The Integumentary System

It’s Professor Dave, let’s look at the integumentary system. With some basics covered regarding two of the types of tissue in the human body, we are ready to start looking at some of the different systems found in the body, and we are going to start with the outermost system, which is called the integumentary system. This is made up of the skin that covers and protects all your other body parts, as well as its derivatives, like certain glands, as well as hair and nails.

Let’s look at the structure and function of these components now. The skin is a remarkable organ, acting as the first line of defense from the elements that surround us. It keeps bacteria out, and water and heat in, so let’s check out the structure of skin.

Our skin is made of two regions, a thin outer layer, which is the epidermis, and a thicker inner layer, called the dermis. The dermis, which is a tough layer of fibrous connective tissue, is vascularized, getting its nutrients from the bloodstream. The epidermis, which is made of epithelial tissue, is avascular, so it gets nutrients by diffusion through the tissue fluid from the dermis.

In addition, below the dermis is the subcutaneous layer, also called the hypodermis. This is made predominately of adipose tissue, and it anchors the skin to other structures below. Now let’s zoom in on the epidermis, the thin outer layer of the skin.

This is made of keratinized stratified squamous epithelium, made of four different cell types in five layers. The deepest layer, the one attached to the dermis, is called the stratum basale, meaning basal layer. This is made of a single row of cells, most of which are called keratinocytes.

These are rapidly dividing all the time, pushing new cells up into the layers above, to help regenerate dead skin, and producing lots of keratin as they do so. This is necessary because millions of dead keratinocytes rub off your skin every day, due to friction, and even more from your hands and feet, so these constantly dividing cells in the stratum basale ensure that a new epidermis forms every few weeks, so that we always have our skin intact. This layer also contains melanocytes, which produce melanin, and tactile cells, which act as the sensory receptor for touch.

Above this is the stratum spinosum, meaning prickly layer. This section is several layers of cells thick, and is full of cells with a weblike system of intermediate filaments attached to desmosomes. They look kind of spiky, so they are sometimes called prickly cells.

Here we can also find dendritic cells, which ingest foreign substances and activate the immune system, which we will talk about later. Next is the stratum granulosum, or granular layer. This is four to six cell layers thick, and in this section, keratinization begins as cells continue to move their way upwards from the basal layer.

This is when the cells get far enough from the dermal capillaries below to receive sufficient nutrients, so the cells fill up with keratin as they die, and they flatten while the organelles disintegrate. This makes the cell tougher and scalier, which allows for the outer layers to better protect the body. Above this is the stratum lucidum, or the clear layer.

This is two or three cell layers thick, made of dead keratinocytes that have become flat and clear. This is where they begin to aggregate into arrays called tonofilaments. And lastly, there is the stratum corneum, or horny layer.

This outermost section is twenty to thirty cell layers thick, and all of these cells are anucleated, meaning the nucleus has disintegrated. So it’s incredible to think that all the cells covering the outside of your body are actually dead, but they are dead in a specialized way, with thick plasma membranes surrounding lots of keratin. These dead cells protect all the living ones inside from all the outside dangers.

With the epidermis covered, let’s go a little deeper and talk about the dermis. This is made of strong and flexible connective tissue, and is full of nerves and blood vessels. This is also where hair follicles begin.

The dermis has two sections, the papillary layer, and the reticular layer. Descending from the epidermis we enter the papillary layer, which is very thin, made of areolar connective tissue and a loose network of collagen and elastic fibers. This leaves room for defensive cells to patrol the area for bacteria that may have made it through the skin.

The word papillary refers to the projections from the surface of this layer, which are called dermal papillae, that stick out into the epidermis above. These contain lots of tactile cells as described previously. In areas where there is lots of friction, like the hands, these papillae sit on mounds called dermal ridges, which cause ridges in the epidermis as well, which are meant to enhance the gripping ability of the fingers, and they are visible as the lines on our fingertips that make our unique fingerprints.

Below this papillary layer sits the reticular layer, which is most of the dermis, and it is made of dense fibrous connective tissue that is arranged irregularly. A network of blood vessels sits below this, just before the hypodermis. What else can we say about the skin?

We mentioned that the stratum basale also contains melanocytes, which produce melanin. This is a pigment molecule, and this is one of the components of the skin that determines its color, and protects the skin from ultraviolet radiation. The other pigments responsible for skin color are carotene, which is yellow-orange, and hemoglobin, which is red when oxygenated.

