basal side, as has been reported for the guinea-pig and human placenta. It remains to be investigated whether these invaginations belong to a continuous transtrophoblastic channel system.

The patterning and onset of opsin expression in vertebrate retinae

A fascinating area of current research for developmental biologists concerns the patterning of complex tissues. The distribution of photoreceptors across the vertebrate retina is an excellent example of patterning in a tractable model system. Recent studies defining photoreceptor distribution in developing and mature tissue have set the stage for mechanistic studies of the control of patterning.

The chronotopic reordering of optic axons

Retinal ganglion cell axons become reordered as they pass through the chiasmatic region of the optic pathway. Studies in carnivores and rodents show that the fiber order established in the optic tract is a chronological index of their arrival time during development and that the cause of the reordering may relate to the changing glial environment, as well as to the spatial and temporal distribution of proteoglycans within the developing pathway. Primate optic axons become similarly reordered, allowing one to predict a developmental sequence of ganglion cell genesis from fiber position within the mature optic tract. Fiber position within the tract also anticipates the pattern of geniculate innervation, but a prominent exception to this rule is found in the prosimian Galago. The chronotopic reordering is found in every mammalian species that has been examined, including eutherians and metatherians, suggesting that the mechanism producing it is evolutionarily conserved.

Anatomy, development and lesion-induced plasticity of rodent corticospinal tract

In this review the current knowledge of the anatomy, development and plasticity of the rodent corticospinal tract is summarised. Recent technical advancements, especially in neuronal tracing methods, have provided much new data concerning the anatomy of the corticospinal tract. The rodent corticospinal axons project to the subcortical nuclei via collateral branches. These collateral branches of corticospinal axons are formed by delayed interstitial budding during early postnatal periods. Corticospinal neurons are generated in the ventricular zone during a short time lag, migrate into the cortical plate, and settle in layer V of the cerebral cortex. The migration of corticospinal neurons is experimentally deranged by prenatal exposure to alcohol or genetically affected by the reeler genetic locus (rl), resulting in generation of ectopic corticospinal neurons. Such experimentally or genetically induced ectopic corticospinal neurons are a good model for examining whether target recognition and path finding are affected by the intracortical position of corticospinal neurons. Some chemical molecules (e.g. L1 and B-50/GAP43) are transiently expressed in the corticospinal tract during the perinatal period, while others (e.g. protein kinase C gamma subspecies and alpha CaM kinase II) are permanently expressed in the adult corticospinal tract. The only chemical marker specific for layer V corticofugal neurons is an antibody to a soluble protein, protein 35. Since the corticospinal tract in the rodent is an easily identified group of fibers situated in the most ventral portion of the dorsal funiculus of the spinal cord and exhibits considerable postnatal development, it has often been utilized in the neurological studies on plasticity and regenerative capacity of the lesioned central nervous system. Recently, it has been clarified that growing corticospinal fibers have the ability to penetrate and traverse across the lesion sites under certain special conditions.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Implantation and decidualization in rodents

This article reviews the main events of embryo-implantation and decidualization in rodents. In common laboratory rodents the embryo attaches to the uterine epithelial lining, usually on days 4 to 6 of pregnancy. A progressive degree of proximity between trophoblast and epithelium occurs until the epithelial cells undergo apoptosis and detach from the basement membrane. During the attachment stage, the spindle-shaped connective tissue cells that underlie the epithelium next to the embryos transform into polyhedral and closely packed decidual cells. Following the epithelial detachment and the breaching of the basement membrane the embryo is thus in direct contact with decidual cells. These cells accumulate organelles associated with synthesis of macro-molecules, intermediate filaments, and eventually lipid droplets and glycogen. Another remarkable feature of decidual cells is the establishment of gap and adherens intercellular junctions. Differentiation of fibroblasts into decidual cells advances antimesometrially and mesometrially, creating in the endometrium several regions of cells with different morphology. The whole phenomenon of decidualization which is normally triggered by the embryo can be artificially induced in pseudo-pregnant or hormonally-prepared animals with the use of diverse stimuli. The uterine epithelium is probably responsible for the transduction of the initial stimulus. Prostaglandins have been shown to be important in the induction of decidualization. More recently other substances such as leukotrienes, platelet-activating factor (PAF), and transforming growth factor (TGF) have been thought to play a role in induction. Much evidence points to prostaglandin production by the decidual cells. New proteins such as a luteotropic factor, desmin, and other molecules were shown to be produced after rat stromal cells undergo decidual transformation. The extracellular matrix of the mouse decidua contains very thick collagen fibrils. Mouse decidual cells are also very active in phagocytosing the thick fibrils, contributing to the remodeling and involution of the decidua that accompanies embryonic growth. Radioautographic data indicates that mouse decidual cells produce and secrete collagen and sulfated proteoglycans.

