Integrating the roles of extracranial lymphatics and intracranial veins in cerebrospinal fluid absorption in sheep

At relatively low cerebrospinal fluid (CSF) pressures, the majority of CSF drainage in 6- to 8-month-old sheep occurs through the cribriform plate into lymphatic vessels in the nasal submucosa. As CSF pressures are elevated, other absorption sites are recruited and these may include transport through arachnoid projections. To test for the transport of CSF directly into the venous sinus, the concentration of a tracer (131I-human serum albumin [HSA]) administered into the CSF compartment was measured in the confluence of the intracranial venous sinuses (torcular) and in the peripheral blood (inferior vena cava). CSF pressures were adjusted to favor absorption. Enrichment of the CSF tracer in the cranial venous system was most evident when the CSF-venous sinus pressure gradients were high. Peak concentration differences occurred 90 s after the CSF pressures were elevated. When pressure gradients approached 30 cm H(2)O, tracer concentrations in the torcular were approximately twofold higher than those observed in peripheral blood. The greatest concentration differences favoring the torcular were obtained when the CSF-venous sinus pressure gradients were elevated to high levels (20- to 40 cm H(2)O) and when CSF access to the paranasal lymphatics and CSF transport into the spinal subarachnoid compartment were prevented. In conjunction with previous studies, these results are compatible with the view that CSF absorption in the adult animal can occur directly into the cranial venous system. However, contrary to the established view, this pathway may represent a secondary system that is recruited to compliment lymphatic transport when global absorption capacity is stressed or compromised.

Lymphatic cerebrospinal fluid absorption pathways in neonatal sheep revealed by subarachnoid injection of Microfil

There is mounting evidence that a significant portion of cerebrospinal fluid drainage is associated with transport along cranial and spinal nerves with absorption taking place into lymphatic vessels external to the central nervous system. To characterize these pathways further, yellow Microfil was infused into the cisterna magna of 2-7-day-old lambs post mortem to perfuse either the cranial or spinal subarachnoid compartments. In some animals, blue Microfil was perfused into the carotid arteries simultaneously. Microfil was observed in lymphatic networks in the nasal mucosa, covering the hard and soft palate, conchae, nasal septum, the ethmoid labyrinth and the lateral walls of the nasal cavity. Many of these lymphatics drained into vessels located on the lateroposterior wall of the nasopharynx and from this location drained to the retropharyngeal lymph nodes. Additionally, lymphatics containing Microfil penetrated the lateral wall of the nasal cavity and joined with superficial lymphatic ducts travelling towards the submandibular and preauricular lymph nodes. In two cases, lymphatic vessels were observed anastomosing with deep veins in the retropharyngeal area. Microfil was also distributed within the nerve trunks of cranial and spinal nerves. The contrast agent was located in longitudinal channels within the endoneurial space and lymphatics containing Microfil were observed emerging from the mesoneurium. In summary, Microfil distribution patterns in neonatal lambs illustrated the important role that cranial and spinal nerves play in linking the subarachnoid compartment with extracranial lymphatics.

Reassessment of the pathways responsible for cerebrospinal fluid absorption in the neonate

OBJECTIVE

In neonatal lambs, the quantitative evidence suggests that a significant volume of cranial CSF drainage is associated with transport along olfactory nerves with absorption primarily into extracranial lymphatics in the paranasal region. Arachnoid granulations appear to be poorly developed at this level of development and their function is unknown. In this report, we tested whether a CSF protein tracer ((131)I-human serum albumin) could transport directly into the superior sagittal sinus of newborn lambs.

METHODS AND RESULTS

The concentration of the tracer administered into the CSF compartment was measured in the confluence of the intracranial venous sinuses (torcula) and in the peripheral blood (inferior vena cava). Enrichment of the CSF tracer in the cranial venous system was most evident when the CSF-venous sinus pressure gradients approached 20-30 cm H(2)O.

CONCLUSION

The data suggests that neonatal CSF can be absorbed directly into the cranial venous system. However, contrary to the classical view, this route may represent an auxiliary system that is recruited to compliment lymphatic transport when intracranial pressures are very high.

