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The Epithelium
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Questions in this quiz (38)
1A fertilized egg divides many times to produce many small cells
- ATrue
- BFalse
2These cells arrange themselves into three germ layers – ectoderm, mesoderm and endoderm
- ATrue
- BFalse
3These cells arrange themselves into three germ layers – ectoderm, mesoderm and endoderm
- ATrue
- BFalse
4A tissue is defined as a group of cells of common origin that perform a common function
- ATrue
- BFalse
5Tissues are building blocks of organs
- ATrue
- BFalse
6A sound knowledge of tissues is essential in understanding organ physiology
- ATrue
- BFalse
7Basic types of Tissues – Epithelium, Connective Tissue, Blood, Muscle Tissue and Nervous Tissue
- ATrue
- BFalse
8Origin of Epithelium
- AMost epithelia arise from ectoderm or endoderm
~Skin and cornea are ectodermal in origin
~Glands (sudoriferous or sweat, sebaceous or oil and mammary) are formed by invagination or evagination of ectoderm
~Endocrine glands are ductless and exocrine have ducts
~Alimentary canal is endodermal in origin
Functional Specialization of Epithelia
~Absorption
~Secretion
~Transport
~Protection
~Excretion
~Sensory Reception - Bnone
- AMost epithelia arise from ectoderm or endoderm
9General Characteristics of Epithelium
- ALittle or no intercellular space
Avascular
Supplied with nerves
Held in position by connective tissue underneath
Basement membrane present
Subject to wear and tear and rapid proliferation - Bnone
- ALittle or no intercellular space
10Clinical Aspect of Epithelium
- AEpithelial cells have high mitotic index
Are exposed to the surface and hence access to pathogens and carcinogens
Most common types of cancerous or malignant tumors originate from epithelial cells
They invade or metastasize to distant tissues
Carcinomas originate from surface epithelium Adenocarcicomas
from glandular epithelium
Some epithelial tumors are benign
Papillomas from surface epithelium
Adenomas from glandular epithelium - Bnone
- AEpithelial cells have high mitotic index
11Classification of Epithelia
- AAccording to cell shape and number of layers
~Simple, stratified and pseudostratified
~Squamous, cuboidal and columnar
~Additional kinds
Transitional or uroepithelium
Germinal epithelium
According to location and function
~Covering or lining epithelium - lines surfaces
~Glandular epithelium – glands - Bnone
- AAccording to cell shape and number of layers
12Classification of Epithelia -Simple Squamous Epithelium
- ASingle layer of flattened cells
Functions – Filtration, absorption, protection
Found in
~Tympanic membrane, Bowman’s capsule, lung alveoli,
~Mesothelium (lines the contents of the closed body cavities such as abdominal, pericardial, and pleural)
~Endothelium (lining of blood and lymphatic vessels)
~Endocardium (lining of ventricles and atria of the heart) - Bnone
- ASingle layer of flattened cells
13Clinical Point -Mesothelioma
- AAggressive tumors of parietal and visceral serous membranes
Caused by exposure to asbestos
Symptoms - shortness of breath, chest pain, accumulation of pleural - Bnone
- AAggressive tumors of parietal and visceral serous membranes
14Classification of Epithelia -Simple Cuboidal Epithelium
- ASingle layer of cells with roughly equal height and width – box like
Functions
~Absorption, secretion
Found in
~Thyroid gland, free surface of ovaries, ducts of glands, kidney tubules - Bnone
- ASingle layer of cells with roughly equal height and width – box like
15Classification of Epithelia -Simple Columnar Epithelium
- ASingle layer of cells that are taller than they are wider
Often have microvilli
Functions: Absorption, secretion
Found in
Lining of hollow organs, digestive tract, uterus, oviduct, bronchi,
gallbladder
Modified simple columnar epithelium
~Goblet cells – mucus secretion, digestive tract
~Simple columnar ciliated – dust protection in respiratory tract; oviduct - Bnone
- ASingle layer of cells that are taller than they are wider
16Classification of Epithelia
- AStratified squamous
~Protection - epidermis, mouth, esophagus, conjunctiva, cornea, vagina, portions of female urethra
~Keratinized – protection – epidermis
~Nonkeratinized – protection – mouth, tongue, vagina
Stratified cuboidal
~Protection – ducts of glands, male urethra
Stratified columnar
~Protection, secretion – male urethra, pharynx, epiglottis
Pseudostratified columnar
~Protection – male urethra, excretory ducts of parotid and other glands
Ciliated pseudostratified columnar
~Dust trapping, moving mucus – greater part of trachea and bronchi, Eustachian tube
Transitional (Uroepithelium)
~Prevent organ rupture – urinary bladder, ureter - Bnone
- AStratified squamous
17Epithelial Polarity
- AEpithelial cells are polarized
Basal and distal surfaces differ in structure
Golgi complex adluminal (toward lumen) or supranuclear (above nucleus)
Other organelles such as mitochondria are away from lumen or below nucleus
Polarity less evident in stratified epithelium - Bnone
- AEpithelial cells are polarized
