HISTOLOGY - EPITHELIAL TISSUE

HISTOLOGY FOR MEDICAL STUDENTS

EPITHELIAL TISSUE

EPITHELIUM IS A CELLULAR LAYER ON THE BASEMENT MEMBRANE

epithelia cover the external surfaces of the body, its serous cavities, outer and inner surfaces of many
internal organs, form secretory units and line excretory ducts of exocrine glands

EPITHELIAL TISSUE TRAITS

cells form layers
there is a basement membrane
cells are tightly linked to each other

cells are polar (apical and basal parts)
no blood vessels
no intercellular matter

high regenerative capacity

CLASSIFICATION OF EPITHELIAL TISSUE

BY ORIGIN BY LOCATION AND FUNCTION
Ectodermal epithelia (epidermal; ependymoglial)
Entodermal epithelia
Coelomic nephrodermal epithelia
Mesodermal epithelia
Mesenchymal epithelia (angiodermal)
Surface epithelia
cover organs from outer and inner surfaces
Glandular epithelia
form secretory portions and line excretory ducts of exocrine glands
BY STRUCTURE
this classification is based on the main principle: cells' relationship to the basement membrane
SIMPLE (NON-STRATIFIED) EPITHELIA STRATIFIED EPITHELIA
all epithelial cells lie on the basement membrane not all epithelial cells lie on the basement membrane
simple squamous
simple cuboidal (low columnar)

simple columnar (cylindrical):
all epitheliocyte nuclei are on the same height from the basement membrane, for epithelium consists of monomorphic (similar) cells

pseudostratified columnar:
epitheliocyte nuclei are on different levels, since epithelium cellular composition is heterogeneous (e.g. goblet, large and small intercalated cells)
villous (brush) - if epithelium possesses cells with apical microvilli
ciliated - if there are cells with cilia
stratified cuboidal or columnar epithelium consists of 2 or 3 layers of columnar or cuboidal cells
stratified squamous nonkeratinized consists of three layers: basal, intermedium (spinosum), and surface
stratified squamous keratinized - five layers are identified: basal, spinosum, granulosum, lucidum and corneum; basal and spinosum layers combine to form stratum germinativum, for these cells are capable of division
stratified squamous epithelia possess nuclear polymorphism: nuclei of the basal layer are elongated and perpendicular to basement membrane, stratum intermedium (spinosum) nuclei are around-shaped, and superficial (granular) cell nuclei are also elongated, but parallel to the basement membrane
transitional epithelium (urothelium) is composed of basal and superficial cells without nuclear polymorphism: all cells' nuclei are around in shape

EXOCRINE GLANDS consist of 2 parts: terminal (secretory) portions and excretory ducts

CLASSIFICATION

simple glands have a non-branching excretory duct	complex have a branching excretory duct
non-branched have non-branching terminal portions	branched have branching terminal portions
by terminal (secretory) portion type: alveolar, tubular, alveolar-tubular
by secretory product: e.g. proteinaceous, mucous, muco-proteinaceous (mixed)

TYPES OF SECRETION

merocrine - a secretory cell is not destroyed in the process of secretion (salivary glands, pancreas)
apocrine - apical end of the cell is partially destroyed in the process of secretion (mammary glands)
- micro-apocrine - secretion is accompanied by destruction of mirovilli (sweat glands)
- macro-apocrine - secretion is accompanied by destruction of apical part of cytoplasm (mammary glands)
holocrine - a cell is totally destroyed in the secretory process (sebaceous glands)

cells form layers there is a basement membrane cells are tightly linked to each other	cells are polar (apical and basal parts) no blood vessels no intercellular matter
high regenerative capacity

BY ORIGIN	BY LOCATION AND FUNCTION
Ectodermal epithelia (epidermal; ependymoglial) Entodermal epithelia Coelomic nephrodermal epithelia Mesodermal epithelia Mesenchymal epithelia (angiodermal)	Surface epithelia cover organs from outer and inner surfaces Glandular epithelia form secretory portions and line excretory ducts of exocrine glands
BY STRUCTURE this classification is based on the main principle: cells' relationship to the basement membrane
SIMPLE (NON-STRATIFIED) EPITHELIA	STRATIFIED EPITHELIA
all epithelial cells lie on the basement membrane	not all epithelial cells lie on the basement membrane
simple squamous simple cuboidal (low columnar) simple columnar (cylindrical): all epitheliocyte nuclei are on the same height from the basement membrane, for epithelium consists of monomorphic (similar) cells pseudostratified columnar: epitheliocyte nuclei are on different levels, since epithelium cellular composition is heterogeneous (e.g. goblet, large and small intercalated cells) villous (brush) - if epithelium possesses cells with apical microvilli ciliated - if there are cells with cilia	stratified cuboidal or columnar epithelium consists of 2 or 3 layers of columnar or cuboidal cells stratified squamous nonkeratinized consists of three layers: basal, intermedium (spinosum), and surface stratified squamous keratinized - five layers are identified: basal, spinosum, granulosum, lucidum and corneum; basal and spinosum layers combine to form stratum germinativum, for these cells are capable of division stratified squamous epithelia possess nuclear polymorphism: nuclei of the basal layer are elongated and perpendicular to basement membrane, stratum intermedium (spinosum) nuclei are around-shaped, and superficial (granular) cell nuclei are also elongated, but parallel to the basement membrane transitional epithelium (urothelium) is composed of basal and superficial cells without nuclear polymorphism: all cells' nuclei are around in shape