∞ generated and posted on 2020.01.13 ∞

Diversity of epithelial tissue and connective tissue making up especially cutaneous membranes.

The tissue membranes making up the integumentary system consist of multiple varieties of epithelial tissue in combination with underlying connective tissue, with the skin in particular possessing multiple layers of cells.

This page contains the following terms: Simple epithelium, Stratified epithelium, Mucous membrane, Basement membrane, Cutaneous membrane, Fibers, Collagen, Elastin, Reticulin, Keratin, Skin, Hypodermis, Subcutaneous, Dermis, Epidermis, Basal stratum, Cornified stratum, Callus

This is a second posting of this video in case you would like a further reminder of what epithelial tissue consists of.

Simple epithelium

Single layer of tightly connected animal cells that serves as a surface covering of other tissues.
The function of simple epithelium is particularly as a selectively permeable tissue layer or instead as a site of release of epithelium-synthesized materials such as hormones (glandular epithelia).

Simple epithelium can be found lining the small intestine as well as in the kidneys, and making up key components of the endocrine system.

It is the thinness of simple epithelium that allows movement of substances both across and out of this tissue.

Links to terms of possible interest: Basement membrane, Epithelial tissue, Simple columnar epithelium, Simple cuboidal epithelium, Simple epithelium, Simple squamous epithelium

The above video is really well done, introducing simple epithelium in its various guises.

Stratified epithelium

Multiple layers of tightly connected animal cells that serve as a surface covering of other tissues.
The function of stratified epithelium particularly is as a protective barrier. The skin, for example, consists of stratified epithelium. It is the relative thickness of stratified epithelium that inhibits chemical or mechanical penetration either into or through this tissue.

Links to terms of possible interest: Basement membrane, Cell nucleus, Epithelial tissue, Simple columnar epithelium, Simple cuboidal epithelium, Simple epithelium, Simple squamous epithelium Stratified columnar epithelium, Stratified cuboidal epithelium, Stratified epithelium, Stratified squamous epithelium,

The above video provides nice graphics of slides presenting stratified epithelium along with a nice discussion towards identification.

Mucous membrane

Antimicrobial barrier often possessing viscous, moist secretions on its surface and covering most body surfaces not directly in contact with the outside environment.
The outer surfaces of vertebrate bodies consist of a combination of skin – along with associated appendages such as hair, scales, or feathers – and mucous membranes. Mucous membranes are notable particularly for their secretion of mucus, which serves as a lubricating, moistening, and antimicrobial agent.

Mucous membranes can be found interior to various body openings, such as the anus, the penis (i.e., the urethra is lined with a mucous membrane), the vagina, the mouth, the nose, etc.

These more closely surface-associated mucous membranes often are associated as well with more internal mucous membranes, such as the lining of the gastrointestinal tract as well as much of the urinary tract and respiratory system. Technically, though, mucous membranes, unlike the skin and its appendages, are not considered to be part of the integumentary system.

Links to terms of possible interest: Epithelium, Goblet cell, Mucus, Mucous membrane, Respiratory tract

The above video contrasts skin and mucous membrane, discussing the latter second. It doesn't go into all that much detail, but it is short and kind of fun.

Basement membrane

Acellular fibrous sheets found underlying epithelial and endothelial tissue.
The basement membrane consists in part of collagen fibers. It serves to anchor overlying cells (i.e., epithelial tissue). They also serve to impede cell movement, thereby contributing to the spatial structure of bodies, including in terms of preventing cancer-cell invasiveness and metastasis.

Basement membranes are selectively permeable barriers, e.g., determining what kinds of materials can move to and from epithelial tissue. This is seen most notably in association with kidney functioning but also functions more generally in allowing nutrients to reach epithelial cells from the blood supply that is found underlying basement membranes.

Links to terms of possible interest: Apical surface, Basal lamina, Basement membrane, Connective tissue, Dermis, Endothelium, Epidermis, Epithelial tissue, Extracellular matrix, Fibroblast, Glandular epithelium, Interstitial matrix, Pseudostratified ciliated columnar epithelium, Skin, Simple columnar epithelium, Simple cuboidal epithelium, Simple squamous epithelium, Stratified squamous epithelium

Nice overview of basement membranes and their functions: epithelium support, selective permeability (so that epithelial cells can be fed since they don't have a blood supply of their own), and as blocks on cancer cell movement to the rest of the body.

