Cell Structure and Tissue Organization: A Study Guide for Anatomy & Physiology

The Cell

Introduction to the Cell

The cell is the fundamental structural and functional unit of all living organisms. It contains various organelles and structures that perform specialized functions necessary for life.

• Organelles are specialized subunits within a cell, each with a specific function.

• Cells are surrounded by a plasma membrane that regulates the movement of substances in and out.

• The cytoplasm is the jelly-like substance within the cell, containing organelles and cytosol.

Cytoskeleton

Structure and Function of the Cytoskeleton

The cytoskeleton is the internal framework of the cell, providing structural support, shape, and facilitating movement. It is a dynamic network of protein filaments and tubules extending throughout the cytoplasm.

• Microtubules: Hollow tubes made of tubulin proteins, about 25 nm in diameter. They maintain cell shape, form the mitotic spindle during cell division, and serve as tracks for organelle movement.

• Microfilaments: Thin filaments composed of actin, about 7 nm in diameter. They support the cell membrane, aid in cell movement, and are involved in muscle contraction.

• Intermediate Filaments: Rope-like fibers, about 10 nm in diameter, providing mechanical strength and helping anchor organelles.

Example: The cytoskeleton enables white blood cells to change shape and move toward sites of infection.

Centrioles

Structure and Role of Centrioles

Centrioles are small, cylindrical organelles found near the nucleus in animal cells. They play a crucial role in organizing microtubules during cell division and in the formation of cilia and flagella.

• Each centriole is composed of a wheel array of nine triplets of microtubules.

• Centrioles are located in the centrosome, the microtubule-organizing center of the cell.

• They help organize the mitotic spindle during mitosis, ensuring proper chromosome separation.

• Centrioles also form the bases of cilia and flagella, structures involved in cell movement.

Example: During mitosis, centrioles duplicate and move to opposite poles of the cell to help organize the spindle fibers.

Cytoplasmic Organelles

Types and Functions of Organelles

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions. They are classified as membranous or nonmembranous.

• Membranous Organelles: Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum (ER), and Golgi apparatus.

• Nonmembranous Organelles: Cytoskeleton, centrioles, and ribosomes.

Example: The mitochondria generate ATP, the cell's main energy currency.

Endosymbiont Theory

Origin of Eukaryotic Organelles

The endosymbiont theory explains how certain organelles, such as mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Mitochondria and chloroplasts have their own DNA and ribosomes, supporting this theory.

• These organelles replicate independently of the cell cycle.

Example: Mitochondria are believed to have evolved from aerobic bacteria.

Nucleus

Structure and Function of the Nucleus

The nucleus is the control center of the cell, containing most of the cell's genetic material and coordinating activities such as growth, metabolism, and protein synthesis.

• Enclosed by a nuclear envelope, a double membrane with nuclear pores for molecular transport.

• Contains nucleoli (sites of ribosome assembly) and chromatin (DNA and associated proteins).

• Controls gene expression and mediates the replication of DNA during the cell cycle.

Nuclear Envelope

• Double membrane structure with nuclear pores for selective exchange of materials.

• Outer membrane is continuous with the rough endoplasmic reticulum (RER).

• Lined by the nuclear lamina, which maintains nuclear shape.

Nucleolus

• Dense, spherical body within the nucleus.

• Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Endoplasmic Reticulum (ER)

Types and Functions of ER

The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis. It is classified as rough or smooth based on the presence of ribosomes.

• Rough ER (RER): Studded with ribosomes; synthesizes proteins for secretion, membrane insertion, or lysosomes.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Example: In liver cells, the SER detoxifies drugs and metabolizes lipids.

Ribosomes

Structure and Function of Ribosomes

Ribosomes are small granules composed of protein and ribosomal RNA (rRNA). They are the sites of protein synthesis.

• Free ribosomes synthesize proteins for use within the cytosol.

• Membrane-bound ribosomes (on RER) synthesize proteins for membranes, lysosomes, or export.

Golgi Apparatus

Structure and Function of the Golgi Apparatus

The Golgi apparatus is a stack of flattened membranous sacs responsible for modifying, concentrating, and packaging proteins and lipids for secretion or delivery to other organelles.

