Question

which brain areas lack a blood-brain barrier and what purpose does this absence observe?

Answer 1

Anatomical structures that constitute Blood-Brain-Barrier :

1. Capillary endothelium
2. Basement membrane of the endothelium
3. Foot processes of asrocytes(which cover 85% of the capillary surface)

Areas that lack Blood-Brain-Barrier are:

• Pineal body
• Neurohypophysis(Posterior lobe of pituitary gland)
• Organum vasculosum of Lamina Terminalis(OVLT)
• Subcommisural organ
• Subfornical organ
• Area postrema

These areas are also called as Circumventricular Organs(CVOs) & these are specialised/strategical areas in the walls of 3rd and 4th ventricles.

Purpose of absence of BBB at these locations or purpose of CVOs:

In these non-barrier regions, the tight junctions between endothelial cells are discontinuous thus allowing entry of molecules. Many of these areas participate in hormonal control.

The circumventricular organs (CVO) are structures that permit polypeptide hypothalamic hormones to leave the brain without disrupting the blood-brain barrier (BBB) and permit substances that do not cross the BBB to trigger changes in brain function.

Examples:

• Subcommisural organ involved in hypertension produced by aldosterone acting on the brain
• Feedback control of corticotropin-releasing hormone (CRH) secretion is exerted by free steroids diffusing into the brain, but substances such as cytokines and angiotensin II act on CVO to produce increase in CRH secretion
• CVO may be involved in the negative feedback control of growth hormone and prolactin secretion.