Human Tissue & Histology Part 1: Organization of the Human Body

Human Tissue & Histology

Part 1: Organization of the Human Body

Tissue

Tissue: A group of similar cells (and cell products) that arise from the same region of the developing human embryo and work together to perform a specific task.

Primary Germ Layers are the rapidly dividing cells of a zygote organized into 3 layers that form epithelial tissue: Endoderm: The innermost germ layer; develops into

parts of the digestive and respiratory system. Mesoderm: The middle germ layer; develops into

bone, muscle, and connective tissue. Ectoderm: The outermost germ layer; develops into

epithelium and nervous tissue

Types of Tissue

Four basic types of tissue: Epithelial: Covers surfaces or lining tubes and hollow

organs in the body, also forms glands Connective: Provides support, protection, and fills

spaces. Helps bind organs together; Stores energy reserves; Useful

in providing immunity

Muscular: When stimulated by nerve impulses, produces movement.

Nervous: Neurons initiate nerve impulses. Neuroglia provide support to neurons.

Epithelial Tissue

Epithelium: Cells tightly arranged in continuous sheets with little intracellular space.Form boundaries between the body’s organs

or protects the body from the external environment

High rate of cell division helps keep the tissue in constant repair

TWO primary categories: Covering & Lining Epithelium Glandular Epithelium

Covering & Lining Epithelium

Forms the outer covering of skin and some organs

Forms the lining of blood vessels, ducts, and body cavities

Lines the interior of most of the body’s systems (respiratory, digestive, reproductive, etc.)

Further subdivided by layer and cell type.

Covering & Lining Epithelium: Layers

3 Layers: Simple: A single layer of cells that functions

in diffusion, osmosis, filtration, secretion, and absorption

Stratified: Composed of two or more layers of cells that protect underlying tissues in locations where there is considerable wear and tear.

Pseudostratified: Contains a single layer of cells; not all cells reach the apical surface and are either ciliated or secrete mucous

Covering & Lining Epithelium: Cell Shapes

Cell Shapes: Squamous: Thin and tile-likeCuboidal: Cube shaped.Columnar: Column shapedTransitional: Change shape

Covering & Lining Epithelium in the Human Body

Simple Squamous Epithelium: Lines the heart and blood vessels (endothelium), air sacks of the lungs, and serous membranes (mesothelium)


Simple Cuboidal Epithelium: Used primarily in secretion and absorption


Simple Columnar Epithelium: Found in the lining of the gastrointestinal tract (nonciliated) and respiratory tract (ciliated)


Stratified Squamous Epithelium: Lines wet surfaces like mouth, epiglottis and vagina (non-Keratinized); superficial skin layer (Keratinized), ureters, and urethra.


Transitional Epithelium: Lines urinary bladder to permit distension

Glandular Epithelium

Glandular Epithelium: The primary function is sectretion.Gland: Any collection of cells that secrete

substances either into the blood, onto the surface, or into the ducts. Two major categories: Endocrine and Exocrine

Endocrine Glands

Endocrine Glands: Secrete hormones directly into the bloodstream to regulate body activities.

Examples:Pituitary GlandAdrenal GlandThyroid Gland

Exocrine Glands

Exocrine Glands: Secrete various materials such as mucous, saliva, and sweat into ducts that then empty onto the surface of a covering or lining epithelium.

Two major categories: Unicellular (like goblet cells) or Multicellular (like sebaceous or oil glands)

Means of Secretion: Merocrine Glands: Use exocytosis to secrete materials from

cells and release secretions synthesized by ribosomes. Ex. Salivary pancreatic glands.

Apocrine Glands: Accumulate secretions on the surface of the cell which are then “pinched off”. Ex. Mammary glands

Holocrine Glands: Accumulate secretions in the cytosol until the entire cell ruptures to release it. Ex. Sebaceous glands on skin.

