EPITHELIUM

Question 1

Division of epithelium according to Function

Answer 1

Overview:

-One of the 4 basic body tissues

• Epithlium
• CT
• Muscle
• Nerve

-Developed from all three germinal layers

• Ectoderm:
  
  • Epithelium of skin and skin appendages
• Mesoderm
  
  • Epithelium of serous membranes (mesothelium): peritoneum, pleural, and pericardial cavities
  • Cells lining the inner surface of blood and lymphatic vessels (endothelium)
• Endoderm: epithelium of the respiratory and gastrointestinal tract

-Contain general characteristics:

• Cells are tightly arranged
  
  • With little extracellular space
  • And connected with intracellular junctions
• Attahced to basal lamina that seperates them from surrounding tissue
• Functional and morphological polarity
  
  • Apical, basal, and lateral domains
  • Cell surface/Cilia, Stereocilia, Microvilli, Basolateral labyrinth.
• High regenerration activity
• The epithelial component of organs consiting of several types of tussue is refered to as parenchyma (e.g hepatocytes in liver) ?????
• The typical intermediate fillament of the epithelial cellular cytoskeleton are cytokeratin fillaments

-Classification of epithelial cells

• According to arrangment of cells
• According to the number cell layers
• According to shape of cells
• According to function

Answer: Division of epithelium according to Function

Covering/Protection:

• Physical protection by covering and lining surfaces, or by being a selective barrier
  
  • Epithelial tissue forms a selective barrier, protecting the underlying organs from mechanical and chemical insults such as intoxication, tearing and infections.
  • This is one reason why epithelia doesn’t have blood vessels, as abrasion could result in tearing of the vessel and bleeding.
  • Epithelia specialized for protection are usually multilayered and have a high cell renewal rate. This means that they repair quickly after injury.
• Localization
  
  • Exterior body surface
    
    • e.g. skin (st. sq. k)
  • Internal body closed cavities
    
    • e.g. urinary bladder – transitional
    • vagina (st. sq)
    • anorectal junction (st. cub)

Absorption:

• Cells with bsorption function might have speciliized apical surfaces to increase increase the absorptive surface area.
• Localization:
  • Columnar epithelium in the small intestine
    
    • ​These cells function to absorb nutrients from the digestive tract, then transport the digested substances into the circulation.
  • Nutrients absorption in the proximal convoluted tubules in the kidney
    
    • that absorbed to the body.
    • (Absorptive function – brush border, basal labyrinth
      
      • Very rich brush border, alkaline phosphate activity. only proximal tubules have alkaline phsphate activity, distal tubules are free of this
    • but also ion-transporting function (Na+/K + pump))

Respiratory

• Alveolar lining, respiratory epithelium is composed of two cell types:
  
  • Pneumocytes type I:
    
    • Extremely squamous cells that line all alveolar surfaces
    • Connected to each other by tight junctions
    • are the main sites for gaseous exchange.
    • Form the blood-air barrier, together with the endothelial cells of the capillaries and the basement membrane between the two cells.
• Pneumocytes type II​
  
  • Septal cells
  • Fewer in number and cuboidal in shape.
  • Contain dense-staining lamellar bodies in their apical cytoplasm.
  • phospholipids, GAG, and proteins.
  • Protruding apical surface is furnished with the short microvilli.
  • Synthesize and secrete a phospholipid-rich product called pulmonary surfactant.
  • Released into the alveolus
  • Surfactant spreads as a thin layer over the surfaces of type I alveolar cells, lowering the alveolar surface tension.
  • The reduced surface tension in the alveoli decreases the force that is needed to inflatealveoli during inspiration.
  • Stabilizes the alveolar diameters
  • Facilitates their expansion
  • Prevents their collapse during respiration
  • by minimizing the collapsing forces
  • Can divide and function as stem cells for type I squamous alveolar cell

Sensory:

• Receive and transduce external stimuli
• Strucutre:
  
  • Consists of sensory cells and supporting cells
    
    • Sensory cells are receptors, convert external stimuli into electric impulses
    • Sensory epithlium of the inner ear consists of sensory hair cells and supporting cells.
      
