Biology Lecture: Tissues, Junctions, and Membranes
Review of Cellular Principles and Osmosis
Osmosis is defined by the movement of water following the solute. Water moves toward the area with a higher concentration of solute.
Scenario: A cell with clinical internal solute concentration of is placed in a beaker with a solute concentration of .
Because the concentration of solute is higher outside the cell, water inside the cell moves out.
The resulting loss of water causes the cell to shrivel like a raisin.
This environment is known as a hypertonic solution.
Translation and Codons:
A codon is a set of three nucleotides that the ribosome reads to add specific amino acids to a protein chain.
The first codon added during translation (the start codon) is AUG.
Introduction to Biological Tissues
A tissue is defined as a group of cells that work together to perform the same function. These groups can be organized or, in some specific cases, unorganized.
All tissues in the body share a common embryonic origin, meaning they develop from the same precursor cells in the embryo that divide and migrate to various locations.
Main Tissue Types vs. Tissue Subtypes:
Main Tissue Type: Refers to the four primary categories: Connective, Nervous, Epithelial, and Muscular.
Tissue Subtype: Refers to more specific, niche classifications within those four main categories.
General Functions of the Four Main Tissue Types
Epithelial Tissue: Primary role is to line body surfaces, cavities, and glands (e.g., the skin).
Connective Tissue: Functions to connect different parts of the body and provide structural support.
Muscular Tissue: Responsible for movement.
Nervous Tissue: Primarily involved in sending and receiving electrical signals.
Cell Junctions
Intercellular junctions are structures that connect two cells together. There are four primary types:
Tight Junctions:
Act like a zipper, completely sealing off the intercellular space between cells.
They form a watertight seal that prevents substances from passing between cells.
Found in the stomach and the bladder to prevent leakage (e.g., preventing stomach acid from digesting internal body tissues).
Gap Junctions:
Act like a gate that can open or close.
They utilize proteins called connexons (also spelled as connections) that allow materials to move in and out between cells when open.
Desmosomes:
Function as anchoring points or "spot welds" between cells.
These connect to the cytoskeleton of the cell, extending deep into the cell structure.
They allow tissues to resist mechanical stresses such as twisting or pulling without ripping. They are found in the skin.
Hemidesmosomes:
Similar in structure to desmosomes but differ in location.
Instead of connecting two cells, they anchor the bottom of a cell to the basement membrane (the line at the bottom of a tissue layer).
Epithelial Tissue Characteristics
Epithelial tissue is organized in very close-knit sheets with cells packed tightly together.
It contains many junctions and is always anchored to a basement membrane.
Avascularity: Epithelial tissue is avascular, meaning it has no direct blood supply or blood vessels. It must obtain oxygen and nutrients through diffusion from underlying tissues.
Classification of Epithelial Tissue
Classification is based on two factors: the shape of the cells and the number of layers.
Shape Classifications:
Squamous: Flattened or squished cells.
Cuboidal: Round or cube-shaped cells.
Columnar: Tall, rectangular cells, resembling columns; taller than they are wide.
Layer Classifications:
Simple: A single layer of cells.
Stratified: Multiple layers of cells.
Epithelial Subtypes and Locations
Simple Squamous Epithelium:
A single row of thin, flat cells forming a thin barrier.
Allows substances to diffuse easily. Found in the alveoli (air sacs) of the lungs for gas exchange (oxygen and carbon dioxide).
Stratified Squamous Epithelium:
Multiple layers of flattened cells designed for protection.
Found in the skin, mouth, esophagus, and vagina.
Keratinized: Includes a layer of dead cells that flake off (shedding), such as the skin.
Non-keratinized: No shedding of dead cell layers. Found in the mouth and vagina.
Simple Cuboidal Epithelium:
A single row of cube-shaped cells.
Effective for secreting substances like mucus. Found in the liver, mammary glands, and salivary glands.
Stratified Cuboidal Epithelium:
Multiple layers of cube-shaped cells.
Involved in the production of sweat, hormones, and sperm. Found in sweat glands, ovaries, and seminiferous tubules in the testes.
Simple Columnar Epithelium:
A single row of tall, narrow cells. Often features microvilli (hair-like projections) on the top surface to move substances (mucus, cerebrospinal fluid).
Functions in secretion and absorption. Found in the respiratory tract, digestive tract (nutrient absorption), kidneys, and uterus.