Epithelial Membranes

Epithelial Membrane: Combination of an epithelial layer and an underlying connective tissue layer. 3 Main Types: Mucous Membranes (Mucosa): Lining of the body

cavities that open to the outside of the body. E.g. lining of the entire digestive, respiratory, and

reproductive tracts. Serous Membranes (Serosa): Cover the organs &

the cavities that contain them Cutaneuous Membranes: Skin!

Synovial Membranes: Lines the cavities of movable joints, secreting a fluid that lubricates the cartilage covering the bones at such joints.

Connective Tissue

Connective Tissue: Supports, strengthens, and binds together the other tissues. Primary site of stored energy (adipose tissue) and

immune response Provides the body its main transport system (blood) Often has a rich blood supply (with some exceptions)

and generally consists of few cells scattered throughout an intercellular matrix or protein fibers and a ground substance which binds the cell together.

Connective Tissue

Connective tissue arises from embryonic cells whose name ends in –blast and –cyte

BLAST cells have a high capacty for cell division and are responsible for making the matrix

Once the matrix is formed, the blast cells mature into CYTE cells designed to maintain the matrix.

6 Types of Connective Tissue

6 Types of Connective Tissue: FibroblastsMacrophagesPlasma CellsMast CellsAdipocytesWhite Blood Cells

6 Categories of Mature Connective Tissue

1. Loose Connective Tissue: Most common type found in the human body. Composed of areolar, adipose, and reticular tissues. Many cells present and fibers loosely intertwined.

1. Areolar: Forms subcutaneous layer that attaches skin to underlying tissues.

2. Adipose: Stores triglycerides (fat). Provides insulation and a source of energy reserves.

3. Reticular: Forms the supporting framework of the liver, lymph nodes, and spleen and binds smooth muscle cells together.


2. Dense Connective Tissue: Has thicker fibers and fewer cells than loose tissue. Due to toughness and pliability, forms tendons and ligaments, providing strong points of attachment. Subdivided into 3 kinds:

1. Dense Regular Connective Tissue: Tissue is tough, silvery-white and pliable – forms ligaments and tendons; contains regularly arranged bundles of collagen fibers

2. Dense Irregular Connective Tissue: Occurs where forces pull from multiple directions; forms pericardium around the heart; heart valves and the dermis of skin; contains closely collagen fibers, irregularly arranged.

3. Elastic Connective Tissue: Easily resumes normal shape after being stretched – makes up lung tissue and walls of elastic arteries; contains branching elastic fibers.


3. Cartilage: Made up of tightly packed collagen and elastic fibers in a rubbery substance known as chondroitin sulfate. Provides support for ears, nose, and trachea, intervertebral disks; makes up the embryonic skeleton. 3 Types of Cartilage:

1. Hyaline Cartilage: Most abundant and weakest type in human body; provides smooth surface for movement at joints; located at articular cartilage & epiphyseal plates.

2. Fibrocartilage: Exists at kneecaps and the discs between the vertebrae to provide support & connectivity; strongest type.

3. Elastic Cartilage: Gives shape and support; found in epiglottis and external ear; provides strength and elasticity.

Hyaline

Fibrocartilage

Elastic


4. Bone Tissue: Known as osseous tissue; composed primarily of collagen and calcium phosphate (provides durability); divides into two categories:

1. Compact Bone: Composed of osteons

2. Spongy Bone: Lacks osteons

Compact Bone

Spongy Bone


5. Blood Tissue: Liquid matrix is blood plasma with formed elements suspended in it.


6. Lymph: Extracellular fluid flowing in lymphatic vessels. Serves a variety of functions.

Muscle Tissue

Muscle Tissue: Consists of muscle fibers that use ATP to generate movement & heat. 3 classifications: Skeletal Muscle Tissue: Striated tissue attaches to bones of

the skeleton, usually by tendons, & is used to produce movement & heat. Under voluntary control.

Cardiac Muscle Tissue: Forms most of the heart wall & tissue; responsible for pumping blood; under involuntary control.

