epithelial tissues

What are tissues?

groups of cells that are similar in structure and perform similar or related functions

How many different types of cells are in the human body?

approximately 200 cells

What are the 4 basic kinds of tissue the body is composed of?

1. epithelial

2. connective

3. muscle

4. nervous

What are tissues formed by?

1. cells

2. molecules of extracellular matrix (ECM), which is an intricate meshwork of proteins and polysaccharides that are secreted by the cell assembled locally

How are organs formed?

by the combination of different tissues in variable proportions

What is the cell type of epithelial tissue?

aggregated polyhedral cell

What is the cell type of connective tissue?

several types of fixed and wandering cell

What is the cell type of muscle tissue?

elongated contractile cells

What is the cell type of nervous tissue?

intertwining elongated processes

how much extracellular matrix do epithelial tissues contain?

a small amount

how much extracellular matrix do connective tissues contain?

an abundant amount

how much extracellular matrix do muscle tissues contain?

a moderate amount

how much extracellular matrix do nervous tissues contain?

none

what is the function of epithelial tissues?

lining body surfaces, body cavities and glandular secretion

what is the function of connective tissues?

support and protection

what is the function of muscle tissues?

movement

what is the function of nervous tissues?

transmission of nervous impulses

how is epithelial tissue formed?

formed from large numbers of closely aggregated cells of a similar type. it has little intracellular substance, leading to stronger adhesion.

what are the 3 embryonic germ layers? what do they form?c

they form the epithelium and are as follows:

1. ectoderm (external)

2. endoderm

3. mesoderm

What is the ectoderm?

outermost surface. lines the body surface, such as the:

• skin

• mouth

• nose

What is the endoderm?

innermost layer of the 3. the lining epithelium of:

• digestive tract

• glands of digestive tract (liver, pancreas)

• respiratory tract

What is the mesoderm?

the middle layer of the 3 layers. lines the:

• vascular system

• closed body cavities

• parts of the urogenital system

What are the two types of epithelium in the body?

2. covering (lining)

3. glandular (secretory)

Covering (lining):

cells are organized in layers that cover the external surface of the body or they line the cavities of the body (mesothelium)

Glandular (secretory)

formed by cells that are specialized to secrete proteins, lipids or carbohydrates. The cells of this type of epithelium are organized

What do epithelial cells form?

membranes

What layer does the epithelial membrane consist of?

it has a layer of epithelial tissue, as well as an underlying layer of connective tissue

What are the two types of epithelial membranes?

1. mucous membrane

2. serous membrane

Mucous membrane:

• also known as mucosa

• there are goblet cells present, which secrete mucus, which helps in lubrication, protection and easy movement of materials. it also prevents tissues from drying

• it lines the body cavities such as respiratory and digestive tracts, which open outside the body

Serous membrane:

• lines the body cavities that don’t open outside the body, such as the lining of the pleural cavity and pericardial membranes.

• these membranes secrete the fluid inside the cavity and are made up of simple squamous epithelium

What are glands made up of? what are the two types of glands?

• they are made up of epithelial cells. the two types are:

  1. exocrine

  2. endocrine

Exocrine glands:

secrete their product into a duct

• ex: goblet cells, sweat glands

Endocrine glands:

• ductless glands

• release their products directly into the blood or intestinal fluid

• ex: hormones secreted by thyroid glands

What are some common characteristics of epithelial cells?

1. they have a polyhedral form

2. they show polarity

3. most epithelial rest on connective tissue, which provides them with nutrition and support, because the epithelium is avascular (no blood vessels). it is bended to the underlying structures

what is the characteristics of epithelium cells?

closely aggregated cells with little to no intercellular spaces (densely packed)

What holds the junctions of the epithelium?

it is held by intercellular junctions

what type of membrane does the epithelium lie on?

a basement membrane

How is the epithelium nourished?

• it is avascular, so it is nourished by diffusion

• rich in sensory nerve endings

What is the renewal rate of the epithelium?

It is high

How is the polyhedral form achieved?

• it is resulted from their close juxtaposition in cellular layers or masses

What does the nuclear form often correspond with?

it often corresponds to the cell shape.

What is the long axis of the nucleus always parallel to?

the long axis of the nucleus is always parallel to the main axis

What are the 3 types of shapes in epithelial tissues?

1. cuboidal (spherical nucleus)

2. squamous (flattened nucleus)

3. columnar (cylindrical nucleus)

what are the 3 surfaces of an epithelial cell?

1. apical (faces outer surface)

2. basal (rests on basement membrane)

3. lateral (on the sides)

What does each surface exhibit?

each surface exhibits special structural modifications to carry out specific functions

How is cell polarity in epithelial cells characterized?

• differences in structure, composition and function between at least two poles of a cell.

