The aim of this chapter is to review histology covered in your Anatomy & Physiology classes-- but only the tissues that appear in the head and neck, and only the parts that are relvant to this course. This is by no means a comprehensive review. Histology is the study of what tissues look like under a microscope. A tissue is a group of cells, all of the same type, working together to perform a function. Because most cells are transparent, microscopists use of a number of different stains to highlight different parts of cells. Shown to the left is an image of a very common staining technique called an H&E stain. These two stains turn molecules with negative charges blue, and positive charges pink. DNA and RNA take up both colors, whick makes nucleuses purple, while most proteins turn pink. Unfortunately, most cells are full of proteins so their cytoplasm turns pink, but the extra-cellular matrix is also full of protein, it turns pink as well. We are therefore often trying to make sense of a sea of pinks with purple spots. On the plus side, if you purchase a histology coloring book, you only need two crayons.
Most of the images you see in histology textbooks use stains such as H&E stains-- this type of technology dates back to the 1800s. Microscope technology has improved since then, such as the image to the left, which comes from a confocal microscope. The confocal microscope uses expensive lasers to generate some very beautiful images, but still relies on stains which are theoretically similar to H&E. Sadly, most textbooks still use the old-fashioned uglier images. Since you will be using neither the newer nor the older microscopes in your line of work, I have illustrated most of the histology images in this book. This should allow us to focus more on the concepts, and spend less time developing the skills necessary to interpret pictures of pink blobs with purple spots.
I presume that in the Anatomy and Physiology classes you took to get into this program, you learned to spot the differences between different types of tissues. We will review those differences. I would ask, though, for you to now keep an eye out for similarities between different tissues (aside from they all look like pinkish blobs with purple spots). If you jumped ahead and read the embryology chapters, you'd have a better idea of what to look for, but for now just keep that thought in the back of your head as we cover the major tissues of the human body.
Enough development to get started on histology:
Classification - the old way, and a better way
Histology categorizes tissues based on what the cells look like in adulthood. If we determined people's families based off what they look like as adults, we might do an OK job of it, or we might make some really bad mistakes. For instance, I might try to lump these cartoon faces shown to the left into two families based primarily on hair and eye color. That is how the 4 major tissues types we are going to review are categorized.
When we learn about embryology, however, we will learn that we could do a better job. We could categorize tissues using cell lineage instead, tracing who the adult cells are related to by following which cell they came from during embryogenesis. Early embryologists stained individual cells and followed them through cell division after cell division to see what they became. Others, such as Hans Spemann, moved cells from one place to another and asked whether they still turned into what was expected, based on their new location. When those embryos grew two heads, Spemann concluded that was "unexpected". To get a better idea how that happened, science had to wait for the discovery that DNA stored genetic information for an organism. Then, developmental biologists like Christiane Nüsslein-Volhard were able to determine the mechanisms by which a single embryonic cell became different types of cells in an adult organism--her organism of choice was flies, and for that work she won the Nobel Prize. Unlike earlier histologists and embryologists who named everything after themselves, we now get fun names like Bicoid, Dickkopf, Frizzled and Sonic Hedgehog. These scientists have followed the lineages of cells, watched what they become, and have determined how the cells make decisions as to when and what to become.
Rather than use the appearance of our cartoon people, if we knew their lineage (you might also see the fancier term ontogeny), we can classify them into two more accurate families, with one sub-family. When I use the word lineage, think family line, where differentiated cells are the youngest generation, and stem cells are their parents and grandparents. But, sadly, this is not how histology is taught, so we're going to look at what adult cells look like and classift them that way. When we do, look to see whether two tissues have distinct borders (different lineages) or blended borders (often the same lineage).
