The Temporalis Muscle

• This is an extensive fan-shaped muscle that covers the temporal region.
• It is a powerful masticatory muscle that can easily be seen and felt during closure of the mandible.

• Origin: floor of temporal fossa and deep surface of temporal fascia.
• Insertion: tip and medial surface of coronoid process and anterior border of ramus of mandible.
• Innervation: deep temporal branches of mandibular nerve (CN V³).

• The temporalis elevates the mandible, closing the jaws; and its posterior fibres retrude the mandible after protrusion.

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.

Mylohyoid Muscle

• Origin: Mylohyoid line of the mandible.
• Insertion: Median raphe and body of the hyoid bone.
• Nerve Supply: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
• Arterial Supply: Sublingual branch of the lingual artery and submental branch of the facial artery.
• Action: Elevates the hyoid bone, base of the tongue, and floor of the mouth; depresses the mandible.

The Masseter Muscle

• This is a quadrangular muscle that covers the lateral aspect of the ramus and the coronoid process of the mandible.

• Origin: inferior border and medial surface of zygomatic arch.
• Insertion: lateral surface of ramus of mandible and its coronoid process.
• Innervation: mandibular nerve via masseteric nerve that enters its deep surface.

• It elevates and protrudes the mandible, closes the jaws and the deep fibres retrude it.

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.

Intramembranous ossification

• Flat bones develop in this way (bones of the skull)
• This type of bone development takes place in mesenchymal tissue
• Mesenchymal cells condense to form a primary ossification centre (blastema)
• Some of the condensed mesenchymal cells change to osteoprogenitor cells
• Osteoprogenitor cells change into osteoblasts which start to deposit bone
• As the osteoblasts deposit bone some of them become trapped in lacunae in the bone and then change into osteocytes
• Osteoblasts lie on the surface of the newly formed bone
• As more and more bone is deposited more and more osteocytes are formed from mesenchymal cells
• The bone that is formed is called a spicule
• This process takes place in many places simultaneously
• The spicules fuse to form trabeculae
• Blood vessels grow into the spaces between the trabeculae
• Mesenchymal cells in the spaces give rise to hemopoetic tissue
• This type of bone development forms the first phase in endochondral development
• It is also responsible for the growth of short bones and the thickening of long bones