• In epithelial cells, these spatial differences allow for the formation of defined apical and basal membranes.

cell polarity:

• organelles and membrane proteins are unevenly distributed. different regions, have different functions and are split as follows

  1. basal pole

  2. apical pole

  3. lateral surfaces

basal pole:

the region facing the connective tissues for nourishment

apical pole:

• the pole opposite to the basal pole

• it faces the lumen or cavities

• it is rich in iron channels, carrier proteins and enzymes

• its main function is regulating water balance

lateral surfaces:

• intervening sides apposed in neighboring cells for communication

apical modification of the lumina surface

luminal surface of epithelial cells may incorporate 3 main types of specialization:

1. cilia

2. microvilli

3. stereocilia

What is the microvilli? (apical)

non-motile, finger-like cytoplasmic projections that arise from the apical surface of epithelial cells.

What is the structure of the microvilli? (apical)

it is a core of actin filaments

What is the function of microvilli? (apical)

it increases the surface area for absorption

What are the sites of the microvilli? (apical)

the cells of the intestine and kidney tubules

What is the stereocilia? (apical)

long, branching microvilli. non-motile

• contains the same structure as microvilli (a core of actin filaments)

What is the function of the stereocilia? (apical)

increases the surface area for absorption, just as microvilli

What are the sites of stereocilia? (apical)

the non-ciliated pseudostratified columnar epithelium of male genital ducts

• ex: the epididymis

What is the cilia / flagella? (apical)

motile, cytoplasmic projections that extend from the cell surface

What is the cilia? (apical)

hair-like processes that are longer than the microvilli

what is the structure of the cilia?

consists of:

• basal body, shaft and rootlets, all formed by microtubules

What is the flagella? (apical)

they resemble cilia in structure but they are much longer and are single for each cell

• ex: flagellum of the sperm

What is the function of the cilia? (apical)

moves fluids / substances

What are the sites of cilia? (apical)

respiratory tracts

cellular interdigitations (lateral specialization):

increases the surface area for transport

intercellular junctions (lateral specialization):

links the neighboring cells together. they are as followings

1. tight junctions

2. adherens junctions

3. desmosome

4. gap junction

Tight junction (occluding junction):

site: at the apical parts of the cells

function: restricts the passage of molecules between the epithelium (barrier)

• ex: epithelial cells of the intestine

communicating junction (gap junction, nexus):

site: in epithelial cells, as well as cardiac and smooth muscle cells

functions:

• permit communication rather than adhesion

• they permit the exchange of molecules (such as ions and amino acids), allowing the passage of signals involved in contraction and communication from one cell to another

what is basal infoldings? (basal modification)

the basal cell membrane is thrown into folds

what is the function of basal infoldings? (basal modification)

it increases the surface area for ion transport

what is the site of basal infoldings? (basal modification)

kidney tubules

What is the site of the basement membrane? What are its components?

• the site is in the interface between the epithelium and the connective tissue

• it is composed of the basal lamina and the reticular lamina

basal lamina:

produced by epithelial cells

reticular lamina:

produced by connective tissue cells

What is are the two components of the basal lamina?

1. Lamina lucida, which is located just beneath the epithelium

2. Lamina densa, which is the dense region of the basal lamina

where is the basal lamina located?

t lies at the interface of epithelial cells and connective tissue

where must the nutrients for the epithelial cells diffuse?

they must diffuse across the (basal) lamina

what never enters an epithelium across the (basal) lamina?

blood capillaries

What enters an epithelium across the (basal) lamina?

Nerves

What is the lamina reticularis composed of?

1. Type I & type III collagen

2. fibronectin

3. type VII & IV collagen

What is the structural attachment of the basement membrane?

the attachment of the epithelial cells to the underlying connective tissue

What is the filtration function of the basement membrane?

it regulates the exchange of macromolecules between the epithelium and the surrounding tissues

What is the tissue scaffold of the basement membrane?

• it direction of the migration of the epithelial cells (re-epithelization) during wound repair

• it acts as a barrier against the passage of malignant cells (cancerous cells)

What is turnover time?

the time that it takes for a cell to divide and pass through the entire epithelium

ex: turnover time for skin

52-75 days

ex: turnover time for gut

4-14 days

ex: turnover time for gingiva

41-57 days

ex: turnover time for cheek

25 days

What has a faster turnover rate? Nonkeratinized buccal epithelium or keratinized gingival epithelium?

Nonkeratinized buccal epithelium turns over faster than keratinized gingival epithelium

what is the epithelial metaplasia?

a reversible conversion of one mature epithelial cell type to another mature epithelial cell type

what causes epithelial metaplasia?

it is generally an adaptive response to stress, chronic inflammation or other abnormal stimuli

How is a tumor caused?

can form in any tissue if there is an uncontrolled growth in cells

carcinoma

a malignant tumor arising an epithelia

squamous cell carcinoma

malignant tumor arising from squamous epithelium

adenoma

a malignant tumor arising from glandular epithelium