CH4 - TISSUES PT1 -Epithelial Tissue (Epithelium)
Tissues: Overview
Tissues are groups of cells that work together with a common function.
Organs in the body are composed of four major tissue types: epithelial tissue (epithelium), connective tissue, muscle tissue, and nervous tissue.
Example: organs like the stomach, heart, and intestines are made up of these four tissues.
The study of tissues is called histology.
In the lab, you will perform histology drawings to depict what you see under the microscope (cell shapes, organization, and how they fit together).
This material is fundamental to understanding anatomy and physiology and will be referenced across topics.
Epithelial Tissue (Epithelium): Introduction
Epithelium is the tissue that covers surfaces or lines cavities.
Major function: protection; forms a barrier from the outside to the inside of the body.
Epithelial tissue has a free surface and a basal surface; the basal surface is attached to a basement membrane.
Epithelia are avascular (they lack blood vessels).
They regenerate rapidly through mitosis, especially near the basement membrane where nutrients from underlying connective tissue are available.
Nerve endings can be present in epithelium, providing sensory functions (e.g., touch in the skin).
Glandular tissue is also made up of epithelial tissue and includes exocrine glands (secrete into ducts) and endocrine glands (secrete into the blood).
Key Cell Junctions in Epithelium
Gap junctions: allow direct communication between adjacent cells via a channel.
Desmosomes: weld-like connections that help cells resist separation and tearing.
Tight junctions: seal the space between cells, preventing fluid passage between them.
These junctions contribute to the tight packing and coordinated function of epithelial layers.
Basement Membrane and Its Functions
The basement membrane is a layer of proteins and glycoproteins.
It is secreted in part by epithelial cells and in part by connective tissue.
Function: anchors epithelial cells to the connective tissue below, providing strong adhesion.
It also serves as a scaffold for regeneration and mitosis, guiding cell division and tissue repair.
Not visible under light microscope at typical magnifications (e.g., around 400\times).
It helps regulate what substances can pass from the connective tissue to the epithelial cells.
Avascularity and Nutrient Supply
Epithelial tissues do not have their own blood vessels (avascular).
They receive nutrients from blood vessels in the underlying connective tissue.
The basement membrane helps regulate nutrient diffusion and provides a barrier to some substances.
The analogy example: a pin piercing the skin will bleed only if it reaches the connective tissue below the epithelium; superficial penetration through the epithelium alone does not reach blood vessels.
Regeneration and Nerve Endings
Epithelial tissues regenerate rapidly via mitosis.
Regeneration is most active near the basement membrane due to proximity to nutrients.
Some epithelia contain nerve endings, which contribute to sensory functions (e.g., touch receptors in the skin).
Glandular Epithelium
Glandular tissue is epithelial tissue.
Exocrine glands secrete into ducts and onto surfaces; endocrine glands secrete directly into the bloodstream.
In exocrine glands, glands and ducts are composed of epithelial tissue; ducts are often lined by simple cuboidal epithelium.
Modes of exocrine secretion:
Merocrine (or merocrine): secretion via exocytosis with little or no loss of cytoplasm. Example: sweat glands.
Apocrine: secretory product accumulates at the apex, which then pinches off, releasing part of the cell along with the secretion. Example: mammary glands producing milk.
Holocrine: the whole cell fills with secretion and disintegrates, releasing the product; the cell is replaced by a new one from below. Example: sebaceous (oil) glands.
Secretion involves membranes and cytosol; holocrine secretion involves whole-cell loss and replacement.
Some exocrine glands are more prone to infection due to the volume and composition of their secretions (e.g., lipids in holocrine glands like sebaceous glands).
Unicellular Glands: Goblet Cells
Goblet cells are unicellular glands that produce mucus; they are named for their goblet-like shape.
Not every mucus-secreting cell is a goblet cell, but goblet cells are common in various epithelia, especially in pseudostratified and columnar types.
