NEET MDS Lessons
Anatomy
- 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.
Initially, four clefts exist; however, only one gives rise to a definite structure in adults.
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1st pharyngeal cleft |
Penetrates underlying mesenchyme and forms EAM. The bottom of EAM forms lateral aspect of tympanic cavity. |
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2nd pharyngeal cleft |
Undergoes active proliferation and overlaps remaining clefts. It merges with ectoderm of lower neck such that the remaining clefts lose contact with outside. Temporarily, the clefts form an ectodermally lined cavity, the cervical sinus, but this disappears during development. |
Skull bones
- 26 bones: 22 bones + hyoid (small bone in neck for swallowing) + 3 auditory ossicles (middle ear: incus, malleus, stapes)
- 21 bones: tightly connected; mandible is freely mobile at temperomandibular joint (synovial)
- connective-tissue interface b/w bones = suture
- bones – mandible = cranium
- cranium
- neurocranium: covers brain anteriorly, laterally and posteriorly
- brain supported by bones of basicranium
- also contributes to interorbital region; b/w eyes and superior to nasal passages
- viscerocranium/splanchnocranium: bones of face
- sutures
- coronal: separates frontal from parietals
- sagittal: separates two parietal bones
- lambdoidal: separates parietal form occipital
- squamosal: b/w temporal and parietal; overlapping sutures
- At birth: 2 frontal bones which eventually fuse; metopic suture disappears
Cranial Cavities: 5 major cavities
Endocranial, left and right orbits, nasal cavities, oral cavity, middle ear cavities
Endocranial cavity
- contains brain, meninges, cerebrospinal fluid, brain’s vascular supply and most proximal portion of cranial nerves
- enclosed by neurocranium and basicranium
- basicranium: foramina for neurovascular bundles
- foramen magnum: spinal cord exit
- floor of endocranial cavity divide into fossae
- anterior: frontal lobes of brain
- middle: pair temporal lobes
- posterior: cerebellum and brainstem
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
BONE
A rigid form of CT, Consists of matrix and cells
Matrix contains:
organic component 35% collagen fibres
inorganic salts 65% calcium phosphate (58,5%), calcium carbonate (6,5%)
2 types of bone - spongy (concellous)
compact (dense)
Microscopic elements are the same
Spongy bone consists of bars (trabeculae) which branch and unite to form a meshwork
Spaces are filled with bone marrow
Compact bone appears solid but has microscopic spaces
In long bones the shaft is compact bone
And the ends (epiphysis) consists of spongy bone covered with compact bone
Flat bones consists of 2 plates of compact bone with spongy bone in-between
Periosteum covers the bone
Endosteum lines marrow cavity and spaces
These 2 layers play a role in the nutrition of bone tissue
They constantly supply the bone with new osteoblasts for the repair and growth of bone
Microscopically
The basic structural unit of bone is the Haversian system or osteon
An osteon consists of a central Haversian canal
- In which lies vessels nerves and loose CT
- Around the central canal lies rings of lacunae
- A lacuna is a space in the matrix in which lies the osteocyte
- The lacunae are connected through canaliculi which radiate from the lacunae
- In the canaliculi are the processes of the osteocytes
- The canaliculi link up with one another and also with the Haversian canal
- The processes communicate with one another in the canaliculi through gap junctions
- Between two adjacent rows of lacunae lie the lamellae, 5-7µm thick
- In three dimensions the Haversian systems are cylindrical
- The collagen fibres lie in a spiral in the lamellae
- Perpendicular to the Haversian canals are the Volkman's canals
- They link up with the marrow cavity and the Haversian canals
- Some lamellae do not form part of a Haversian system
- They are the:
- Inner circumferential lamellae - around the marrow cavity
- Outer circumferential lamellae - underneath the outer surface of the bone
- Interstitial lamellae - between the osteons
Endosteum
Lines all cavities like marrow spaces, Haversian- and Volkman's canals
Consists of a single layer of squamous osteoprogenitor cells with a thin reticular CT layer underneath it
Continuous with the inner layer of periosteum
Covers the trabeculae of spongy bone
Cells differentiate into osteoblasts (like the cells of the periosteum)
Periosteum
Formed by tough CT
2 layers
Outer fibrous layer: Thickest, Contains collagen fibres,
Some fibres enter the bone - called Sharpey's fibres
Contains blood vessels.
Also fibrocytes and the other cells found in common CT
Inner cellular layer
Flattened cells (continuous with the endosteum)
Can divide and differentiate into osteoprogenitor cells
spindle shaped
little amount of rough EPR
poorly developed Golgi complex
play a prominent role in bone growth and repair
Osteoblasts
Oval in shape, Have thin processes, Rough EPR in one part of the cell (basophilic)
On the other side is the nucleus, Golgi and the centrioles in the middle, Form matrix
Become trapped in the matrix
Osteocytes
Mature cells, Less basophilic than the osteoblasts, Lie trapped in the lacunae, Their processes lie in the canaliculi, Processes communicate with one another through gap junctions, Substances (nutrients, waste products) are passed on from cell to cell
Osteoclasts
Very large, Multinucleate (up to 50), On inner and outer surface of bone, Lie in depressions on the surface called Howships lacunae, The cell surface facing the bone has short irregular processes
Acidophylic
Has many lysosomes, polyribosomes and rough EPR
Lysosomal enzymes are secreted to digest the bone
Resorbs the organic part of bone
Histogenesis
Two types of bone development.
- intramembranous ossification
- endochondral ossification
In both these types of bone development temporary primary bone is deposited which is soon replaced by secondary bone. Primary bone has more osteocytes and the mineral content is lower.
The Palate
- The palate forms the arched roof of the mouth and the floor of the nasal cavities.
- The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.
The Hard Palate
- The anterior bony part of the palate is formed by the palatine process of the maxillae and the horizontal plates of the palatine bones.
- Anteriorly and laterally, the hard palate is bounded by the alveolar processes and the gingivae.
- Posteriorly, the hard palate is continuous with the soft palate.
- The incisive foramen is the mouth of the incisive canal.
- This foramen is located posterior to the maxillary central incisor teeth.
- This foramen is the common opening for the right and left incisive canals.
- The incisive canal and foramen transmit the nasopalatine nerve and the terminal branches of the sphenopalatine artery.
- Medial to the third molar tooth, the greater palatine foramen pierces the lateral border of the bony palate.
- The greater palatine vessels and nerve emerge from this foramen and run anteriorly into two grooves on the palate.
- The lesser palatine foramen transmits the lesser palatine nerve and vessels.
- This runs to the soft palate and adjacent structures.
Connective Tissue
Functions of Connective tissue:
→ joins together other tissues
→ supporting framework for the body (bone)
→ fat stores energy
→ blood transports substances
Connective tissue is usually characterized by large amounts of extracellular materials that separate cells from each other, whereas epithelial tissue is mostly cells with very little extracellular material. The extracellular substance of connective tissue consists of protein fibers which are embedded in ground substance containing tissue fluid.
Fibers in connective tissue can be divided into three types:
→ Collagen fibers are the most abundant protein fibers in the body.
→ Elastic fibers are made of elastin and have the ability to recoil to original shape.
→ Reticular fibers are very fine collagen fibers that join connective tissues to other tissues.
Connective tissue cells are named according to their functions:
→ Blast cells produce the matrix of connective tissues
→ Cyte cells maintains the matrix of connective tissues
→ Clast cells breaks down the matrix for remodeling (found in bone)