 This video will cover part two of an introduction to histology. As we go, we will cover the following objectives. Name and describe the various types of epithelia and indicate their functions and locations. Define glandular epithelia and distinguish between endocrine and exocrine glands. Describe how multicellular exocrine glands are classified structurally. Epithelia are classified based on the number of layers as either simple with just one layer of cells or stratified with more than one layer of cells. Epithelia are also classified by the shape of the cells. Specifically, in a stratified epithelium, you focus on the shape of the cells in the apico layer. If we start at the top left here, we see a simple squamous epithelium is just one layer of flat cells that have a height that is less than their width. Whereas a simple cuboidal epithelium is one layer of round cells that have a cube shape with their width approximately equal to their height and then a columnar epithelium, a simple columnar epithelium is a single layer of tall cells where the height of the cell is greater than the width. We have a stratified squamous epithelium that's multiple layers of cells and the cells at the apical surface are flattened with a height less than the width. A stratified cuboidal epithelium is multiple layers of cells that are round cells and a stratified columnar epithelium is multiple layers of cells that are tall cells. Specifically, the cells at the apical surface are taller cells in a stratified columnar epithelium. Here we can see an example of a simple squamous epithelium in the lung. The air sacs of the lung called alveoli are formed from simple squamous epithelium and the inner lining of blood vessels called endothelium is also formed from a simple squamous epithelium. The smallest blood vessels are formed out of only the endothelium with a small amount of surrounding connective tissue These are called capillaries and capillaries have this very thin simple squamous epithelium to allow diffusion of nutrients and waste in and out of the blood. Here in the lungs oxygen diffuses from the alveoli into the blood and carbon dioxide diffuses from the blood into the air of the alveoli. Here's another example of a simple squamous epithelium. This is the mesothelium or the simple squamous epithelium found in a cirrus membrane lining the visceral body cavities. This is a superficial view looking at the epithelium from the surface so the cells have the appearance of fried eggs laid side by side where we can see the nucleus is like the yolk of the egg whereas if we were to look at these cells from the side they would be very thin when we look at them from the the surface of the epithelium here they have a wide width. Here is an example from a section of the skin from the cutaneous membrane we're focused on part of the dermis the connective tissue underlying the epithelium of the epidermis and in the dermis there's numerous blood vessels and the lining of the blood vessel the endothelium the inner lining of the blood vessel is formed from a simple squamous epithelium and the function is to allow the diffusion of nutrients and waste across the blood vessel lining. Here's another example of a blood vessel with a simple squamous epithelium forming the inner lining or endothelium and this is an image taken from the kidney where there's numerous blood vessels in the kidney that are important for the function of the kidney to process blood through three mechanisms filtration secretion and reabsorption reabsorption is moving chemicals from the the filtrate liquid that is going to become urine back into the blood so that they're not lost from the body in the urine and secretion would be to move chemicals out into the filtrate to join what will become urine. Here we can see another example of an image from the kidney with simple squamous epithelium the structure in the center here that's around shape is a renal corpuscle a structure in the kidney where the process of filtration occurs where liquid is forced out of the blood and the outer layer of the renal corpuscle the parietal layer of the glomerular capsule or bowman's capsule is a simple squamous epithelium liquid would then flow into tubules of a nephron where that liquid is processed by reabsorption and secretion the tubules known as proximal and distal convoluted tubules are made of a simple cuboidal epithelium and here we can see another example of the nephron in the kidney that has a simple cuboidal epithelium and also a simple squamous epithelium forming the thick and thin segments of the nephron loops also known as loops of henley that are important for processing the filtrate through reabsorption in order to pull water and sodium and potassium and chloride back from the filtrate into the blood so that it's not lost in the urine the nephron loops have a a thick segment made of simple cuboidal epithelium that has the machinery in order to regulate reabsorption and secretion and the thin segments enable diffusion of water as the filtrate moves down through the nephron loop water is able to diffuse out of the filtrate and then return to the blood by diffusing through a simple squamous epithelium here's another example of a simple cuboidal epithelium this one is in the thyroid gland the thyroid gland is an example of an endocrine gland that is a gland that produces a secretion that travels into the body that travels through the blood as a hormone which will regulate the function of other organs in the body the follicle cells the simple cuboidal epithelium is made of follicle cells that produce a thyroid hormone and first produce a storage form of the thyroid hormone that's stored inside the follicle and then can be broken down and