Chapter 4, Tissue, the Living Fabric. The next level of structural organization after the cellular level is tissues. Tissues are defined as a collection of structurally similar cells working together to achieve a common function. The four main classes of human tissue are nervous, muscle, epithelial, and connective tissue, each of which have their own defined locations and functions in the body.
With respect to location, nervous tissue can be found in the brain, spinal cord, and nerves. Muscle tissue can be found anchored to bones. in the heart, or in the walls of hollow organs. Now, epithelial tissue can be found in the skin, in the external surface of organs.
Epithelial tissue can also be found lining closed ventral body cavities, as well as the interior of hollow organs. With respect to function, nervous tissue participates in control and communication. Muscle tissue generates contractions to produce movement. Epithelial tissue covers and lines various body structures and participates in filtration, secretion, as well as absorption. Lastly, connective tissue provides support or reinforcement and unites other tissue types together.
This brings us to the first checkpoint question of this lecture recording. This brings us to a discussion of epithelial tissue, also known alternatively as epithelium. The plural form of epithelium is known as epithelia. There are two main types of epithelial tissue.
Designated by location, the first main type of epithelial tissue is referred to as covering and lining epithelia, which line the interior of closed ventral body cavities and the interior wall of hollow organs such as blood vessels, the stomach, and the trachea. Covering and lining epithelia also form the external surface of organs located inside ventral body cavities, such as the heart, lungs, and urinary bladder. Lastly, covering and lining epithelia also forms the skin. which separates the body's internal environment from the external environment.
The second type of epithelial tissue is referred to as glandular epithelia, which forms glands. Glands are tissues that are specialized in the secretion of products such as hormones or aqueous fluid. Examples of glands include, but are not limited to, sweat glands, salivary glands, the pancreas, and the thyroid gland. This brings us to the next checkpoint question of this lecture recording.
We've previously established... Fat tissues are comprised of a collection of similar cells. Thus, epithelial tissue is defined as a collection of similar epithelial cells.
The two criteria or rules by which we name and classify epithelial tissue are the number of epithelial cell layers present and the shape or morphology of those epithelial cells. If one single layer of epithelial cells is present, then the epithelium is termed simple. If there is more than one layer of epithelial cells present, then the epithelium is termed stratified.
Stratified translates to multi-layered, in other words, to be arranged into many strata or into many layers. There are three different types of cell shapes or cell morphologies, squamous, cuboidal, and columnar. Squamous cells are flattened and scale-like. Cuboidal cells are cube-like.
Columnar cells are tall and column-like. Standard naming convention. or the complete nomenclature for epithelial tissue, requires that both of the two criteria are accounted for in the name. For instance, in this bottom left figure, we have a single layer of flattened epithelial cells, thus the complete name for this epithelial tissue. is simple squamous epithelium.
In the bottom middle figure, here we have a single layer of cube-like epithelial cells. Thus, the complete name for this epithelial tissue is simple cuboidal epithelium. In this bottom right figure, here we have a single layer of tall or elongated epithelial cells. Thus, the complete name for this epithelial tissue is now simple columnar epithelium. Now, each epithelial cell can have up to two sides or surfaces, an apical surface, a basal surface, or even both apical and basal surfaces.
Now, the apical surface of an epithelial cell is considered free or unattached because it either directly faces the external environment of the body, such as atmospheric air, or the apical surface of an epithelial cell can also face a cavity or space within a hollow organ called the lumen. For instance, the apical surface of epithelial cells in our skin is directly exposed to and it faces the external environment of the body, that is atmospheric air. When we glance at our skin surface, we are viewing the free apical surface of those epithelial skin cells.
In another example, the apical surface of epithelial cells lining the interior wall of hollow blood vessels face the lumen, which fills with blood contents. As we can see, the apical surface of an epithelial cell can either face the external environment, such as atmospheric air, or the lumen inside a hollow organ. This is what's considered the free apical surface of an epithelial cell. The fact that it is unattached. and freely faces the external environment or the lumen of a hollow organ.
