Norman R. Saunders

Engaging neuroscience to advance translational research in brain barrier biology

The delivery of many potentially therapeutic and diagnostic compounds to specific areas of the brain is restricted by brain barriers, of which the most well known are the blood–brain barrier (BBB) and the blood–cerebrospinal fluid (CSF) barrier. Recent studies have shown numerous additional roles of these barriers, including an involvement in neurodevelopment, in the control of cerebral blood flow, and — when barrier integrity is impaired — in the pathology of many common CNS disorders such as Alzheimers disease, Parkinsons disease and stroke.

Barrier mechanisms in the developing brain.

The adult brain functions within a well-controlled stable environment, the properties of which are determined by cellular exchange mechanisms superimposed on the diffusion restraint provided by tight junctions at interfaces between blood, brain and cerebrospinal fluid (CSF). These interfaces are referred to as “the” blood–brain barrier. It is widely believed that in embryos and newborns, this barrier is immature or “leaky,” rendering the developing brain more vulnerable to drugs or toxins entering the fetal circulation from the mother. New evidence shows that many adult mechanisms, including functionally effective tight junctions are present in embryonic brain and some transporters are more active during development than in the adult. Additionally, some mechanisms present in embryos are not present in adults, e.g., specific transport of plasma proteins across the blood–CSF barrier and embryo-specific intercellular junctions between neuroependymal cells lining the ventricles. However developing cerebral vessels appear to be more fragile than in the adult. Together these properties may render developing brains more vulnerable to drugs, toxins, and pathological conditions, contributing to cerebral damage and later neurological disorders. In addition, after birth loss of protection by efflux transporters in placenta may also render the neonatal brain more vulnerable than in the fetus.

Development of the choroid plexus

Mammalian choroid plexuses develop at four sites in the roof of the neural tube shortly after its closure, in the order IVth, lateral, and IIIrd ventricles. Bone morphogenetic proteins and tropomyosin are involved in early specification of these sites and in early plexus growth. Four stages of lateral ventricular plexus development have been defined, based on human and sheep fetuses; these depend mainly on the appearance of epithelial cells and presence or absence of glycogen. Other plexuses and other species are probably similar, although marsupials may lack glycogen. Choroid plexuses form one of the blood‐brain barrier interfaces that control the brains internal environment. The mechanisms involved combine a structural diffusion restraint (tight junctions between the plexus epithelial cells) and specific exchange mechanisms. In this review, it is argued that barrier mechanisms in the developing brain are different in important respects from those in the adult brain, but these differences do not necessarily reflect immaturity of the system. Absence of a barrier mechanism or presence of one not found in the adult may be a specialisation that is appropriate for that stage of brain development. Emphasis is placed on determining which mechanisms are present in the immature brain and relating them to brain development. One mechanism unique to the developing brain transfers specific proteins from blood to cerebrospinal fluid (CSF), via tubulocisternal endoplasmic reticulum in plexus epithelial cells. This results in a high concentration of proteins in early CSF. These proteins do not penetrate into brain extracellular space because of “strap” junctions between adjacent neuroependymal cells, which disappear later in development, when the protein concentration in CSF is much lower. Functions of the proteins in early CSF are discussed in terms of generation of a “colloid” osmotic pressure that expands the ventricular system as the brain grows; the proteins may also act as specific carriers and growth factors in their own right. The pathway for low molecular weight compounds, which is much more permeable in the developing choroid plexuses, appears also to be a transcellular one, rather than paracellular via tight junctions. There is thus good evidence to support a novel view of the state of development and functional significance of barrier mechanisms in the immature brain. It grows in an environment that is different from that of the rest of the fetus/neonate and that is also different in some respects from that of the adult. But these differences reflect developmental specialisation rather than immaturity. Microsc. Res. Tech. 52:5–20, 2001.

BARRIER MECHANISMS IN THE BRAIN, II. IMMATURE BRAIN

1. It is widely believed that ‘the’ blood–brain barrier is immature in foetuses and newborns.

Barriers in the brain: a renaissance?

Barrier mechanisms regulate the exchange of molecules between the brains internal milieu and the rest of the body. Correct functioning of these mechanisms is critical for normal brain activity, maintenance and development. Dysfunctional brain barrier mechanisms contribute to the pathology of neurological conditions, ranging from trauma to neurodegenerative diseases, and provide obstacles for successful delivery of potentially beneficial pharmaceutical agents. Previous decades of research have yielded insufficient understanding for solving brain barrier problems in vivo. However, an awakening of interest and novel approaches are providing insight into these mechanisms in developing and dysfunctional brain, as well as suggesting new approaches to circumventing brain barrier mechanisms to get therapeutic agents into the central nervous system.

