Skin histology

The skin covers and helps protect the body from its environment.It’s also known as the integument, which is part of the integumentary system.The integumentary system also consists of structures that are derivatives of the skin, such as nails, hair, and glands within the skin, like sweat glands.The skin can be split into two general types: thick skin and thin skin.Thick skin has a much thicker outer layer of epidermis that helps to protect the body in places where it’s regularly exposed to slightly harsher physical conditions.That’s why it’s typically found in places such as the palms of the hands and soles of the feet.Thick skin can also be differentiated from thin skin because it doesn’t have hair follicles or oil-producing glands called sebaceous glands.It does still contain sweat glands though.The overall thickness is often more than 5mm, whereas thin skin is typically only between 1-2mm thick.Thin skin has all the same structures as thick skin except it has a thinner epidermis and additional structures such as hair follicles and sebaceous glands.If we compare histologic images of thick skin and thin skin at low magnification, we can see the three main layers that make up the skin.At the top of the image is the surface epidermal layer or epidermis that’s significantly thicker in thick skin.Hair follicles are only found in thin skin, and as we can see in this image, they’re located in the middle layer called the dermis or dermal layer.The third and deepest layer is called the hypodermis, which consists mainly of adipose and loose connective tissue, although this image of thick skin doesn’t include the hypodermis.Now, if we look at just the epidermis, there are sub-layers that make up the epidermis.Starting from the base of the skin is the stratum basalis, spinosum, granulosum, lucidum, and corneum at the surface.The Stratum Lucidum can only be seen histologically in very thick skin.If we take a closer look at the epidermis of both thick skin and thin skin, we can see that the stratum basalis is only a single-layer of cuboidal or columnar germ cells.These cells are attached to a basement membrane that’s normally too thin to see under the microscope.These cells have a high level of mitotic activity, dividing to produce all the cells that migrate up to the surface of the skin.In both thick and thin skin, the cells within the stratum spinosum synthesize a fibrous protein called keratin, which is why the cells themselves are called keratinocytes, even though they also produce other proteins as well.This image of thick skin was taken while using a 60x oil immersion objective.Oil immersion objectives allow for higher resolution images to be seen through the microscope.All the skin cells of the epidermis are keratinocytes except for the germ cells, which are found in the stratum basalis.The keratinocytes are attached to each other by strong cell-to-cell structures called desmosomes, which are more easily seen in this layer compared to the layers above the stratum spinosum.The desmosomes are found in between the cells and look like tiny needles or spines.As the keratinocytes mature, they’re pushed up closer to the surface by newly formed cells below and gradually become flatter in appearance as well.Above the stratum spinosum, is a layer of 3-5 flattened cells that contain dense basophilic kerato-hyalin granules within their cytoplasm, which is why this layer is called the stratum granulosum.In thick skin, the lighter or more lucid layer of cells seen above the stratum granulosum is the stratum lucidum, which is typically only 2-3 cells thick.The keratinocytes in this layer will have lost their nuclei and organelles, although they’re still attached to one another by desmosomes.This layer won’t be seen above the granulosum in tissue sections taken from thin skin.Now, the stratum corneum is the outermost layer of dead keratinocytes from the underlying stratum lucidum.So these cells also don’t have nuclei or organelles, but in this layer, the cells are almost completely filled with keratin filaments.This layer consists of approximately 15-30 layers of the squamous keratinocytes.Over time, the desmosomes in this layer gradually break down, which causes the cells to shed or desquamate from the surface of the skin.In thick skin, the stratum corneum is often thicker than all of the other epidermal layers combined, whereas in thin skin, this layer is much thinner when compared to the rest of the epidermis.Alright, let's move onto the dermis or dermal layer that sits below the epidermis.The junction between the two layers is irregular, forming many projections of the dermal layer called dermal papillae.The corresponding invaginations of the epidermal layer are called epidermal ridges, and the combination of the papillae and ridges is referred to as the papillary layer of the dermis.The dermal papillae help improve adhesion between the epidermal and dermal layers.This image is from thick skin.If we compare it to thin skin at the same magnification, the dermal papillae in thin skin are noticeably smaller, which makes the papillary layer thinner as well.Sensory receptors called Meissner’s corpuscles are found in the dermis.These elliptical structures are found inside dermal papillae and are positioned perpendicular to the surface of the skin.They function as mechanoreceptors, which means they sense or respond to physical pressure or