released from the storage form and in order to travel throughout the body in the blood and the thyroid hormone will function to stimulate metabolism in order to increase metabolic rate producing heat to warm our body here's an example of a simple columnar epithelium found lining the mucus membrane of the digestive tract in the intestines this is a non-siliated simple columnar epithelium there are not cilia on the apical surface of these cells but there are small projections called microvilli on the apical surface that increase the surface area the function of this epithelium is the secretion of mucus and absorption of nutrients and the tall columnar shaped cells provide more protection but also an increased amount of cytoplasm with proteins that are important for secretion and absorption another example of a simple columnar epithelium is the uterine tubes the uterine tubes or fallopian tubes attach to the uterus and carry the the oocyte which becomes an ovum and then a zygote during fertilization which will then travel through the uterine tube into the uterus to implant and develop into a fetus the uterine tube simple columnar epithelium is a ciliated simple columnar epithelium meaning the apical surface has cilia that function to beat back and forth and move mucus along the surface which helps with the movement of the oocyte zygote through the uterine tube before and after fertilization here we see an example of a stratified squamous epithelium this is a keratinized stratified squamous epithelium found in the cutaneous membrane in the skin this is the epidermis the keratin forms numerous layers of dead cells at the apical surface making this a distinctive epithelium numerous layers of flat dead cells are held onto the apical surface because they're held together with keratin the function of the stratified squamous epithelium is to resist abrasion to protect underlying tissue and so it can be found in the skin where it's a keratinized stratified squamous epithelium and examples of non keratinized stratified squamous epithelium are found in mucus membranes of the mouth and esophagus rectum in the vagina here's a light micrograph of the non keratinized stratified squamous epithelium from the esophagus and so you can see there's still flat cells at the apical surface and there's numerous layers of cells but there's much there's many fewer layers of cells at the apical surface there's not many layers of dead cells attached to the apical surface because the esophagus mucus membrane is a non keratinized stratified squamous epithelium here's a stratified cuboidal epithelium so there's more than one layer usually two layers of cells in a stratified cuboidal epithelium and the cells have a round shape this is an example from the duct of a salivary gland so that this is an exocrine gland a gland that produces a secretion that travels out of the body so the function of the stratified cuboidal is important for secretion for carrying an exocrine product like sweat out of the body sweat is secreted onto the skin in order to help cool the body here's another example of an exocrine gland this one has a stratified columnar epithelium so you can see there's more than one layer of cells and the cells at the apical surface have a tall shape a columnar shape and again the the function is for secretion this is an exocrine gland the salivary gland that produces saliva that travels out through the ducts of the salivary glands into the oral cavity in order to lubricate the food and participate in there are enzymes that help participate in digestion perform chemical digestion to help break down our food and release nutrients from the food a pseudo stratified epithelium is an epithelium that at first appears to have more than one layer but if you carefully examine the outline of each cell you'll see that all of the cells contact the basal lamina or the the basement membrane so it's actually just one layer of cells so it's a false stratified epithelium pseudo stratified means false stratified this is the ciliated pseudo stratified columnar epithelium found lining the trachea it's also found lining the nasal cavity and perinesal sinuses so the respiratory mucus membrane has a ciliated pseudo stratified columnar epithelium there are cilia at the apical surface that function to move mucus across the surface in order to carry foreign particles out of the respiratory tract so although it appears like there's multiple layers some of the cells are shorter and some of the cells are taller and so there's multiple rows of nuclei but only one layer of cells in a pseudo stratified columnar epithelium and this is the example from the respiratory mucus membrane lining the trachea a transitional epithelium is an epithelium that can stretch to look either like the apical cells have a round cuboidal shape or a flattened squamous shape the transitional epithelium is found lining the mucus membranes of the urinary tract this is an example from the urinary bladder where some of the cells have a round shape some of them are flattened it is a stratified epithelium there's multiple layers of cells and most of the cells have a cuboidal shape but the apical surface cells have a variety of shapes some are stretched out to look more squamous in appearance and some are domed up to look more round in appearance depending on the state of that epithelium whether it's stretched or not and so the function is to allow stretching of the urinary tract and here's another example of the transitional epithelium in the urinary tract this is from the ureter the tubes that connect from the kidney down to the urinary bladder the transitional epithelium with some cells that appear squamous and some cells that appear cuboidal has