1. Exocrine Gland
The exocrine gland is a gland that secretes its products onto the apical surface via epithelia ducts or tubes connected to the apical surface.
2. Endocrine Gland
The endocrine gland is a gland that secretes its products basally, thus the secretion goes through the basal lamina, moves into the underlying tissue and enters the vascular system. Endocrine glands are ductless.
Paracrine Glands - Paracrine glands are similar to endocrine glands but their secretions reach the target cells by diffusion through the extracellular space. The secretory products of paracrine glands are not delivered to their target tissue via ducts or the bloodstream.
Tuesday, February 7, 2012
Glandular Epithelia
Glandular Epithelia are epithelia that forms the secretory portions of glands and their ducts.
Thursday, February 2, 2012
Stratified Epithelium
1. Stratified Squamous Epithelium
Stratified Squamous Epithelium consists of squamous(flattened) epithelial cells arranged in layers upon the basement membrane. A single layer is in contact with the basement membrane while the other layers adhere to one another in order to maintain structural integrity. Despite being referred as squamous, many cells within the layers may not be flattened; this is due to the conventional naming of epithelia based on the cell type at the surface. In the deeper layers, the cells may be columnar or cuboidal.
The stratified squamous epithelium is well suited to areas of the body subjected to constant abrasion. Since this is the thickest, layers can be sequentially sloughed off and replaced before the basement membrane is exposed. Hence, the main function of this type of epithelia is for protection.
The stratified squamous epithelium may either be keratinized or non-keratinized. Keratin is a tough protective protein found at the apical surface.
The non-keratinized stratified squamous epithelium include the oral cavity, vagina, oesophagus, etc. The non-keratinized type must be kept moist by bodily secretions to prevent them from drying out.
The keratinized stratified squamous epithelium include the outer layers of the skin.
2. Stratified Cuboidal Epithelium
Stratified cuboidal epithelium is a rare type of epithelial tissue which consists of cube-shaped cells that are arranged in mmultiple layers.
The chief function of this type of epithelial tissue is to provide protection.
They are found in the larger ducts of various glands such as the sweat glands as well as the mammary glands.
3. Stratified Columnar Epithelium
Stratified columnar epithelium is a rare type of epithelia. The upper layer of this type of epithelia consists of column-shaped cells while the lower layers are made up of cube-shaped cells. They also contain specialized secretory cells called goblet cells that stores mucus.
The main function of this type of epithelia is for protection and secretion of mucin(mucin is a heavily glycosilated protein which helps in forming chemical barriers, lubrication and cell signalling).
The stratified columnar epithelia are found in the intralobular ducts in the salivary glands, the male urethra, and the female'r uterus.
4. Trnsitional Epithelium
The transitional epithelium or also known as the 'urothelium' is a type of epithelial tissue that consists of multiple layers of epithelial cells that can contract and expand. These cells appear cuboidal in shape with a domed apex when the organ or tube they reside in is not stratched. When the bladder is filled, the compress and the cells become stratched giving them a squamous appearance.
The main function of this epithelia is for protection and distensibility.
The transitional epithelia is found in the inner lining of the urinary bladder, ureter and part of the urethra.
Stratified Squamous Epithelium consists of squamous(flattened) epithelial cells arranged in layers upon the basement membrane. A single layer is in contact with the basement membrane while the other layers adhere to one another in order to maintain structural integrity. Despite being referred as squamous, many cells within the layers may not be flattened; this is due to the conventional naming of epithelia based on the cell type at the surface. In the deeper layers, the cells may be columnar or cuboidal.
The stratified squamous epithelium is well suited to areas of the body subjected to constant abrasion. Since this is the thickest, layers can be sequentially sloughed off and replaced before the basement membrane is exposed. Hence, the main function of this type of epithelia is for protection.
The stratified squamous epithelium may either be keratinized or non-keratinized. Keratin is a tough protective protein found at the apical surface.
The non-keratinized stratified squamous epithelium include the oral cavity, vagina, oesophagus, etc. The non-keratinized type must be kept moist by bodily secretions to prevent them from drying out.
The keratinized stratified squamous epithelium include the outer layers of the skin.
2. Stratified Cuboidal Epithelium
Stratified cuboidal epithelium is a rare type of epithelial tissue which consists of cube-shaped cells that are arranged in mmultiple layers.
The chief function of this type of epithelial tissue is to provide protection.
They are found in the larger ducts of various glands such as the sweat glands as well as the mammary glands.
