Epithelial Tissues

Chapter 2. Epithelial tissues

Main contants  1.

The feature of epithelium  2. Types of epithelium  3. The form and distribution of different  covering epithelium  4. The form of glandular epithelium  5. Epithelial specialization

The feature of epithelium  1.

Form,composition and distribution (one or more layers

of cells tightly joined together, very little intercellular  substance, lining inner or outside of some organs and our  human body)  2. Polarization (free surface, basal surface) 3. Avascularity (except stria vascularis) 

4. Innervation

 The

forms

– Epithelial cells are varied, ranging from high columnar to

cuboidal to low squamous cells and including all intermediate forms. Their common polyhedral form is accounted for by their juxtaposition in cellular layers or masses. A similar phenomenon might be observed if a large number of inflated rubber balloons were compressed into a limited space.

 Epithelial

cell nuclei

– distinctive appearance, varying from spherical to elongated

or elliptic in shape. The nuclear form often corresponds roughly to the cell shape; thus, cuboidal cells have spherical nuclei and squamous cells have flattened nuclei. The long axis of the nucleus is always parallel to the main axis of the cell.

 Polarity – ie, epithelium has apical surface and a basal surface that rests on a

basal lamina. Since blood vessels do not normally penetrate an epithelium, all nutrients must pass out of the capillaries present in the underlying lamina propria. These nutrients and precursors of products of the epithelial cells then diffuse across the basal lamina and are taken up through the basolateral surface of the epithelial cell. – Receptors for chemical messengers that influence the activity of epithelial cells (eg, hormones, neurotransmitters) are localized in the basolateral membranes. In absorptive epithelial cells, the apical cell membrane contains, as integral membrane proteins, enzymes such as disaccharidases and peptidases, which complete the digestion of molecules to be absorbed.

Renewal Of Epithelial Cells  Epithelial

tissues are labile structures whose cells are renewed continuously by means of mitotic activity. This renewal rate is variable. It can be fast in such tissues as the intestinal epithelium, which is replaced every 2 days; or slow, as in the pancreas, where tissue renewal takes about 50 days. In stratified and pseudostratified epithelial tissues, mitosis occurs within the germinal layer, those cells closest to the basal lamina.

 Innervation – Most epithelial tissues receive a rich supply of

sensory nerve endings from nerve plexuses in the lamina propria. Everyone is aware of the exquisite sensitivity of the cornea, the epithelium covering the anterior surface of the eye. This sensitivity is due to the great number of sensory nerve fibers that ramify between corneal epithelial cells.

 Metaplasia – Under certain physiologic or pathologic conditions, one

type of epithelial tissue may undergo transformation into another epithelial type. This process is called metaplasia (Gr. metaplasis, transformation). – Examples – In heavy cigarette smokers, the ciliated pseudostratified epithelium lining the bronchi can be transformed into stratified squamous epithelium. – In individuals with chronic vitamin A deficiency, epithelial tissues of the type found in the bronchi and urinary bladder are gradually replaced by stratified squamous epithelium.

 

All Epithelia Have A Basal Lamina 20-100 nm thick type 4 collagen, laminin, proteoglycans

Basement Membrane Is a Thickened Basal Lamina Kidney glomeruli Lung alveoli and trachea

Types of epithelium 1. Covering epithelium 2. Glandular( 腺 ) epithelium 3. Sensory epithelium 4. Germinal epithelium 5. Myoepithelium

Covering Epithelium squamous Simple

cuboidal columnar pseudostratified squamous keratinized( 角化 )

Stratified ( 复层 )

nonkeratinized transitional

epithelium

Covering Epithelium 1

11

6

3

2

5

4

Simple epithelium can, according to cell shape, be squamous, cuboidal, or columnar. The endothelium lining blood vessels and the mesothelium lining certain body cavities are examples of simple squamous epithelium . Although endothelial and mesothelial cells present the same appearance in the light microscope, they should not be considered as one cell type differently localized. They differ not only in their embryologic origin and ultrastructural morphologic characteristics but also in their pathologic responses. Therefore, they react differently to several types of insults and even produce different types of tumors. An example of cuboidal epithelium is the surface epithelium of the ovary , and an example of columnar epithelium is the lining of the small intestine .

Simple squamous epithelium  Cell:

(surface view)  Flattened  Irregular in shape  Nuclear

oval  randomly located 

Cell (lateral view) Cytoplasm thin, not clear Nuclear Rod-like Distribution Outer surface Inner surface

Simple cuboidal epithelium  Cell

Cuboidal  Nucleus  Spheric ,  Centrally-located 

 Distribution  Renal

tubules;

Thyroid gland

Simple columnar epithelium Form Tall prisms Polygonal outline Ovoid, basally-located nucleus Goblet cell(in some organs) Distribution Gastrointestinal tract; The gall bladder The uterus etc .

figure

Pseudostratified columnar epithelium Form High columnar cell Fusiform cell Basal cell Goblet cell Distribution Respiratory system Male reproductive system

Stratified Epithelium  Stratified

squamous epithelium  Transitional epithelium

Stratified squamous epithelium  Keratinized Nonkeratinized Distribution:

skin, esophagues

Nonkeratinized Stratified squamous epithelium Form     

Surface: Flattened Middle: Polygonal Basal: Cuboidal or low columnar (connective tissue papilla)