Now, the basal surface of an epithelial cell is not free, but instead is anchored or attached to a structure known as the basement membrane, hence the term basal. surface. The basement membrane is a structure that adheres epithelial tissue to underlying connective tissue. Now, in simple epithelium, note that each individual epithelial cell has a free exposed apical surface and a basal surface that is attached to the basement membrane. In contrast, in stratified epithelium, note that the layer of epithelial cells facing the lumen or external environment only has a free apical surface but lacks a basal surface.
attached to the basement membrane. For this reason, we can refer to this layer as the apical layer of epithelial cells. In the same figure, note that the middle layers of epithelial cells lack both a free apical surface and a basal surface. attached to the basement membrane. So the middle layers of epithelial cells have neither an apical surface nor a basal surface.
Lastly, we can note that the layer of epithelial cells is attached directly to the epithelial cells. to the basement membrane have a basal surface but lack a free apical surface. For this reason, we can refer to the layer of cells here as the basal layer of epithelial cells.
Therefore, This epithelial tissue cannot be classified as simple because in order to be classified as simple epithelial tissue, each individual epithelial cell must have an apical surface and a basal surface. In this figure illustrating stratified epithelium, we can also note that the apical layer of cells are more squamous or flattened in shape, whereas the basal layer of cells are more cuboidal or columnar in shape. In stratified epithelium, when there are several layers of cells with different shapes or morphologies, we default to naming the entire epithelium after the shape or morphology of the apical cells.
And we disregard the shape of the basal cells in the final name of the epithelium. Therefore, the complete name of this epithelium is stratified squamous epithelium. Again, when dealing with stratified epithelium and Layers of cells with different shapes or morphologies, we default to naming the epithelium after the apical layer of cells, which is squamous. Again, always remember to combine both criteria. the number of cell layers and cell shape to arrive at the complete nomenclature for epithelial tissue.
In order to view epithelial tissue microscopically, there are two ways in which a thin slice of epithelial tissue can be removed from a body. structure. In the first method, we can cut along the transverse plane of a body structure, such as our microscopic kidney tubules. Then we can remove a thin slice, and the resulting piece we're able to view. is called a transverse or cross-section of those kidney tubules.
The second method of tissue preparation involves cutting along the natural longitudinal direction of those kidney tubules. This is also known as the lengthwise direction. then removing a thin slice, and the resulting piece we're able to view is called a longitudinal section of those kidney tubules. Now recall that in order to be classified as simple epithelial tissue, each individual epithelial cell must have an apical surface and a basal surface. In the top microscope image here, we can see that each individual epithelial cell has an apical surface facing the lumen and a basal surface attached to the basement membrane.
So despite the presence of two layers of epithelial cells in this image, these layers are not stacked. and arranged on top of each other in a stratified arrangement. Instead, what we're observing is a longitudinal section of a single layer of cube-like epithelial cells lining the interior wall of a kidney tubule. In other words, we're observing simple cuboidal epithelium, not stratified cuboidal. epithelium.
In the bottom microscope image, we can see that each individual epithelial cell also has an apical surface facing the lumen and a basal surface attached to the basement membrane. Again, what we're observing here is a transverse or cross-section of many individual kidney tubules, each lined by a single layer of cube-like epithelial cells. So despite the presence of multiple rings of epithelial cells, these cells again are not stacked and arranged on top of each other in a stratified arrangement.
In other words, we're also observing Simple cuboidal epithelium here, not stratified cuboidal epithelium. This brings us to a discussion of the features or characteristics unique to epithelial tissue. We've previously established that epithelial cells can possess up to two surfaces, an apical surface.
a basal surface or both apical and basal surfaces. This is known as the feature of polarity, which translates to opposing or opposite. Epithelial cells are said to exhibit polarity because they can possess an apical surface that is free and an opposing basal surface that is attached.
