Episode 20: Hereditary spherocytosis
Author: Dr. Suneet Sood
Editor: Dr. Suneet Sood
Narrators: Alan Koay, Thong Yi Kun
Why so glum?
No, there’s something on your mind
It’s that Professor Rahman. He asks the strangest questions. And I have to come up with an answer by tomorrow.
What strange question?
Well, I was presenting a case of hemolytic anemia, and the patient had hereditary spherocytosis. Prof Rahman sked what are spherocytes, so I told him that normally our red cells are biconcave, but in HS they become spheres, so we call them spherocytes. I also told him that HS may be autosomal dominant or autosomal recessive.
Then he asks, “What’s the benefit of biconcave red cells over spheres?” And I told him that the biconcave structure allows the cells to be deformable when they pass through the spleen.
That’s also correct, so what was Prof Rahman’s issue?
So then he asks if there are any other benefits of a biconcave shape, and I didn’t know.
Do you know?
Yes, I do.
I’ll tell you. But go on. What else did Prof Rahman say?
He asked, “What defect causes the RBC to become a spherocyte?”
And then he asks me this really weird question. What’s the relationship of the surface area of a sphere to its volume, and how is that relevant in HS?
And who knows the answer to such strange questions? Do you know?
Okay. Well, first, the benefits of the biconcave shape are two. One, as you pointed out, the biconcave shape allows deformability. The RBCs can easily pass through the splenic cords of Billroth. The second advantage of a biconcave shape is that it provides a much larger surface area to the RBC. You remember your geometry, don’t you?
During our maths classes in school, we learnt that a sphere has the smallest surface area for a given volume. Obviously the biconcave shape gives the cell a much larger surface area, which allows for better oxygen absorption.
Ah! Of course!
The second question is, why does the RBC become a spherocyte? Well, in HS, there is an inborn defect of cell membrane proteins, particularly proteins called spectrin and Ankyrin. The defect makes the cell membrane unstable and results in a loss of surface membrane area. So HS is primarily a deficiency of RBC surface membrane area.
Oh, and a loss of surface membrane area means that the cell has to occupy the smallest possible surface area for its volume, and become spherical!
True. Interesting, isn’t it, how the laws of geometry intrude into the pathology of hereditary spherocytosis?
Not at all interesting. I prefer geometry to stay back in school, where it belongs.
Diez-Silva M, Dao M, Han J, Lim C-T, Suresh S. Shape and Biomechanical Characteristics of Human Red Blood Cells in Health and Disease.MRS Bull. 2010 May; 35(5): 382–388.
Rajpoot HC. Why sphere minimizes surface area for a given volume? Stack Exchange, 2015, https://physics.stackexchange.