Epithelia
The epithelia is characterized by
1) cells resting on a basal lamina
2) has a free surface
3) blood vessels do not penetrate basal lamina
4) epithelial cells have well developed cell junctions
5) cells exhibit polarity (apical, basal ends etc)
6) there is little intercellular space and substance.
The epithelia has the functions of absorption, secretion, excretion, transport, sensory reception, protection.
Epithelia is classified based on shape of the cells, and layering as well as any surface specializations.
Simple Squamous
A
N
Lung Tissue
Simple squamous epithelia of the lung tissue is shown here to emphasize the thin wafer-likeness of the squamous cell. the width is many times the height. The thinness of these cells facilitates the transfer of gas in the lungs or fluids in the kidney etc.
The alveolar sac (A) is lined with simple squamous cells, can see the single nuclei (N) in the layer of cells.
Simple Cuboidal
L
Kidney
Thyroid
Simple cuboidal epithelia can be found in many places, in the kidney it lines the proximal and distal convoluted tubules surrounding a lumen (L).
These cells are also found creating follicles in the ovary as well as the thyroid pictured here.
Cuboidal cells are as tall as they are wide with a central nucleus. the relative thickness can vary with state of the gland (low cuboidal when inactive, tall cuboidal when active).
These cells often contain the cellular machinery that enables a variety of functions to be carried out - secretion, ion exchange.
This cell type is characteristic of glands
Simple Cuboidal
N
C
Intestine
A
B
N
Cuboidal cells (C) are at least 3-4x the cell width. Nucleus (N) is nearest to the basal end (B). the apical end (A) is usually specialized (Cilia in the oviduct, microvilli in the intestine, rich in RER and Golgi etc.)
What might this form mean for function
Stratified Squamous
add new pic of palm skin
c
d
Esophagus
Formed from layers of flattened squamous cells this type of epithelia is found in areas of constant abrasion, it can be keratinized as in the skin or non keratinized as in the vagina and esophagus
cells next to the basal lamina are more rounded (c), towards the surface they become flattened (d), you can tell the width of the cells increases as you move towards the lumen because of the space between nuclei increases. They function in protection from abrasion and desiccation. Where else might you find this type of epithelia?
Stratified cuboidal
A
B
Ovary
Ovarian follicles are composed of an oocyte (B) surrounded by granulose cells that are cuboidal in shape and grow to form many layers (A). The free layer of the stratified epithelia is next to the oocyte.
This type of epithelia can also be found in sweat and sebaceous glands.
Transitional Epithelia
Lumen
E
Lamina Propria
Bladder Relaxed
Lumen
E'
Lamina Propria
Bladder Stretched
You can see the difference between expanded and contracted transitional epithelia, this type of tissue has two layers; the epithelium and the mesothelium
The apical cells are rounded with a scalloped lining when the bladder is relaxed (E), as the bladder is stretched the number of layers will decrease and the apical cells will become flattened (E').
Pseudostratified Epithelia
Lumen
A
B
This epithelia has the appearance of stratification but is really only 1 layer thick. All the cells are attached to the basement membrane. We can see this in the upper respiratory tract of the trachea.
The nuclei are at different level and the cells lack uniformity (they are neither cuboidal, squamous etc.), some cells lose their connection to the free surface, these are the supporting cells (B) and often look pyramidal. The tall superficial cells (A) cover these and can have apical specializations.