Histology Chapter 4 Introduction to Tissues

Histology

is the study of normal structures of tissues.

Tissues

are a group of structurally and functionally related cells and their external environment that together perform common functions.

All tissues share two basic components:

Consist of discrete population of cells that are related in structure and function

Have a surrounding material called

 extracellular matrix (ECM)

Four primary tissue types are defined by kind and number of cells, amount and composition of ECM, and their specific functions:

1. Epithelial tissues (epithelia), Connective tissues, Muscle tissues, Nervous tissues

Epithelial tissues 

1. tightly packed sheets of cells with no visible ECM; cover and line all body surfaces and cavities; specialized epithelia form glands that manufacture secretions such as sweat, saliva, or chemical messengers called hormones

Connective tissues

connect all other tissues in body to one another; ECM is a prominent feature for most connective tissue types with cells scattered throughout; bind, support, protect, and allow for transportation of substances

Muscle tissues

capable of generating force by contracting; little ECM between cells

Nervous tissues 

consist of cells capable of generating, sending, receiving messages, and cells that support this activity all within a unique ECM

Extracellular matrix

 composed of substances in a liquid, thick gel, or solid that surround cells of a tissue; consist of two main components, ground substance and protein fibers

ECM performs a variety of functions:

Provides tissue with strength to resist tensile (stretching) and compressive forces

Directs cells to their proper positions within a tissue and holds those cells in place

Regulates development, mitotic activity, and survival of cells in a tissue

Ground substance 

 makes up most of ECM and consists of extracellular fluid (ECF or interstitial fluid)

Glycosaminoglycans (GAGs)

 examples are chondroitin sulfate (small) and hyaluronic acid (enormous)

Proteoglycans 

are GAGs bound to a protein core (structure resembles bottle brush)

Thousands of proteoglycans bind to a very long GAG such as hyaluronic acid, forming huge proteoglycan “aggregates” 

Cell-adhesion molecules (CAMs) 

made up of different types of glycoproteins

Adhere cell to cell and cells to surroundings; hold everything in place within ECM 

CAMS bind to cell surface proteins as well as protein fibers and proteoglycans; maintain normal tissue architecture 

Protein fibers

 embedded within ground substance; long molecules composed of multiple fibrous subunits with a ropelike structure; enormous tensile strength; three protein fiber types are found within ECM

Collagen fibers,

make up 20–25% of all proteins in body; composed of multiple repeating subunits that form a white fibrous protein; resistant to tension (pulling and stretching forces) and pressure

Elastic fibers

composed of protein elastin surrounded by glycoproteins; extensibility allows fiber to stretch up to one and a half times resting length without breaking; once stretched, fibers return to resting length (called elasticity)

Reticular fibers

thin, short collagen fibers; form a meshwork or scaffold that supports cells and ground substance of many tissues; form a weblike structure in organs like spleen that help trap foreign cells

Cell junctions 

another way cells bind to one another where neighboring cell’s plasma membranes are linked by integral proteins; three major types of cell junctions are: Tight junctions 

1. Desmosomes 

2. Gap junctions 

Tight junctions

also known as occluding junctions, hold cells closely together such that space between is impermeable to movement of macromolecules

Seal may not be complete allowing for

 leakage in some tissues

 found between cells in blood vessels

 prevent blood from exiting vessels                                                                                                                                                                                                                                 

Desmosomes

composed of linking integral proteins; allow for materials in extracellular fluid to pass through space between cells

Increase strength of a tissue by holding cells together so mechanical stress is more evenly distributed

 intermediate filaments

Integral “linker” proteins are attached to…of cytoskeleton for structural reinforcement

Gap junctions 

are small pores formed by protein channels between adjacent cells that allow small substances to flow freely between each cell’s cytoplasm

Epithelial tissues 

found on every internal and external body surface; barriers between body and external environment; line organs and fluid-filled cavities; serve following additional functions

Protection Epithelial

 shield underlying tissues from mechanical and thermal injury

Immune defenses Epithelial

form physical barriers; prevent invasion by microorganisms; house cells of immune system enhancing protective function

Secretion

form glands that produce substances like hormones and oils; secreted into blood or through ducts respectively

Transport into other tissues

 selectively permeable membranes; certain substances are able to cross these barriers by passive or active transport and enter other tissues

