The distribution of plasma proteins during early embryonic development in the sheep

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Medications for pregnant women: A balancing act between the interests of the mother and of the fetus

Drug entry into the adult brain is controlled by efflux mechanisms situated in various brain barrier interfaces. The effectiveness of these protective mechanisms in the embryo, fetus and newborn brain is less clear. The longstanding belief that "the" blood-brain barrier is absent or immature in the fetus and newborn has led to many misleading statements with potential clinical implications. Here we review the properties of brain barrier mechanisms in the context of drug entry into the developing brain and discuss the limited number of studies published on the subject. We noticed that most of available literature suffers from some experimental limitations, notably that drug levels in fetal blood and cerebrospinal fluid have not been measured. This means that the relative contribution to the overall brain protection provided by individual barriers such as the placenta (which contains similar efflux mechanisms) and the brain barriers cannot be separately ascertained. Finally, we propose that systematic studies in appropriate animal models of drug entry into the brain at different stages of development would provide a rational basis for use of medications in pregnancy and in newborns, especially prematurely born, where protection usually provided by the placenta is no longer present.

Physiology and molecular biology of barrier mechanisms in the fetal and neonatal brain

Properties of the local internal environment of the adult brain are tightly controlled providing a stable milieu essential for its normal function. The mechanisms involved in this complex control are structural, molecular and physiological (influx and efflux transporters) frequently referred to as the 'blood-brain barrier'. These mechanisms include regulation of ion levels in brain interstitial fluid essential for normal neuronal function, supply of nutrients, removal of metabolic products, and prevention of entry or elimination of toxic agents. A key feature is cerebrospinal fluid secretion and turnover. This is much less during development, allowing greater accumulation of permeating molecules. The overall effect of these mechanisms is to tightly control the exchange of molecules into and out of the brain. This review presents experimental evidence currently available on the status of these mechanisms in developing brain. It has been frequently stated for over nearly a century that the blood-brain barrier is not present or at least is functionally deficient in the embryo, fetus and newborn. We suggest the alternative hypothesis that the barrier mechanisms in developing brain are likely to be appropriately matched to each stage of its development. The contributions of different barrier mechanisms, such as changes in constituents of cerebrospinal fluid in relation to specific features of brain development, for example neurogenesis, are only beginning to be studied. The evidence on this previously neglected aspect of brain barrier function is outlined. We also suggest future directions this field could follow with special emphasis on potential applications in a clinical setting.

Estradiol stimulates progenitor cell division in the ventricular and subventricular zones of the embryonic neocortex

Two distinct populations of cerebral cortical progenitor cells that generate neurons during embryogenesis have been identified: radial glial cells and intermediate progenitor cells. Despite advances in our understanding of progenitor cell populations, we know relatively little about factors that regulate their proliferative behaviour. 17-beta-Estradiol (E2) is present in the adult and developing mammalian brain, and plays an important role in central nervous system processes such as neuronal differentiation, survival and plasticity. E2 also stimulates neurogenesis in the adult dentate gyrus. We examined the role of E2 during embryonic cortical neurogenesis through immunohistochemistry, in situ hybridization, functional enzyme assay, organotypic culture and in utero administration of estradiol-blocking agents in mice. We show that aromatase, the E2 synthesizing enzyme, is present in the embryonic neocortex, that estrogen receptor-alpha is present in progenitor cells during cortical neurogenesis, that in vitro E2 administration rapidly promotes proliferation, and that in utero blockade of estrogen receptors decreases proliferation of embryonic cortical progenitor cells. Furthermore, the E2 inhibitor alpha-fetoprotein is expressed at high levels by radial glial cells but at lower levels by intermediate progenitor cells, suggesting that E2 differentially influences the proliferation of these cortical progenitor cell types. These findings demonstrate a new functional role for E2 as a proliferative agent during critical stages of cerebral cortex development.

Fetuin expression in the dorsal root ganglia and trigeminal ganglia of perinatal rats

Fetuin, a fetal plasma glycoprotein, has been shown previously to be present in sub-populations of neurons in the developing central and peripheral nervous system. To gain a more complete description of the time course of the appearance of fetuin during neurogenesis we have examined fetuin immunoreactivity, and the presence of fetuin mRNA, in the developing rat trigeminal and dorsal root ganglia. Fetuin immunoreactivity and its mRNA were first seen at embryonic day 15 in the trigeminal ganglia, and at embryonic day 16 in dorsal root ganglia. In both trigeminal and dorsal root ganglion, fetuin appeared to be present up until around the time of birth, and then again between postnatal days 3 and 16. The results suggest that fetuin first appears at around the time that ganglion cell axons reach their central targets, which is also approximately when the cell-death period begins. The proportion of ganglion neurons that were fetuin immunoreactive at different ages was inversely related to the amount of cell death that is known to occur in these populations, thus it seems that fetuin is more likely to be associated not with dying cells, but with those that survive the cell-death period.

