Department of Histology and Embryology, P.J. Šafárik University, Medical Faculty, Košice

EPITHELIAL TISSUE: Sylabus for foreign students

Author: MVDr. Štefan TÓTH, PhD.

Revised by: prof. MUDr. Eva Mechírová, CSc.

Tissues – def. are aggregates or groups of cells organized to perform one or more specific functions

● despite its complexity, the human body is composed of only 4 basic types of tissues:

1. EPITHELIAL TISSUE, 2. CONNECTIVE TISSUE

3. MUSCULAR TISSUE, 4. NERVE TISSUE

○ these tissues, which are formed by cells and several structures and substances generally called – the extracellular (intercellular) matrix (ECM)

Main characteristics of epithelia

1. Epithelia are closely associated aggregations of cells with almost no free intercellular substance.

2. Epithelia derive from the ectoderm, mesoderm, and endoderm. 3. Epithelia line and cover all body surfaces. 4. Epithelia lack a direct blood and lymphatic supply. Nutriens are delivered by diffusion. 5. Epithelial cells are strongly attached one to another by intercellular tight, gap, and

communicating junctions. 6. Epithelial cells are polarized structures (apical, basal and lateral surfaces) 7. Basal surface of epithelial cells rests on a basement membrane. 8. Most epithelial cells renew continuously by mitosis, regenaration.

The basic functions of epithelia

● the functions of epithelia are very different

1.) Covering and lining → protection – e.g. epidermis of skin

2.) Absorption – e.g. intestinal epithelium

3.) Transport of material – e.g. trachea and bronchi

4.) Secretion – e.g. the epithelial cells of glands

5.) Gas exchange - e.g. lung alveolus

6.) Gliding between surfaces – e.g. pleura and peritoneum

Polarity of epithelial cells

● is an important morphological feature of epithelial cells

● the epithelial cells are polarized structures

● they have a free – apical and a basal surface that rest on a basement membrane and lateral surface

● different composition of their membranes → specialized functions

● this is important for orientation and distribution of the cells & their cellular organelles

Basement membrane

Fig. Basement membrane composition

● basement membrane (BM) separate epithelia from underlying supporting connective tissue and is never penetrated by blood vessels

● the term BM is used to specify a homogenous eosinophilic and PAS-positive layer visible with the light microscope

● the BM is therefore thicker and usually formed by the fusion of either two basal laminae or basal lamina and a reticular lamina

Functions of BM – 1. provides structural support 2. involved in the control of epithelial growth and differentiation 3. necessary for cellular interactions

Basal lamina

● is a sheet-like extracellular structure

● this structure is visible only with the electron microscope !!!

● under EM, the basement membrane consists of 2 layers:

1. basal lamina, 2. reticular lamina

● basal lamina is composed from 2 parts:

a) lamina lucida or lamina rara – electron-lucent structure (light part)

b) lamina densa – electron-dense layer (darker part)

● the main components of basal laminae are – collagen type IV, laminin, proteoglycan heparansulfate, entactin, fibronectin

● reticular lamina is closely associated with basal lamina and is composed of collagen type III (reticular fibers), collagen type VII (anchoring fibers) and fibrilin

Lateral cell surface specializations – Intercellular junctions

1. Impermeable (occluding) – tight junctions: Zonula occludens

2. Adhering (anchoring) junctions: Zonula adherens and Macula adherens

(desmosome)

3. Communicating junction: Nexus (gap junction)

Fig. Lateral cell surface specializations (1) Zonula occludens – tight junction

● the most apical of the junctions

● "zonula " – junction form bands completely encircling each cell

● " occludens " – refers to the membrane fusions that close off the intercellular space

● divides cell surface into apical and basolateral part and separates the luminal space from the intercellular space

● the major components of zonula occludens are transmembrane proteins called – claudin and occludin which make tight contact across the intercellular space – creating a seal

(2) Zonula adherens – adherent junction

● this junction also encircles the cell immediately below the zonula occludens

● provides for the firm adhesion of one cell to its neighbors

● adhesion is mediated by transmembrane glycoproteins - E-cadherins

● cadherins bind the protein catenin which is linked by means of actin-binding proteins (α-actinin, vinculin) to actin microfilaments

● actin-binding proteins form the electron dense plaques on the cytoplasmic surface of the epithelial cells, where the actin microfilaments are attached

● numerous actin microfilaments form part of terminal web

(3) Macula adherens, Desmosome – adherent junction

● represents a major anchoring cell-to-cell junction that provides a particularly

very strong attachment

● this junction resembles a single "spot-weld", desmosome is a disk-shaped symmetric structure

● between cell membranes is electron-dense material (cadherin proteins) → "desmosome central plate"

● on the cytoplasmic site of each cell membrane these cadherin-type proteins inset into a dense attachment plaque of anchoring proteins e.g. - plakoglobin, desmoplakin

● anchoring proteins bind cytokeratin intermediate filaments

ZO- zonula occludens

ZA- zonula adherens

D- macula adherens

N- nexus

Fig. Desmosome

(4) Nexus, gap junction – communicating junction

● occur almost anywhere along the lateral surface

● gap junction consists of an accumulation of transmembrane channels or pores in a tightly packed array

● the proteins of nexus, called connexins, form hexagonal complexes called connexons

● each connexon has a central hydrophilic pore (Ø 1,5 nm)

● gap junctions permit the rapid exchange cells of small molecules (e.g. ATP, ADP,

AMP, K+, nucleotides)

● gap junctions are also called low-resistance junctions

Specializations of apical surface/domain:

A- Microvilli B- Stereocilia C- Cilia (Kinocilia) MICROVILLI ● minute, finger-like extensions of the apical cell membrane ● present on many epithelial cells ● within microvillus are bundles of actin microfilaments, cross-linked to each other and to the surrounding plasma membrane by actin-bundling proteins ● brush or striated border – numerous, uniform and closely packed microvilli ● function: increase the surface of absorptive capacity of epithelial cells STEREOCILIA ● unusually long, slender, branching cytoplasmic proccesses ● are not widely distributed among epithelia ● limited to the epididymis, the proximal part of the ductus deferens of ♂ reproductive system, and the sensory (hair) cells of the inner ear ● stereocilia can not move (immotile microvilli) ● stereocilia are supported by internal bundles of actin microfilaments that are cross-linked by fimbrin ● functions: a) ♂ reproductive system – have absorptive function b) sensory hair cells in the inner ear - function as mechanoreceptors CILIA (kinocilia) ● elongated, highly motile structures on the apical surface of epithelial cell ● cilia are motile structures – there are two types of contractile proteins – tubulin and ciliary dynein ● their structure having 2+9 configuration (axoneme) – central pair of microtubules surrounded by 9 pairs (dublets) of peripheral microtubules ● inserted into basal bodies (modified centrioles) ● function: rapid back-and-forth movement and, motile cilia are capable of moving fluid and particles along epithelial surfaces ● examples: in respiratory passages – trachea, bronchi

Fig. Motile cilia – detail molecular structure

A- Terminal part, B- Major-free part, C- Intermediate (transitional) part, D- Basal root, E- Striated

rootlet, 1- Axonema, 2- Central sheath + Central pair of microtubules doublet, 3- Microtubule

doublet, 4- Outer and Inner dynein arm, 5- Radial spoke, 6- Nexin bridge, 7- Microtubule, 8-

Protofilaments

A

B

C

D

E

1

2

3

4

5

6

7

8

Fig. Axoneme of motile cilium – cross section

1- cell membrane

2- peripheral microtubular doublet

3- dynein arm

4- nexin bridge

5- radial spoke

6- central sheath

7- central pair of microtubules