Apical surfaces can either be completely bare or covered by projections such as microvilli or cilia. Micro is derived from the Greek term for tiny, and villi translates to shaggy hair. Microvilli is alternatively assigned the name brush border because of its resemblance to the tip of a paintbrush underneath.
the microscope. The function of microvilli is to increase the surface area available for nutrient absorption of ingested food in the small intestine. For example, the presence of microvilli on the apical surface of absorptive intestinal cells increases the surface area available for nutrient absorption by 600 times compared to the same number of absorptive cells without microvilli.
Now, cilia are longer hair-like projections that can be found on the apical surface. of epithelial cells lining the trachea, bronchi, and nasal cavity of the respiratory tract, as well as the uterus or uterine tubes of the reproductive tract. The function of cilia is to propel material along the apical surface of cells.
such as mucus, along the apical surface of cells in the respiratory tract. Cilia can also propel materials such as reproductive cells along the apical surface of cells in the reproductive tract. Another feature of epithelial tissue is that it's comprised of very densely packed epithelial cells with very tight contacts. There are lots of epithelial cells, but very little material in between those cells. The material in between cells is termed extracellular matrix.
Epithelial tissue is said to have a very high cell-to-matrix ratio. Now, epithelial tissue is also avascular. Here, the prefix a translates to without or lack of, and vascular translates to blood vessels. Therefore, epithelial tissue is lacking in blood vessels. On the other hand, epithelial tissue is innervated, which means to be supplied with nerves.
In summary, epithelial tissue lacks blood supply, but does have nerve supply. Lastly, epithelial tissue is very mitotically active. and thus has a high rate of regeneration. In other words, epithelial cells are very active in cellular division, so the rate of epithelial cell turnover is quite high compared to those of other tissues.
This brings us to our next checkpoint question. For the following types of epithelial tissue, we will note their microscopic features, functions, and locations in the body. Now, simple squamous epithelium is characterized by a single layer of thin, flattened cells. Since we've...
Previously established that structure informs function, a single layer of cells is ideally located and best suited for locations where materials such as gases, water, and ions are transported across membranes. Examples of these locations include the air sacs of the lungs, blood vessels, as well as kidney structures. In contrast, a stratified arrangement of cells or cells thicker in shape would actually hinder the passage of substances and therefore stratified arrangements of cells or cell shapes thicker than squamous will not be well-suited to the same body locations.
In areas lined with simple squamous epithelium, protection is not deemed a priority. Simple squamous epithelium also comprises the double-layered serous membrane of closed ventral body cavities. Additionally, simple squamous epithelium is responsible for secreting friction-reducing fluid known as serous fluid.
There are two specialized terms for simple squamous epithelium located in specific locations. Simple squamous epithelium in three areas, the lymphatic vessels, blood vessels, and the heart can be assigned the specialized term endothelium. Simple squamous epithelium forming any of the three serous membranes, the pericardium pleura and peritoneum.
can be assigned the specialized term mesothelium. Now simple cuboidal epithelium is characterized by a single layer of cube-like cells. Since cuboidal epithelial cells are thicker in comparison to simple squamous cells, These cuboidal cells are more selective in terms of which substances can be transported and allowed passage across membranes. For example, cuboidal cells in the kidney tubules function in secretion as well as absorption.
Absorption is the movement of necessary substances out of urine and into blood to be used and metabolized by our cells. Secretion, on the other hand, involves the movement of excess substances out of blood and into urine for elimination from the body. Simple cuboidal epithelium can also be found in small glands of the body.
such as the female ovaries and also in the thyroid gland. These are some locations of simple cuboidal epithelia in addition to the kidney tubules. Now simple Columnar epithelium is characterized by a single layer of tall cells with nuclei arranged at the same level.