Cutaneous membrane

Skin as defined from the perspective of its being a tissue membrane.
Cutaneous membranes are one of the four types of tissue membranes. Cutaneous membranes in combination with mucous membranes make up the vast majority of the external covering of our body tissues.

Links to terms of possible interest: Cutaneous membrane, Dermis, Epidermis, Hair, Integumentary system, Skin, Skin appendages

This video reminds us about tissue membranes, which in terms of cutaneous membrane makes up the integumentary system.


Elongated extracellular matrix proteins produced by connective tissue.
These fibers include those of various collagen protein types, reticulin (which is a variation on collagen), and elastin (which does not consist of collagen). In addition there is keratin, which is produced particularly by epithelial tissue in cutaneous membranes and which is not exactly found extracellularly but instead within dead cells, though keratin also is found within living cells more generally.

The above video provides an introduction to different types of fibers making up fibrous connective tissue, collagen, elastin, and reticulin.


Most abundant protein in mammals, connects together tissues and is the primary constituent of connective tissue.
Collagen fibers are a key constituent of connective tissues, providing tensile strength, that is, resistance to excessive stretching as well as resistance to tearing. This is similar to the functioning of the cables that help to hold up suspension bridges or the rebar that serves to reinforce concrete used in the construction of buildings and roads.

These roles can be most easily appreciated in terms of the functioning of ligaments and tendons, which contain large amounts of collagen fiber (ligaments and tendons can be thought of as the principle macroscopic "ropes" or "cables" serving to hold bodies together).

It is cells called fibroblasts that are responsible for the production of collagen, where collagen otherwise serves as a key constituent of extracellular matrix.

Links to terms of possible interest: Collagen, Collagen fiber, Extracellular matrix, Fibroblast, Protein, Space-filling model

This is a brief video that focusses mainly on collagen's structure – though what then is collagen if not structure? – but has great graphics.


Connective tissue protein that is capable of fairly substantial stretching and contraction while exerting force towards return to its original size.
Elastin is important both for supplying a suppleness to tissues and for the sake of storing mechanical energy. The skin, for example, returns to its original state after being pulled or pinched, or indeed simply as the underlying body moves about. This returning action to a large extent is due to the action of elastin.

Elastin is found in the walls of arteries, which allows arteries to both stretch in response to the beating of the heart and to store the pressure exerted by that heartbeat (thus evening out blood pressure between individual heartbeats).

The stretching of our lungs upon breathing in is a consequence in lung tissue of an extensive presence of elastin, which imposes its "stretchiness" on the inhaling lung. So too the substantial expansion and then voiding of the urinary bladder is a function of elastin found within the bladder walls.

Links to terms of possible interest: Conformation, Cross-link, Elastic fiber, Elastin, Protein,

This goes into much more detail then we will tend to care about, but a short discussion of elastin is followed by description of its synthesis/formation.


Crosslinked collagen fibers that create a fine mesh that serves as connective tissue internal support of various soft internal tissues within animal bodies.
The tissues that are supported by reticulin fibers include bone marrow, the liver, and lymph nodes. Basically these tissues and organs consist of spongy, semi-solid material that nonetheless is resistant to flowing as well as resistant to tearing internally.

This resistance is a consequence of this fine mesh of collagen fibers that form a microscopic, extracellular, reticular network of collagen "cables" within these tissues. The word "reticular", by the way, describes something as being netlike, intricate, and/or entangled.

Links to terms of possible interest: Blood vessel, Collagen fibers, Reticular fibers, μm

The above video allows you to gain an appreciation of the presence of reticulin fibers in the liver.

The above video allows you to gain an appreciation of the presence of reticulin fibers in lymph nodes.


Major protein constituent of the outer layer of skin, nails, hair, reptilian scales, feathers, hooves, claws, beaks, baleen, horns including those of rhinoceros, and porcupine quills.
Keratin is a water-insoluble fibrous structural protein that is an important constituent of the connective tissue of tetrapods (i.e., amphibians, reptiles, mammals, and birds) but which nonetheless is found in chordates generally (fish, lancelets, and tunicates). The intermediate filaments making up cell cytoskeletons consist of keratin protein. Keratin is produced in abundance by cells called keratinocytes.