Localization of alpha-fetoprotein in developing chick amniotic membrane

The aim of this work was to investigate the localization of alpha-fetoprotein (AFP) in amniotic membrane (AM). By using the immunoperoxidase technique in several developmental stages, which reflected the changes of structure of the AM germinal layers, AFP was detected earliest in 7-day AM and localized selectively in the ectodermal cell layer. This was the only developmental stage at which AM occurred as a two-layer structure, ectoderm and somatic mesoderm, and was AFP-positive. In the zone of fusion of the AM with the inner wall of the allantoic sac, cystlike cavities were observed which were markedly immunoreactive to AFP. In those membranes where fusion had consolidated and a four-layer structure could be distinguished: ectoderm, somatic mesoderm, splanchnic mesoderm and endoderm, AFP was localized in the ectodermal cells and in the splanchnic mesoderm resulting from the inner wall of the allantoic sac. Both mesodermal layers could be distinguished by means of the AFP immunoreaction since AFP labelled the splanchnic, but not the somatic mesoderm. At later developmental stages, e.g. 18-day, the AM had a three-layer structure and AFP was localized selectively throughout the splanchnic mesoderm. The disappearance of the somatic mesoderm coinciding temporarily with the disappearance of AFP from the ectodermal cells, suggests that the presence of AFP in such cells could depend on some factors related to the somatic mesoderm.

Expression and distribution of fetuin in the developing sheep fetus

Tissue distribution and developmental expression of fetuin were studied in the sheep fetus from embryonic day (E) 30 to adult (gestational period is 150 days). The presence of fetuin was demonstrated immunocytochemically using anti-fetuin antibodies; in situ hybridisation using short anti-sense oligonucleotide probes labelled with digoxigenin was used to study the ability of the developing tissue to synthesise fetuin, and reverse transcription-polymerase chain reaction (RT-PCR) was used to estimate the level of fetuin mRNA in selected tissues. Tissue distribution of fetuin was widespread in the younger fetuses (E30 to E40). The most prominent presence due to in situ synthesis was demonstrated in the liver, central nervous system (CNS) including anterior horn cells, dorsal root ganglia and in skeletal muscle cells. Other developing tissues and organs that showed evidence of fetuin synthesis and presence of the protein included mesenchyme, kidney, adrenal, developing bone, gut, lung and heart. In the immature liver (E30-40) there was a strong signal for fetuin mRNA in hepatocytes and also in numerous haemopoietic cells; the proportion of these latter cells that was positive for fetuin mRNA increased between E30 and E40. Only some hepatocytes and a proportion of the haemopoietic stem cells were immunoreactive for fetuin itself at E30-40; immunoreactive hepatocytes were more frequently observed in the more mature outer regions of the developing liver. Lung and gut contained scattered fetuin-positive epithelial cells, especially at E30; a weak fetuin mRNA signal could be detected above background in many of these cells up to E40, but not at E60-E115 or in the adult. Particularly at E30 to E40, mesenchymal tissue both within organs such as the gut and lung and around forming bone and skeletal muscle contained cells that were positive for fetuin mRNA. Mesenchyme at these ages was also very strongly stained for fetuin protein, much of which may reflect fetuin in tissue extracellular spaces and be derived from the high concentration in plasma. By E80 fetuin mRNA was mainly present in the liver and the CNS; staining of the muscle tissue was becoming less pronounced. However in developing bone tissue, staining of chondrocytes for fetuin mRNA was still prominent in older (E80) fetuses; there was also fetuin protein staining of chondrocytes at the growing surfaces of bones and in bone marrow at this age. In the adult, weak immunocytochemical staining for fetuin itself was present in hepatocytes, but the mRNA signal was barely above the threshold limit of detection.(ABSTRACT TRUNCATED AT 400 WORDS)

Human fetuin/alpha 2 HS glycoprotein in colloid and parenchymal cells in human fetal pituitary gland

An immunohistochemical study was undertaken, in an attempt to identify the acidic glycoprotein(s) present in colloid and in parenchymal cells in human fetal pituitary gland. As the colloid has been proposed to represent disintegrating cells, a series of antibodies against plasma glycoproteins and plasma proteins was applied; their presence intracellularly would generally be an indicator of plasma membrane leakage in dying parenchymal cells. In tissue sections from 9- to 20-week-old fetuses, the colloid showed prominent staining with an antibody to human fetuin/alpha 2 HS glycoprotein. Anti-alpha 2-HS glycoprotein labelled parenchymal cells in pars anterior and intermedia. Apart from a minor immunoreactivity for alpha 1 beta glycoprotein, no other plasma glycoprotein was seen in colloid or parenchymal cells. An antibody against bovine fetuin showed staining of the colloid and of some parenchymal cells in pars distalis and intermedia; the plasma and stroma of the pituitary gland were unstained. In contrast, the anti-human plasma protein antibodies all stained the stroma. The presence of alpha 2 HS glycoprotein in parenchymal cells and absence of other plasma glycoproteins imply integrity of the parenchymal cell plasma membrane. Thus, alpha 2 HS glycoprotein is either synthesized locally or taken up specifically in the parenchymal cells, which are proposed to participate in the formation of colloid. It is suggested that alpha 2 HS glycoprotein is part of a homeostatic system, which controls remodelling and physiological cell death during development.