Clear or transparent mucus-secreting goblet cells can be found wedged between the simple columnar epithelial cells. Simple columnar epithelial cells function in the absorption of nutrients from ingested food, as well as the secretion of mucus and digestive enzymes. The simple Columnar epithelial cells found along the digestive tract, spanning from the region of the stomach all the way to the anal canal, are non-ciliated. Now, on simple columnar epithelial cells lining bronchi, the apical surface is covered with cilia.
The cilia help propel mucus along the respiratory tract. The apical surface of simple columnar cells lining the uterus and uterine tubes are also covered by cilia. Here, cilia help propel reproductive cells along the reproductive tract.
This brings us to pseudostratified columnar epithelium. Here, the term pseudostratified translates to false layering, where the columnar epithelial cells and Their nuclei are positioned at different heights or levels, giving the false impression of stratification. Here, pseudostratified columnar epithelium is actually only comprised of a single layer of cells. So we have simple epithelial cells masquerading as or pretending to be stratified.
All cells in pseudostratified columnar epithelium touch or make contact with the basement membrane, but not all cells reach the apical surface. Clear or transparent mucus-secreting goblet cells can also be seen wedged between the pseudostratified columnar epithelial cells. In the upper respiratory tract, cilia found on the apical surface of pseudostratified columnar epithelial cells help propel mucus along the upper epithelial cell. respiratory tract. Now, there are also pseudostratified columnar epithelial cells without cilia, which can be found in a male's sperm-carrying duct.
This brings us to our next checkpoint question. Now, stratified squamous epithelium is comprised of several epithelial cell layers where the basal cells adjacent to the basement membrane are more cuboidal or columnar in shape, and the free apical cells are flattened or squamous in shape. We default to naming this entire epithelium after the shape of the apical cells, which are squamous. Now, since a multi-layered or stratified arrangement of cells provides protection, stratified squamous epithelium is best suited for locations subject to a lot of abrasion, friction, and an overall high degree of pressure. Now, epithelial cells are continually lost from the free apical surface.
And to keep up with the constant wearing away of cells at the apical surface, this loss requires the rapid continuous mitotic division of basal cells in order to produce a steady supply of new cells that will replace and replenish those apical cells lost at the surface. Thus, basal cells in stratified squamous epithelium are very mitotically active, very active in cell division. The two varieties of stratified squamous epithelium are cratinized and non-cratinized. A structure that is deemed cratinized will contain and be filled with the protein known as keratin. A structure that is deemed non-keratinized will not contain nor be filled with the protein keratin.
Now keratin is a waterproofing protein whose presence renders a body structure dead or non-living. For instance, we do not experience pain when hair and nails are cut because the presence of keratin protein in hair and nails renders these structures dead or non-living. The only example of keratinized stratified squamous in the human body is the outer layer of the skin called the epidermis.
Here in this image of keratinized stratified squamous epithelium, we can see that the superficial or apical layer of epithelial cells is completely absent or lacking in dark staining nuclei. Nuclei are absent in the apical cells because the apical cells have been invaded or filled with keratin protein. Thus, these apical cells are rendered dead and no longer alive. Note that as the replacement basal cells are gradually pushed closer towards the apical surface, their shape changes from cuboidal or columnar to squamous. as they transform into dead, non-nucleated, keratinized epithelial cells.
Therefore, the younger epithelial cells are located in the basal layers. near the basement membrane. And the oldest epithelial cells can be found in the apical layers near the free surface.
Now, non-keratinized stratified squamous epithelium is characterized by the continuous presence of nuclei from the basal layers all the way through to the apical layers. There is no invasion by keratin protein, thus non-keratinized stratified squamous epithelium is considered living tissue. Examples of non-keratinized stratified squamous include the moist mucous membrane linings of the esophagus, the oral cavity, and the vaginal tract, all of which are vulnerable to abrasion and friction from the external environment. For instance, the walls of the oral cavity or esophagus are susceptible to friction and abrasion introduced by ingested foods or liquids.