Keratinized skin is a waterproof outer covering that takes on its tough form due to the presence of keratin and which represents a defining characteristic of reptiles, mammals, and birds. This keratin serves in addition to collagen and elastin towards maintaining the structural integrity of skin.

The production of the keratinized layer – which consists of stratified squamous epithelial tissue – is called cornification. The end result of cornification is the production of "cells" that consist of little more than bags of keratin. These cells make up the bulk of the epidermis and are continuously shed, i.e., as dead skin cells.

Links to terms of possible interest: Cortex, Cortical cell, Cuticle, Hair, Hair shaft, Intermediate filament, Keratin, Macrofibril, Matrix, Protein

The above video provides a nice overview of the place of keratin and their producing cells, keratinocytes, in physiology, that is, rather than in terms of the biochemistry of keratin.


Outer covering of vertebrate animals which serves as a barrier to both parasite invaders and the leaking of water, nutrients, and colloidal substances out of bodies.
Skin, in other words, is something that is associated with animals with backbones (vertebrates, but not invertebrates) and which serves as a barrier to a number of things including organisms found in an animal's environment (particularly microorganisms but also various invertebrate organisms). The skin also serves as a barrier to potentially noxious compounds that also can be found especially outside of the body as well as substances that the skin instead retains within the body (e.g., such as electrolytes and water).

The skin is often described as the largest organ of the body and this is true in terms of overall size and mass. It also is a key constituent of the integumentary system. While still serving as an outer covering, skin is not necessarily also a tough, waterproof outer covering in all animals, i.e., as we may view our own skin versus, for example, that of a salamander, or indeed of a fish. Our skin has these latter properties because it represents a special kind of skin that is described as keratinized (as in keratinized skin), which in turn is a characteristic of the amniotes (that is, the reptiles, mammals, and birds).

Our skin consists of two plus one layers. Starting from the outside and going in, these are the epidermis, the dermis, and the hypodermis.

Links to terms of possible interest: Cutaneous membrane, Histology, Mucous membrane, Serous membrane, Synovial membrane, Tissue membrane

An introduction to the integumentary system but particularly the skin.

The above video provides a nice overview of what the skin is all about.

The above video provides a really nice overview of the layers/cells associated with skin. Where the narrator states "three types of tissue" it would perhaps have been better to have stated "three types of fibers".

The above video provides an overview of the anatomy as well as physiology and microbiology of the skin.".

There is an advertisement towards the end of the above video but it provides a nice, short overview of what skin is all about nonetheless.

The above video is a general overview of skin cancers.

The above video is the second in the skin cancer series that begins with the immediately previous video.


The underlying layer of the skin.
To remember this, recall that a hypodermic needle delivers materials to beneath the skin, and indeed potentially to the hypodermis. That is, the hypodermis is found beneath the rest of the skin.

The hypodermis serves as the connector between the skin and underlying tissues. This connection is not tight but instead can be described as loose, with the hypodermis in fact containing what is known as loose connective tissue. This looseness allows the skin and underlying body to move somewhat independently of each other, though the connective tissue assures that this movement is not completely independent. Your skin thus has the properties that you are aware of, i.e., it is a flexible outer covering that allows substantial freedom of body movement but nonetheless pretty much remains in place.

The hypodermis also contains your skin-associated adipose tissue, that is, the fatty deposits that are found directly beneath your skin's dermis and which in fact make up a substantial fraction of your total fat stores (and which also serve as an example of loose connective tissue). When one attempts to become "ripped" or to display a "six pack" (i.e., as one's "abs"), it is the hypodermic fat stores that must be reduced, through dieting and exercise, so as to more directly display the underlying muscle

Links to terms of possible interest: Fat reserves, Hypodermis, Hypodermis fat cell layer

The above video provides a nice over of the hypodermis, including in terms of subcutaneous fat.


Alternative name for hypodermis.
Subcutaneous literally means below or under the cutaneous membrane, that is, the lowest layer of the skin. The term superficial fascia is also equivalent, meaning surface connective tissue.

There are a number of things to glean from this video, most notably there is an MRI that visualizes subcutaneous fat, not that it needs all that much visualization since if you can pinch it, and it's body fat, then it is subcutaneous fat; also the point is made that subcutaneous fat, unlike visceral fat, is removable surgically, i.e., via liposuction, not that I am in any way suggesting that this might be preferable to reducing caloric intake and exercising!


