Veterinary Histology and Embryology

STRATIFIED EPITHELIA

• Epithelia are classified on the basis of the number of cell layers and the shape of the cells in the surface layer.
• If there is only one layer of cells in the epithelium, it is designated simple.
• If there are two or more layers of cells, it is termed stratified.
• Cells in the surface layer are, as a rule, described according to their height as squamous (scale- or plate-like), cuboidal or columnar.
• The cells are arranged in more than one layer is known as stratified epithlum

Stratified squamous epithelium

• It is the main protective epithelium of body and has several cell layers.
• The number of layers of the epithelium varies in different regions but shape and the arrangement of cells are characteristic.Two types of this epithelium are recognized
  • Keratinizing and
  • Non-keratinizing.
• Deepest layer consists of columnar cells resting on an indistinct basement membrane is called as stratum cylindricum.
• Above this the cells become polyhedral and are usually larger than the basal cells. As the free surface unit is approached the cells are more flattened and at the surface they become squamous. The deeper cells of the basal polyhedral layers (stratum germinativum) are young soft cells with large nuclei rich in chromatin and a finely granular cytoplasm. The intercellular bridges are prominent, giving the cell a prickly appearance (prickle cells) and hence this layer is called stratum spinosum. Mitosis seen in these layers. 
• In the keratinizing type above the stratum germinativum there is stratum granulosum consisting of 2 - 5 rows of flattened; rhomboid cells with basophilic kerato hyalin granules in the cytoplasm.
• Above this is the stratum lucidum consisting of a few row of flattened cells devoid of any nuclei, appearing as a translucent homogenous highly refractive band, in which cell boundaries cannot be made out. The cytoplasm in these cells contains droplets of eleidin.
• Above the stratum lucidum is the most superficial layer the stratum corneum. The thickness of this varies in different locations. It is composed of dead scales with no nuclei and is formed of keratin.
• In the non-keratinizing type, stratum granulosum, stratum lucidum and stratum corneum are usually absent. The extent of the charges in the superficial cells varies with the location and the environment of the epithelium. The epidermis of the skin for instance, is subjected to phagus. Its surface cells are non-nucleated, scale-like and keratinized. The keratinized surface cells being flattened. The process of keratinization in epidermis is so important for bodily protection and is probably dependent on insufficient nutrition of the upper cell layers. The dead scale-like cells are constantly cast off to be replaced by cells from the deeper strata. In man this is slow and continuous process, in some of the lower vertebrates (snakes) there is periodic shedding of the whole superficial layer of the epidermis.

Distribution

• Stratified suqamous epithelium covers the entire body and the orifices of the cavities opening upon it.
• It lines the mucous membrane of the digestive tract from the mouth to the secretory portion of the stomach, and the anal canal, anterior surface of the cornea, bulbar conjuctiva, lacrimal canaliculi, vaginal vestibule, glans penis and elsewhere.

Stratified cuboidal epithelia

• This type of epithelium is comparatively rare. 
• It the most superficial layer of cells are columnar
• They are located in larger excretory ducts of some glands, palpebral conjuctiva of the horse and dog. 

  • The top cells are cuboidal.
  • They are seen in ducts of sweat glands, cells lining antrum of the ovarian follicles.

Stratified columnar epithelia