We will discuss this molecule in more depth later in the series. That wraps things up for the skin, so let’s just briefly touch on the other components of the integumentary system, which are called skin appendages. First, there is hair.

Of course most of us have lots of hair on our heads, but there is also hair all over the body, including eyelashes and nose hairs, and these all have specific protective functions. So what is a hair made of exactly? As it turns out, a hair is a flexible strand made largely of dead, keratinized cells.

This is hard keratin, which is a bit different from the soft keratin found in cells of the epidermis, which makes them more durable, and not as flaky. Hairs are produced by hair follicles. These possess a root, which is the part deep inside the follicle where keratinization is happening, and a shaft, the part closer to the surface of the skin and then extending outside the body, where keratinization is complete.

The hair itself consists of three layers of cells. The innermost is the medulla, containing large cells and soft keratin. Next is the cortex, which is several layers of flattened cells.

And lastly there is the cuticle, which is a single layer of overlapping cells, the most keratinized cells in the hair. As for the follicle the hair sits in, this also has some structure to it. In general, a hair follicle is a pocket that folds down from the surface of the epidermis down into the dermis, about four millimeters below the surface.

The deep end then expands slightly to form a hair bulb. A bundle of nerve endings attach to the bulb and act as a receptor, responding to any bending of the hair and alerting the brain in case an insect is there, or something of the like. A little bit of dermal tissue called a hair papilla protrudes into the bulb, which supplies signals and nutrients to the hair so it can grow.

The wall of each follicle has a few layers. The outermost is the peripheral connective tissue sheath, derived from the dermis. Next is the glassy membrane, derived from the basal lamina.

And then there is the innermost epithelial root sheath, derived from the epidermis. This last section has an external part and an internal part, which thins as it approaches the bulb. The cells that actively divide are found in the hair matrix, which push existing cells upwards as they divide, causing the hair to grow.

In addition, each follicle has an arrector pili. This is a small bundle of muscle cells that can contract and pull the follicle in such a way that the surface of the skin dimples out, producing what we refer to as goose bumps, when cold or afraid. Our body hair is pretty sparse so this no longer serves much purpose, but for much furrier mammals it is an important defense mechanism for trapping heat and intimidating enemies.

Hair can either be vellus hair, which is pale and fine, or terminal hair, which is darker and more coarse, like hair of the eyebrows and scalp. The nails found on our fingers and toes are also part of the integumentary system. These are products of a modification of the epidermis.

Just like with hair, in contrast to the soft keratin of the skin, nails contain hard keratin, making them great tools for scratching or picking up objects. Each nail has a free edge, which is the very tip, then a body, which is most of what we see, and then a proximal root, which is embedded in the skin. The part of the epidermis the nail sits on is called the nail bed, and the nail grows out of the nail matrix, which pushes the nail outwards across the nail bed as these cells divide.

There are also skin folds overlapping the borders of the nail which are called nail folds, sitting on the lateral and proximal borders, and the latter extends onto the nail as the eponychium. At the edge of the finger is the hyponychium, where dirt tends to collect. The last part of the integumentary system we must mention is the vast collection of glands that can be found.

First up are the sweat glands, also known as sudoriferous glands. These are found almost everywhere on the surface of the skin, totalling up to around three million. There are two types of sweat glands.

The first is called eccrine, or merocrine sweat glands. Most of them are of this type, and it consists of a coiled tube. Secretion occurs in the dermis, and the resulting fluid, or sweat, travels through the tube towards a funnel-shaped opening called a pore.

Sweat is ninety-nine percent water, but it contains some salts and metabolic wastes, among other things. Sweat is also secreted by apocrine sweat glands, which are far fewer, found only in certain areas, and secrete fat and protein components along with the normal mixture, which is the cause of body odor. Ceruminous glands and mammary glands are also types of apocrine glands, which produce earwax and breast milk.

Beyond sweat glands there are sebaceous glands, also known as oil glands. These are branched alveolar glands that secrete sebum, which is made of oily lipids. This will soften and lubricate hair and skin, slowing water loss and killing certain bacteria.

So that covers the basics of the integumentary system, which consists of skin, hair, nails, and glands. This system acts a barrier, separating what’s outside of us from what’s inside of us. It has the ability to repair quickly, regulate body temperature, and respond to stimuli outside of the body, among other things.

Now that we have this system covered, let’s head inside the body and check out the rest.