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

The skull, the skeleton of the head, is the most complex bony structure in the body because it:

1. Encloses the brain, which is irregular in shape;
2. Houses the organs of special senses for seeing, hearing, tasting, and smelling; and
3. Surrounds the openings in to the digestive and respiratory tracts.

• In the anatomical position, the skull is oriented so that the inferior margin of the orbit (eye socket) and the superior margin of the external acoustic meatus (auditory canal) are horizontal. This is called the orbitomenial plane (Frankfort plane).

• The term cranium (L. skull) is sometimes used when referring to the skull without the mandible (lower jaw), but the cranium is often used when referring to the part of the skull containing the brain.
• The superior part is the box-like structure called the calvaria (cranial vault, brain case); the remainder of the cranium, including the maxilla (upper jaw), orbits (eyeball sockets) and nasal cavities, forms the facial skeleton.
• The term skullcap (calotte) refers to the superior part of the calvaria, which is removed during autopsies and dissections. The inferior aspect of the cranium is called the cranial base.

Tongue 
Appears at 4th week.
Musculature derived from mesoderm of occipital somites.  Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
              Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
              Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue.  Midline frenulum continues to anchor tongue to floor of mouth.

Thyroid Gland

Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum).  It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development.  Thyroid gland descends to a point just caudal to laryngeal cartilages. 

Facial structures (general)

a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines

Nose

At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour.  Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP.  Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes).  These nasal prominences form the nose.

Mouth 

Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch.  The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence.  The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion.  Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).

Nasolacrimal structures

Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove.  Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm.  The epithelial cord canalizes to form the nasolacrimal duct.  The upper end of the duct widens to form the lacrimal sac.  After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.  
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.

Secondary (hard) palate

Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences.  These appear at the 6th week.  They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.  
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate.  At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate.  The incisive foramen marks the midline between the primary and secondary palate.

External Ear

The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear).  These fuse to form the definitive auricle.  At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.  
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.

Middle Ear

The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).  
The first arch cartilage forms the malleus and incus.  The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes.  The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.

Inner Ear

The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes).  The placodes invaginate to form otic vesicles (otocytes).  The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.

Cochlear Duct

Derived from an outgrowth of the saccule during the 6th week.  The outgrowth penetrates the surrounding mesenchyme in a spiral fashion.  The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct.  They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.  

Semicircular canals

The utricle is initially three flattened outpocketings, which lose the central core.  From this three semicircular canals are forms, each at 90 degree angles from one another.  Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule. 

Cardiac Muscle

Fibres anastomose through cross bridges

Fibres are short, connected end to end at intercalated discs, also striated,  contract automatically

Light microscopic Structure:

Short fibres connected at intercalated disks,  85 - 100 µm long,  15 µm

same bands as in skeletal muscle,  1 or 2 nuclei - oval and central,  in perinuclear area is a sarcoplasmic reticulum, intercalated discs lie at the Z line

Electron microscopic structure:

 Between myofibrils lie the mitochondria,  2,5 µm long mitochondria,  dense cristae

and are as long as the sarcomere, fibres have more glycogen than skeletal muscle fibres

myofilaments, actin and myosin are the same as in skeletal muscle,  the sarcoplasmic reticulum differs in that there is no terminal sisterna. The sarcotubules end in little feet that

sit on the T-tubule

Intercalated Disc:

on Z lines,  fibres interdigitate,

 3 types of junctions in the disc

Transverse Part:

zonula adherens

desmosomes

Lateral Part:

Gap junctions (nexus) - for impulse transfer

Mechanism of Contraction:

slide - ratchet like in skeletal muscle, certain fibres are modified for conduction,  Impulses spread from cell to cell through gap junctions,  Purkinje cells are found in the AV bundle

they have less myofibrils,  lots of glycogen and intercalated discs

Connective tissue coverings:

Only endomycium in cardiac muscle,  Blood vessels, lymph vessels and nerves lie in the endomycium

Levator Palpebrae Superioris Muscles

• This is a thin, triangular muscle that elevates the upper eyelid.
• It is continuously active except during sleeping and when the eye is closing.

• Origin: roof of orbit, anterior to the optic canal.
• Insertion: this muscle fans out into a wide aponeurosis that inserts into the skin of the upper eyelid. The inferior part of the aponeurosis contains some smooth muscle fibres that insert into the tarsal plate.
• Innervation: the superior fibres are innervated by the oculomotor nerve (CN III), and the smooth muscle component is innervated by fibres of the cervical sympathetic trunk and the internal carotid plexus.