The middle layer of the skin.
It is the dermis that contains much of the skin's collagen and elastin as well as reticular fibers. The cells that make up this layer are the fibroblasts, which are responsible for producing these various fibrous proteins, and also cells that play important roles in the immune protection of the body (i.e., various white blood cells including those white blood cells known as macrophages).

The dermis has a substantial blood supply such that if you scratch your skin and it doesn't bleed then you haven't reached the dermis whereas if you scratch or slice yourself relatively shallowly into the skin and it does bleed, then you have reached the dermis.

It is within the dermis that both hair follicles and the various skin glands are found. Our bodies are also able to generate vitamin D from sunlight via a chemical reaction that occurs within the dermis.

Links to terms of possible interest: Basement membrane, Cutaneous membrane, Dermal papilla, Dermis, Eccrine sweat glands, Epidermis, Epithelial peg, Hypodermis, Interdigitate, Keratin, Skin

The above video is a nice introduction to what the dermis of the skin is all about.


The outer layer of the skin.
The epidermis is a relatively thin layer of tissue (epithelial tissue) that consists of a number of layers. The basal stratum is found at the bottom, attaching the epidermis to the dermis, and cornified stratum is found at the very top, defining the surface of the skin.

It is the epidermis that displays continuous cell growth and dead-cell shedding, with new cells generated at the bottom of the epidermis, in the basal stratum, and dead cells shed from the top (from the cornified stratum). In between, keratinocytes undergo cornification where they are converted to what essentially are bags of keratin. Also contained within the epidermis are melanocytes.

The source of nutrients and oxygen for the epidermis, as well as the means by which wastes are removed, is via the blood supply that is found within the underlying dermis, as epithelial tissue does not contain blood vessels since, at least logically, these would serve as breaks within the otherwise protective barrier that epithelial tissue consists of.

Links to terms of possible interest: Basal cell, Basal stratum, Cornification, Cornified cell, Desmosomes, Epidermis, Granular cell, Spinous cell

The above video considers in particular the attachment between the dermis and the epidermis, the dermal-epidermal junctions.

Basal stratum

The lower-most layer of the epidermis.
The basal stratum, or stratum basale, is the source of keratinocytes to the rest of the epidermis. Specifically, these are the cells that divide into keratinocytes and thus serve as the keratinocyte stem cells.

The basal stratum contains other cell types beside these keratinocyte progenitor cells, and these are the melanocytes as well as a type of immune system cell called Langerhans cells. In addition, the basal stratum contains cells that effect touch reception, called Merkel cells.

The above video provides a nice, diagrammatic glimpse of the basal stratum at 1:06 but otherwise talks about cancers of the basal stratum, that is, basal cell carcinoma (and it does a good job of talking about these skin cancers).

Cornified stratum

The upper-most layer of the epidermis.
The cornified stratum, or stratum corneum, consists entirely of dead keratinocytes, described as corneocytes, ones in which the majority of the cell's interior has been replaced with keratin fibers. It is this layer in particular that provides the "horny" as well as waterproof nature of the keratinized skin of amniotes (reptiles, birds, and mammals).

The ongoing loss of cells from the cornified stratum is described as desquamation and is a characteristic of mammals but not of reptiles, which instead periodically molt their cornified stratum (e.g., such one sees with the skin shedding of snakes).

The pace of cell movement from the basal stratum through to the cornified stratum, that is, as the epidermis and cornified stratum are continuously renewed, takes roughly two weeks. This process together is known as cornification.

This above video is nicely, professionally produced – and brief! – look at what the upper layer of our skin is all about and good for. Note the mention of stem cells.


Hardened as well as thickened regions of epidermis that develop in skin given substantial use.
Calluses form when skin is exposed to repeated, relatively low levels of friction or pressure. If relatively high levels of friction are applied, then blisters instead will form.

Calluses can be viewed as a means by which skin can morphologically adapt/physiologically adapt to excessive use, resulting in skin that is more resistant to damage in the course of such use. Calluses also can be viewed as horny, keratinized protective plates serving as a form of localized armor.

If we're going to talk calluses, we might as well talk guitars and this video discusses callus formation, callus care, and nail care, as well as repair, on one's fret hand.