      • Hair cells are mechanoreceptors. Apical surface carries highly organized system of sterocilia and one kino- cilium. Stereocilia are rigid structures and have mechanical-gated ion channels on their tops.
      • Flection of stereocilia to kinocilium causes excitation (depolarization of the plasma membrane)
• Classification according stimulus:
  
  • Photoreceptors: rods and cones of the retina
  • Chemoreceptors: olfactory cells (smell, olfaction) and taste cells
  • Mechanoreceptors: hair cells of the inner ear (auditory /sound perception/ and vestibular /balance/ systems)
• Classification according structure:
  
  • Primary sensory cells have axon
    
    • Rods, cones, and olfactory cells
  • Secondary sensory cells dont have an axon
    
    • Hair and taste sensory cells
• Location: Rods and cons in retina, olfactory in nasal cavity, taste buds, harry cells in the acoustic apparatus in the inner ear.

Myoepithelial cells

• Contractile cells
• Shape:
  
  • Very flat, squamous cells lying between the basement membrane and the secretory cell.
• Surrounding secratory passages of glands and their ducts
  
  • Facilliate the transportation of the secretion from glands
  • Contains Actin and Myosin Myofilaments- BUT it is an epithelial cell, not in a muscle tissue!
• Location:
  
  • Surround the glandular epithelial cells in:
    
    • the salivary glands,
    • eccrine and apocrine glands in the skin,
    • mammary gland,
    • dilatator pupillae muscle

Germinative/Spermiogennic

Males:

• Germinal epithelium is also known as the wall of the seminiferous tubule within the testis.
• Contents: germ line cells, supporting Sertoli cells
• The innermost layer of the testicle.
• Function:
  
  • The seminiferous epithelium of testis produces spermatozoa.
  • Supporting Sertoli cells (part of a seminiferous tubule and helps in the process of spermatogenesis; that is, the production of sperm).

Females:

• The ovarian surface epithelium, also called the germinal epithelium of Waldeyer
  
  • Despite its name, the germinal epithelium is not the site of germ cell differentiation.
• Layer of simple squamous-to-cuboidal epithelial cells covering the outer layer of the ovary - peritoneum ????????

Urothelium (transitional epithelium)

• Definition:
  
  • Stratified epithelium with cells of variable height and shape
• Structure:
  
  • There are three cell layers (stratum basale, stratum intermedium, superficial layer) between the basement membrane and the surface.
  • Empty bladder: multinucleated umbrella cells are hemispherical
  • Full bladder: umbrella cells are flattened
• Properties:
  
  • Adapts to variations in pressure/volumetric load; permeability barrier for urine and substances dissolved in urine
• Distribution
  
  • Throughout the urinary tract: renal pelvis, ureter, bladder, proximal urethra
• Exceptions: parts of the male urethra

7. Secretory/Glandular epithelium

Question 2

Basic Division according to structure – Question 10

According to arrangement/type of cells:

Answer 2

Overview:

-One of the 4 basic body tissues

• Epithlium
• CT
• Muscle
• Nerve

-Developed from all three germinal layers

• Ectoderm:
  
  • Epithelium of skin and skin appendages
• Mesoderm
  
  • Epithelium of serous membranes (mesothelium): peritoneum, pleural, and pericardial cavities
• Cells lining the inner surface of blood and lymphatic vessels (endothelium)
• Endoderm: epithelium of the respiratory and gastrointestinal tract

-Contain general characteristics:

• Cells are tightly arranged
  
  • With little extracellular space
  • And connected with intracellular junctions
• Attahced to basal lamina that seperates them from surrounding tissue
• Functional and morphological polarity
  
  • Apical, basal, and lateral domains
  • Cell surface/Cilia, Stereocilia, Microvilli, Basolateral labyrinth.
• High regenerration activity
• The epithelial component of organs consiting of several types of tussue is refered to as parenchyma (e.g hepatocytes in liver) ?????
• The typical intermediate fillament of the epithelial cellular cytoskeleton are cytokeratin fillaments

-Classification of epithelial cells

• According to arrangment of cells
• According to the number cell layers
• According to shape of cells
• According to function

1. Plane/Planer/Epithelium sitting in a surface, covering function on a surface,
2. Trabecular Epithelium forming trabecula’s, pancreas (isle of Langerhans), Hepatocytes as well.
3. Reticular Epithelium Thymus gland, holding T Cells, only in the thymus gland. Cells are organized in three-dimensional network (stroma of thymus behind the sternum, epithelium of crypts in tonsils, stellate reticulum of enamel organ). We can tell its epithelium due to the presence of desmosomes.