Structural interactions of trophoblast and uterus during hemochorial placenta formation

It is argued that the successful placental arrangements are those that decrease the exposure of the trophoblast to the efferent side of the cellular immune response. Examples are taken from three different groups with hemochorial placentation, to show that their placental morphology places most of the trophoblast in contact with maternal blood, not maternal connective tissue. In addition, maternal cells of the junctional area are modified either before contact with trophoblast (rat) or just after trophoblast invasion (primate), or the region of contact is limited as in the armadillo, in which maternal blood sinuses are expanded to form intervillous spaces.

Patterning of the barrel field in somatosensory cortex with implications for the specification of neocortical areas

The adult neocortex, a distinct region of the mammalian cerebral cortex, is characterized by numerous anatomically and functionally distinct areas. Many of the connectional and architectural features that distinguish areas in the adult neocortex are not evident in the immature neocortex. A central issue in understanding neocortical area differentiation is determining the relative contributions of genetic and epigenetic factors in the emergence of area-specific features during neocortical development. A model system for this issue has been the rodent somatosensory cortex, which uniquely contains "barrels," anatomically evident functional groupings of cortical neurons and thalamocortical afferents that, in the tangential plane of cortex, are arranged in a pattern that reflects the distribution of vibrissae on the rodent body surface. Here, we address the role of thalamocortical afferents in the differentiation of barrels and their patterning in the context of discussing the specification of neocortical areas.

Comparison of staging systems for the gastrulation and early neurulation period in rodents: a proposed new system

Because there is no standard developmental staging system for the early postimplantation period of rodent embryos, investigators must now choose between a variety of systems that differ significantly. We have reviewed many of these staging systems and have summarized the ambiguities within them and the inconsistencies among them. In order to compare systems, we first obtained a consensus of the order of developmental events from the literature, and then attempted to fit existing systems into this order taking into account inconsistencies in terminology and blurred borderlines between stages. We were able to do this for most systems but not all because some were too divergent. We found that inconsistencies in definition of some terms, such as "primitive streak stage" and those used to describe the early neurulation process (neural plate, neural groove, neural folds, and head fold) cause much confusion. In order to develop an unambiguous system which can be used by all investigators, we propose to modify Theiler's system, which is one of the most commonly used systems but is not defined precisely during the early postimplantation period. We suggest making subdivisions of the original stages as follows: 1) stage 8 into 8a and 8b, by the degree of extension of the proamniotic cavity into the extraembryonic region; 2) stage 10 into 10a and 10b, by the completion of amnion formation; 3) stage 11 into 11a, 11b, and 11c, by the appearance of neural folds and foregut pocket. After Stage 12, the number of somite pairs can be used to precisely stage embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for interferons in early pregnancy

In order to survive, the developing conceptus must interrupt the normal ovarian cycle of the mother and extend the production of progesterone by the corpus luteum. An unusual Type 1 interferon (IFN), related structurally to the IFN-alpha molecule and produced in massive amounts for only a few days by the first epithelium (trophectoderm) of the preimplantation conceptus, has been implicated as the antiluteolytic agent in sheep and cattle. IFN-alpha therapy during this critical period can also improve pregnancy success in sheep. It remains unclear, however, whether the trophoblast IFN have specialized biological properties or whether they are unique merely in the timing, magnitude and site of their expression.

Development, morphology, and function of the yolk-sac placenta of laboratory rodents

A review of current knowledge of the unusual structure and several functions of the yolk-sac membranes of common laboratory rodents, viz., rats, mice, hamsters, guinea pigs and gerbils, enables a better assessment of the significance of this maternofetal exchange system in the experimental production of congenital anomalies. The anatomy of both visceral and parietal walls of the rodent yolk-sac placenta--specifically the anatomical relationships of each wall with maternal and with other fetal tissues--depends on the mode of origin and subsequent development of the yolk sac in these several species. Accordingly, the developmental biology of the rodent yolk sac is described. Since both fine structure and anatomical relationships also determine in large measure the functioning of the membrane as a whole in the absorption of selected materials either for intracellular digestion or for cellular translocation and transport to the developing embryo, the anatomy of the yolk sac is considered in detail. Similarly, since available evidence strongly suggests that teratogenic agents induce perturbations in the cellular mechanisms that control these several functions of the yolk-sac placental system in the production of birth defects, additionally an account is given of the cell biology of the membrane, i.e., endocytosis and targeting/trafficking of materials either for digestion within the epithelium at the maternal surface of the visceral yolk sac or for translocation across the yolk-sac membrane as a whole.