The 4 main tissue types
Epithelial
Connective Tissue
Muscle
Nervous
Epithelia
Location of epithelia
An epithelium is composed of a bunch of epithelial cells, who usually connect to one another in a sheet. There is very little ECM in an epithelium, just cells. The cells are anchored together by anchoring junctions, which holds them in a sheet. Tight junctions ensure that the only things that travel from one side of the epithelium to the other pass through the epithelial cells, and therefore must either be lipid-soluble, or be recognized by transport proteins on the apical and basolateral plasma membranes. An epithelium therefore has apical-to-basolateral polarity. The apical surface faces the outside of the body, and the basolateral inward. The "outside" can mean the outer surface of our skin, or inner surfaces, such as lining the oral cavity, stomach, and bladder. Every surface of the human body is an epithelium. Therefore, under the microscope, if you see an empty space, an epithelium will border that space. Inside of a sweat duct? Epithelium. Inside of a blood vessel? Epithelium. All other tissues must be deep to an epithelium-- even enamel is made by an epithelium, so if anyone tells you enamel is a connective tissue because it is looks similar to bone tissue, they haven't studied embryology, and need to be educated. Try using words like ontogeny.
s
Epithelial thickness
Epithelial tissue are good at healing because they contain numerous stem cells capable of undergoing mitosis (you can tell by the presence of visible chromosomes, rather than chromatin). For a thick epithelium, stem cells reside in the deepest layer. Epithelia cannot be terribly thick, however, because they are avascular (without their own blood vessels). Nutrients for an epithelium must diffuse from underlying connective tissue, which is why in the image to the left, the epithelial cells on the apical side lack nuclei. They are dead, but still connected to the rest of the epithelium. The down-side to being good at mitosis is it means those cells are a few steps closer to becoming cancerous. Many people regularly expose themselves to high doses of carcinogens found in alcoholic beverages and tobacco (chewing tobacco in particular), which lead to oral cancers.
In addition to membranes, glands are made of epithelial cells. Exocrine glands, such as the salivary glands, are usually made of a single layer of epithelial cells, rolled up into tubes called ducts. These ducts bring exocrine secretions to a surface of the body. Endocrine glands, however, are different. They are not arranged in a sheet, but are amorphous clusters of epithelial cells. Endocrine glands aren't avascular, either. Blood vessels grow right into the epithelia of an endocrine gland. This is important because endocrine glands secrete hormones directly into the blood.
How epithelia are classified
Number of layers
Shape of cells
simple
squamous
stratified
cuboidal
columnar
Epithelia can be classified based on two criteria. With a few exceptions, epithelia will have a name from the first column and a name from the second column. Simple epithelia have just one layer of cells, while stratified epithelia have more than one. Squamous means the cells are flat, like fried eggs. Cuboidal means square-ish in appearance (cells are 3D, but look 2D under the microscope). Columnar means the cells are tall.
Simple squamous epithelia
If you look at the lining of a capillary, you might be able to see the simple squamous epithelium that lines the inner surface. These cells, called endothelial cells, are functionally really cool cells, even if they are hard to see. Normally, endothelial cells keep blood inside of a capillary, but they can become damaged, such as during probing or flossing.
Simple cuboidal epithelia
Simple cuboidal epithelia line most exocrine gland ducts, such as sweat and salivary glands. In the image to the left, you can see two ducts coming out towards the camera, and one duct running side-to-side. Each is surrounded by a simple cuboidal epithelium. Where two ducts get close together, it is still simple cuboidal epithelium, just two seperate layers of it. The capillary we saw two sections above is lined by flat cells to allow for diffusion of nutrients out of the capillary, but these salivary ducts have thicker cells to keep saliva in the duct.
Simple columnar epithelia
Simple columnar epithelia will not be found in the oral cavity of your patients, but we will see some this term when we focus on the Inner Enamel Epithelium that differentiates into enamel-producing ameloblasts. When we do, the cell shape won't be important, other than it allows scientists to find the IEE in a developing tooth.
