Fatty cell membranes affect fixative penetration by causing what effect on aqueous fixatives?

• A. Cause many aqueous fixatives to penetrate poorly
• B. Cause rapid penetration of fixatives
• C. Have no impact on fixative penetration
• D. Promote uniform fixation

Answer: (A)

Fatty cell membranes present a diffusion barrier to water-based fixatives. Because these fixatives are hydrophilic, they move into tissue mainly by diffusion, and the lipid-rich interior of the membrane slows or blocks their entry. As a result, many aqueous fixatives penetrate poorly into fatty tissues, especially in thicker samples, leading to slow and uneven fixation. To improve penetration, thinner tissue sections or fixatives containing organic solvents are often used to help cross the lipid barrier.