18Epithelial Plasma Membrane
- AApical PM – rich in glycolipids, cholesterol, H+ ATPase, hydrolytic enzymes, transport proteins for release of secretory products
Lateral PM – has Na+ / K+ ATPase and receptors for hormones and neurotransmitters
Basal PM – has binding sites for basal lamina - Bnone
- AApical PM – rich in glycolipids, cholesterol, H+ ATPase, hydrolytic enzymes, transport proteins for release of secretory products
19Basal PM – has binding sites for basal lamina
- ADepends on a family of glycoproteins called cell adhesion molecules (CAM) or Integrins
One such protein cadherin is responsible for mutual cell - cell recognition
Six or more surface glycoproteins identified - one class is Ca++ dependent
Other molecules called ligands bind to CAM such as fibronectin, laminin and collagen - Bnone
- ADepends on a family of glycoproteins called cell adhesion molecules (CAM) or Integrins
20Cell Cohesion -Junctional Complexes
- AAll maintain cell osmotic pressure
- BZonula Occludens or Tight Junctions
~A belt around PM of adjacent cells found among columnar epithelial cells - CZonula Adherens
~Found among columnar epithelial cells - DDesmosomes or Macula Adherens
~Found among columnar epithelial cells and consist of central lamina, attachment plaques, transmembrane linkers and tonofilaments
21Cell Communication
- AGap Junctions or Nexus
~Involved in electrical coupling and hormonal actions
~Found commonly in nerve and muscle cells
~Consist of six subunits of proteins
~Contain hydrophilic pores
~Allow ions, water, cAMP etc. - Bnone
- AGap Junctions or Nexus
22Basal Lamina
- AFunctions - physical support by attachments; ultrafiltration (kidney endothelium)
Consist of two layers
1. Lamina rara or lucida adjacent to epithelium – consists of
proteoglycans laminin and type IV collagen secreted by epithelium
2. Lamina densa adjacent to connective tissue – consists of type VII
collagen and fibronection - Bnone
- AFunctions - physical support by attachments; ultrafiltration (kidney endothelium)
23Surface Specializations
- ABrush border or Microvilli
~Functions – absorption, secretion
~Found in digestive system and kidney tubules
~Consist of actin, villin and glycocalyx - BStereocilia
~Function – absorption
~Found in pseudostratified columnar epithelium of epididymis
~Are non-motile cilia - CCilia
~Functions – rapid to and fro movement, transport of fluid over the
~surface of respiratory tract
~50 – 100 per cell and are motile
~Have a core structure called axoneme
~Axoneme consists of long microtubules
~Microtubules arranged in 9 doublets + 2 singlets
~Tubules slide over each other for movement - DFlagella
~Function – rapid and propelling movement
~Found in spermatozoa
~Usually one / cell – longest in mammalian spermatozoa
- ABrush border or Microvilli
24Epithelial Renewal
- AEpithelium is subject to wear and tear
~Skin – keratinization and shedding of cells
~GI – very active loss and renewal
~Glands and respiratory passages – loss is rare – cells are long lived
~Loss of cells is balanced by active mitotic proliferation - Bnone
- AEpithelium is subject to wear and tear
25Absorptive Epithelia
- ASome organs require an absorptive epithelium composed of cells specialized for uptake of ions and small molecules
Contain carrier and channel proteins
Have brush border and multiple infoldings of basal cell membrane to increase surface area - Bnone
- ASome organs require an absorptive epithelium composed of cells specialized for uptake of ions and small molecules
26Secretory Epithelia
- ASecretion vs Excretion
~Secretion is uptake of substances from blood and transformation into complex forms within cells for a later release – examples – hormones, gastric juices
~Excretion is simple diffusion of excretory (waste) products across epithelial membranes – example – CO2
~Cells or associations of cells specialized for secretion are called glands - Bnone
- ASecretion vs Excretion
27Kinds of Glands
- AExocrine
~Release their products into a system of ducts opening onto an internal or external surface – example – digestive glands
Endocrine
~Release their products into blood or lymph for transport to target cells – example – hormones
Paracrine or Paraneurones
~Release their products into extracellular spaces – transport by simple diffusion to target cells in the vicinity - Bnone
- AExocrine
28Synthesis and Release of Protein Secretory Products
- AGlands can be stimulated to synthesize and release proteins
Gland cells contain zymogen granules in their cytoplasm - Bnone
- CProtein synthesis – flow of information – central dogma of molecular biology
~DNA (Replication) --> (Transcription) (RNA) --> (Translation) --> ribosomes --> proteins
- AGlands can be stimulated to synthesize and release proteins
29Mechanisms for the Release of Secretory Products
- AMerocrine
~Release of products from cell membrane while cell being intact – example goblet cells, sweat glands - BApocrine
~Involves loss of a part of apical cell cytoplasm – less common – example – mammary glands - CHolocrine