multiple layers of cells the majority of the cells have a cuboidal shape but some of the cells are more flattened some are more tall and some are more round and this stretching allows the the change in shape of cells as this epithelium stretches is what accounts for the variety of cell shapes found in the transitional epithelium a gland is one or more cells that produce secretions endocrine glands produce secretions into the body that travel through the bloodstream which are hormones and the hormones secreted by endocrine glands are chemical messages that regulate the functions of other cells and organs throughout the body for example the thyroid gland is an endocrine organ the pituitary gland and the adrenal glands the gonads testes and ovaries are other examples of endocrine glands and we'll study those in more detail when we do the endocrine system. Exocrine glands produce secretions onto a surface through a duct either onto a mucous membrane or out onto the skin examples of exocrine glands include the sweat glands and salivary glands which are multicellular exocrine glands and there's also unicellular exocrine glands the example shown here is a goblet cell a unicellular exocrine gland found in mucous membranes that produces mucous and the apical region of the cell contains numerous secretory vesicles filled with mucin a glycoprotein that gives mucous a thick and slippery consistency the secretory vesicles don't stain very well with the normal staining of H&E stain for histology and that gives a white round shape to the apical region of a goblet cell which has the appearance of a goblet glass hence the name goblet cell here we see a light micrograph of a mucous membrane from the digestive tract that's been stained specially to show the goblet cells with a purple stain and so here we can see the the goblet shape is shown with a purple stain with this tissue that was specially stained so that the mucin proteins would look purple. Exocrine glands can be unicellular as in the goblet cell or they can be multicellular glands and a multicellular gland has a glandular portion that produces the secretion and a duct portion that connects to the surface. The example shown here is a sebaceous gland and the sebaceous gland has what's known as a simple branched alveolar structure the alveolar structure means that there's a a pocket shape an expanded pocket shape to the secretory unit and it's a branched alveolar structure so there's more than one pocket that comes together and it's a simple branched alveolar structure because all of the pockets come together into one duct that connects out to the surface. The secretion of the sebaceous gland is an oil known as sebum that functions to condition and lubricate the hair in the skin and helps to protect against infection. So multicellular exocrine glands can be classified by their structure based on the structure of the duct. If the duct is unbranched it's a simple structure whereas a branching duct structure is known as a compound branched structure is a compound structure to the duct and then the secretory unit can have a tubular shape that is in a elongated tube shape it can have an alveolar shape which is a pocket or a sac shape it could be there can be a combination between a tubular and alveolar and there can be a branched secretory unit so we can have a branched secretory unit with some regions that are tubular and some regions that are alveolar. So we'll start by looking at the simple branched alveolar example the sebaceous gland where we can see there's more than one secretory unit that has a pocket shape or sac shape that come together into a single duct that connects to the surface then we'll look at the example of the intestinal glands that have a simple tubular structure so there's one elongated secretory unit that connects to a single duct america and sweat gland it's a sweat gland found in the skin that produces sweat to cool the body and this has a simple coiled tubular structure and so one elongated secretory unit that forms a coil and connects to a single duct with no branching then there are simple branched tubular glands found in the stomach esophagus tongue and the first segment of the small intestine the duodenum and these glands have a secretory unit with an elongated tubular shape with multiple elongated tubular shapes that come together so there's a simple branched tubular structure where there's branching to the secretory unit but only one duct no branching to the duct structure. Now below that we see the compound ducts where there is branching in the duct structure over here in the mammary gland has a compound alveolar structure where there's multiple levels of branching there's branching in the duct system and then the secretory unit branches into multiple sacs or alveolar shapes so that's a compound alveolar structure a compound tubular alveolar structure is where there's multiple levels of branching the duct system branches and then connects to multiple elongated secretory units as well as sac-shaped secretory units so tubular and alveolar structure a compound tubular alveolar structure is seen in the salivary glands as well as glands in the respiratory passages and in the pancreas in the exocrine ducts of the pancreas the pancreas can function both as an exocrine gland and an endocrine gland but the exocrine glands in the pancreas have this compound tubular alveolar structure and then a compound tubular structure where there's branching in the duct that connects down to multiple elongated secretory units this is found in the mucus glands of the mouth as well as glands in the reproductive tract the bubble urethro glands in the male reproductive tract as well as the seminiferous tubules of the testes