Classification

Epitheliums can be classified on appearance or on function

Classification based on appearance

- Simple - one layer of cells

- Pseudostratified - looks like more than one layer but is not

- Stratified - more than one layer of cells

Simple epitheliums

Simple squamous epithelium

 Cells are flat with bulging or flat nuclei.  Lines the insides of lung alveoli and certain ducts in the kidney

 Forms serous membranes called mesothelium that line cavities like: pericardial ,  peritoneal,  plural

 Lines blood vessels - known as endothelium

Simple cuboidal epithelium

It appears square in cross section,  Found in: - Ducts of salivary glands,  Follicles of the thyroid gland,  Pigment layer in the eye,  Collecting ducts of the kidney, In the middle ear is ciliated type.

Simple columnar

• Lines the gastrointestinal tract from the stomach to the anal canal,  Some columnar cells have a  secretory function – stomach, peg cells in the oviduct,  Some columnar cells have microvilli on their free border (striated border) – gall bladder, duodenum
• Microvilli increase the surface area for absorption
• Some columnar cells have cilia – oviduct, smaller bronchi
• Cilia transport particles

Pseudostratified

Appears as stratified epithelium but all cells are in contact with the basement membrane.  Has a thick basement membrane. Different cell types make up this epithelium,  Cells that can be found in this type of epithelium are:

• Columnar cells with cilia or microvilli.
• Basal cells that do not reach the surface.
• Goblet cells that secrete mucous.
• Found in the trachea, epididymus, ductus deferens and female urethra

Stratified epithelium

Classified according to the shape of the surface cells

Stratified squamous epithelium

Has a basal layer that varies from cuboidal to columnar cells that divide to form new cells. Two types are found:

Keratinized:  Mostly forms a dry covering, The middle layers consists of cells that are forming- and filling up with keratin. The superficial cells form a tough non living layer of keratin,  Keratin is a type of protein,  The skin is of this type has  thick skin - found on the hand palms and soles of the feet,  thin skin - found on the rest of the body

Non-keratinized:  Top layer of cells are living cells with nuclei  Forms a wet covering,  The middle layers are polyhedral,  The surface layer consists of flat squamous cells

• Is found in:  mouth,  oesophagus,  vagina

Stratified cuboidal epithelium

Found: - in the ducts of sweat glands

Stratified columnar epithelium

Found at the back of the eyelid (conjunctiva)

Transitional epithelium

- Sometimes the surface cells are squamous, sometimes cuboidal and sometimes columnar

- The superficial cells are called umbrella cells because they can open and close like umbrellas, when the epithelium stretch and shrink

- Umbrella cells can have 2 nuclei

- Found in the bladder and ureter

- The forehead is formed by the smooth, broad, convex plate of bone called the frontal squama.
- In foetal skulls, the halves of the frontal squama are divided by a metopic suture.
- In most people, the halves of the frontal bone begin to fuse during infancy and the suture between is usually not visible after 6 years of age.
- The frontal bone forms the thin roof of the orbits (eye sockets).
- Just superior to and parallel with each supraorbital margin is a bony ridge, the superciliary arch, which overlies the frontal sinus. This arch is more pronounced in males.
- Between these arches there is a gently, rounded, medial elevation called the gabella; this term derives from the Latin word glabellus meaning smooth and hairless. In most people, the skin over the gabella is hairless.
-The slight prominences of the forehead on each side, superior to the superciliary arches, are called frontal eminences (tubers).
- The supraorbital foramen (occasionally a notch), which transmits the supraorbital vessels and nerve is located in the medial part of the supraorbital margin.
- The frontal bone articulates with the two parietal bones at the coronal suture.
-It also articulates with the nasal bones at the frontonasal suture. At the point where this suture crosses the internasal suture in the medial plane, there is an anthropological landmark called the nasion . The depression is located at the root of the nose, where it joins the cranium.
- The frontal bone also articulates with the zygomatic, lacrimal, ethmoid, and sphenoid bones.
In about 8% of adult skulls, a remnant of the inferior part of the metopic (interfrontal) suture is visible. It may be mistaken in radiographs for a fracture line by inexperienced observers.
- The superciliary arches are relatively sharp ridges of bone and a blow to them may lacerate the skin and cause bleeding.
- Bruising of the skin over a superciliary arch causes tissue fluid and blood to accumulate in the surrounding connective tissue, which gravitates into the upper eyelid and around the eye. This results in swelling and a "black eye".
- Compression of the supraorbital nerve as it emerges from its foramen causes considerable pain, a fact that may be used by anaesthesiologists and anaesthetists to determine the depth of anaesthesia and by physicians attempting to arouse a moribund patient.

The Oropharynx

• The oral part of the pharynx has a digestive function.
• It is continuous with the oral cavity through the oropharyngeal isthmus.
• The oropharynx is bounded by the soft palate superiorly, the base of the tongue inferiorly, and the palatoglossal and palatopharyngeal arches laterally.

• It extends from the soft palate to the superior border of the epiglottis.

The Palatine Tonsils

• These are usually referred to as "the tonsils".
• They are collections of lymphoid tissue the lie on each side of the oropharynx in the triangular interval between the palatine arches.