Alpha fetoprotein and albumin gene transcripts are detected in distinct cell populations of the brain and kidney of the developing rat

We report the cellular localization of alpha-fetoprotein (AFP) and albumin (ALB) gene transcripts in rat kidney and brain as detected by in situ hybridization on tissue sections with [35S]-labelled alpha-fetoprotein and albumin cDNA probes. Both types of mRNA were present in distinct cell populations of the developing kidney and brain. In the kidney, both gene transcripts were distributed over all developing tubular cells in the 20-day-old fetus. During the first 3 weeks of life, a gradual decrease in the expression of AFP and ALB mRNA was apparent, the rate of decrease being greater on proximal tubules than on the other tubular cells. From the 4th week onwards, a weak signal for both mRNAs persisted in the majority of the tubular cells. In the brain, all neuronal cells expressed both genes. Transcript cellular distribution was mainly cytoplasmic during fetal and early postnatal life and became predominantly nuclear at 3, 4 and 5 weeks, suggesting that posttranscriptional mechanisms are involved in the control of AFP and ALB gene expression at these stages. In the adult brain no significant signal was recorded thereafter. Coexpression of AFP and ALB transcripts by specific cell types, together with their gradual disappearance concomitant with postnatal organ maturation, suggests a possible role for these proteins in terminal differentiation processes of tubular and neuronal cells.

Serum proteins enhance aggregate formation of dissociated fetal rat brain cells in an aggregating culture

Dissociated fetal rat brain cells (Day 14.5 of gestation) reaggregated into small cell clusters and formed large aggregates in a medium supplemented with serum or dialyzed serum in an aggregating culture. In contrast, only small aggregates were produced in a serum-free medium. The present results indicated that albumin, fetuin, transferrin, and alpha 1-antitrypsin enhanced the aggregate formation. Small aggregates produced in a serum-free medium elongated neurites when they were cultured within a collagen gel matrix. Total DNA per flask was almost the same in small and large aggregates. Thus, these serum proteins may well play an important role in the adhesion of small cell clusters and cause the formation of large aggregates in this short-term aggregating culture.

Transferrin and iron requirements of embryonic mesoderm cells cultured in hydrated collagen matrices

Very early embryonic mesoderm cells were taken from the primitive streak-stage chick embryo and cultured in a matrix of type I collagen in the presence of serum. Previous work has shown that under these conditions cells do not leave the explant and move in the collagen in the absence of supplemented avian transferrin. Cells explanted onto tissue culture plastic in the presence of serum do not require this transferrin supplement. These observations were investigated further by culturing cells in collagen in the presence of the lipophilic iron chelator, ferric pyridoxal isonicotinoyl hydrazone (FePIH), which can replace transferrin as an iron-delivery agent. Under conditions in which FePIH could effectively stimulate chick embryo myoblast growth, no such long-term stimulation was obtained with the early mesoderm cells in collagen. This suggested that for mesoderm cells, FePIH could not replace transferrin. Antibody to the transferrin receptor and to transferrin itself inhibited growth of myoblasts in collagen and on plastic, and of mesoderm cells in collagen. Mesoderm cells on plastic, however, were refractory to the presence of the antibody directed to the receptor and seemed to show a low dependency on transferrin-delivered iron under these conditions, inasmuch as antiserum to transferrin itself only caused a partial inhibition of outgrowth. The results suggest that mesoderm cells in collagen require transferrin for both iron uptake and for another unspecified function. It is consistent with the results to propose that transferrin binding might modulate the cells' attachment to collagen, thus influencing outgrowth. The distribution of the actin cytoskeleton in mesoderm cells actively migrating in collagen, such as in the presence of transferrin, suggests a stronger attachment to the collagen than nonmigrating cells.