If you trust my skills in histology, feel free to skip this entire paragraph. But if you are looking at my illustration to the left, you may wonder why I drew fewer nucleusses than are visible in the actual histology picture. Did I mistakenly give you a picture of a stratified columnar epithelium and try to pass it off as simple? No. When we say an epithelium is simple, it means one cell thick from apical-to-basolateral. But when we cut a tissue sample to view under the microscope, our slice may be several cells thick from side-to-side. And when we lay the tissue sample on its side, the cells that were running side-to-side are now sitting on top of one another. I have illustrated what it should look like if we had the ideal 1-cell-thick slice of tissue. This might be a good time to bring up perspective. Epithelia form flat sheets, and here we are looking at a (folded) sheet in cross section. Imagine slicing through your bed and looking at it from the side view. You could make out the individual, single layers of your bed sheet and comforter. Furthermore you might note that while each is a single layer, the bedsheet is thin and the comforter is thicker. Under the miroscope, we're doing the same thing, but unfortunately cells are semi-transparent, so it gets messy. When we call this a simple epithelium, that means there is only one pink cell between the white space (here, the lumen of the intestines) and the very light-blue space (the connective tissue, where nutrients can be absorbed into blood vessels). Those cells, like the comforter, are thicker than cells we looked at above. But we can also see some columnar cells that were connected side-to-side in this cimple epithelium stacked on top of one another. Next time, you should probably just trust me.
Stratified squamous epithelia
Stratified squamous epithelia can be found in the outer lining of the skin, and the inner lining of the oral mucosa. While we have different names for the skin and the oral mucosa, if we were to name them based off of embryology, rather than superficial appearance, they would both contain a stratified squamous epithelium. The epidermis is a keratinized stratified squamous epithelium. Keratin is a tough, water-resistant protein made by the main type of epithelial cell in this tissue, the keratinocyte. The stratified squamous epithelium changes at the vermilion border to less-keratinized, and the stratified squamous epithelium of the oral mucosa is either less or not-at-all keratinized. Based on that information, should I classify the vermilion zone as an extension of the labial mucosa, or as part of the skin and lips? Or is there a line in the vermilion zone that divides them into a skin half and an oral mucosa half? It's times like this I could use the advice of an embryologist to inform me I shouldn't be looking for magical lines in adult tissues, nor draw some sort of imaginary border between the skin, the vermilion and the oal mucosa based on color. If you ever change careers an go into proctology, you can use this same information down there.
Stratified cuboidal, stratified columnar and transitional epithelium
You might spot some stratified cuboidal epithelium in a duct of larger exocrine glands, but I can easily describe these tissues: not appearning in this textbook.
Pseudo-stratified epithelia
Ciliated pseudo-stratified columnar epithelium, or as most people say, pseudo-stratified epithelium, can be found in upper portions of the respiratory tract, such as lining the nasal cavity and the para-nasal sinuses. This tissue definitely deserves a different name from the stratified squamous epithelia of the skin, labia and oral mucosa. However, I'm not convinced it needed to break the naming rules I outlined above. It has more than one layer, but it is hard to count how many. So someone called this tissue pseudo-stratified.
The big fat blue cells, who don't have cilia, are named goblet cells. These cells produce mucus. Goblet cells synthesize mucous proteins within their rER and secrete them, where they attract water and become mucus. The blue coloring of the goblet cells tells us that mucous proteins do not carry the same ionic charges as most proteins found within the epithelial cells of the pseudo-stratified epithelium. Due to this difference in color, they are classified as a distinct cell-- a unicellular gland found within the pseudo-stratified epithelium. But what does our friend the embryologist say? Well, the columnar cells can't become goblet cells, and the goblet cells can't become columnar cells, but they do share a common ancestor (or stem cell), so we probably should consider the goblet cells a part of the pseudo-stratified epithelium. Does it matter? No, but questions like these will matter later in the book, it's good to start practicing now.
