HUMAN TISSUES

The human body is composed of four levels of organization: cells, tissues, organs and body systems.
Cells of similar structure & function are organized into tissues.
There are four primary types of tissue in the human body:
epithelial
connective
muscle
neural.

Epithelial Tissue

Epithelial tissue covers most internal & external surfaces of the body. In addition, epithelial tissue also forms many glandular structures as well.
Epithelium consists almost exclusively of cells with very little extracellular material between them.
Epithelial tissue often has one free surface that is not associated with other cells and a basal surface which is attached to a basement membrane.
Epithelial tissue are avascular. Blood vessels do not penetrate the basement membrane to the reach the epithelium. Therefore, all gases, nutrients & waste products must diffuse across the basement membrane to & from blood vessels in the underlying connective tissue.

Classification of Epithelium

Epithelial tissue are classified according to the arrangement of cell layers & cell shape.
Arrangement of layers:
Simple epithelium- consists of a single layer of cells with each cell extending from the basement membrane to the free surface.
Stratified epithelium- consists of more than one layer of cells & only one of which is adjacent to the basement membrane.
Pseudostratified epithelium- consists of a single layer of cells that are all attached to the basement membrane yet only some of the cells reach the free surface.

Squamous- cells are flatten or scale-like.
Cuboidal- cells are cube-shaped and are similar in width & height.
Columnar- cells are tall and thin.

Function of Epithelial Tissu

Functionally, epithelial tissues play an important role in diffusion, filtration, secretion, absorption, protection, movement of mucus and are also capable of great stretching

Connective Tissue

Structure of Connective Tissue

The essential characteristic that separates connective tissue from the other three tissue types is the presence of an abundant amount of extracellular matrix or material between its cells.
The extracellular matrix has three major components:
Protein fibers (ie. Collagen, reticular & elastin)
Ground substances consisting of nonfibrous proteins and other materials

Classification of Connective Tissue
The extracellular matrix is the basis for the classification of connective tissue into the three following subgroups:
Matrix with mainly protein fibers (ie. Adipose)
Matrix with protein fibers and ground substance (ie. Bone)
Matrix with mainly fluid (ie. Blood)

Adipose Tissue
Structure: Consists of adipocytes which are specialized to store triglycerides & lipids. Cells are so full of lipids that the nucleus is pushed to the periphery of the cell.
Function: Thermal insulator, energy storage, supports & protects underlying structures.

Bone
Bone is a hard connective tissue that consists of living cells ( osteocytes) & mineralized matrix. Bone tissue is classified into two types:
Compact (Dense) bone
Structure: Hard bony matrix predominates with almost no space between the many thin layers of bone.
Function: Provides great strength & support, protects delicate structures and works in conjunction with skeletal muscles to generate movement.
Location: Outer portion of all bones and the shafts of long bones.

Blood

Structure: Various blood cells and a fluid matrix.
Function: Transports O2, CO2, hormones, nutrients & waste products. Protects the body from infections, and is involved in temperature regulation and the clotting process.

Muscle Tissue

Structure of Muscle Tissue
Muscle tissue consists of long, thin cells or fibers known as myocytes which are constructed to generate a force for contraction.
Based on location and certain structural & functional characteristics, muscle tissue is classified into three categories:

Smooth Muscle Tissue
Structure: Cells are spindle-shaped, nonstriated fibers with one centrally located nucleus.
Function: Regulates the size of organs, forces fluid through tubes, controls the amount of light entering the eye. Under involuntary control.
Location: Walls of hollow organs, glands & iris of the eye.

Skeletal Muscle Tissue
Structure: Cells are large, long, striated & cylindrical with multiple peripherally located nuclei in each cell.
Function: Movement of the body, posture & heat production. Under voluntary control.
Location: Attached to bones by tendons.

Cardiac Muscle Tissue

Structure: Cells are cylindrical, branching and are striated with a single, centrally located nucleus. Cells are connected to one another largely by specialized gap junctions called intercalated disks.
Function: Pumps blood. Under mostly involuntary control.
Location: Heart

Nerve Tissue

Structure of Nerve Tissue
Nerve tissue is composed of highly specialized tissue called neurons.
Neurons are composed of three major parts:
Cell body- contains the nucleus & is the site of general cellular function.
Dendrites- receives electric signals (action potentials) and then conduct these signals toward the cell body.
Axons- conducts action potentials away from the cell body.
Structure: Relatively large cells in a variety of shapes. Characterized mostly by cell processes.
Function: Conducts action potentials, store data and integrate & evaluate data.
Location: Primarily in the CNS yet some cell processes extend to all parts of the human body.