Sensation

 most associated with a rich nerve supply; detects changes in internal and external environments; taste buds are examples of specialized sensory epithelial cells

tight junctions

Epithelial tissues consist of tightly packed cells linked together by…

desmosomes

Structural arrangement make sheets of cells fairly impermeable and resistant to physical stresses and mechanical injury

avascular 

Epithelial tissues are… limits their thickness

 basement membrane

ECM is found beneath cells in a thin

Basal lamina

 ECM synthesized by epithelial cells; consists of collagen fibers and ground substance

Reticular lamina

synthesized by underlying connective tissue; consists of reticular fibers and ground substance

Epithelial tissue 

• classified based on two criteria: number of cell layers and shape of cells in those layers; both have functional significance

 Simple epithelia

consist of a single cell layer

Stratified epithelia

consist of more than one cell layer

Squamous cells 

are flattened

Cuboidal cells

 are short

Columnar cells

are tall and elongated

Covering and lining epithelia

found on inner and outer body surfaces; each cell shape can be found in varying thicknesses in broad, flat sheets

Covering and lining epithelia

membranes, basement membrane

Simple epithelia

one cell-layer thick, adapted for transportation of substances between different tissues 

Some have microvilli 

 for increased surface area; some have cilia for moving substances through hollow organs

Simple epithelia

one cell-layer thick, adapted for transportation of substances between different tissues 

Simple squamous epithelium

very thin single layer of cells with a “fried egg” appearance; adapted for rapid diffusion of substances like oxygen, carbon dioxide, fluids, and ions; found in air sacs of lung, specific segments of kidney tubules, and lining blood vessels

Simple cuboidal epithelium

single layer of cube-shaped cells with large central nucleus; thin enough for rapid substance diffusion; found in segments of renal tubules, respiratory passages, ducts of many glands, and thyroid gland

Simple columnar epithelium 

a single layer of rectangular-shaped cells with nuclei located in basal portion of cell; often has microvilli on apical plasma membrane (drastically increases surface area for absorption of substances) or cilia 

Pseudostratified columnar epithelium

appears to be layered because nuclei are found at various heights, but only one cell-layer thick with basal plasma membranes firmly in contact with basement membrane; found in segments of respiratory tract and nasal cavity; ciliated

Stratified epithelium 

more than one layer of cells; best suited as protective barriers in locations subjected to high degrees of mechanical stress

Keratinized stratified squamous epithelium

apical cellular layers are
dead; lack nuclei; filled with protein keratin; makes tissue tough and resistant to friction; well adapted for outer layers of skin 

Nonkeratinized stratified squamous epithelium

apical cellular layers retain nuclei; still alive; found in regions subjected to mechanical stress where surface must remain moist; mouth, throat, esophagus, anus, and vagina

Stratified cuboidal epithelium

rare in humans, consists of two cell layers and lines ducts of sweat glands 

Stratified columnar epithelium 

also rare in humans; consists of only a few layers; apical cell layer is columnar and basal cell layer is cuboidal; found in male urethra, cornea of eye, and in ducts of certain glands like salivary glands

Transitional epithelium

only found in urinary system; lines interior of kidney, ureters, urinary bladder, and urethra; basal cell layers are cuboidal while apical cell layers are dome-shaped when tissue is relaxed; ability of apical cells to flatten contributes to ability of urinary tissues to stretch

Gland 

 structure of epithelial origin that synthesizes and secretes a product from designated secretory cells

Arise from epithelial tissue that migrated into deeper connective tissue instead of remaining at surface 

Products are released by two mechanisms:

Endocrine, Exocrine

Endocrine glands

secrete their products, usually hormones, directly into the bloodstream without the use of ducts: Allows products to have widespread systemic effects on distant cells in different areas of body

Exocrine glands

release products onto apical surfaces of epithelium located on external surface of body or lining a hollow organ that opens to outside of body:

Products, secreted from gland through an epithelial-lined duct, have only local effects on cells in general vicinity

Goblet cells

most common unicellular exocrine gland; found in digestive and respiratory tracts; secrete mucus, a thick sticky
liquid that protects
underlying epithelium

exocrine glands

 are multicellular glands made up of clusters of secretory cells arranged in different ways