Connective tissue (CT)
General characteristics of connective tissue
Many textbooks will tell you connective tissue connects 2 other tissues . That's like saying your smart-phone connects you to other people. It is technically true, but glosses over the fact these could be people you know, people you don't know, or people you don't want to know. It ignores the fact you could be communicating in real-time with people on the other side of the world, or accessing information from the near sum-of-all human knowledge (the internet), or reading the sophmoric rant of some disgruntled knuckle-dragger who, years ago, posted a rude comment on a youtube video recording of a histology class lecture (also the internet. Congratulations, Steve, you made it into my book... REVENGE IS MINE). So far I've tried to keep you, the reader, safe from extraneous information about histology that will never be relevent to you. Here I must go into more detail than your average undergraduate histology class. Do you trust me?
Connective tissues come from little segments of tissue in an embryo, found sandwiched between two layers of epithelium, called somites (shown to the left). Because of their shared lineage, connective tissues share a number of common features. The average connective tissue has few cells, and is mostly composed of Extra-Cellular Matrix. That matrix include glycoproteins and polysaccharides made by the cells, which attract water to form the gel matrix of ground substance. It also includes the more visibly-appealing fibers. Some important ground substance molecules we covered earlier are Fibronectin and Hyaluronic Acid. The three main types of visible fibers are collagen, reticular and elastic fibers. Connective tissues are usually highly vascular, meaning they contain blood vessels.
There are a number of different cell types found within a connective tissue. The stem cells are called Mesenchymal Stem Cells (MSCs). MSCs are extremely important cells (here is some further reading), and since you can find them in adult tissues they are also called a type of adult stem cell. These cells are capable of undergoing mitosis to produce more adult stem cells, which can differentiate into fibroblasts, lipoblasts, chondroblasts, osteoblasts, hemocytoblasts, myoblasts and neuroblasts. Thus they can form most connective tissues, including bone, cartilage, blood (both red and white blood cells), plus muscle and neural tissue. That's pretty much everything except epithelia, which MSCs can form as well after going through a transition.
Indian Muntjac fibroblast cells By Michael W. Davidson is liscended CC BY 2.0In a basic connective tissue, MSCs divide and differentiate into fibroblasts, the cells that blast out all the fibers and ground substance. There may be other cells found in a connective tissue, including adipocyes, red and white blood cells, or other cells that have emigrated from their tissue of origin. In a mature connecgive tissue, the fibroblasts are sometimes called fibrocytes, which fits with our regular nomenclature for a cell going through stages of differentiation. More frequently, however, they are called fibroblasts and we don't worry about whether they are actively blasting out fibers at the time or taking a rest.
Human Mesenchyme By Jpogi at Wikipedia - Public Domain CC0
Mesenchyme
Mesenchyme is the first type of connective tissue we make. It turns into other connective tissues. It is an embryonic tissue that has yet to decide what it will become when it grows up. It is composed mainly of MSCs and some mucousground substance. There is a special type of mesenchyme that we will run into, called ecto-mesenchyme. The lineage of ecto-mesenchyme is not from MSCs, but from a special type of neuronal cell called a Neural Crest cell. Neuro-mesenchyme forms other connective tissues I have yet to mention: dentin, cementum and the periodontal ligament.
Areolar connective tissue
Areolar CT is the quintessential connective tissue, or the most boring depending on how you look at it. It contains a little of everything the other connective tissues have: cells, ground substance, and all 3 fibers. Because it has a fair amount of ground substance, it is an ideal tissue to occupy places where blood vessels might need space to grow in the future. Hence areolar CT is found in regions that are highly vascular. This is why you can find a small layer of areolar connective tissue directly underneath nearly every epithelium, including the stratified squamous epithelium of the various types of oral mucosa.
Dense irregular connective tissue
Underneath the areolar CT of the oral mucosa is dense irregular connective tissue. It contains fibroblasts which make the bulk of the tissue: collagen fibers, the strongest of the 3 fiber types. In a dense irregularCT, the collagen fibers point in all directions. This makes this tissue particularly strong in all directions. You can also find dense irregular CT (along with its buddy, areolar CT) in the dermis of the skin. Why in both places, you ask? lineage.