~Involves complete loss of cells discharging product – example – oil glands, spermatozoa
- AMerocrine
30Classification of Exocrine Glands
- AUnicellular
~Goblet cells are the only examples
Multicellular
~Sheet of epithelium or secretory sheet
~Gastric mucosa
Intraepithelial gland
~Nasal mucosa, male urethra
Simple Exocrine Glands – open by one duct
~Simple tubular – crypts of Lieberkuhn
~Simple coiled tubular – sweat glands
~Simple branched tubular – uterine, esophageal and Brunner’s glands
~Simple acinar – no example in mammals
~Branched acinar – sebaceous or oil glands
Compound Exocrine Glands
~According to shape
~Tubular – mucus glands of oral cavity
~Acinar or alveolar - salivary glands
~Saccular – mammary and prostate glands
~According to nature of secretion
~Mucous – thick secretion
~Serous – watery secretion
~Mixed – both – example – salivary glands - Bnone
- AUnicellular
31Histological Organization of Exocrine Glands
- AEnclosed in sacs and divided into lobes and lobules
Have connective tissue, blood and lymphatic vessels, nerve supply and ducts and ductules
Connective tissues works as stroma (supporting cells or tissue) and epithelium as parenchyma (functional cells or tissue) - Bnone
- AEnclosed in sacs and divided into lobes and lobules
32Control of Exocrine Secretion
- APhysiological (Nervous)
~Varies gland to gland - mostly via Autonomic Nervous System - BHormonal
~Involves gastrin, cholecystokinin, secretin - CDual
~Involving both physiological and hormonal - DExamples
~Smell – entirely nervous through vagus nerve
~Food placed in stomach – both nervous and hormonal
- APhysiological (Nervous)
33Endocrine Glands
- ADuctless – release hormones into blood
Hormones stimulate target cells elsewhere
Develop by evagination of epithelia and lose connection
Hormones – amino acids, proteins, glycoproteins or steroids
Examples – pituitary, thyroid, parathyroid, pancreas, adrenal, pineal, testes, ovaries and placenta - Bnone
- ADuctless – release hormones into blood
34Cytology of Endocrine Glands
- APolypeptide (Protein) Secreting Endocrine Glands
~Cells rich in ribosomes, rough endoplasmic reticulum (RER) and Golgi complex
~Cells capable of storing hormones before release
~Example – pancreas – islets of Langerhans – insulin - BSteroid Secreting Endocrine Glands
~Little RER, few ribosomes, rich in SER, abundant lipid droplets, numerous mitochondria
~Store very little of hormones, store the precursor cholesterol
~Examples – adrenal, ovaries, testes
- APolypeptide (Protein) Secreting Endocrine Glands
35Receptors (R) and Mechanism of Hormone (H) Action
- AProtein or Water Soluble Hormones
~Hormone (first messenger) binds to membrane R and G protein
~Binding activates membrane adenylate cyclase (AC)
~AC hydrolyzes an ATP to cAMP (second messenger)
~cAMP activates intracellular enzyme kinase to initiate a chain of events for cellular response
~Example – conversion of glycogen to glucose by liver cells by hormone epinephrine from adrenal glands - BSteroid Hormones
~No R on PM – H enters cytoplasm and binds to intracellular R
~H-R complex binds to DNA and activates genes for cellular response
~Examples – testosterone, estrogens and progesterone
- AProtein or Water Soluble Hormones
36Relation of Endocrine Cells (EC) to Blood and Lymphatic System
- AEC are richly vascular with extremely thin endothelium
Both blood and lymph are capable of hormone transport
Essentially no barrier – very thin basement membrane of EC and narrow intercellular spaces
Most hormones are released into blood, the only example for lymph are thyroid and testes - Bnone
- AEC are richly vascular with extremely thin endothelium
37Hormonal Control Mechanisms
- ASimplest Form
~Negative feedback system – output inhibits input
~Rise in blood sugar level stimulates pancreas to release insulin
~Insulin causes uptake of glucose into cells
~Resulting low blood sugar level inhibits the release of insulin
~PTH mobilizes Ca++ from bone and increased Ca++ inhibits release of PTH - BComplex Form
~Here the EC are under the control of nervous system
~Here output increases output
~Example – milk ejection in mammary glands
~Suckling of the baby stimulates posterior pituitary to release oxytocin
causing milk ejection – positive feedback
- ASimplest Form
38Diffuse Endocrine System
- ATwo systems to coordinate body activities – nervous and endocrine
There is now a new discipline called Neuro-endocrinology which involves paraneurons
Some substances act both as neurotransmitters and hormones – examples – norepinephrine, vasopressin (ADH)
Chemical messengers in diffuse system are called neuropeptides
20 identified – examples – enteroendocrine or gastroenteropancreatic endocrine cells, gustatory cells, bronchial epithelium - Bnone
- ATwo systems to coordinate body activities – nervous and endocrine
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- Quiz on The Epithelium, created by Ruben De Santiago on 22/08/2021.
- Author
- Ruben De Santiago
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- English
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