chapter 4 tissue review

Histology and Tissue Types

Definition and Major Tissue Types

Histology is the study of tissues, which are groups of similar cells working together to perform specific functions. The four major types of tissues in the human body are:

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nervous tissue

Example: Muscle tissue enables movement, while nervous tissue transmits signals throughout the body.

Embryonic Germ Layers and Tissue Origins

Germ Layers and Associated Tissues

During embryonic development, three germ layers give rise to all tissues:

• Ectoderm: forms epithelial and nervous tissues

• Mesoderm: forms muscle and connective tissues

• Endoderm: forms epithelial tissues

Additional info: Most organs are derived from combinations of these layers.

Sectioning Techniques

Types of Sections

To study tissues microscopically, specimens are cut in different planes:

• Longitudinal section

• Cross/transverse section

• Oblique section

Example: Cross sections are commonly used to view blood vessels.

Characteristics of Epithelial Tissue

General Features

Epithelial tissue covers body surfaces and lines cavities. Key characteristics include:

• Cellularity: tightly packed cells

• Desmosomes/Gap junctions: specialized cell junctions

• Basement membrane: supports epithelium

• Avascular: lacks blood vessels

• Regeneration: rapid cell turnover

Classification of Epithelium

Layers and Shapes

Epithelial tissues are classified by the number of layers and cell shape:

• Layers: Simple (one layer), Stratified (multiple layers)

• Shapes: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall, rectangular)

Types of Epithelium: Structure, Function, and Location

Simple Squamous Epithelium

• Structure: Single layer, flattened "egg fried" cells

• Function: Filtration, diffusion, osmosis, secretion

• Location: Air sacs of lungs, blood vessels

Simple Cuboidal Epithelium

• Structure: Single layer, cube-shaped cells

• Function: Secretion, absorption

• Location: Kidney tubules, glands

Simple Columnar Epithelium

• Structure: Single layer, tall rectangular cells

• Function: Secretion, absorption

• Location: Stomach, intestines, gallbladder

Stratified Squamous Epithelium

• Keratinized:

  • Structure: Apical surface lacks nuclei, filled with keratin

  • Function: Protection, hydration

  • Location: Outer layer of skin

• Non-keratinized:

  • Structure: Apical cells are nucleated

  • Function: Protection against abrasion, maintains moist surface

  • Location: Mouth, vagina, esophagus

Pseudostratified Epithelium

• Structure: Appears stratified, but all cells touch basement membrane

• Function: Protection, secretion, movement of mucus

• Location: Trachea, upper respiratory tract

Transitional Epithelium

• Structure: Multiple layers, basal cells cuboidal, apical cells dome-shaped

• Function: Stretching, protection

• Location: Urinary bladder

Connective Tissue: Types and Characteristics

Major Types

Connective tissue supports, binds, and protects organs. Four major types are:

• Loose connective tissue

• Dense connective tissue

• Cartilage

• Bone

• Blood

Common Characteristics

• Structure: ECM (protein fibers and ground substances)

• Function: Connecting, binding, support, protection, transport

Types of Connective Tissue: Structure, Function, and Location

Loose Connective Tissue

• Areolar: Fibroblasts, ground substance, all 3 fibers; loosely binds organs, holds tissue fluids; found around organs, binds skin to deeper organs

• Adipose: Adipocytes; fat storage, insulation, protection; found in breasts, hypodermis, yellow bone marrow

• Reticular: Reticular fibers; supports organs; found in red bone marrow, spleen

Dense Connective Tissue

• Dense regular: Parallel collagen fibers; tightly binds body parts; found in tendons, ligaments

• Dense irregular: Collagen fibers in multiple directions; tension, durability; found in dermis, periosteum of bone

• Elastic CT: Elasticity; found in vocal cords

Bone

• Structure: Osteoblasts, osteocytes, osteoclasts, ECM (collagen fibers & calcium phosphate crystals)

• Function: Support, protection, framework

• Location: Compact bone (shafts, beneath periosteum), spongy bone (ends of long bones)

Blood

• Structure: ECM (plasma), red and white blood cells

• Function: Transport nutrients, waste, gases, clotting

Nervous Tissue

Structure and Function

• Structure: Neurons, neuroglial cells

• Function: Support, protect, nourish neurons

• Location: Brain, nerves

Muscle Tissue: Types and Characteristics

Comparison Table

The three types of muscle tissue differ in structure, function, and location:

Tissue	Structure	Function	Location
Skeletal	Long, cylindrical, striated, multiple nuclei	Movement	Bones
Cardiac	Short, wide, branching, striated, intercalated discs, 1-2 nuclei	Pumps blood through the heart	Heart
Smooth	Thin, smooth, 1 nucleus, joined by gap junctions	Moves materials through internal organs	Walls of hollow organs (stomach, intestines)

Cellular Junctions

Types and Functions

• Tight junctions: Hold cells tightly, prevent substance leakage (found in blood vessels)

• Desmosomes: Button-like links, subject to stress (found in skin)

• Gap junctions: Small pores formed by protein channels, allow communication (found in cardiac muscle)

Glandular Epithelium

Classification and Secretion

• Exocrine glands: Use ducts, secrete onto epithelial surfaces (e.g., sweat, sebaceous, mammary glands)

• Endocrine glands: Secrete hormones into bloodstream

• Modes of secretion: Holocrine (cell rupture), Merocrine (exocytosis), Apocrine (cell apex pinches off)

• Branching: Unicellular (goblet cells), Multicellular (clusters of cells)

Membranes: Types and Locations

Cutaneous, Mucous, and Serous Membranes

• Cutaneous membrane: Keratinized stratified squamous & loose/dense irregular CT (skin)

• Mucous membrane: Epithelial layer, loose CT (digestive tract)

• Serous membrane: Simple squamous & loose CT (pleural, pericardial cavities)

Tissue Repair: Regeneration and Fibrosis

Processes and Steps

• Regeneration: Replacement of damaged/dead cells with new cells of the same type

• Fibrosis: Fibroblasts produce collagen to fill injury, forming scar tissue

Bone Repair Steps

1. Hematoma formation: Blood clots form at injury site

2. Soft callus formation: Fibroblasts and chondroblasts create soft callus

3. Hard callus formation: Osteoblasts form hard callus (primary bone)

4. Bone remodeling: Hard callus replaced with secondary bone

Additional info: Osteoblasts and chondroblasts are essential for bone healing and remodeling.