This brings us to our next checkpoint question. Now, stratified cuboidal epithelium is not common or frequently occurring in the body, but it can be found in some sweat, breast, or mammary glands, as well as some salivary glands. Stratified cuboidal epithelium is generally two cell layers thick. Next, we have stratified columnar epithelium, which is also found in short supply throughout the body, although it can be found in small amounts in the pharynx or throat and the male urethra. The male urethra is the tube-like duct or passageway for the exit of urine out of the male body.
Here in the photomicrograph of stratified columnar epithelium, we can conclude that there are two distinct layers or rows of cells, okay, both the top and bottom layer. of cells contain nuclei positioned at the same height or level. This is in contrast to pseudostratified columnar epithelium where we observed only a single layer of cells with the nuclei positioned at varying or different heights.
This brings us to our next checkpoint question. Now, transitional epithelium is found in organs such as the urinary bladder and urethra. that undergo stretch or expansion. The term transitional translates to change.
So in transitional epithelium, apical cells can transition or change shape to allow the organ to undergo stretch or expansion during increased internal pressure. from urine. For instance, when the urinary bladder is not filled to capacity with urine, it is considered to be in a relaxed state. So the apical cells are more rounded, dome-shaped, or bulbous, as we can see in the photomicrograph here.
However, as urine continues to fill and accumulate inside the bladder, the apical cells will stretch and become flattened or squamous-like. When these apical cells become compressed and squamous-like, this allows the urinary bladder to expand. and thus accommodate the increased pressure from the urine contents within. This brings us to our next checkpoint question.
Glands are broadly classified into two groups, either endocrine or exocrine, based on their site of product release, whether the product is released directly into the bloodstream or a duct. Now, a duct is defined as a tube-like canal. or passageway. Now, endocrine glands, by definition, release hormone products directly into the blood.
Here, endo translates to inside or within. In other words, Endocrine glands do not release their hormone products into a duct, but instead directly into the blood. As a result, endocrine glands are referred to as duct-less glands.
Examples of endocrine glands include, but are not limited to, the thyroid gland as well as the adrenal glands. In this figure, we can observe endocrine cells secreting hormone product directly into a blood capillary. Now, exocrine glands secrete their non-hormone aqueous products into a duct that is continuous with the external environment of the body.
Here, exo translates to outside or external. For example, sweat glands release sweat product into a duct, which transports and eventually empties the sweat product onto the surface of the skin. into a duct, out through a sweat pore, and onto the surface of the skin.
Here, the surface of the skin is considered the body's external environment. Likewise, breast or mammary glands will release milk product into a duct, which transports and eventually empties that milk product into the external environment. However, it's important to note that even body sites physically located inside of the body are still considered part of the body's external environment.
For instance, the lumen or space inside of hollow organs, such as the stomach, oral cavity, esophagus, trachea, or intestines, are all classified as part of the body's external environment. This is because the space or cavity inside of those structures is continuous with and can be traced to the exterior of the body. For example, mucus glands found in the lining of the esophagus secrete mucus product into the lumen or the cavity of the esophagus to help lubricate.
food passage. So here, the site of mucus product release is the lumen of the esophagus, which is physically located inside of the body. But because the lumen of the esophagus can be traced to the exterior of the body, it must still be considered part of the body's external environment.
Therefore, mucous glands, because of their site of product release into the external environment, must be classified as exocrine and not endocrine glands. Lastly, both Endocrine and exocrine glands are classified as either unicellular or multicellular, depending on the relative number of cells comprising the gland. When the gland is comprised of a single epithelial cell, It is classified as unicellular.
Here, uni translates to one. One of the chief examples of unicellular glands in the body is the mucus-secreting goblet cell, where the lone single goblet cell itself is the entire gland. When the gland is comprised of multiple or several epithelial cells, it is classified as multicellular.
Here, multi translates to many or several. The majority of endocrine and exocrine glands are multicellular. This brings us to our final checkpoint question.