Changes in blood-brain barrier permeability to large and small molecules following traumatic brain injury in mice

The entry of therapeutic compounds into the brain and spinal cord is normally restricted by barrier mechanisms in cerebral blood vessels (blood–brain barrier) and choroid plexuses (blood–CSF barrier). In the injured brain, ruptured cerebral blood vessels circumvent these barrier mechanisms by allowing blood contents to escape directly into the brain parenchyma. This process may contribute to the secondary damage that follows the initial primary injury. However, this localized compromise of barrier function in the injured brain may also provide a ‘window of opportunity’ through which drugs that do not normally cross the blood–brain barriers are able to do so. This paper describes a systematic study of barrier permeability in a mouse model of traumatic brain injury using both small and large inert molecules that can be visualized or quantified. The results show that soon after trauma, both large and small molecules are able to enter the brain in and around the injury site. Barrier restriction to large (protein‐sized) molecules is restored by 4–5 h after injury. In contrast, smaller molecules (286–10 000 Da) are still able to enter the brain as long as 4 days postinjury. Thus the period of potential secondary damage from barrier disruption and the period during which therapeutic compounds have direct access to the injured brain may be longer than previously thought.

Barriers in the immature brain.

Abstract1. The term “blood–brain barrier” describes a range of mechanisms that control the exchange of molecules between the internal environment of the brain and the rest of the body.2. The underlying morphological feature of these barriers is the presence of tight junctions which are present between cerebral endothelial cells and between choroid plexus epithelial cells. These junctions are present in blood vessels in fetal brain and are effective in restricting entry of proteins from blood into brain and cerebrospinal fluid. However, some features of the junctions appear to mature during brain development.3. Although proteins do not penetrate into the extracellular space of the immature brain, they do penetrate into cerebrospinal fluid by a mechanism that is considered in the accompanying review (Dziegielewska et al., 2000).4. In the immature brain there are additional morphological barriers at the interface between cerebrospinal fluid and brain tissue: strap junctions at the inner neuroependymal surface and these and other intercellular membrane specializations at the outer (pia–arachnoid) surface. These barriers disappear later in development and are absent in the adult.5. There is a decline in permeability to low molecular weight lipid-insoluble compounds during brain development which appears to be due mainly to a decrease in the intrinsic permeability of the blood–brain and blood–cerebrospinal fluid interfaces.

Monodelphis domestica (grey short-tailed opossum): an accessible model for studies of early neocortical development.

SummaryThe development of the neocortex of the marsupial Monodelphis domestica has been studied from birth until adulthood. Monodelphis is born after a gestational period of 14 days, a time when the neocortex is still at a two-layered “embryonic’ stage of development, that is equivalent to a 13–14 day rat embryo or 6 week human embryo. The cortical plate does not begin to appear until 3 to 5 days postnatal. Thus the whole of neocortical development is a postnatal phenomenon in this species, as has been previously described in other marsupials. The general pattern of development of the characteristic layers of the immature neocortex and the subsequent development of a six-layered adult neocortex is similar to that found in eutherian species. However there are some differences. The depth of the immature cortical plate when compared to the thickness of the neocortical wall is less than in eutherians and the subplate zone is much deeper in Monodelphis; this transient subplate zone consists of widely spaced rows of cells that are aligned parallel to the cortical surface. Unlike eutherians there appears to be no secondary proliferative zone in the subventricular zone of the dorso-lateral neocortical wall.Maturation of the neocortex is apparent by 45 days postnatal and by 60 days (around the time of weaning) the characteristic six-layered adult neocortex is clearly present. The neuronal marker PGP 9.5 was used to define neuronal populations in the adult brain. The density of neurons in Monodelphis appears to be considerably less than in eutherians such as the rat. The suitability of postnatal Monodelphis for studies of neocortical development is discussed.

The development of the human blood-brain and blood-CSF barriers

The development of the human blood‐brain and blood‐CSF barriers

Degeneration and regeneration in the nervous system

1. Repair after Spinal Cord Injury: A Clinical Perspective 2. Recovery from Injury in the Immature Mammalian Spinal Cord 3. Intrinsic Determinants of Differential Axonal Regeneration by Adult Mammalian Central Nervous System Neurons 4. Inflammation and the Glial Scar: Factors at the Site of Injury that Influence Regeneration in the Cental Nervous System 5. Intrinsic Neuronal and Extrinsic Glial Determinants of Axonal Regeneration in the Injured Spinal Cord 6. Evolutionary Hierarchy of Optic Nerve Regeneration: Implications for Cell Survival, Axon Outgrowth and Map Formation 7. Regeneration in the Central Nervous System: Mechanisms and Strategies for Enhancement 8. What Types of Bridges will Best Promote Anoxal Regeneration Across an Area of Injury in the Adult Mammalian Spinal Cord? 9. Use of Cell/Polymer Hybrid Structures as Conduits for Regenerative Growth in the Central Nervous System 10. Neural Stem Cell: Regulation and Potential Use in Neuronal Regeneration 11. The Low Affinity Neurotrophin, p75: A Multifunction Molecule with a Role in Nerve Regeneration? 12. Regeneration in the Peripheral Nervous System 13. The Role of Macrophages in Degeneration and Regeneration in the Peripheral Nervous System 14. The Response of the Somatosensory System to Peripheral Nerve Injury