Illnesses involving the Levator Palpebrae Superioris

• In third nerve palsy, the upper eyelid droops (ptosis) and cannot be raised voluntarily.
• This results from damage to the oculomotor nerve (CN III), which supplies this muscle.

• If the cervical sympathetic trunk is interrupted, the smooth muscle component of the levator palpebrae superioris is paralysed and also causes ptosis.
• This is part of Horner's syndrome.

The Rectus Muscles

• There are four rectus muscles (L. rectus, straight), superior, inferior, medial and lateral.

• These arise from a tough tendinous cuff, called the common tendinous ring, which surrounds the optic canal and the junction of the superior and inferior orbital fissures.
• From their common origin, these muscles run anteriorly, close to the walls of the orbit, and attach to the eyeball just posterior to the sclerocorneal junction.

• The medial and lateral rectus muscles attach to the medial and lateral sides of the eyeball respectively, on the horizontal axis.
• However, the superior rectus attaches to the anterosuperior aspect of the medial side of the eyeball while the inferior rectus attaches to the anteroinferior aspect of the medial side of the eye.

The Oblique Muscles

The Superior Oblique Muscle

• This muscle arises from the body of the sphenoid bone, superomedial to the common tendinous ring.
• It passes anteriorly, superior and medial to the superior and medial rectus muscles.
• It ends as a round tendon that runs through a pulley-like loop called the trochlea (L. pulley).
• After passing though the trochlea, the tendon of the superior oblique turns posterolaterally and inserts into the sclera at the posterosuperior aspect of the lateral side of the eyeball.

The Inferior Oblique Muscle

• This muscle arises from the maxilla in the floor of the orbit.
• It passes laterally and posteriorly, inferior to the inferior rectus muscle.
• It inserts into the sclera at the posteroinferior aspect of the lateral side of the eyeball.

Muscles of the Soft Palate

The Levator Veli Palatini (Levator Palati)

• Superior attachment: cartilage of the auditory tube and petrous part of temporal bone.
• Inferior attachment: palatine aponeurosis.
• Innervation: pharyngeal branch of vagus via pharyngeal plexus.
• This cylindrical muscle runs inferoanteriorly, spreading out in the soft palate, where it attaches to the superior surface of the palatine aponeurosis.
• It elevates the soft palate, drawing it superiorly and posteriorly.
• It also opens the auditory tube to equalise air pressure in the middle ear and pharynx.

The Tensor Veli Palatini (Tensor Palati)

• Superior attachment: scaphoid fossa of medial pterygoid plate, spine of sphenoid bone, and cartilage of auditory tube.
• Inferior attachment: palatine aponeurosis.
• Innervation: medial pterygoid nerve (a branch of the mandibular nerve).
• This thin, triangular muscle passes inferiorly, and hooks around the hamulus of the medial pterygoid plate.
• It then inserts into the palatine aponeurosis.
• This muscle tenses the soft palate by using the hamulus as a pulley.
• It also pulls the membranous portion of the auditory tube open to equalise air pressure of the middle ear and pharynx.

The Palatoglossus Muscle

• Superior attachment: palatine aponeurosis.
• Inferior attachment: side of tongue.
• Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.
• This muscle, covered by mucous membrane, forms the palatoglossal arch.
• The palatoglossus elevates the posterior part of the tongue and draws the soft palate inferiorly onto the tongue.

The Palatopharyngeus Muscle

• Superior attachment: hard palate and palatine aponeurosis.
• Inferior attachment: lateral wall of pharynx.
• Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.
• This thin, flat muscle is covered with mucous membrane to form the palatopharyngeal arch.
• It passes posteroinferiorly in this arch.
• This muscle tenses the soft palate and pulls the walls of the pharynx superiorly, anteriorly and medially during swallowing.

The Musculus Uvulae

• Superior attachment: posterior nasal spine and palatine aponeurosis.
• Inferior attachment: mucosa of uvula.
• Innervation: cranial part of accessory through the pharyngeal branch of vagus, via the pharyngeal plexus.
• It passes posteriorly on each side of the median plane and inserts into the mucosa of the uvula.
• When the muscle contracts, it shortens the uvula and pulls it superiorly.