Monoclonal antibody that binds to both the prenatal and postnatal astroglia in rodent cerebellum

A monoclonal antibody (MAb), generated by immunizing mice with homogenized guinea pig cerebellum, labeled cerebellar astroglia including perikarya, radial fibers and veil-like processes in adult rats, mice and guinea pigs. Cell bodies and processes of the immature radial glia in the ventricular neuroepithelium of fetal mice cerebellum were definitely stained by the MAb on the 14th day of gestation. The astroglial components continued to show selective immunoreactivity to the MAb after the 14th day of gestation.

[Development of the rete testis in the prenatal ontogeny of rodents and effect of prolactin and thyrotropin on its cellular differentiation]

The development of rete testis in the rat, rabbit and guinea pig foetuses has been studied, as well as the influence of prolactin and thyrotropin on differentiation of its cells. It was shown that the rete testis tubules, as well as the seminiferous tubules develop from sex cords, which were derived from coelomic epithelium cells and gonocytes. The development of seminiferous tubules and rete testis was described at various stages of prenatal ontogenesis. Thyrotropin and prolactin exert different effects on differentiation of the rete testis cells: the former increases the mitotic activity of gonocytes and the latter increases that of epithelial cells and enhances degenerative processes in primary germ cells.

Foetal meiosis in the testis of the rodent Octodon degus

Different stages of meiotic prophase have been studied in foetal testes of the rodent, Octodon degus, using the light and electron microscope. Special attention was focused on the ultrastructural morphology of these meiotic cells in comparison to pre-spermatogonia of foetal testes and meiotic spermatocytes of the adult male testis. Meiosis occurs in only a few cells located among fibroblasts of the tunica albuginea or in the region of the gonadal blastema. The foetal meiotic process resembles adult meiosis in its ultrastructural characteristics; typical pachytene synaptonemal complexes and leptotene or diplotene axial elements appear associated to the chromatin. This process occurs at the same foetal age that meiosis commences in the ovary, thus reinforcing the idea that both meiosis-inducing and meiosis-preventing substances are secreted in both sexes. The intra-or extracordonal localization of the germ cells would be an important factor in determining the cells' response to these substances.

Embryonic susceptibility of Microtus ochrogaster (common prairie vole) to phenyl mercuric acetate

Pregnant M. ochrogaster received single intraperitoneal (i.p.) doses of 0.06, 0.125, 0.25, 0.5, 1,2, or 5 mg phenyl mercuric acetate (PMA/kg of body weight on day 8, 9, or 10 of gestation or 0.5 mg PMA/kg on day 7, 8, 9, 10, 11, or 12 of gestation. No toxicity was exhibited, and no abnormal fetuses were observed in any group. An embryocidal effect that depended on dose and stage of development was determined by increased numbers of resorption sites; resorption sites were dose-dependent, and the embryocidal effect decreased as the embryo matured. A dose of 0.06 mg PMA/kg of body weight produced no resorptions when given on day 8 or day 10 of gestation; 0.125 mg/kg produced no resorptions if given on day 10; 0.5 mg/kg resulted in live fetuses and resorptions in the same uterus when given on days 7 through 11; the same dose on day 12 produced all live fetuses; and 1, 2, or 5 mg PMA/kg given on day 8, 9, or 10 of gestation caused total embryo resorption. The i.p. LD50 of PMA in Microtus ochrogaster adult females was 10 mg/kg of body weight.

Implantation and early postimplantation development of the bank vole Clethrionomys glareolus, Schreber

The development of the bank vole Clethrionomys glareolus is described from implantation to the formation of the foetal membranes. The embryonic development of this species combines features of primitive rodent species, for example Geomys bursarius and highly specialized ones, for examples Mus musculus. The egg-cylinder is formed by invagination into the blastocoelic cavity of the inner cell mass and polar trophoblast overlying it; this resembles in many respects the early stages of development of primitive species. The fully formed egg-cylinder, however, resembles that of the mouse and the formation of foetal membranes is also similar to that in Muridae. It is concluded that in the bank vole and also in other rodents, the extra-embryonic ectoderm of the egg-cylinder is derived from the polar trophoblast rather than from the inner cell mass.

Preimplantation development in vivo and in vitro in bank voles, Clethrionomys glareolus, treated with PMSG and HCG

Ovulation can be induced in the bank vole by PMSG and HCG and takes place 9 1/2 to 11 1/2 hr after administration of HCG. The number of eggs obtained varied from 1 to 30 (average 9.3). The highest ovulation rates were observed when PMSG and HCG were administered 41 to 43 1/2 hr apart. The embryos attained the blastocyst stage 96 hr after HCG injection. Bank vole eggs can be cultured in chemically defined media beginning with the eight-cell stage; only 25% of four-cell eggs developed to blastocysts in vitro.