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Appendices: Introduction to Histology for first-time learners

Histology – Interpreting 2D Sections of 3D Structures

Willie Wu and Athena Li

Learning Objectives

By the end of this section, you should be able to:

  • Identify the three major anatomical planes of section (transverse, sagittal, and coronal) and predict how a hollow structure would appear in each.
  • Define the “apical” and “basal” surfaces of epithelial tissue using the lumen and basement membrane as consistent landmarks, regardless of image orientation.
  • Use the presence of a lumen to correctly orient yourself in a micrograph of a tubular structure.

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.
a long tube (nephron) is drawn with different cuts along different planes. There are surrounding images to demonstrate what the tube would look like when cut along different planes.
The cross sectional views of a tube – The cross sectional views of a tube, taken from the sagittal (longitudinal), horizontal (transverse, axial), frontal (coronal), and oblique planes. Image created by Sarah Pinault under a CC BY 4.0 license.

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.
There is a blue bucket that is upright and a red bucket that is upside down with not bottom. There are 3 ants positioned: Ant A is between the ground and the blue bucket's bottom. Ant B is between the upside down mouth of the red bucket and the ground. And Ant C is on the rim of the blue bucekt
Ants positioned near buckets – Ant A is between the ground and the upright blue bucket’s bottom. Ant B is between the ground and the mouth of the upside down red bucket. Ant C is on the rim of the upright blue bucket

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

Transitional epithelial tissue stained with H&E, making the nuclei of each cell a deep purple, surrounded by cytoplasm of differing shades of pink/purple. The apical surface is highlighted in blue, next to the empty space of the lumen. The basal surface is the bottom of the image as the nuclei of deeper tissue structures is obvious.
Transitional Epithelial bladder tissue stained with H&E – The apical surface is highlighted in blue, next to the empty space of the lumen. The basal surface is the bottom of the image as the nuclei of deeper tissue structures is obvious. (Slide ID: Path 304 022, Image ID: 1548 – Bladder, Guinea pig)

 

Pseudostratified Epithelial tissue stained with H&E, making the nuclei of each cell a deep purple, surrounded by cytoplasm of differing shades of pink/purple. The apical surface is highlighted in green and red, next to the empty space of the lumen. The basal surface is the left side of the image as the nuclei of deeper tissue structures is obvious.
Ciliated Pseudostratified Epithelial Tissue from Trachea – Epithelial lung tissue is stained with H&E, making the nuclei of each cell a deep purple, surrounded by cytoplasm of differing shades of pink/purple. The apical surface is highlighted in green with specialized apical features outlined in red. The apical surface is next to the empty space of the lumen on the right of the image. The basal surface is the left of the image as the nuclei of deeper tissue structures is obvious. (Slide ID: Path 304 032, Image ID: 1559 – Trachea & Esophagus, pig)

Key Takeaways

  • Histology requires interpreting 2D sections of 3D objects. The plane of section (transverse, sagittal, coronal) determines the shape you see.
  • Use gross anatomical directions (anterior, superior, etc.) only when the body’s overall orientation is clear.
  • For epithelial tissue, always use intrinsic landmarks: the apical surface faces the lumen, and the basal surface faces the basement membrane.
  • Do not assume the top of the image is “up” in the body. Rotate your mental model to align the lumen correctly.

 

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Pathology Copyright © 2022 by Jennifer Kong, Zoe Soon, and Helen Dyck is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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