Module 1 Lecture 2

Epithelial tissue

Functions of ET

Selective barriers (limit or aid transfer), Secretory (onto a free surface), Protective (especially from abrasion)

Structure of ET

Arranged in continuous sheets as single or multiple layers, held together by cell junctions, contains nerves but not blood vessels (avascular), exchange of nutrients and wastes occur by diffusion from vessels in CT

Cell junctions

Tight, Adherens, Desmosome, Gap, Hemidesmosomes

Lateral junctions

Tight, Adherens, Desmosome, Gap

Basal junction

Hemidesmosome

Actin

Microfilament, bundles beneath cell membrane and cytoplasm; strength, alter shape; link cytoplasm to membrane; tie cells together; muscle contraction

Keratin

Intermediate filaments, strength

Structure and function of tight junctions

Individual sealing strands, transmembrane, more strands = tighter junction, many proteins involved but key proteins are Claudins and Occludins, joins cytoskeletons of adjacent cells via ZO-1 to actin, keeps cell polarity by preventing migration of proteins between apical and basal surfaces, lots found in stomach, intestines, and bladder, electrically tight

Structure and function of adherens junctions

‘Belt desmosome’ or “adhesion belts (zonula adherens)’ in other epithelial cells, other cells are less continuous and are called adhesion plaques, have a plaque layer of proteins on the inside of the cell to join actin to cadherins, cadherins span the gap and catenins link cadherin to actin, prevent cell separation from tension forces like contractions

Structure and function of desmosome junctions

Lateral wall, have plaque, resist shearing forces, cadherin spans the gap and binds to desmoplakin, links cell surface to keratin, keratin spans from one desmosome to another on the other side for structural integrity, they bind muscle cells, common in skin epithelium and cardiac cells of the heart

Structure and function of gap junctions

Direct connection between cells, 6 connexIn protein molecules form 1 connexOn or a hemichannel, 2 hemichannels form a gap junction, allow up to ~1kDa small molecules through

Linking proteins

Cadherin, integrin, and catenin proteins

Anchoring proteins

Actin and keratin filaments

Structure and function of hemidesmosome junction

Connect epithelia to BM, links keratin to BM, integrin is the linker protein instead of cadherin, binds to Laminin in the BM and to keratin in cytoplasm

Structure of BM

Basal lamina - secreted by epithelial cells, contains collagen, laminin, other proteoglycans, glycoproteins, etc

Reticular lamina - produced by underlying CT tissue known as fibroblasts, contains fibrous proteins like fibronectin, collagen, etc.

Therefore BM is between ET and CT

Functions of BM

Supports overlying epithelium, provides a surface which epithelial cells migrate during growth and wound healing, acts as a physical barrier, participates in the filtration of substances in the kidney

Types of ET

Covering and lining epithelia - outer covering and inner lining

Glandular epithelia - secretory portion of glands

Simple arrangement

Single layer, for secretion, absorption, filtration, diffusion

Stratified arrangement

Two or more layers, for protection

Psuedostratified arrangement

appears to have a multiple layers judged by position of nuclei but not all cells react the apical surface and all cells are in contact with BM, making it simple, for secretion

Shape of ET

Squamous - flat and thin, aids in diffusion

Cuboidal - tall as wide, aids in secretion and absorption

Columnar - taller than wide, aids in secretion and absorption

Transitional - a stratified epithelium in which in can change shape from cuboidal to flat, allows for stretch

Structure and functions of Simple squamous epithelium

Most delicate epithelium, where there filtration (kidney); diffusion (lung); and secretion where slippery surface needed (outer layer of serous membranes), looks like fried eggs

Where can mesothelium be found

a layer of simple squamous epithelial cells covering serous membranes (peritoneum, pericardium, pleura) of an adult e.g. Visceral cavity linings

Where can endothelium be found

lines inside of heart and the blood and lymphatic vessels, lines cardiovascular and lymphatic systems, inside of the eye

Where can simple squamous epithelium be found

Bowman’s capsule of kidney and alveoli of lungs

Structure and function of simple Cuboidal epithelium

One cell layer thick, looks like cubes, for secretion and absorption

Where can simple cuboidal epithelium be found

Pancreas ducts, parts of kidney tubules, smaller ducts of many glands, secretory chambers of thyroid, anterior surface of lens, pigmented epithelium at posterior of retina, secretory part of some glands