SIMPLE

Plane Epithelium: always start with simple /stratified.  How many layers of cells are lying on the basement membrane? • Simple epithelium: one layer of cells on the basement membrane. 1. Simple Squamous epithelium: alveoli, capillaries, outer sheet of bowman capsule. Function: Diffusion 2. Simple cuboidal: loop of helne, ovaries duct, thyroid gland follicles. Function: Secretion 3. Simple columnar epithelium: gall bladder, stomach, intestines, the uterus Function: Secretion and Absorption 4. Pseudostratified columnar: looks like, false, simple epithelium, ciliated Nasal, trachea, bronchi, and larynx: considered to be a simple epithelium!!

STRATIFIED

Stratified Epithelium - Stratified squamous epithelium: 1. Non keratinizing: oral cavity, vagina, esophagus, rectum, vocal cords larynx 2. Keratinizing flat dead cells with no nucleus: Dermis, on the skin. - Stratified columnar epithelium: Best example is the big part in the male urethra?? Transitional zones: anorectal junction, uterine cervix junction (transition into non k stratified squamous epithelium)

TRANSITIONAL

From the pelvic part of kidney: Ureter, Urinary bladder, Urethra considered a simple epithelium and not stratified because all cells are connected to the basal membrane. They can change their approach, turning very flat and coming back to “high” position. Umbrella cells covering the transitional epithelium. - Lines urinary passages, is a special form of the covering epithelium. - according of arrangement of cells: pseudostratified epithelium - Is impermeable for salts and water, is able to adapt to the distension of organ. Adaptability of epithelium to distension is enabled by a unique structure of luminal cell membrane that exhibits modified areas – plaques. Plaques appear to be more rigid and thicker (12 nm) and actin filaments are attached to the inner surface of plaques. In the undistended epithelium plaques in fold inward the cytoplasm and appear as fusiform vesicles– their lamina, however, are in continuity with the cell surface. As organ distends, the fusiform vesicles unfold and become part of the surface as the The cell stretches and flattens.

Question 3

Types of secretory cells, classification of glands

Secretory Epithelium

Answer 3

Classification according to secretion type

1. Endocrine Glands:

• The secreted hormones are released in to the blood!
• They are transported by the blood to the target cells, the hormones are produced by the endocrine cells.
  
  • *thyroid (follicular)
  • *Adrenal cortex, ant lobe of hypophysis (Trabecular endocrine gland)

2. Exocrine glands:
   * The secret is released and transported by a system of ducts to the target place System of Ducts?

Exocrine – Digestive enzymes to GI tract, Sweat glands, salivary glands

3. Amphicrine secretion: endocrine and exocrine *Heterotypic: in one organ, pancreas *homotypic: in one cell, hepatocytes (biles, IGF1, insulin type growth factor)
4. Paracrine secretion
5. Neurocrine secretion
6. Autocrine secretion (immune)

classification according to mechanism of secretion

Exocrine Glands:

a. Merocrine gland – secrete products by means of exocytosis (form of active transport in which a cell transports molecules, such as proteins, out of the cell (Exocytosis) by expelling them in an energy-using process.) – Mucous, serous. - Mixed Exocrine gland: will contain serous cells and mucous cells!  Very important to recognize when looking at the slide. Staining will be HE, Mucus will be stained lighter! b. Apocrine gland – involves loss of a large membrane-enclosed portion of apical cytoplasm. This apical portion of the cell may subsequently break down to release its contents during passage into the duct. c. Holocrine gland – secretion is produced by the breaking down of the mature secretory cells themselves as they complete differentiation which involves becoming filled with product.

