Ear ossicle, malleus: easy description and important points

Ear ossicles : malleus, incus ,stapes : short and easy description

Ear osssicles :

The middle ear cavity contains a chain of three small bones the malleus , the incus and the stapes .

Conduction of sound through the middle ear to internal ear is transmitted by these three ossicles. Each ear ossicles is ossified from one ossification centre in the fourth month of intra-uterine life and they attain adult size at birth .

Malleus :

It is the largest bone of middle ear.

Shape: like a round-headed club

Parts:

Head: it lies in the epi- tympanic part of middle ear cavity. Head articulate with the incus forming a siddle shaped synovial joint

Neck:  it situated in between head and handle and lies just medial to pars flaccida of tympanic membrane. Its medial side is related with chorda tympani nerve

Handle: it is attached with medial side of tympanic membrane and its upper part receives insertion of tendon of tensor tympani muscles.

Anterior process: situated just below the neck: it receives attachment of anterior ligament which extends up to spine of sphenoid. This ligament derived from perichondrium of 1^st pharyngeal arch cartilage.

Lateral process: mallear folds are attached to the apex of the lateral process.

Incus

Shape : like premolar tooth

Development : 1^st pharyngeal arch

Parts :

Body : articulate with head of malleus

Short process : directed posteriorly

Long process : it is situated parallel to the upper part of handle of malleus and tip of incus forms a knob known as lentiform nodule which articulate with head of stepes . 

Stepes:

Shape : it looks like stirrup

Development :2^nd pharyngeal arch

Parts :

Head : articulate with lentiform nodule of incus

Neck : receive insertion of stepedius muscle

Anterior and posterior limbs

Base or foot- plate : it is attached with fenestra vestibule by annular ligament

What are the structure seen through auroscope

Lateral process of malleus

Long process of incus

Two small muscles are associate with ear ossicles

1.      Stapedius :

Attachement : origin : interior of hollow pyramid

Insertion : neck of stepes

Nerve supply :  facial nerve

Functions: contraction of this muscle draw the stapes laterally thus tilting its foot plate in the oval window.

 it prevent excessive movement of stapes so its paralysis leads  to uninhibited movement of stapes and excessive acuteness of hearing (hyperacusis) 

SN: paralysis of the stapedius causes an abnormally increased power of

2.      Tensor tympani:

Attachment : origin : bony part of auditory tube

Insertion : handle of malleus

Nerve supply : mandibular division of trigeminal nerve

Functions: contraction of this muscle draw the handle of malleus medially (inward) which makes tympanic membrane more concave laterally  and tense. It prevent wide excursions of the ear ossicles and potential damage to the inner ear when exposed to loud sounds

          Functions of both muscles: damp high frequency vibrations 

Histological features of hard palate

Histology of hard palate :
 The hard palate is a thin horizontal bony plate of the skull, located in the roof of the mouth.
Formations:  It is formed by the palatine process of the maxilla and horizontal plate of palatine bone.

It has two surfaces:

1. nasal surface and 
2. oral surface 

functions : it is important for feeding and speech
 

Histology of nasal surface of hard palate:

1. It is lined by pseudo-stratified ciliated columnar epithelium.
2. This epithelium contains intraepithelial gland.
3. Sub-epithelial connective tissue contains glands, blood vessels and lymph vessels.
4.  The epithelium and the sub-epithelial connective tissue are collectively known as the mucoperiosteum, which is firmly attached to the bony shelf of the palate.

Histology of oral surface of hard palate 

Parts of oral surface of hard palate: 

1. Anteriorly fatty part 

2. Posteriorly glandular part   

 

The oral surface of hard palate  is covered by masticatory mucosa,  it is a specialized mucosa of oral cavity 

 

Masticatory mucosa :

It is found in the gingival (gums) and the hard palate. 
From superficial to deep it has three layers

1.      Lining epithelium: Keratinized stratified squamous epithelium and in some place parakeratinized

Keratinized epithelium of hard palate likes skin but stratum lucidum layer is absent.

Criteria of parakeratinized stratified squamous epithelium :

·         Superficial cells have nuclei like keratinized stratified squamous epithelium but nuclei are pyknotic (highly condensed) remain until the cell is exfoliated and

·         cytoplasm is not intensely stain by eosin

  Rete ridges are present in epithelium they are deep invagination of epithelium with

   subepithelial connective tissue  

2.      Lamina propria is formed by connective tissue contain dense collagen fibres, blood vessels and nerves. Hard palate's lamina propria  is also containing large deposits of adipose tissue at its anterior part and mucous gland at its posterior part.
3.  Reticular layer of dense connective tissue : Deep to lamina propria contain reticular fibre rich dense connective tissue which firmly bind the palatine mucosa to the periosteum. 

Glands in general : easy and brief description

Glands:

Definition: glands are specialized epithelium which are formed by proliferation of epithelium into the underlying tissues

Parts of a gland:

Parenchyma: glandular portion which responsible for secretion

Stroma: it is the supporting portion of a gland which is formed by connective tissue

Classification of glandular epithelium

 According to presence or absence of duct: glands are two types: exocrine gland (gland with duct) and endocrine gland (gland without duct)

According to the mode of secretion :gland are three types

•         Merocrine: secretory granules leave the cell by exocytosis.  exm: pancreatic acinar cells secretion

•         Apocrine: secretion discharge with parts of apical cytoplasm. exm: lactating mammary glands   

•         Holocrine: secretory product is shed with the whole cell. exm: sebaceous gland                       

According the nature of secretion : glands are three types

•         Mucous : viscous secretin. exm: goblet cells

•         Serous: thin secretion. exm: parotid gland contain only serous acini

•         Seromucous: exm: submandibular glands

Distribution of different types of exocrine glands:

 According to the number of cells. Exocrine glands are two types

1) Uni-cellular: goblet cells

2) Multi-cellular: they are two types’ simple (single duct) and compound (more than one duct)

Simple (single duct)	Compound (more than one duct)
Simple tubular: G.Of L.intestine Simple acinar: G.Of penile part of urethra Simple coiled tubular : Sweat gland Simple bran. Acinar: Sebaceous gland Simple bran. Tubular: G of pylorus	Com. Bra. Tubular: Brunner’s gland Com. Acinaer g: pancreas Com. Tubuloacinar: Submandibular gland

Distribution of different types of endocrine glands:

Cord or clump type: ex: Adrenal glands

Follicular type: ex: Thyroid