Pseudostratified Columnar Epithelium:
"Pseudo" means fake; it appears stratified because nuclei are at different levels, but every cell reaches the basement membrane, making it a single layer.
Primarily found in the respiratory tract and parts of the urethra.
Transitional Epithelium:
Cells have the unique ability to change shape, transitioning from round/cuboidal to flat/squished.
Found in the bladder and ureters. As the bladder fills with urine, the cells flatten to allow for expansion while tight junctions prevent urine leakage.
Connective Tissue Characteristics
Connective tissue is diverse and has the most subtypes of the four main types.
Structure: Cells are spread far apart, separated by a space called the matrix.
Vascularity: Generally very vascular (rich blood supply), with the notable exception of cartilage.
Fibers within the Matrix:
Collagen: Provides strength to the tissue and skin.
Reticular: Net-like fibers that form a framework (similar to chicken wire).
Elastic: Provides stretch and allows tissues to "bounce back" to their original size.
Connective Tissue Subtypes
Fibrous Connective Tissue:
Loose Areolar: Highly vascular; found beneath the skin and between muscles. It provides blood and nutrients to the avascular epithelial tissue above it via diffusion.
Loose Reticular: Forms the structural framework (stroma) for lymphatic organs, including lymph nodes, spleen, thymus, and bone marrow.
Dense Regular: Fibers are organized, parallel, and nicely lined up. Found in tendons and ligaments.
Dense Irregular: Fibers are densely packed but unorganized (swirls and waves). This allows the tissue to withstand unpredictable stresses from many directions. Found deep in the skin layers.
Adipose Tissue (Fat):
Appears like soap bubbles under a microscope. Each cell contains a large lipid droplet that pushes the nucleus and organelles against the plasma membrane.
Used for insulation, cushioning, energy storage, and heat production (especially in infants).
Cartilage:
The only avascular connective tissue. It is stiff and provides shape and protection.
Cells are called chondrocytes and reside in small spaces within the matrix.
Hyaline Cartilage: Very stable and rigid; resists bending. Found in joints, ribs, trachea (windpipe), and larynx (voice box).
Fibrocartilage: Contains many fibers; resists compression and shock. Found in intervertebral discs.
Elastic Cartilage: Contains elastic fibers for flexibility. Found in the ear and epiglottis.
Bone:
Organized into concentric circles (rings), similar to a cut tree trunk. Cells are very close with no visible space between them.
Compact Bone: Densely packed with no spaces.
Spongy Bone: Contains spaces within the bone tissue.
Blood:
Consists of red blood cells, white blood cells, and platelets.
The matrix of blood is the liquid plasma.
Body Membranes
Membranes are composed of epithelial and connective tissues. There are four types:
Mucous Membranes: Line openings to the outside (mouth, nose).
Serous Membranes: Line internal cavities (pleura, pericardium, peritoneum).
Cutaneous Membrane: The skin.
Synovial Membranes: These contain only connective tissue (no epithelial tissue) and are found in joint cavities.
Glandular Epithelium and Secretion
Glands are made of epithelial tissue and come in various shapes (tubular, branched).
Exocrine Glands: Use a duct (passageway) to secrete products typically outside the body or into a cavity (e.g., sweat, tears, oil).
Endocrine Glands: Ductless glands that secrete products (hormones) directly into the bloodstream (e.g., thyroid, pituitary, adrenal glands).
Methods of Glandular Secretion
Eccrine (Merocrine) Secretion:
The cell packages products into a vesicle using the Golgi apparatus.
The vesicle fuses with the membrane and releases the product via exocytosis.
Apocrine Secretion:
The cell releases its product by breaking off a portion of the cell itself.
Found in mammary glands (milk contains parts of the mammary cells).
Holocrine Secretion:
The cell produces products until it is full and then explodes, releasing the product along with dead cell fragments.
Found in oil (sebaceous) glands of the skin and hair.
Questions & Discussion
How can one test for the avascularity of epithelial tissue?
If a safety pin is stuck through the very top layers of the skin and it does not bleed, you have only pierced the epithelial tissue. If it bleeds, you have reached the underlying vascular areolar connective tissue.
What are the identification requirements for the exam?
There are approximately three identification questions.
Students should specifically be able to recognize what skin looks like under a microscope.
Students must distinguish between a "main tissue type" (e.g., Epithelial) and a "tissue subtype" (e.g., Simple Squamous) when answering identification questions.
What is the scheduled time for the return from break?
The lecture resumes at .