Histology – Interpreting 2D Sections of 3D Structures

Introduction: The Perspective Problem

Just as the word “MOM” can look like “WOW” depending on your viewpoint, a histological structure can look dramatically different based on how it is sliced. This chapter addresses a fundamental skill in histology: learning to interpret two-dimensional (2D) sections on a microscope slide while mentally reconstructing their three-dimensional (3D) reality.

Planes of Section: How We Slice the Specimens

Tissue specimens are processed and sliced into extremely thin sections. The most common planes of section are defined by standard anatomical terminology (the diagram below is an excellent example of the different cross sectional views of the same tube):

• Transverse (Horizontal or Axial) Section: A cut made perpendicular to the long axis of a structure or the body. A cross-section of a tube would appear circular.
• Sagittal (Longitudinal) Section: A cut made parallel to the long axis, dividing a structure into left and right parts. A section through a tube would appear as two parallel lines.
• Coronal (Frontal) Section: A cut made parallel to the long axis, dividing a structure into anterior (front) and posterior (back) parts.

Anatomical Directions vs. Histological Landmarks

It is critical to distinguish between gross anatomical directions and microscopic landmarks.

• When Anatomical Directions Apply: Terms like anterior/posterior (front/back) and superior/inferior (up/down) only make sense in the context of the body’s anatomical position. For example, in a section of the torso, you could identify that the abdominal aorta is anterior to the vertebral body.
• When They Do Not Apply: In an isolated tissue biopsy (e.g., a piece of skin or liver), there is no inherent “up” or “down.” The tissue can be rotated arbitrarily on the slide. Relying on these terms in such contexts will lead to unnecessary confusion.

The Key to Orientation: Apical and Basal Surfaces

For many tissues, especially epithelia, the correct way to orient yourself is by identifying intrinsic structural landmarks, not external directions. All epithelial tissue has polarity, defined by two surfaces:

• The Apical Surface: The “free” surface that faces an external environment or an internal open space (a lumen).
• The Basal Surface: The attached surface that rests on a thin, non-cellular layer called the basement membrane.

The single most reliable rule for orientation is this: find the lumen. The apical surface always faces the lumen.

Practical Application: The “Bucket Analogy”

Now, let’s look at the image below and apply the concept of apical and basal surfaces. The ground represents the basement membrane, and both the blue and red buckets are empty, which represent the lumen whereas the ground represents the basement membrane/basal layer.

• From Ant A’s perspective, the apical (lumen) surface would be if it looked ‘up’ from the bottom of the bucket, into the empty bucket. Similarly, the basal surface for Ant A would be the ground under its legs.
• From Ant B’s perspective, the same goes: the apical surface is if Ant B looked up into the empty space of the bucket, even though the bucket bottom encloses Ant B. The basal surface is still the ground under its legs
• From Ant C’s perspective, there is no basal surface. Ant C is perched between two open spaces: the air above and the empty bucket below.

So let’s apply this to the following histology slides!