Dense regular connective tissue
Like dense irregular CT, dense regular CT is mostly collagen fibers, only the fibers run parallel. That makes this tissue strong in one direction. You mostly find dense regular CT between muscle and bone, or between bone and bone. The only place in this book where we will find dense regular CT is the periodontal ligament, between bone and cementum.
If you trust my histology skills by now, you can skip this paragraph. The rest of you might be thinking the collagen fibers look wavy, not parallel. Well, when someone sliced this section of a tendon, they had to saw through the tissue using a razor blade. That blade pushed the fibers in one direction, then pulled them back, and pushed again as the person was was cutting as thin of a slice as they could. If they wanted better results, they should have used an expensive device such as a microtome, cryotome, vibratome or vibraphone (just kidding, that last one is a percussion instrument). I bring this up because many textbooks suggest histology is a nearly-perfect representation of what is found in the human body, when in reality there are often numerous artifacts, or errors, that histologists have learned to ignore when they see them. That's why they used to pay me the big bucks in academic research, it was my job to find the best pictures with the fewest artifacts, because most of my audience wouldn't ignore them. That's why I've chosen to illustrate most of the images used in this book. Do you trust me yet?
Adipose connective tissue
Adipose tissue is a type of connective tissue. It is special in that is has very little ECM. It contains mostly adipocytes, the mature cells that store triglycerides. It also contains MSCs, which if a person needs more adipose tissue, will divide and differentiate into adipocytes. Technically, the MSCs differentiate into a lipoblast first, but this would only be important for the sake of staying consistent with the names of other connective tissue cells. Maybe if they had named the cells adipoblast and adipocyte, we'd use the -blast name more frequently, but someone messed that up.
Most textbooks place adipose tissue next to areolar CT and categorize them both "loose", on account of their low number of visible fibers. That's fine if you like categories based on appearances, but you don't need to call either of them "loose". It is purely descriptive and does not correspond to anything physiological. We find adipose tissue in many places throughout the body, either between two other tissues, or embedded within another tissue (such as the adipose you find in bacon). On the other hand, areolar CT is found next to dense irregular CT, which is why I've deviated from the grouping found in most textbooks.
Bone connective tissue
Bone (or osseus) is the last type of connective tissue I will cover in detail. In fact, because of its similarity to dentin, cementum and enamel, I will cover bone tissue in a fair amount of detail. Like any connective tissue, bone tissue comes starts as MSCs, which differentiate into an intermediate stem cell called an osteo-chondro-progenitor cell, which can in turn decide to become an osteoblast or a chondroblast, depending on what signals it receives. It can also remain a stem cell within the connective tissues surrounding bones, the periosteum (superficial) and endosteum (deep). Osteoblasts secrete collagen fibers and ground substance, which later mineralizes, trapping the osteoblasts inside of a lacuna. At this point, they differentiate into osteocytes and maintain the bone tissue. Next, a new set of osteoblasts lay down another layer of bone tissue around the previous one. This creates either the concentric layers of bone tissue found in the osteons of compact bone, or the thinner spiral layers of bone tissue found in the trabeculae of spongy bone.
Fluorapatite by Von Benjah-bmm27 - own work, Public Domain CC0The extra-cellular matrix of bone tissue is roughly 1/3 collagen fibers and 2/3 mineral. The mineral component is a mixture of Ca2+ ions, which react with phosphate (PO43-) to form a hard crystalline matrix. Because most cells are full of phosphate, Ca2+ levels in the cytoplasm must always be kept very low, so if a cell like an osteoblast wishes to store Ca2+, it will do within its smooth Endoplasmic Reticulum. The Ca2+PO43- can further react with water and small amounts of fluoride(F-) to form a crystal named calcium hydroxyapatite. Dentin, enamel and cementum will have ECM very similar to bone tissue, only with varying amounts of collagen versus mineral components.
