Chapter 4The Tissue Level of Organization

What is a Tissue?

A tissue is a group of cells Common embryonic origin Function together to carry out

specialized activities Hard (bone), semisolid (fat), or liquid

(blood)Histology is the science that deals

with the study of tissues.Pathologist specialized in

laboratory studies of cells and tissue for diagnoses

4 Types of Tissues

Epithelial▪ Covers body surfaces and lines hollow organs,

body cavities, duct, and forms glands Connective▪ Protects, supports, and binds organs.▪ Stores energy as fat, provides immunity

Muscular▪ Generates the physical force needed to make

body structures move and generate body heat Nervous▪ Detect changes in body and responds by

generating nerve impulses

Development of Tissues

Tissues of the body develop from three primary germ layers: ▪ Ectoderm, Endoderm, and Mesoderm

Epithelial tissues develop from all three germ layers

All connective tissue and most muscle tissues drive from mesoderm

Nervous tissue develops from ectoderm

Cell Junctions

Contact points between the plasma membranes of tissue cells 5 most common

types:▪ Tight junctions▪ Adherens

junctions▪ Desmosomes▪ Hemidesmosomes▪ Gap junctions

Tight Junctions

Web-like strands of transmembrane proteins Fuse cells together Seal off passageways

between adjacent cells▪ Common in epithelial

tissues of the stomach, intestines, and urinary bladder

▪ Help to retard the passage of substances between cells and leaking into the blood or surrounding tissues

Adherens Junctions

Dense layer of proteins called plaque Resist separation of cells

during contractile activities

Located inside of the plasma membrane attached to both membrane proteins and microfilaments of the cytoskeleton

Transmembrane glycoproteins called cadherins insert into the plaque and join cells

In epithelial cells, adhesion belts encircle the cell

Desmosomes

Contain plaque and cadherins that extends into the intercellular space to attach adjacent cells together Desmosome plaque

attaches to intermediate filaments that contain protein keratin

Prevent epidermal cells from separating under tension and cardiac muscles cells from pulling apart during contraction

Hemidesmosomes

Resemble half of a desmosome Do not link adjacent

cells but anchor cells to the basement membrane

Contains transmembrane glycoprotein integrin

Integrins attach to intermediate filaments and the protein laminin present in the basement membrane

Gap Junctions

Connect neighboring cells via tiny fluid-filled tunnels called connexons Contain membrane

proteins called connexins

Plasma membranes of gap junctions are separated by a very narrow intercellular gap (space)▪ Communication of

cells within a tissue▪ Ions, nutrients, waste,

chemical and electrical signals travel through the connexons from one cell to another

Epithelial Tissues

Epithelial tissue consists of cells arranged in continuous sheets, in either single or multiple layers Closely packed and held tightly together Covering and lining of the body Free surface

3 major functions: Selective barrier that regulates the movement of

materials in and out of the body Secretory surfaces that release products onto the

free surface Protective surfaces against the environment

General Features of Epithelial Cells

Surfaces of epithelial cells differ in structure and have specialized functions Apical (free) surface ▪ Faces the body surface, body cavity, lumen, or

duct Lateral surfaces▪ Faces adjacent cells

Basal surface▪ Opposite of apical layer and adhere to

extracellular materials

General Features of Epithelial Cells

Basement membrane Thin double extracellular layer that serves as the

point of attachment and support for overlying epithelial tissue

Basal lamina▪ Closer to and secreted by the epithelial cells▪ Contains laminin, collagen, glycoproteins, and

proteoglycans Reticular lamina▪ Closer to the underlying connective tissue▪ Contains collagen secreted by the connective

tissue cells

Epithelial Cells

Epithelial Tissues

Own nerve supply Avascular or lacks its own blood supply Blood vessels in the connective tissue

bring in nutrients and eliminate waste High rate of cell division for renew and

repair Numerous roles in the body (i.e.

protection and filtration)Covering and lining epithelium

▪ Outer covering of skin and some internal organs

Glandular epithelium▪ Secreting portion of glands (thyroid, adrenal,

and sweat glands)

Covering and Lining Epithelium Normally classified according to:

Arrangement of cells into layers Shapes of cells

Covering and Lining Epithelium

Arrangement of cells in layers Consist of one or more layers depending on function Simple epithelium▪ Single layer of cells that function in diffusion, osmosis,

filtration, secretion, or absorption Pseudostratified epithelium▪ Appear to have multiple layers because cell nuclei at

different levels▪ All cells do not reach the apical surface

Stratified epithelium▪ Two or more layers of cells that protect underlying

tissues in areas of wear and tear

Different Types of Covering and Lining Epithelium

Cells vary in shape depending on their function Squamous▪ Thin cells, arranged like floor tiles▪ Allows for rapid passage of substances