• Receives transport vesicles from the ER at the cis face.

• Processes and sorts proteins, then dispatches them from the trans face in secretory vesicles.

Peroxisomes

Structure and Function of Peroxisomes

Peroxisomes are small, membrane-bound organelles containing enzymes such as oxidases and catalases. They detoxify harmful substances and neutralize free radicals.

• Oxidases break down fatty acids and amino acids, producing hydrogen peroxide.

• Catalases convert hydrogen peroxide into water and oxygen.

• Protect cells from oxidative damage.

Lysosomes

Structure and Function of Lysosomes

Lysosomes are spherical organelles containing digestive enzymes. They break down ingested materials, old organelles, and cellular debris.

• Digest bacteria, viruses, and toxins.

• Break down nonfunctional organelles and tissues.

• Release calcium from bone when needed.

Tissues

Overview of Tissues

Tissues are groups of similar cells that perform a common function. The human body has four primary tissue types, each with specialized roles.

• Epithelium: Covers body surfaces and lines cavities.

• Connective tissue: Supports, protects, and binds other tissues.

• Nervous tissue: Transmits electrical impulses.

• Muscle tissue: Produces movement.

Development of Tissues: Germ Layers

During embryonic development, three primary germ layers form and give rise to all adult tissues.

• Ectoderm: Forms nervous tissue, integument, and some connective tissues.

• Mesoderm: Forms muscle, most skeletal tissues, cardiovascular and lymphatic systems.

• Endoderm: Forms linings of major internal organ systems.

Additional info: Epithelial tissue can arise from all three germ layers, while muscle and connective tissue arise from mesoderm, and nervous tissue from ectoderm.

Histological Preparation of Tissues

Steps in Tissue Processing

To study tissues under a microscope, they must be properly prepared through several steps:

1. Grossing: Selecting and trimming tissue specimens.

2. Fixation: Preserving tissue structure using chemicals like formalin.

3. Dehydration: Removing water using alcohol.

4. Clearing: Replacing alcohol with a substance like xylene.

5. Impregnation: Infiltrating tissue with paraffin wax.

6. Embedding: Placing tissue in a mold with wax to solidify.

7. Sectioning: Cutting thin slices for slides.

8. Staining: Applying dyes to highlight structures.

Epithelial Tissue

Characteristics and Functions

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is classified by cell shape and number of layers.

• Cells fit closely together, forming continuous sheets.

• Always has a free (apical) surface and a lower (basal) surface attached to a basement membrane.

• Avascular (lacks blood vessels) but regenerates easily if well nourished.

• Functions include protection, absorption, filtration, and secretion.

Naming Epithelial Tissues

Epithelia are named based on the number of cell layers and the shape of the cells at the apical surface.

1. Determine thickness: Simple (one layer) or Stratified (multiple layers).

2. Determine cell shape: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall).

3. Combine thickness and shape for the name (e.g., simple squamous epithelium).

4. Add special features (e.g., cilia, goblet cells) if present.

Classification of Epithelium

Type	Layers	Shape	Location/Function
Simple Squamous	1	Flat	Lines body cavities, lungs, capillaries; allows diffusion/filtration
Simple Cuboidal	1	Cube	Glands, ducts, kidney tubules, ovaries; secretion/absorption
Simple Columnar	1	Tall	Digestive tract; absorption, secretion (often with goblet cells)
Pseudostratified Columnar	1 (appears multi-layered)	Varied	Respiratory tract; secretion, movement of mucus (often ciliated)
Stratified Squamous	Multiple	Flat at surface	Skin, mouth, esophagus; protection from abrasion
Stratified Cuboidal/Columnar	Multiple	Cube/Column	Rare; ducts of large glands
Transitional	Multiple	Varies with stretch	Urinary bladder, ureters; allows stretching

Special Considerations in Epithelial Identification

• Always identify the lumen and basement membrane first.

• Staining techniques can alter the appearance of tissues.

• Transitional epithelium can be confused with other types due to its variable shape.

• Pseudostratified and stratified columnar epithelia may look similar; careful examination is needed.

• Always use the full name: thickness + shape + epithelium + accessory structures (if any).