Collagen fibers run parallel within one layer of bone tissue, but run 90° in the next layer. While the ineral component of bone and tooth tissues provides strength, the collagen provides the ability to bend or flex, readucing the chances the tissue will shear off under stress. Collagen fibers, therefore, have a function similar to the rebar in reinforced concrete. The lower percentages of collagen found in enamel makes it harder and more resistant to caries than dentin or cementum, but more succeptible to abfraction.
Bones are surrounded by a layer of dense regular connective tissue called the periosteum. This tissue is a continuation (not a seperate structure) of the same tissue of tendons and ligaments. The periosteum also contains osteo-progenitor cells, the semi-differentiated cells that came from Mesenchymal Stem Cells, as well as osteoblasts and osteoclasts (coming up). Collagen fiber bundles from this layer named Sharpey's fibers penetrate the superficial layers of compact bone. This makes a very strong conneection between bone tissue and tendons or ligaments.
Another important cell found in the periosteum and endosteum is the osteoclast. This cell is not derived from the osteo-chondro-progenitor cell that gives rise to osteoblasts and osteocytes. The lineage of an osteoclasts traces to the bone marrow, from a stem cell that also gives rise to red and white blood cells. An osteoclast a close relative of white blood cells called macrophages, more than it is to bone cells. Osteoclasts de-mineralize bone tissue, releasing Ca2+ into the bloodstream. This is important because muscle and nervous tissue cannot function without some Ca2+, which our diet cannot provide regularly. Hence, bone tissue can be though of as a calcium storage organ, and in fact that is how I teach it in my Anatomy class.
Osteoclast activity is very important during the exfoliation of teeth, as bone tissue is removed to loosen the connection between bone and tooth. Osteoclasts activity is also key to the mechianism by which orthodontia works. Lastly, osteoclasts play an important role in maintaining bone health-- this may seem counter-intuitive, because they destroy bone tissue. Bone tissue is constantly being repaired by a group of osteoblasts and osteoclasts working together, known as a remodelling unit. Because compact bone tissue is dense, there is little room for cells to work. Osteocytes can repair small amounts of damage, but larger amounts of damage would build up over time. To prevent this, remodelling units constantly work to remove bone tissue and replace it with fresh bone tissue. The remodelling units cannot find damaged bone tissue, they simply keep removing and replacing bone tissue. Physical stress on bone tissue causes osteoblasts to work a bit harder, which leads to increased bone density. Loss of osteoblast-stimulating hormones (such as estrogen) can make osteoblasts work a bit slower than the osteoclasts, which can lower bone density and potentially lead to osteoporosis.
Bones form either by endochonral ossification, or by intra-membranous ossification. The two processes are very similar, with the exception that endochondral ossification begins with a hyaline cartilage model, whereas intra-membranous ossification begins with a dense connective tissue model. Most of the skull develops by intramembranous ossification, except the mandible and maxilla, which use both. I will cover endochondral ossification.
Most of the skeleton starts off not as bones, but as cartilage. Cartilage is typically avascular, but when chondrocytes receive the correct signals, blood vessels can grow into the ground substance. The growth of new blood vessels into a tissue is called angiogenesis. Aniogenesis allows MSCs to migrate into the central area and differentiate into osteoblasts and begin replacing cartilage tissue with bone tissue. This first site of ossification is named the primary ossification center. As remodelling occurs at the primary ossification center, blood vessels grow into the epiphyses and begin the same process at what are named the secondary ossification centers. Ultimately, most of the cartilage is removed, except in two important places. First, cartilage may remain between the parimary and secondary ossification centers, leaving growth plates (such as in bones of the arms and legs). Cartilage also remains at the very ends, to produce the articular cartilage found in a synovial joint, such as the tempero-mandibular joint. Intra-membranous ossification, because it begins with a dense connective tissue, leaves fontanels (soft-spots) between bones until ossification is complete, rather than cartilaginous connections.