A fetuin-related glycoprotein (alpha 2HS) in human embryonic and fetal development

The human plasma protein, alpha 2HS glycoprotein, has an amino acid composition very similar to that of fetuin, the major protein in fetal calf and lamb serum. Immunohistochemical studies of human fetuses (6-33 weeks gestation) showed that alpha 2HS glycoprotein and fetuin have similar distributions in developing brain and several other tissues, e.g., bone, kidney, gonads, gastrointestinal tract, respiratory and cardiovascular systems. There were notable differences in the liver and thymus in the distribution of the two proteins. Fetuin and alpha 2HS glycoprotein are present in plasma and cerebrospinal fluid of both human and sheep fetuses; their concentrations are reciprocally related: in human plasma and cerebrospinal fluid alpha 2HS glycoprotein concentration is high and fetuin low; the reverse is the case in sheep fetuses. Estimates of the concentration of alpha 2HS glycoprotein in human fetal cerebrospinal fluid and plasma were obtained. It is suggested that alpha 2HS glycoprotein may play a role in developing tissues, especially in the human fetus, similar to that of fetuin in other species.

An immunocytochemical demonstration of alpha 2HS-glycoprotein in the developing neocortex of the rat

The presence of the plasma protein alpha 2HS-glycoprotein (alpha 2HS) has been demonstrated in the developing rat neocortex, using biotin-streptavidin immunocytochemistry. alpha 2HS was observed in the neocortex on embryonic day 20 but not earlier. At this age it was present in cells of the subplate and of the intermediate layer and in layer I anteriorly. Between postnatal day 5 and day 10 alpha 2HS-positive cells were found in greater number in various cortical layers and in fibres in layer I. At postnatal day 28 no alpha 2HS-positive cells or fibres could be found in the neocortex. alpha 2HS has been reported to be very closely related to fetuin which is a fetal plasma protein found in cells of the developing cortex in the sheep and the pig. It is suggested that these glycoproteins may be important in some aspects of early neocortical differentiation.

The distribution of native albumin and foreign albumin injected into lateral ventricles of prenatal and neonatal rat forebrains

Several plasma proteins are found within the cells of the developing brain of many species, with a distribution pattern which changes during development, but the origin of such proteins is in dispute. The experiments described here were designed to test the hypothesis that some developing brain cells are able to take up plasma proteins. The distribution of the plasma protein albumin has been studied in the rat forebrain from the 14th day of gestation until birth. Although present within the cerebrospinal fluid and plasma from the earliest age studied, albumin was not seen within cells of the developing forebrain until day 16E or 17E. A foreign protein, sheep albumin, was injected into the ventricles at days 14E, 16E, 18E, 20E and on the day of birth. Sheep albumin can be detected in the presence of rat albumin because the antibody to sheep albumin does not cross-react with rat albumin. The sheep albumin was taken up very rapidly into cells of the ventricular zone at the later but not the earlier ages, thus mimicking the distribution of the naturally occurring rat albumin. After the foreign albumin had been left within the ventricle for several hours, some of the cells of the cortical plate also contained the protein, again mimicking the normal distribution of albumin. These findings suggest that the presence of albumin within cells of the developing rat forebrain can largely be attributed to uptake rather than synthesis.

The distribution of plasma proteins in the neocortex and early allocortex of the developing sheep brain

The histogenesis of the cerebral neocortex and early allocortex of the sheep has been described and, using an immunohistochemical technique, five plasma proteins have been identified in the telencephalic wall and their distribution followed during its differentiation. The development of the neocortex was studied from 18 days gestation, when the neural tube was still open, to 120 days, when the adult structure was established. A primordial plexiform layer was formed above the ventricular zone by 25 days and by 35 days this layer was divided by the differentiating cortical plate into an outer marginal zone and an inner subplate zone. The appearance of the subventricular and intermediate zones by 50 days gestation completed the formation of the neocortical layers. The differentiation of the allocortex was generally less advanced than the neocortex up to 40 days gestation, when the primordium of the pyramidal layer was beginning to develop. The five plasma proteins identified, fetuin, alpha-fetoprotein, albumin, transferrin and alpha 1-antitrypsin, are quantitatively the most important in the csf and plasma of the sheep fetus. Fetuin was the earliest plasma protein to be detected in the brain and it was also the most widespread; positive staining for this protein was seen in cells and fibres of all layers as they differentiated and could still be identified in some mature neurons at 120 days. alpha-Fetoprotein and albumin had a limited distribution, appearing in cells in the developing cortical plate for a short period early in gestation (35-40 days), but mainly confined to the ventricular zones later and barely detectable by 80 days gestation. Transferrin appeared to have a different distribution, being detected in fibres first in the primordial plexiform layer and then in the marginal and subplate zones, only later being identified in cells of the cortical plate. From their distribution it is suggested that fetuin and transferrin may play an important role in the differentiation of the cortex and the establishment of correct connections between fiber systems and migrating cells at certain stages of development. alpha 1-Antitrypsin was only found in a few cells during a restricted period of gestation. All five plasma proteins were identified in precipitated csf and plasma at most ages examined, although at 18 days gestation albumin, transferrin and alpha 1-antitrypsin and at 120 days, alpha-fetoprotein, could not be detected.(ABSTRACT TRUNCATED AT 400 WORDS)