movement of the skin, but they’re also specialized to detect light touch and low frequency vibrations better than the mechanoreceptors found in deeper layers of the dermis.When textured objects are moved across the skin, they create the low-frequency vibrations that are detected by Meissner’s corpuscles.Alright, let’s zoom back out to the low magnification images of thick and thin skin.The papillary layer is the outer portion of the dermis that includes the dermal papillae, but the rest of the dermal layer is called the reticular layer.This layer is much thicker and contains a lot of dense irregular connective tissue, but this is also where many of the specialized structures of the skin are also located, such as sweat glands and sensory receptors called Pacinian corpuscles.In addition to these structures, thin skin will also have hair follicles, sebaceous glands, and arrector pili within the reticular layer.The eccrine sweat glands are the coiled tubular glands and ducts that produce and secrete sweat onto the surface of the skin.When seen histologically, we can only see cross-sections of the tubular structures, as a result the gland looks like a collection of circular structures that each have a central lumen.The walls of the sweat glands consist of a simple or stratified cuboidal epithelium.These sweat glands are found in both thick and thin skin.In this image of thin skin, we can see the secretory component of the sweat gland in the bottom of the image as well as part of the ducts as they travel to the surface of the skin.Because the ducts are not producing fluid, the ductal cells have a cytoplasm that’s more dense.This means that the ducts will typically stain a darker purple when compared to the secretory component of the sweat glands.The Pacinian corpuscles are the mechanoreceptors in the dermis that are better at sensing course touch, pressure, and high frequency vibrations when compared to the Meissner’s corpuscles.Sensing high frequency vibrations allow the skin to distinguish the difference between different fine surface textures, such as the difference between paper and glass.The Pacinian corpuscles are large round structures that have 20 to 60 concentric layers or lamellae of modified Schwann cells.That’s why these corpuscles are also often called lamellar corpuscles.The lamellae surround a central inner pulp that contains unmyelinated axons.In this image, we can also see that the corpuscles are also surrounded by an outer capsule of connective tissue.Alright, let’s move on to hair follicles,which are only found in thin skin.The base of each strand of hair and hair follicle has a hair bulb, which contain specialized basal epithelial cells that produce the keratinocytes that eventually become the hair shaft as well as the inner root sheath.Hair shafts are made up of dead keratinocytes that have lost the majority of their cellular detail.The inner root sheath immediately surrounds the hair shaft within the hair follicle and is split further into two sublayers.The outer layer is called Henle’s Layer, which is a layer of simple cuboidal cells, and the inner layer is called Huxley’s Layer, which consists of a single or double layer of flat cells.Surrounding the inner root sheath is the external root sheath, which contains layers of cells that are continuous with the epidermis.Separating the external root sheath from the dermis is a hyaline layer without cells or an “acellular” layer called the glassy membrane.The dermal papilla is the connective tissue and network of capillaries that help bring nutrients to the growing basal cells and projects into the base of the bulb.In this image of a hair follicle, we can also see the sebaceous glands nearby that produce and secrete an oily and highly lipid fluid called sebum.The cells of the sebaceous gland are called sebocytes.They have a central nucleus and their cytoplasm is filled with small fat droplets that give the cell a foamy appearing cytoplasm.The glandular cells then drain into converging ducts that typically empty into the upper portion of the hair follicle.Just below the base of the sebaceous gland, we can see part of a small muscle.These muscles are able to move the hair follicles they’re attached to, causing the hairs to stand on their end when a person is cold or scared.

layer of flat cells? Surrounding the inner route sheet is the external root sheath which contains layers of cells that are continuous with the epidermis.

Separating, the external route sheet from the dermis is a Highland Laird without cells or an, a cellular layer called the glassy membrane.

The dermal, papilla is the connective tissue and network of capillaries that helps bring nutrients to the growing basil cells and projects into the base of the bulb in this image of a hair follicle.

We can also see the sebaceous glands nearby that produce and secrete an oily and highly lipid fluid called sebum, the cells of the sebaceous.

Glands are called Subway sites. They have a central nucleus and their cytoplasm is filled with small fat droplets that give the cell a foamy appearing cytoplasm.

The glandular cells then drain into converging ducts that typically empty into the upper portion of the hair follicle just below the base of the sebaceous gland.

We can see part of a small muscle. These muscles are able to move the hair follicles.

They're attached to causing the Harris to stand on their end when a