When bone tissue is injured, it goes through a healing process similar to endochondral ossication. Perhaps you remember I mentioned wound healing recapitulates development? Here is an example of an adult tissue re-using the same mechanism to heal that it used during embryogenesis to grow in the first place. Similar to an injury to the skin, bone tissue first goes through an inflammatory process, and damaged blood vessels form clots. External blood clots are known as scabs, but inside the body they are named a hematoma. A blood clot contains an ECM fiber named fibrin that acts as a scaffold. This scaffold allows blood vessels to grow into the area, by angiogenesis. This in turn allows MSCs to migrate into the injured area, leave the blood and migrate along the scaffold, and differentiate into chondrocytes. The chondrocytes replace the fibrous hematoma with fibrocartilage-- this cartilage step is really the only step that is different from wound repair in the skin. Unlike the skin, bone tissue growth needs a cartilage model first. With this new cartilage scaffold, more MSCs migrate and differentiate into osteoblasts, and begin replacing the cartilage with bone tissue. Later, spongy bone tissue is remodelled so that it roughly matches the compact and spongy layers that were there previously-- only the former site of injury will be a little bit thicker once it is repaired.
Alloplastic particulate graft by Coronation Dental Specialty Group is liscensed CC BY SA 3.0Knowing the steps of endochondral ossification and bone fracture repair is important in instances where we want to stimulate or mimic the natural healing process. For instance, some patients will lose a significant amount of bone tissue with prolonged periodontitis. If enough bone tissue is lost, a surgeon may use a bone tissue graft to replace it. Bone tissue from a cadaver can be ground up, destroying the cells but leaving the collagen and minerals. Other sources may include a patient's own bone tissue (no one actually needs all of their ribs or fibulas more than they need and intact set of jaw bones and teeth). This is spread into the injured site. What has been grafted is just the ECM, not an entire organ, and it acts as a scaffold, not as a replacement. The patient's MSCs migrate through the scaffold and begin the healing process. The grafted scaffolding does not have to come from actual bone tissue, however. It can be ground-up dentin, fibrin, or synthetic polymers printed using a 3-D printer. The scaffolds, synthetic or biological, are ultimately replaced, which is why you may see this referred to as Guided or Resorbable Tissue Regeneration. Later we will learn that the scaffold is just one part of the healing process, ther are also important signals that guide MSC migration, division and differentiation. If you wish to know more, I have a short youtube video that covers this new technology in more detail, aimed at 200-level Anatomy and Physiology students.
Blood, Reticular, and Lymph Connective Tissue
There are a number of other connective tissues covered in histology classes, I will cover them very briefly. Blood is a type of connective tissue. Reticular fibers are found in areas where blood cells tend to rest, such as in lymphatic organs, and together this is called reticular connective tissue.
There are some other forms of connective tissue that aren't typically covered in histology classes, and I would like to mention one of them: scar tissue. Scar tissue is made by fibroblasts and is very similar to dense regular CT, except the collagen fibers in scar tissue are highly cross-linked. This makes scar tissue very strong, but also reduces its mobility. Scar tissue can be formed wherever there are large injuries to an organ-- fibroblasts can often fill up the space more quickly than waiting for regeneration to occur. Once formed, scar tissue is very difficult to remodel, so it is more-or-less permanent.