Cuboidal▪ As tall as they are wide, shaped like cubes or

hexagons▪ May have microvilli▪ Function in secretion or absorption

Different Types of Covering and Lining Epithelium

Columnar▪ Much taller than they are wide, like columns▪ May have cilia or microvilli▪ Specialized function for secretion and

absorption Transitional▪ Cells change shape, transition for flat to

cuboidal▪ Organs such as urinary bladder stretch to

larger size and collapse to a smaller size

Simple Epithelium

Simple squamous epithelium Simple cuboidal epithelium Simple columnar epithelium (nonciliated

and ciliated) Pseudostratified columnar epithelium

(nonciliated and cilated)

Simple squamous epithelium

Single layer of cells that resembles a tiled floor on the surface▪ Nucleus is

centrally located and appears flattened oval or sphere

Found at sites for filtration or diffusion

Covering and Lining Epithelium

Endothelium The type of simple squamous that lines

the heart, blood vessels, and lymphatic vessels

Mesothelium The type of epithelial layer of serous

membranes such as the pericardium, pleura, or peritoneum

Unlike other epithelial tissue, Both are derived from embryonic mesoderm

Simple cuboidal epithelium

Cuboidal shaped cells Cell nuclei round and centrally located Found in thyroid gland and kidneys Functions in secretion and absorption

Simple columnar epithelium

Column shaped cells

Oval nuclei at near base

Nonciliated and ciliated

Nonciliated simple columnar epithelium

Contains columnar cells with microvilli at their apical surface and goblet cells▪ Secreted mucus

serves as lubricant for the lining of digestive, respiratory, reproductive and urinary tracts

▪ Also prevents the destruction of the stomach lining by acidic gastric juices

Ciliated simple columnar epithelium

Columnar epithelial cells with cilia at the apical surface▪ In respiratory tract,

goblet cells are interspersed among ciliated columnar epithelia

▪ Secreted mucus on the surface traps inhaled foreign particles. Beating cilia moves particles to the throat for removal by coughing, swallowing, or sneezing

▪ Cilia also moves oocytes to the uterine tubes

Covering and Lining Epithelium

Pseudostratified columnar epithelium Appears to have

several layers due to nuclei are various depths

All cells are attached to the basement membrane in a single layer but some do not extend to the apical surface

Ciliated cells secrete mucus and bear cilia

Nonciliated cells lack cilia and goblet cells

Stratified Epithelium

Two or more layers of cells Specific kind of stratified epithelium

depends on the shape of cells in the apical layer Stratified squamous epithelium Stratified cuboidal epithelium Stratified columunar epithelium Transitional epithelium

Stratified Squamous Epithelium

Several layers of cells that are flat in the apical layer▪ New cells are pushed up toward apical layer▪ As cells move further from the blood supply they dehydrate,

harden, and die Keratinized form contain the fibrous protein keratin ▪ Found in superficial layers of the skin

Nonkeratinized form does not contain keratin ▪ Found in mouth and esophagus

Stratified Cuboidal Epithelium

Fairly rare type of epithelium Apical layers are cuboidal Functions in protection

Stratified columnar epithelium

Also very uncommon Columnar cells in apical layer only Basal layers has shorten, irregular shaped cells Functions in protection and secretion

Transitional Epithelium

Found only in the urinary system Variable appearance In relaxed state, cells appear cuboidal Upon stretching, cells become flattened and appear

squamous Ideal for hollow structure subjected to expansion

Glandular Epithelium: Endocrine Glands

Secretions, called hormones, diffuse directly into the bloodstream

Function in maintaining homeostasis

Glandular Epithelium: Exocrine Glands

Secrete products into ducts that empty onto the surfaces of epithelium

Skin surface or lumen of a hollow organ Secretions of the exocrine gland include mucus,

sweat, oil, earwax, saliva, and digestive enzymes Examples of glands include sudoriferous (sweat)

glands

Structural Classification of Exocrine Glands

Multicellular glands are categorized according to two criteria: Ducts are branched or unbranched Shape of the secretory portion of the gland▪ Simple gland duct does not branch▪ Compound gland duct branches▪ Tubular glands have tubular secretory parts▪ Acinar glands have rounded secretory parts▪ Tubuloacinar glands have both tubular and

rounded secretory parts

Structural Classification of Exocrine Glands

Functional Classification of Exocrine Glands

End of Chapter 4, Part 1