Module 4.1 Introduction to Tissues

Histology

Histology is the study of the normal structures of tissues. It is a foundational discipline in anatomy and physiology, providing insight into how tissues are organized and function within the body.

Types of Tissues

There are four primary tissue types in the human body, each defined by the kind and number of cells, the amount and composition of extracellular matrix (ECM), and their specific functions:

• Epithelial tissues (or epithelia)

• Connective tissues

• Muscle tissues

• Nervous tissues

The Extracellular Matrix (ECM)

The extracellular matrix (ECM) is composed of substances in a liquid, thick gel, or solid state that surround the cells of a tissue. The ECM consists of two main components:

• Ground substance

• Protein fibers

Three major protein fibers are found within the ECM:

• Collagen fibers

• Elastic fibers

• Reticular fibers

Cell Junctions

Cell junctions are connections between neighboring cells' plasma membranes, linked by integral proteins. The three major types are:

• Tight junctions

• Desmosomes

• Gap junctions

Module 4.2 Epithelial Tissues

Functions of Epithelial Tissues

Epithelial tissues are found on every internal and external body surface, acting as barriers between the body and the external environment or between organs and fluid-filled cavities. Their functions include:

• Protection

• Immune defenses

• Secretion

• Transport into other tissues

• Sensation

Components and Classification of Epithelia

• Epithelial tissues are avascular (lack blood vessels).

• There is little ECM between epithelial cells. ECM is found beneath the cells in a structure called the basement membrane, which has two components:

  • Basal lamina

  • Reticular lamina

• Epithelial tissue is classified based on:

  • Number of cell layers:

    • Simple epithelia

    • Stratified epithelia

  • Shape of the cells:

    • Squamous cells

    • Cuboidal cells

    • Columnar cells

Covering and Lining Epithelia

These are found on inner and outer body surfaces. Each cell shape can be found in varying thicknesses in broad, flat sheets called membranes (which include the basement membrane):

• Simple squamous epithelium

• Simple cuboidal epithelium

• Simple columnar epithelium

• Pseudostratified columnar epithelium

• Stratified squamous epithelium

• Stratified cuboidal epithelium

• Stratified columnar epithelium

• Transitional epithelium

Glandular Epithelia

A gland is a structure of epithelial origin that synthesizes and secretes a product from designated secretory cells. Glands are classified as:

• Endocrine glands

• Exocrine glands

  • The goblet cell is the most common unicellular gland.

  • Exocrine glands secrete via two methods:

    • Merocrine secretion

    • Holocrine secretion

Module 4.3 Connective Tissues

Functions of Connective Tissues

Connective tissues are a diverse group with various functions, including:

• Connecting and binding

• Support

• Protection

• Transport

Connective Tissue Proper

Also known as general connective tissue, this group is widely distributed in the body and connects tissues and organs. It is composed of the following cell types:

• Fibroblasts

• Adipocytes

• Phagocytes

• Other immune system cells

The four basic types of connective tissue proper are:

• Loose connective tissue

• Dense connective tissue (subdivided into:

  • Dense irregular connective tissue

  • Dense regular connective tissue

  • Dense regular elastic connective tissue

  )

• Reticular tissue

• Adipose tissue

Obesity is the condition of having excess adipose tissue in proportion to lean body mass.

Specialized Connective Tissues

These have more specific functions and include:

• Cartilage (with cell types: chondroblasts and chondrocytes; can be hyaline, fibrocartilage, or elastic cartilage)

• Bone (with cell types: osteoblasts, osteocytes, osteoclasts)

• Blood (with cell types: erythrocytes, leukocytes, platelets)

Module 4.4 Muscle Tissues

Muscle Tissue Structure and Function

Muscle tissues are specialized for contraction. The main component is the muscle cell or myocyte, which is excitable and can respond to electrical or chemical stimulation.

Types of Muscle Tissue

• Skeletal muscle tissue

• Cardiac muscle tissue (contains intercalated discs with gap junctions and tight junctions)

• Smooth muscle tissue

Module 4.5 Nervous Tissues

Structure and Function

Nervous tissue makes up most of the brain, spinal cord, and nerves. It is composed of:

• Neurons: cells capable of sending and receiving electrical messages

• Neuroglial cells: support neurons

Module 4.7 Membranes

Types of Membranes

Membranes are thin sheets of one or more tissues that line a body surface or cavity. Types include:

• Serous membranes (serosae): line body cavities and secrete serous fluid

• Synovial membranes: line cavities surrounding freely movable joints

• Cutaneous membranes: external surface of the body (skin)

• Mucous membranes: line body surfaces open to the outside

Module 4.7 Tissue Repair

Tissue Repair Processes

• Regeneration: damaged or dead cells are replaced with cells of the same type

• Fibrosis: fibroblasts fill in gaps left from injury with collagen, forming scar tissue

The ability of tissues to repair depends on their capacity for mitosis:

• Epithelial tissues typically undergo regeneration

• Stem cells can regenerate tissue

• Smooth muscle tissue usually regenerates; cardiac and skeletal muscle tissues generally heal by fibrosis

• Nervous tissue generally undergoes fibrosis