Muscle tissue
General characteristics of muscle tissue
Muscle tissue, like connective tissue, is derived from somites. The stem cell that can differentiate into one of the several types of muscle cells is called a myosatellite cell. Given the correct set of extra-cellular signals, myosatellite cells differentiate into skeletal, smooth or cardiac muscle cells. Skeletal muscle cells are long multi-nucleate cells that were once hundreds of myoblasts that fuse together. They are often confusingly referred to as muscle fibers, but they are definitely cells, not molecules like the protein and glycoproteinfibers found within the ECM of connective tissue. In fact, they aren't the only multi-nucleate cell you read about in this chapter, so it perplexes me as to why this cell type needs a special name (whose name confuses me). In adult muscle tissue, some myosatellite cells remain, ready to undergo mitosis and fuse to form larger muscle cells in reponse to exercise. We will see skeletal muscle in the tongue and underneath regions of oral mucosa, but I will not be covering smooth or cardiac muscle tissue.
Muscle tissue is composed primarily of muscle cells, connected by layers of collagen fibers. The layers are thin between cells, and thicker on the outer surface of a muscle. These layers of collagen fibers ultimately become the dense regular connective tissue of a tendon. It is often difficult to determine exacatly where muscle tissue ends and connective tissue begins, because of the collagen fibers in both tissues. By extension, it is also difficult to tell where the tendon ends and the periosteum begins. Some histologists will draw lines between these tissues, but the junction is better described as a blending. This is because both muscle tissue and connective tissue have the same lineage, from the mesenchyme in somites. For people studying musculo-skeletal diseases like Muscular Dystrophy, there is interest in learning how to instruct MSCs to become myosatellite cells-- especially considering the larger number of MSCs in the bone marrow compared to myosatellite cells in the muscle of a child or adult.
Nervous tissue
Desert rose by Tarzerind is liscensed CC By SA 4.0
General characteristics of nervous tissue
Shown to the left is a confocal image of neural tissue. What I like about this image, and is often absent from images of neural tissue, is that this one shows all of the cells. The typical image of neural tissue uses stains to highlight neurons, leaving out the glia. What you should see at this time is neural tissue is composed primarily of cells, and has very little ECM. While the brain especially has a huge number of blood vessels inside of it, technically they do not actually run within the neural tissue. Instead, there is a thin blood-brain-barrier separating the brain from blood vessels. Normally we wouldn't call neural tissue avascular, but I think I can here to make a point: neural tissue shares a lot in common with epithelial tissue. By now, I hope you have guessed why: neural and epithelial tissue share the same lineage.
3 major ebryonic tissues
Gastrulation
When an embryo implants in the uterus, it is a hollow ball of identical, omnipotent stem cells that resemble a simple cuboidal epithelium. But soon, some of these epithelial cells migrate inwards, in a very important process called gastrulation. At this stage, we now have two distinct layers of cells, one on the outside and one on the inside, lining what will ultimately become the gut. Next, cells from the outer layer change and lose contacts with neighboring cells, and migrate into the middle. As this happens, they stop looking like epithelial cells and begin to look like mesenchymal cells. This produces three layers of cells in an embryo. The three layers are listed below, along with what they will form in the adult:
3 embryonic layers
Cell fate
Ectoderm
Epithelium of skin and oral mucosa, neural tissue
Mesoderm
Connective & muscle tissue
Endoderm
Epithelial lining of hollow organs
I have now sneakily introduced you to two important terms used in development. The first is cell lineage, which means where a cell came from (in the past), and now cell fate, which means what that cell can become (in the future). If I didn't think your liscence exam would include questions on the 4 major tissue types, I would have started the chapter with this section and instead taught you the three major tissue types, with neural being a sub-family of epithelia (you'll see why later). I'd also have put connective and muscle tissue belonging to the same lineage, rather than separate them. But I can see why academia hasn't changed the way it teaches histology. Things change so quickly in science that it can be hard to keep track of it all-- the benefit of teaching histology a more correct way hasn't justified the effort it would take-- effort that might be better spent keeping up with medical advances and the like. It's too bad, really, it would make answering questions like "what are these neural crest cells doing in my epithelium?" Why, moving back in with their parents! But this, I think, is more than enough basic histology for a dental hygienist, it is time to move on to the histology of the oral cavity.