Membrane modifications

Microvilli, cilia, goblet cells, keratin

Structure of simple columnar epithelium

Rectangular, nuclei often elongated and near the base of the cell, more cytoplasm so more organelles, more metabolically active than squamous cells, can either be ciliated or non-ciliated

Structure and function of Non-ciliated simple columnar epithelium

Have microvilli on apical surface, have goblet cells interspersed, increases surface area for absorption, secretion, and lubrication (mucus goblet cells)

Where can non-ciliated simple columnar epithelium be found

Lines gut mucosa from stomach to anus, ducts of many glands, gallbladder

Structure and function of Ciliated simple columnar epithelium

Have goblet cells, cilia on apical surface, synchronous movement assists motility of mucus and foreign objects and oocytes

Where can ciliated simple columnar epithelium be found

Some bronchioles, uterine fallopian tubes, sinuses, central canal of spinal cord, ventricles of brain

Structure and function of Stratified squamous epithelium

Cells form layers like plywood, lower layers may look cuboidal or columnar, cells furthest from nutrition are thinner and less active, for areas where mechanical or chemical stresses are severe, both types protect against microbes, has keratin where mechanical stress and dehydration are an issue, makes the surface tough and waterproof, can either be keratinised or non-keratinised, non-keratinised require secretions from glands

Where can keratinised stratified squamous epithelium be found

Skin

Where can non-keratinised stratified squamous epithelium be found

Mouth, throat, tongue, oesophagus, anus, and vagina

Types of psuedostratified columnar epithelium

Ciliated - Cilia on some cells, secretes mucus from goblet cells, to secrete mucus and move it

Non-ciliated - No cilia and goblet cells, for absorption and protection

Where can ciliated pseudostratified columnar epithelium be found

Most of upper airways

Where can non-ciliated pseudostratified columnar epithelium be found

Larger ducts of glands, epididymis, part of male urethra

Function of Stratified cuboidal epithelium

For protection, limited secretion/absorption functions, relatively rare

Where can stratified cuboidal epithelium be found

Ducts of adult sweat glands, oesophageal glands, male urethra

Functions of stratified columnar epithelium

For protection and secretion, rare

Where can stratified columnar epithelium be found

Parts of urethra, some large glands such as oesophageal glands, anal mucosal membrane, part of conjunctiva of the eye

Function of stratified transitional epithelium

Allows for expansion and contraction

Where can stratified transitional epithelium be found

Bladder, some parts of the ureter and urethra

Function of glandular epithelia

The function of all glandular epithelia is secretion

Function of Endocrine glands

Secrete directly into blood via interstitial fluid, generally distant strong effects

What are some endocrine glands

Pituitary glands, pineal glands, thyroid glands, parathyroid glands

Function of exocrine glands

Secrete into ducts that empty onto the surface of covering and lining epithelia, generally local effects

What shapes can exocrine glands be

Simple tubular, simple coiled tubular, simple branched tubular, simple alveolar (acinar or flask-like), simple branched alveolar, compound tubular, compound alveolar (acinar), and compound tubuloalveolar

What exocrine glands are simple tubular shape

Intestinal glands

What exocrine glands are simple coiled tubular shape

Merocrine sweat glands and wax glands

What exocrine glands are simple branched tubular shape

Gastric glands, mucous glands of oesophagus, tongue, and duodenum

What exocrine glands are simple alveolar (acinar or flask-like) shape

A stage in the embryonic development of simple branched glands

Structure of exocrine glandular epithelia

Single cell glands (e.g. goblet cells) or multicellular glands

What exocrine glands are simple branched alveolar shape

Sebaceous (oil) glands

What exocrine glands are compound tubular shape

Mucous glands in the mouth, bulb-urethral glands in the male reproductive system, seminiferous tubules of testes

What exocrine glands are compound alveolar (acinar) shape

Mammary glands

What exocrine glands are compound tubuloalveolar shape

Salivary glands, glands of respiratory passages, pancreas