Circumventricular Organs

Dr Sanjoy Sanyal MBBS, MS (Surgery), MSc (Royal College of Surgeons of Edinburgh),

ADPHAProfessor and Course Director of Neuroscience – FCM-III Neurology

No Blood Brain Barrier (BBB)

Paul Ehrlich (1882) After IV injection of Trypan

Blue, most Brain areas did

not get stained These areas are inside the BBB Some areas of Brain did get stained These structures are outside the

BBB; i.e. they do not have a BBB

Micro-Structure of BBB Tight Junctions (Claudin

/ Occludin) between

Endothelial Cells They form a continuous

Lipid Bi-layer Continuous Basement

Membrane outside Astrocyte Podocytes

form complete circle

Red arrows indicate lack of permeability

Micro-Structure of Non-BBB Highly Vascularized Fenestrations of

Endothelial Cells Permeable

Basement Membrane

Astrocyte Podocytes do not form complete circle

Red arrows indicate permeability

Exception: Sub-Commissural Organ has Non-fenestrated Capillaries

Non-BBB – Role of Tancytes Tancytes line the

Floor of 3rd Ventricle Body of Tancyte is in

contact with CSF Multiple Foot-Process

extend towards

adjacent Capillary Provides conduit

between CSF and

Blood

Tancytes play important role in certain Non-BBB areas

Circumventricular Organs Most Non-BBB

areas are around 3rd Ventricle, and one in 4th Ventricle

They are called Circumventricular Organs

They are bathed by CSF and are also in contact with Capillaries

Median Eminence of Tuber Cinereum

Organum Vasculosum of Lamina Terminalis

Sub-Fornical Organ Pineal Gland Sub-Commissural

Organ Area Postrema Wall of Optic Recess Posterior Lobe of

Pituitary

Functions of CV Organs

Chemosensitive Organs Allow ‘sampling’ of Blood Endocrine Feedback and

Regulation Homeostasis Metabolic Regulation Fluid Electrolyte Balance Detect Toxins in Blood CSF Circulation(?)

1. Median Eminence of Tuber Cinereum ME-TC#1 Tancytes line the Floor of

3rd Ventricle and ME-TC Has Arcuate Nucleus that

detects Endocrine Feedback to Hypothalamus via the Tancytes

Releases RH/IH, P-IH, and maintains Basal Levels of Hormones (FSH-LH)

In Floor of 3rd Ventricle: Raised Central Part of a Convex gray matter in the Ventral-most part of Hypothalamus #1

2. Organum Vasculosum of Lamina Terminalis (OV-LT) #2 Has Osmoreceptors Regulates ADH

release by

Supraoptic Nucleus Provides Vascular

Outlet for LH-RH by

Pre-optic and GH-IH

by Suprachiasmatic

(Both are adjacent

to LT)

A.k.a. Vascular

Organ of

Lamina

Terminalis #2

3. Subfornical Organ (SFO) #3 On inferior surface of

Fornix, at level of

Foramen of Monroe (#3) Angiotensin II Receptors Neurons from SFO

project to SO Nucleus

and OV-LT (Both are

Osmoreceptors) Fluid Electrolyte balance

4. Pineal Gland #4 Pinealocytes

(Epiphyseal Cells) #4 Highly Vascular Fenestrated Capillary Release Melatonin

into 3rd Ventricle Transported by

Tancytes in Floor of 3rd Ventricle

Act on Pituitary Gland

5. Subcommissural Organ #5 Special Epithelial /

Glial Cells Non-fenestrated

Capillaries Functions: (a)

Circulation of CSF

(Hydrocephalus); (b)

Clearance of certain

compounds (?)

Below Posterior Commissure #5

At 3rd Ventricle–Aqueduct of Sylvius Junction

6. Area Postrema Modified Astrocyte-

like Neurons Fenestrated

Capillaries 5-HT3 Receptors

(Excitatory Ionotropic) Induces Vomiting from

Morphine, Blood 5-HT Gets Afferents from

CTZ and GIT Plays Role in Food

Intake / CVS Control

In Floor of 4th Ventricle, Between Vagal Triangle and Inferolateral margin of Rhomboid Fossa, just Rostral to Obex