Multicellular glands are classified according to structure of their ducts and shape of clusters of secretory cells

Duct structure – simple glands

ducts don’t branch

 compound glands

branched ducts

Clusters of secretory cells are arranged in three possible configurations

tubular

long and straight or coiled

 acinar

spherical

tubuloacinar

(duct structure) with both tubular and acinar sections

exocrine glands

are multicellular glands made up of clusters of secretory cells arranged in different ways

Multicellular glands are classified according to structure of their ducts and shape of clusters of secretory cells

Duct structure – simple glands

ducts don’t branch;

compound glands

branched ducts

Clusters of secretory cells are arranged in three possible configurations

tubular

acinar 

spherical

 tubuloacinar

(with both tubular and acinar sections)

Merocrine secretion

used by
majority of exocrine glands in
body including salivary and
sweat glands; secretory cells
package products in secretory
vesicles for release by
exocytosis into ducts

Holocrine secretion

used
by sebaceous gland in skin
to secrete sebum; secretory
cells accumulate product
in cytosol; only release
product when cell
ruptures and dies

Connective tissues

are divided into two basic groups that differ in their cell types and ECM components: 

  Connective tissue proper 

 Specialized connective tissue 

Connecting and binding

anchor tissue layers in organs and link organs together

Support 

bone and cartilage support weight of the body

Protection 

bone tissue protects certain internal organs and cartilage and fat provide shock absorption; components of immune system are found throughout connective tissues

Connective tissue functions: Transport

blood is a fluid connective tissue that is main transport medium in body

 general connective tissue

 Widely distributed in body

Connect tissues and organs to one another

Components of internal architecture of some organs

Cells of connective tissue proper – resident cells 

 permanently inhabit tissue in which they are found; migrant cells migrate into different areas of body depending on situation; cells in connective tissue 

Cells of connective tissue proper – resident cells

Fibroblasts

Adipocytes

Mast cells

Phagocytes

Other immune system cells

Fibroblasts

most common resident cell

Mature cells that have properties of an immature “blast” cell

Adipocytes (fat cells)

found in many different connective tissues; cytoplasm of each cell is filled with a single large lipid inclusion

Mast cells 

 largest resident cell

Immune system cells filled with cytosolic inclusions

 inflammation 

(protective
response that activates immune system)

Phagocytes, macrophages

also immune system cells; can ingest foreign substances, microorganisms, and dead or damaged cells by…

Other immune system cells

 can migrate in and out of connective tissues depending on body’s
needs

types of connective tissue proper: 

1. Loose connective tissue 

2. Dense connective tissue 

3. Reticular tissue 

4. Adipose tissue 

Loose connective tissue

mostly ground substance, with all three types of protein fibers, fibroblasts, and occasionally adipocytes, suspended in ground substance

Found beneath epithelium of skin, in membranes lining body cavities, and within walls of hollow organs

1. Dense connective tissue (fibrous connective tissue)

Dense irregular connective tissue, Dense regular connective tissue, Dense regular elastic connective tissue (elastic tissue

Dense irregular connective tissue

 predominantly disorganized collagen bundles

Strong and resists tension in all three planes of movement

Found in high tension areas like dermis (deep to skin) and surrounding organs and joints

Dense regular connective tissue 

Predominantly organized into parallel collagen bundles; resistant to tension in one plane

Found in tendons and ligaments that are subject to tension in one plane of movement

Dense regular elastic connective tissue (elastic tissue)

Mostly parallel-oriented elastic fibers with randomly oriented collagen fibers

Found in walls of organs that must stretch to perform their function, such as large blood vessels and certain ligaments

Reticular tissue

composed mostly of reticular fibers produced by fibroblasts (reticular cells); form fine networks that can support small structures like blood and lymphatic vessels 

Also found in lymph nodes and spleen; form weblike nets that trap old and foreign cells

Forms part of basement membrane that supports all epithelia and internal structure of liver and bone marrow

1. Adipose tissue (fat tissue)

1. consists of fat-storing adipocytes and surrounding fibroblasts   and ECM; adipocytes can increase in size to point where fibroblasts and ECM are scarcely visible; functions include:

Fat storage (major energy reserve of body)

Insulation (retains warmth)

Shock absorption and protection