Introduction to Histology

The study of tissue, histology, is challenging for many learners. Not only are there new terms and countless examples of stained slides to view, there is an inherent skill in “visual literacy” that needs to be developed. As an educator to many students in many many health care fields, I have noticed that students mostly find challenges with recognizing cells and that a clump of those cells form patterns which can be interpreted with other nearby patterns forming a ‘message’ of sorts written into the tissue. This is similar to how one learns to read: recognizing the individual letters, then small clumps of letters to form short words, and then interpreting a string of short words in a sentence. Thus, this chapter is to help learners develop visual literacy by introducing various hints, tricks, and examples that have helped students Willie Wu (UBC – Interdisciplinary Oncology Program), Athena Li (UBC – Vancouver Bachelors of Medical Laboratory Sciences) and Sarah Pinault (UBC Okanagan – Bachelor of Science) learn and succeed in histology.

This chapter is structured based on how one approaches literacy – by gradual steps which build upon each other. Feel free to approach this chapter based on where your particular challenge is.

What is Histology?

When you look at your hand, you see skin, fingernails, and perhaps veins. But what if you cold zoom in, far beyond what your eye can see? You would discover a hidden universe of intricate structures that are perfectly organized to make your hands functional. This is the world of histology (from the Greek histos, meaning “tissue,” and logia, meaning “study”) – the study of the microscopic structure of biological tissues.

Histology is the essential bridge between two sciences that you may already know:

• Cell Biology: the study of individuals cells and their organelles
• Anatomy and Physiology: the study of organs and how they work

If cell biology gives us the bricks (cells), and physiology tells us what the house does (function), then histology shows us how the bricks are arranged into walls and rooms (tissues) to make that function possible. It answers the question:” How is this organize build to its job?”

Form Follows Function

Before we look at any cells, we must learn the most important rule in all of histology: Form Follows Function!

The shape/form of a cell, tissue, or organ is never random. It is a direct, elegant consequence of the job/function that it needs to perform.

• Think of a spoon and a knife. You can tell their function just by their shape. A spoon is shaped to scoop while a knife is shaped to cut.
• Thank of a neuron. It has long, branch-like extensions because its job is to carry electrical signals over long distances and connect to many other neurons. Its form is perfect for its function.
• What about a red blood cell? It is a tiny, smooth, concave disc. This shape is ideal for squeezing through the narrowest capillaries and carrying oxygen efficiently.

As you learn histology, never just memorize the shape. Always ask yourself:” What is this structure’s job, and how does this form make that job easier?” This mindset will help transform histology from memorization into a critical thinking and analysis.

The Four Basic Tissues

Imagine you are building a complex model. You do not start with thousands of unique pieces, instead you start with a few basic types of pieces, such as beams, plates, bricks and gears. The human body is built the same way! While there are over 200 different cell types, they are organized into just Four Basic Types of Tissues. Every organ in your body is a combination of these four tissues.

Now, let’s take a close look at each tissue type briefly:

1. Epithelial tissue: This is your body’s border control. If forms the lining of your digestive tract, the surface of your skin, the ducts of your glands and so many more. Its cells are tightly packed together, like tiles in a mosaic, which forms a protective barrier. It has a “top” (apical surface) that faces the outside world or a hollow space, and a “bottom” (basal surface) that attaches to a underlying foundation. This tissue type will be explain in detail: (link).
2. Connective tissue: This is your body’s support system. It is the most abundant and diverse tissue type. Its key feature is the extracellular matrix (ECM), which is a network of fibers and gel-like substances that the cells produce. The cells themselves are often few and far between, living within this matrix. Connective tissue includes everything from solid bone and flexible cartilage to the liquid blood flowing in your veins.
3. Muscle tissue: This is your body’s machinery. Made of long, excitable cells that can contract, muscle tissue is what allows you to move, pump blood, and digest food. Its function is clearly reflected in in its form: the cells are elongated (skeletal and cardiac muscle cells are striated and relatively long; smooth muscle cells are spindle-shaped and shorter) and packed with proteins (e.g. actin and myosin) that slide past each other to generate mechanical force.
4. Nervous tissue: This is your body’s communication network. Found in the brain, spinal cord and nerves, it is designed for rapid signalling. Neurons look unlike any other cells, with long projections (axons and dendrites) that allow them to send and receive messaged over long distances.

The Big Picture: From Tissues to You

These four tissues do not exist in isolation. They work together to form organs!

Let’s take your stomach as an example:

• Its inner lining is Epithelial Tissue that secretes acid and absorbs nutrients.
• Beneath the epithelium is the Connective Tissue that supports the epithelium and contains blood vessels.
• Surrounding the Connective Tissue is the layers of Muscle Tissue that churn the food.
• Woven throughout is Nervous Tissue that controls the muscle contractions and gland secretions.

The hierarchy of “Cells -> Tissues -> Organs -> Organ Systems” is the architectural organization of the human body. Histology gives you the perspective to see different levels of organization, revealing the beautiful and logic patterns that keep you alive!