3. Stratified Columnar Epithelium
Stratified columnar epithelium is a rare type of epithelia. The upper layer of this type of epithelia consists of column-shaped cells while the lower layers are made up of cube-shaped cells. They also contain specialized secretory cells called goblet cells that stores mucus.
The main function of this type of epithelia is for protection and secretion of mucin(mucin is a heavily glycosilated protein which helps in forming chemical barriers, lubrication and cell signalling).
The stratified columnar epithelia are found in the intralobular ducts in the salivary glands, the male urethra, and the female'r uterus.
4. Trnsitional Epithelium
The transitional epithelium or also known as the 'urothelium' is a type of epithelial tissue that consists of multiple layers of epithelial cells that can contract and expand. These cells appear cuboidal in shape with a domed apex when the organ or tube they reside in is not stratched. When the bladder is filled, the compress and the cells become stratched giving them a squamous appearance.
The main function of this epithelia is for protection and distensibility.
The transitional epithelia is found in the inner lining of the urinary bladder, ureter and part of the urethra.
Wednesday, February 1, 2012
Simple Epithelium
1. Simple Squamos Epithelium
Simple squamos(pavement) epithelium consists of a single layer of flat, thin cells. Simple squamos epithelium have horizontally flattened, elliptical nuclei which corresponds to the form of the cell.
The thin nature of this type of epithelium makes it ideal for small molecules(such as respiratory gases) to pass through its membranes through diffusion and filtration.
The simple squamos epithelia is found lining the alveaoli as well as the glomerulus, where rapid diffusion is a required.
2. Simple Cuboidal Epithelium
Simple Cuboidal Epithelium consists of a single layer of cube-shaped cells, that have a spherical nucleus in the centre.
The simple cuboidal epithelium constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes. They also offer some degree of protection(by actively or passively pumping material in and out of the lumen) and function in absorption and secretion.
The simple cuboidal epithelia are found lining the surfaces of the ovary, nephron and the walls of the kidney tubules.
3. Simple Columnar Epithelium
Simple Columnar Epithelium consists of a single layer of tall column-shaped cells with an elongated nuclei that is located near the base of the cell.The simple columnar epithelia may or may not have cilia, thus dividing them into two categories; ciliated and non-ciliated.
The primary function of the simple columnar epithelium is protection(for instance, this epithelium in the stomach and digestive tract serves as an impermeable barrier against bacteria). The simple columnar epithelia can also specialize to secrete mucus(a mixture of glyprotein and water) which is a slippery substance produced by the mucus membranes. The cilia on the ciliated simple columnar epithelium helps sweep the mucus which traps infectious agents in a certain direction.
The simple columnar epithelia are found lining the stomach and digestive tract as well as the wall of the uterine tube where currents generated by the beating of cilia propel the egg cell towards the uterus.
4. Pseudostratified Columnar Epithelium
Pseudostratified Columnar Epithelium consists of a singe layer of cells with nuclei located at different levels making it seem as though as it is stratified; this is the main reason why it is called pseudostratified(pseudo=false) which literally means falsely stratified. These epithelia may be ciliated or non-ciliated or may contain stereocilia.
The pseudostratified columnar epithelia function in secretion and absorption. The ciliated type are usually involved with the sweeping of mucus.
The ciliated pseudostratified columnar epithelia are found lining the trachea as wee as certain parts of the upper respiratory tract. The non-ciliated pseudostratified columnar epithelia are found in the membraneous part of the male vas deferens. The pseudostratified columnar epithelia with stereocilia are located in the epididymis(part of the male reproductive system). The stereocilia are distinct from cilia but are both closely related.
Simple squamos(pavement) epithelium consists of a single layer of flat, thin cells. Simple squamos epithelium have horizontally flattened, elliptical nuclei which corresponds to the form of the cell.
The thin nature of this type of epithelium makes it ideal for small molecules(such as respiratory gases) to pass through its membranes through diffusion and filtration.
The simple squamos epithelia is found lining the alveaoli as well as the glomerulus, where rapid diffusion is a required.
2. Simple Cuboidal Epithelium
Simple Cuboidal Epithelium consists of a single layer of cube-shaped cells, that have a spherical nucleus in the centre.
The simple cuboidal epithelium constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes. They also offer some degree of protection(by actively or passively pumping material in and out of the lumen) and function in absorption and secretion.