Cardiac muscle cells are branched & attach to each other by thick circles of plasma membrane known as intercalculated discs

Smooth Muscle Tissue: Spindle-shaped fibers; under involuntary control; found along the hallow internal structures, mainly blood vessels, intestines, stomach, uterus, airways, etc. Primarily responsible for moving materials through these structures.

Skeletal

Cardiac

Smooth

Nervous Tissue

Nervous Tissue: Senses stimuli & converts them into nerve impulses or signals to different parts of the human body. Two kinds of nerve cells: NeuronNeuroglia

Neuron

Neuron: AKA Nerve cell; responsible for converting stimuli into impulses. Impulses travel only one direction along the cell.

Made up of 3 Major Parts: Cell Body: Contains the nucleus and organelles Dendrites: Multiple branched tapering projections that receive

impulses from other cells and move up toward the cell body. Axon: Single, cylindrical projection that carries the signals away

from the cell body and to the next neuron or other tissue. Nerve impulse always moves from the receiving

dendrite, through the cell body, and out the axon to the axon terminals, synapse, and the next dendrite.

Neuroglia

Neuroglia: AKA Glia. Comprise about half of the central nervous system & serve a variety of important roles.

More on these later!

Cell Junctions

Cell Junctions: Places where cells anchor to one another to form seals; some form channels that allow molecules & ions to pass from cell to cell.

5 Primary Types of Cell Junctions: Adherens JunctionsDesmonomesGap JunctionsHemidesmosomesTight Junction

Adherens Junctions

Adherens Junctions: Contain proteins on the inside of the cell membrane called plaque.Provides a strong attachment between

adjacent cells by means of cadherins (glycoproteins which bind together to anchor cells next to each other).

Desmosomes

Desmosomes: Like adherens junctions, they provide a means of holding two cells tightly togetherUse keratin filamentsFound in the epidermis (outer layer of the

skin) & between cardiac muscle cells

Gap Junctions

Gap Junctions: Create intercellular channels that permit passage of small molecules & ions from one cell to another.Essential for the spread of nerve & muscle

impulses In cardiac muscle & smooth muscle of the GI

tract as well as some synapses in the brain

Hemidesmosomes

Hemidesmosomes: Resemble desmosomes, though their primary responsibility is to anchor other cells to the basement membrane (the extracellular layer between epithelium & connective tissue).

Tight Junctions

Tight Junctions: Strands of proteins that fuse cell membranes together, thus controlling the passage of molecules & ions through the space in between cells. Found in the lungs & tissues lining the

stomach, intestines, and urinary bladder.

Tissue Repair

Tissue Repair: The process by which damaged, worn, or dead cells are replaced. The rate & capacity for repairing damaged cells varies with the kind of tissue. Epithelial Tissue: Undergoes lots of wear & tear and

often has a high capacity for self-repair. Stem Cells: Immature, undifferentiated cells that can divide

to replace damaged cells. Sometimes mature, undifferentiated cells can divide to do the same.

Connective Tissue: Capacity for renewal depends a large part on the blood supply. Bone, for example, renews cells easily, whereas cartilage does not.

Muscle & Nervous Tissue: Have poor capacity for repair, mainly due t the lack of stem cells available.

Tissue Repair

Two primary types of tissue repair: Regeneration: Injured tissue is replaced through cell

division of the same type of cells (known as parenchyma).

Fibrosis: Fibroblasts synthesize collagen & other materials to form scar tissue.

If damage is extensive, both processes take place to create a growing connective tissue called granulation tissue which forms the stroma (framework) for new cells to move in & protects against bacteria.

Ailments That Prompt Tissue Repair

Cellulitis: A bacterial infection of the skin & underlying tissues.


Abscesses: Inflamed infection of tissues as a result of pus accumulation.


Ulcers: Open sores that result when inflamed tissue sloughs off from the surface of an organ.

Documents

Human Tissue & Histology Part 1: Organization of the Human Body