Question 4

Cell surface specializations

Structure of intercellular junctions

Cell: functional and morphological polarity * not a real question

Basal Lamina - Basement membrane

REVISE!!!

Answer 4

The basal domain of epithelial cells is characterized by several

• Features:
  
  • The basement membrane is a specialized structure located next to the basal domain of epithelial cells and the underlying connective tissue stroma.
  • Cell-to-extracellular matrix junctions anchor the cell to the extracellular matrix; they are represented by focal adhesions and hemidesmosomes.
  • Basal cell membrane infoldings increase the cell surface area and facilitate morphologic interactions between adjacent cells and extracellular matrix proteins

Basement membrane

• The basal lamina is the structural attachment site for overlying
• epithelial cells and underlying connective tissue.
• basal lamina or, sometimes, lamina densa.
  
  • discrete layer of electron-dense matrix material 40- to 60-nm thick between the epithelium and the adjacent connective tissue (Fig. 5.28) called the
    
    • Network of fine, 3- to 4-nm filaments composed of laminins,
    • a type IV collagen molecule,
    • various associated proteoglycans and glycoproteins.
• the lamina lucida (also about 40 nm wide)
  
  • Between the basal lamina and the cell is a relatively clear or electron-lucent area,
  • . The area outlined by the lamina lucida contains extracellular portions of CAMs, mainly fibronectin and laminin receptors.
  • These receptors are members of the integrin family of transmembrane proteins.
• The basal lamina in nonepithelial cells is referred to as the
• external lamina.
  
  • Muscle cells, adipocytes, and peripheral nerve supporting cells exhibit an extracellular electron-dense material that resembles the basal lamina of epithelium.
    
    • This material also corresponds to a PAS-positive staining reaction, as described earlier
    • Although the term basement membrane is not ordinarily applied to the extracellular stainable material of these nonepithelial cells in light microscopy, the terms basal lamina or external lamina are typically used at the EM level.

The basal lamina contains molecules that come together to form a sheetlike structure.

• proteins that can be classified into four groups:
  
  • collagens, laminins, glycoproteins, and proteoglycans.
  • These proteins are synthesized and secreted by the epithelial cells and other cell types that possess an external lamina.
• Collagens.
  
  • At least three types of collagen species are present in the basal lamina; they represent a fraction of the approximately 28 types of collagen found in the body.
  • The major component, comprising 50% of all basal lamina proteins, is type IV collagen.
  • Two nonfibrillar types of collagens, type XV collagen and type XVIII collagen, are also found in the basal lamina.
    
    • Type XV collagen plays an important role in stabilizing the structure of the external lamina in skeletal and cardiac muscle cells,
    • whereas type XVIII collagen is mainly present in vascular and epithelial basal laminae and is believed to function in angiogenesis.
  • In addition, type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina
• • Laminins.
  
  • These cross-shaped glycoprotein molecules (140 to 400 kilodaltons) are composed of three polypeptide chains.
    
    • They are essential in initiating the assembly of the basal lamina.
    • Laminins possess binding sites for different integrin receptors in the basal domain of the overlying epithelial cells.
    • They are involved in many cellto– extracellular matrix interactions.
• • Entactin/nidogen.
  
  • This small, rodlike sulfated glycoprotein (150 kilodaltons) serves as a link between laminin and the type IV collagen network in almost all basal laminae.
• Proteoglycans.
  
  • Most of the volume of the basal lamina is probably attributable to its proteoglycan content.
  • Proteoglycans consist of a protein core
    
    • to which heparan sulfate (e.g., perlecan, agrin), chondroitin sulfate (e.g., bamacan), or dermatan sulfate side chains are attached.
  • Because of their highly anionic character, these molecules are extensively hydrated.
  • They also carry a high negative charge; this quality suggests that proteoglycans play a role in regulating the passage of ions across the basal lamina.
    
    • perlecan (400 kilodaltons).
      
      • binding to laminin, type IV collagen, and entactin/nidogen.
      • Agrin (500 kilodaltons) is another important molecule found almost exclusivelythe glomerular basement membrane of the kidney.