Distribution:

esophagus

Keratinized stratified squamous epithelium



Form 1.Keratinized cell layer 2.stratum lucidum 2.Granular layer: Flattened cell 3.Middle layer: Polygonal cell 4.Basal layer: Cuboidal or low columnar cell



Distribution: skin

        

Transitional epithelium  The

shape and layer of the epithelium may be changeable, and depend on the contraction or stretching of urinary bladder

 Distribution:

Urinary System

Transtional epithelium (in nondistented urinary bladder)

Epithelium Large

cuboidal

Dome(2

is thicker

shape

nucleus in some cells)

Low

columnar cell

Transitional epithelium 变移上皮 (in distented urinary bladder)

The thickness of epithelium became thinner Layer of epithelium is reduced Cells became flattened

Glandular epithelium Secretory cells mucus-secreting cell

exocrine

serous-secreting cell

glands

steroid-secreting cell

endocrine

endocrine cell

glands

Serous-secreting cell （浆液性细胞）　  Form   

Cuboidal ,triangle Basophilic cytoplasm oval nucleus

 EM  

Zymogen granule RER, Golgi complex

Mucus-secreting cell （粘液性细胞） Form   

cuboidal, columnar, goblet Neutrophilic cytoplasm Flat nucleus, triangle,

EM  

Glycoprotein granule RER, Golgi complex

Types and Structure of gland  1.Exocrine

and endocrine glands

secretory  portion  Exocrine gland (acinus)  duct 

mucus serous SC mixed

Secretion models  Merocrine: membrane-bound granules released by 

exocytosis; pancreas, salivary glands

 Apocrine:

secretions released as droplet with part of cytoplasm from apical surface of the cell; mammary glands and apocrine sweat glands

  

 Holocrine: secretions released by bursting of 

whole cell; sebaceous glands

Functions of Epithelia  1.

Covering and lining surfaces (skin)  2. Absorption (intestine)  3. Secretion (glands of skin & GI tract)  4. Sensation (taste buds, olfactory epith)  5. Contractility (myoepithelium of glands)

Epithelial specialization

Specialization Of free Surface 1.Microvilli

( 微绒毛）

a. Finger-like projection of cell  membrane  b. Microfilament ( 微絲）  c. Terminal web （终未网） Function: Expands the surface area 

 Distribution:

small intestine

Microvilli （微绒毛）

2.Cilia （纤毛）  Elongate

and mobile projection  Microtubules （微管）  Basal body 　（基粒）  Function:  Produce

a forward-moving wave  1.Remove the inhale granules 　  2.Move the ova( 卵 ) through oviduct( 输卵管 )  Distribution:  Respiratory system and oviduct etc.

Cilia ( 纤毛 ）

Specialization of Intercellular surface  Tight

junction or zonula occludens  （紧密连接）  Intermediate junction or zonula adherens  （中间连接）  Desmosome 　（桥粒）  Gap junction 　（逢隙连接）

Specialization of Intercellular surface – The lateral membranes of epithelial cells

exhibit several specializations that form intercellular junctions. These junctions serve not only as sites of adhesion but also as seals to prevent the flow of materials through the intercellular space and to provide a mechanism for communication between adjacent cells. The various junctions are usually present in a definite order from the apex toward the base of the cell.

Intercellular junction – Several membrane-associated structures promote cell

aggregation and contribute to cohesion and communication between cells. Epithelial cells are extremely cohesive, and relatively strong mechanical forces are necessary to separate them. This quality of intercellular adhesion is especially marked in epithelial tissues that are usually subjected to traction and pressure. This is due in part to the binding action of the glycoproteins, which are integral membrane proteins of the plasma membrane, and of a small amount of intercellular proteoglycan. Some glycoproteins lose their adhesiveness in the absence of calcium.

Intercellular junction 

Tight junction ( 紧密连接 )



Linear fuse of outer membrane of neighboring cell



Intermediate junction ( 中间连 接)



Band-shaped, Dense plaque, microfilement



Desmosome ( 桥粒 )



Disk-shaped, dense plaque, microfilement, media line



Gap junction ( 缝隙连接 )



6 subunits, channel ( open or close )



Tight junction

Tight junctions, or zonulae occludentes, are the most apical of the junctions. Zonula refers to the fact that the junction forms a band completely encircling the cell, while occludens alludes to the membrane fusions that close off the intercellular space. One to several of these fusion sites may be observed, depending on the epithelium under observation After cryofracture (Figure 4.7 ), the replicas show anastomosing ridges and grooves that form a netlike structure corresponding to the fusion sites observed in conventional thin sections. The number of ridges and grooves, or fusion sites, has a high correlation with the leakiness of the epithelium. Epithelia with one or very few fusion sites (eg, proximal renal tubule) are more permeable to water and solutes than are epithelia with numerous fusion sites (eg, urinary bladder). Thus, the principal function of the tight junction is to form a more or less tight seal that prevents the flow of materials between epithelial cells (paracellular pathway) in either direction (from apex to base or base to apex.

Tight junction

Intermediate junction and desmosome

Gap junction

Gap junction( 缝隙连接） connexin 连接素 , 体

connexon 连接小

Junctional complex （连接复合体）

Tight junction

Intermediate junction desmosome

Plasma membrane infolding ( 质膜内褶）