Goblet cells are widespread in the respiratory tract (lining with ciliated pseudostratified epithelium) and in the digestive tract (where mucus aids in lubrication and passage of material).
The mucus produced by goblet cells traps debris and protects underlying tissues.
Epithelial Tissue Naming Convention
Epithelia are described by two main criteria:
Shape of the cells at the free surface: columnar, cuboidal, or squamous.
Number of cell layers: simple (one layer) or stratified (more than one layer).
Examples:
Simple columnar: tall, column-shaped cells; nucleus typically near the basal region; lines many ducts and the digestive tract; functions in secretion and absorption.
Simple cuboidal: cube-shaped cells; lines kidney tubules; functions in secretion and absorption.
Simple squamous: flat, one-layered cells; excellent for diffusion; lines alveoli of lungs and blood vessels (endothelium when lining vessels).
Stratified squamous: multiple layers of flat cells at the surface; provides strong protection; found in mouth, esophagus, vagina, and anus.
Stratified cuboidal/columnar: less common; found in certain ducts.
Special cases to know:
Pseudostratified: appears multi-layered due to nuclei at different levels, but every cell contacts the basement membrane; often ciliated and contains goblet cells; lines the respiratory tract.
Transitional epithelium: cells change shape as the organ stretches; seen in the bladder; top layer can appear squamous when stretched or more columnar when relaxed.
Special notes on location and reasoning:
Endothelium: simple squamous epithelium that lines the inside of blood vessels.
Mesothelium: simple squamous epithelium lining the abdominal and thoracic cavities (peritoneum, pleura, and pericardium).
The two-word naming is typically: shape + layer type (and, for stratified tissues, the shape of the cells at the free surface).
Common Epithelial Tissue Locations and Functions
Simple squamous: diffusion and filtration; locations include alveolar air sacs in the lungs and the inner lining of blood vessels.
Endothelium (special case): lines blood vessels.
Mesothelium (special case): lines body cavities such as the peritoneum, pleura, and pericardium.
Simple cuboidal: absorption and secretion; locations include kidney tubules.
Simple columnar: absorption and secretion; locations include the digestive tract lining.
Stratified squamous: protection in areas subject to abrasion; locations include mouth, esophagus, vagina, and anus.
Stratified other shapes (rare): stratified cuboidal and stratified columnar occur in certain ducts.
Transitional epithelium: stretchability; location includes the bladder and portions of the ureters.
Summary: Why This Matters
Epithelium forms protective barriers, enables selective transport (absorption and secretion), and lines many organs and cavities.
The arrangement (simple vs stratified) and the surface shape (columnar, cuboidal, squamous) determine function and location.
Glandular EP tissue forms glands; understanding the secretion modes explains how different glands release their products and why some glands are more prone to infection.
The basement membrane and avascularity are key constraints and enablers of epithelial function and tissue repair.
Quick Visual Checks (to help with histology drawings)
Simple columnar: tall cells, nucleus near base, often with goblet cells.
Simple cuboidal: cube-shaped cells, centrally placed nucleus.
Simple squamous: flat cells, single layer, nucleus often bulging in the center.
Pseudostratified: appearance of multiple layers, but all cells touch the basement membrane; often ciliated and mucus-secreting (goblet cells).
Transitional: layers can vary in shape; top surface can appear cuboidal or squamous depending on stretch.
Stratified squamous: multiple layers that become progressively flattened toward the surface.
Gland ducts: typically lined with simple cuboidal epithelium in ducts; glandular cells on the side.
Lab and Study Notes
You will be drawing tissues and noting cell shapes, layer organization, surface features, the basement membrane, and the relationship to underlying connective tissue.
Remember the functional reasons behind tissue locations: diffusion in squamous epithelia (lungs, vessels), protection in stratified squamous (mouth, esophagus, vagina, anus), absorption/ secretion in columnar and cuboidal epithelia (digestive tract, kidneys).
Always connect the tissue type to its location and function for easier recall.