The simple cuboidal epithelia are found lining the surfaces of the ovary, nephron and the walls of the kidney tubules.
3. Simple Columnar Epithelium
Simple Columnar Epithelium consists of a single layer of tall column-shaped cells with an elongated nuclei that is located near the base of the cell.The simple columnar epithelia may or may not have cilia, thus dividing them into two categories; ciliated and non-ciliated.
The primary function of the simple columnar epithelium is protection(for instance, this epithelium in the stomach and digestive tract serves as an impermeable barrier against bacteria). The simple columnar epithelia can also specialize to secrete mucus(a mixture of glyprotein and water) which is a slippery substance produced by the mucus membranes. The cilia on the ciliated simple columnar epithelium helps sweep the mucus which traps infectious agents in a certain direction.
The simple columnar epithelia are found lining the stomach and digestive tract as well as the wall of the uterine tube where currents generated by the beating of cilia propel the egg cell towards the uterus.
4. Pseudostratified Columnar Epithelium
Pseudostratified Columnar Epithelium consists of a singe layer of cells with nuclei located at different levels making it seem as though as it is stratified; this is the main reason why it is called pseudostratified(pseudo=false) which literally means falsely stratified. These epithelia may be ciliated or non-ciliated or may contain stereocilia.
The pseudostratified columnar epithelia function in secretion and absorption. The ciliated type are usually involved with the sweeping of mucus.
The ciliated pseudostratified columnar epithelia are found lining the trachea as wee as certain parts of the upper respiratory tract. The non-ciliated pseudostratified columnar epithelia are found in the membraneous part of the male vas deferens. The pseudostratified columnar epithelia with stereocilia are located in the epididymis(part of the male reproductive system). The stereocilia are distinct from cilia but are both closely related.
Epithelial Tissue(General)
When describing the epithelial tissue, take note of the characteristic, function and the location of that specific type of epithelial tissue.
In general, the epithelial tissue comprises of closely packed cells that may be of one or more layers. They are so closely packed till there is no intracellular spaces between them; There is only a small amount of intracellular substance.
The epithelial tissue cover and line the internal and external surfaces of the body. Endothelium on the other hand is a term used to describe epithelial tissue that occurs on the surfaces on the interior of the body.
In general, the epithelial tissue comprises of closely packed cells that may be of one or more layers. They are so closely packed till there is no intracellular spaces between them; There is only a small amount of intracellular substance.
The epithelial tissue cover and line the internal and external surfaces of the body. Endothelium on the other hand is a term used to describe epithelial tissue that occurs on the surfaces on the interior of the body.
Monday, January 30, 2012
Classification of Epithelia
The epithelial tissue is classified based on the number of layers that they possess and their shape.
For the number of layers; If the number of layers, n=1 then it is simple. If the number of layers, n>1, then it is striated. For pseudostratified(pseudo means false), it is actually a single layer, that 'appears' like as if it has more than one layer.
For the shape; Squamos(from the Latin word squama which means scale) describes flat, thin epithelial cells. Cuboidal describes epithelial cells that have a cube-like shape. Columnar on the other hand describes tall, elongated cells with a stretched nucleus.
Characteristics of Epithelial Tissue
The cells of the epithelial tissue have what we call 'polarity'. This means that the cells have both an apical and basal surface. The basal surface of the cell is attached to the basal lamina of the basement membrane. Note that the apical surface may bear microvilli(microscopic cellular membrane protrusions that increase the area of a cell) and cilia(latin for 'eyelashes' which are hair-like structures that help moves the cell around or tiny substances around the cell).
The cells of the epithelial tissue are also closely packed. These cells are held together by intercellular connections. If you notice the diagram above, you find that the epithelial tissue is supported by the reticular lamina(a thin extracellular layer that lies below the basal lamina which comprises mainly of collageneous fibers).
The epithelial tissue is avascular. It also rapidly divides via mitosis, thus, damaged or dead cells are quickly(not so fast but fast enough) replaced by new ones.
Sunday, January 29, 2012
Basic Tissues
A tissue is a level of organization that is the intermediate of a cell and organ. A group of cells that are not necessarily identical but work together as an assemblage are organized into tissues. Tissues are grouped into four basic types and they are the epithelial, connective, nervous and muscle(note that for muscle we have the skeletal, smooth and cardiac muscle tissue) tissue. These tissues associate and interact with each other to form organs that carry out specialized functions.The following is a diagram illustrating the various tissue types in the body:
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