• The palatine tonsils vary in size from person to person.
• In children, the palatine tonsils tend to be large, whereas in older persons they are usual small and inconspicuous.
• The visible part of the tonsil is no guide to its actual size because much of it may be hidden by the tongue and buried in the soft palate.

Eye 

At week 4, two depressions are evident on each of the forebrain hemispheres.  As the anterior neural fold closes, the optic pits elongate to form the optic vesicles.  The optic vesicles remain connected to the forebrain by optic stalks. 
The invagination of the optic vesicles forms a bilayered optic cup.  The bilayered cup becomes the dual layered retina (neural and pigmented layer)
Surface ectoderm forms the lens placode, which invaginates with the optic cup.
The optic stalk is deficient ventrally to contain choroids fissure to allow blood vessels into the eye (hyaloid artery).  The artery feeds the growing lens, but will its distal portion will eventually degenerate such that the adult lens receives no hyaloid vasculature.
At the 7th week, the choroids fissure closes and walls fuse as the retinal nerve get bigger.
The anterior rim of the optic vesicles forms the retina and iris.  The iris is an outgrowth of the distal edge of the retina.
Optic vesicles induces/maintains the development of the lens vesicle, which forms the definitive lens.  Following separation of the lens vesicle from the surface ectoderm, the cornea develops in the anterior 1/5th of the eye.
The lens and retina are surrounded by mesenchyme which forms a tough connective tissue, the sclera, that is continuous with the dura mater around the optic nerve.  
Iridopupillary membrane forms to separate the anterior and posterior chambers of the eye.  The membrane breaks down to allow for the pupil
Mesenchyme surrounding the forming eye forms musculature (ciliary muscles and pupillary muscles – from somitomeres 1 and 2; innervated by CN III), supportive connective tissue elements and vasculature.

Eyelids

Formed by an outgrowth of ectoderm that is fused at its midline in the 2nd trimester, but later reopen.

Endochondral ossification

• A cartilage model exists
• Through intramembraneous ossification in the perichondrium a collar of bone forms around the middle part of the cartilage model
• The perichondrium change to a periostium
• The bone collar cuts off the nutrient and oxygen supply to the chondrocytes in the cartilage model
• The chondrocytes then increase in size and resorb the surrounding cartilage matrix until only thin vertical septae of matrix are left over
• These thin plates then calcify after which the chondrocytes die
• The osteoclasts make holes in the bone collar through which blood vessels can now enter the cavities left behind by the chondrocytes
• With the blood vessels osteoprogenitor cells enter the tissue
• They position themselves on the calcified cartilage septae, change into osteoblasts and start to deposit bone to form trabeculae
• In the mean time the periosteum is depositing bone on the outside of the bone collar making it thicker and thicker
• The trabeculae,consisting of a core of calcified cartilage with bone deposited on top of it, are eventually resorbed by osteoclasts to form the marrow cavity
• The area where this happens is the primary ossification centre and lies in what is called the diaphysis (shaft)
• This process spreads in two directions towards the two ends of the bone the epiphysis
• In the two ends (heads) of the bone a similar process takes place
• A secondary ossification centre develops from where ossification spreads radially
• Here no bone collar forms
• The outer layer of the original cartilage remains behind to form the articulating cartilage
• Between the primary and the secondary ossification centers two epiphyseal cartilage plates remain
• This is where the bone grows in length

• From the epiphyseal cartilage plate towards the diaphysis a number of zones can be identified:

 Resting zone of cartilage

 Hyaline cartilage

 Proliferation zone

 Chondrocytes divide to form columns of cells that mature.

Hypertrophic cartilage zone

 Chondrocytes become larger, accumulate glycogen, resorb the surrounding matrix so that only thin septae of cartilage remain 

Calcification and degeneration zone

The thin septae of cartilage become calcified.

The calsified septae cut off the nutrient supply to the chondrocytes so subsequently they die.

Ossification zone.

Osteoclasts make openings in the bone collar through which blood vessels then invade the spaces left vacant by the chondrocytes that died.

Osteoprogenitor cells come in with the blood and position themselves on the calcified cartilage

septae, change into osteoblasts and start to deposit bone.

 When osteoblasts become trapped in bone they change to osteocytes.

Growth and remodeling of bone

Long bones become longer because of growth at the epiphyseal plates

They become wider because of bone formed by the periosteum

The marrow cavity becomes bigger because of resorbtion by the osteoclasts

Fracture repair

When bone is fractured a blood clot forms

 Macrophages then remove the clot, remaining osteocytes and damaged bone matrix

The periosteum and endosteum produce osteoprogenitor cells that form a cellular tissue in the fracture area

 Intramembranous and endochondral ossification then take place in this area forming trabeculae.

Trabeculae connect the two ends of the broken bone to form a callus

Remodelling then takes place to restore the bone as it was

Joints

The capsule of a joint seals off the articular cavity,  

The capsule has two layers

 fibrous (outer)

synovial (inner)

The synovial layer is lined by squamous or cuboidal epithelial cells,  Under this layer is a layer of loose or dense CT, The lining cells consists of two types:

- A cells

- B cells

They secrete the synovial fluid

They are different stages of the same cell, They are also phagocytic., The articular cartilage has fibres that run perpendicular to the bone and then turn to run parallel to the surface