Functional impact of lymphangiogenesis on fluid transport after lymph node excision

When a lymph node is excised, lymphangiogenesis occurs to maintain flow in the affected area. However, a complex network of small vessels replaces the node and these newly formed vessels might increase resistance to lymph transport. To test this in sheep, the popliteal lymph node from one hind limb was removed surgically. The contralateral node was left intact. After 4 to 6 weeks (a period that allowed regenerated vessels to restore flow), a prenodal lymphatic vessel in each limb was cannulated with a polyethylene catheter to permit saline infusion into the node or lymphatic regeneration site. Infusion pressures were monitored from t-pieces inserted between the infusion pump and the point of entry of the catheters in the prenodal ducts. We observed that the flow rate versus perfusion pressure relationships were significantly different in the 2 experimental preparations (node intact limbs, n = 13; node excised limbs, n = 10). In the limbs undergoing lymphangiogenesis, much higher infusion pressures were required to generate a given flow rate. Additionally, the regenerated lymphatic network provided a significantly increased resistance to flow. The data suggested that lymphangiogenesis restored fluid continuity to some extent in the area occupied originally by the popliteal lymph node. However, the transport properties exhibited by the newly formed lymphatics were insufficient to restore flow parameters to their original state.

Cerebrospinal fluid transport: a lymphatic perspective

The textbook view that projections of the arachnoid membrane into the cranial venous sinuses represent the primary cerebrospinal fluid (CSF) absorption sites seems incompatible with many clinical and experimental observations. On balance, there is more quantitative evidence suggesting a function for extracranial lymphatic vessels than exists to support a role for arachnoid villi and granulations in CSF transport.

Does neonatal cerebrospinal fluid absorption occur via arachnoid projections or extracranial lymphatics?

Arachnoid villi and granulations are thought to represent the primary sites where cerebrospinal fluid (CSF) is absorbed. However, these structures do not appear to exist in the fetus but begin to develop around the time of birth and increase in number with age. With the use of a constant pressure-perfusion system in 2- to 6-day-old lambs, we observed that global CSF transport (0.012 +/- 0.003 ml x min(-1) x cmH(2)O(-1)) and CSF outflow resistance (96.5 +/- 17.8 cmH(2)O x ml(-1) x min) were very similar to comparable measures in adult animals despite the relative paucity of arachnoid villi at this stage of development. In the neonate, the recovery patterns of a radioactive protein CSF tracer in various lymph nodes and tissues indicated that CSF transport occurred through multiple lymphatic pathways. An especially important route was transport through the cribriform plate into extracranial lymphatics located in the nasal submucosa. To investigate the importance of the cribriform route in cranial CSF clearance, the cranial CSF compartment was isolated surgically from its spinal counterpart. When the cribriform plate was sealed extracranially under these conditions, CSF transport was impaired significantly. These data demonstrate an essential function for lymphatics in neonatal CSF transport and imply that arachnoid projections may play a limited role earlier in development.

Comparison of cerebrospinal fluid transport in fetal and adult sheep

We quantified cerebrospinal fluid (CSF) transport (conductance) and CSF outflow resistance in late-gestation fetal and adult sheep using two methods, a constant pressure infusion method and a bolus injection technique into the lateral ventricles. No significant differences in CSF conductance (fetus 0.013 +/- 0.002, adult 0.014 +/- 0.003 ml x min(-1) x cm H(2)O(-1)) or CSF outflow resistance (fetus 83.7 +/- 9.8, adult 84.7 +/- 19.7 cm H(2)O x ml(-1) x min) were observed. To confirm CSF transport to plasma in fetal animals, (125)I- or (131)I-labeled human serum albumin (HSA) was injected into the lateral ventricles. The tracer entered fetal plasma with an average mass transport rate of 1.91 +/- 0.47% injected/h (n = 9). In two fetuses, we monitored the tracer appearance in plasma and cervical and thoracic duct lymph after injection of radioactive HSA into the ventricular CSF. As was the case in adult animals, fetal tracer concentrations increased in all three compartments over time, with the highest concentrations measured in lymph collected from the cervical lymphatics. These results 1) indicate that global CSF transport parameters in the late-gestation fetus and adult sheep are similar and 2) suggest an important role for extracranial lymphatic vessels in CSF transport before birth.