School of Anatomy and Human Biology - The University of Western Australia
|Blue Histology - Integumentary System|
Lab Guides and Images
Skin, thick - Major Layers - H&E and trichrome
Skin, thick & thin - Epidermis - H&E and trichrome
Skin, thick & thin - Dermis - H&E and van Gieson & elastin
Sebaceous Gland - trichrome, H&E
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The skin or cutis covers the entire outer surface of the body. Structurally, the skin consists of two layers which differ in function, histological appearance and their embryological origin. The outer layer or epidermis is formed by an epithelium and is of ectodermal origin. The underlying thicker layer, the dermis, consists of connective tissue and develops from the mesoderm. Beneath the two layers we find a subcutaneous layer of loose connective tissue, the hypodermis or subcutis, which binds the skin to underlying structures. Hair, nails and sweat and sebaceous glands are of epithelial origin and collectively called the appendages of the skin.
The skin and its appendages together are called the integumentary system.
Skin, thick - H&E, trichrome
The epidermis is a keratinised stratified squamous
epithelium. The main function of the epidermis is to protect the body
from harmful influences from the environment and against fluid loss. Five structurally
different layers can be identified:
Variations in the thickness of the epidermis (~0.1 mm in thin skin, 1 mm or more in thick skin) are mainly the result of variations in the thickness of the stratum corneum, although the other layers also vary in thickness. Cells of the epidermis of the skin will at some time of their life keratinise and are collectively also called keratinocytes.
Keratinisation should not be used as a synonym for the formation of the stratum corneum: other stratified squamous epithelia may become keratinised but may not form a stratum corneum in which cells join to form a horny cell membrane.
Skin, thin - H&E and Skin,
The most superficial part of the epidermis is formed by the stratum corneum. Nuclei are not visible in this layer. Cell outlines may be visible at high magnification or, in the form of artefacts, as cracks or clefts in the stratum corneum. The stratum granulosum is formed by a single layer of very dark and flattened cells in thin skin. Several layers of cells containing keratohyalin granules are visible in thick skin. Polyhedral cells with clear outlines form the stratum spinosum. The stratum basale is formed by a single layer of cuboidal or columnar cells and delimits the epidermis from the dermis.
At high magnification, the basal cytoplasm of the basal cells seem to interdigitate with the underlying dermis. Similar to the dermal papilla, this irregular border at the cellular level, the dermal-epidermal junction, anchors individual basal cells firmly to the underlying dermis.
Identify and draw the epithelium in thick and thin skin. Identify in your drawing as many of the layers of the epidermis as possible.
The red and yellow hues of the skin are due to haemoglobin in the red blood cells, which pass through the capillaries beneath the epidermis, and carotene, which accumulates in fat cells found in the dermis and hypodermis.
The brown colour component is due to melanin, which is produced in the skin itself in cells called melanocytes (typically 1000-2000 / sqr. mm). These cells are located in the epidermis and send fine processes between the other cells. In the melanocytes, the melanin is located in membrane-bound organelles called melanosomes. The cell bodies of melanocytes are difficult to distinguish in ordinary LM preparations, because the melanosomes are located mainly in the processes of the cells.
Melanocytes can transfer melanin to keratinocytes - mainly to the basal cells. The fine processes of melanocytes may invade keratinocytes and bud-off part of the melanocyte cytoplasm, including the melanosomes, within the keratinocytes. Melanin protects the chromosomes of mitotically active basal cells against light-induced damage.
Pigmentation is not just under the control of light. Hormones produced by the pituitary and the adrenal glands also affect pigmentation. Diseases of these two endocrine organs often result in changes of pigmentation of the skin.
Although melanocytes are also ectodermal in origin,
they are derived exclusively from the neural crest of the embryo,
from where they migrate to all other parts of the body.
are another cell type found within the epidermis. Morphologically they are not unlike melanocytes, but functionally they are more closely related to macrophages. They are important in immune reactions of the epidermis. Their fine processes form a network between the cells of the epidermis and phagocytose antigens which have entered the epidermis. Langerhans cells may only be temporary residents of the skin. If they have come into contact with an antigen, they can migrate to regional lymph nodes, where they initiate an immune response.
are, like Langerhans cells, a group of cells functioning in the immune system. Some of them will be present in the epidermis. Together with Langerhans cells they are sometimes referred to as SALT, i.e. skin-associated lymphoid tissue.
The dermis is the thick layer of connective tissue to which the epidermis is
attached. Its deepest part continues into the subcutaneous tissue without a
sharply defined boundary. Its thickness is for this reason difficult to determine
but 1-2 mm is a good guestimate for "average" skin. The dermis may be divided
into two sublayers (again without a sharp boundary):
Kraissl lines have been defined in living humans. They not always coincide with the cleavage lines, which Langer defined (Langer's cleavage lines) about a century before Kraissl in cadavers.
Elastic fibres are found in both the papillary (fine fibres) and reticular
(coarse fibres) layers.
They can not be identified in H&E stained sections.
Skin, thin - H&E and Skin,
thick - van Gieson & elastin
How easy it is to differentiate between the papillary and reticular layer of the dermis depends on the preparation - you may have to look at several preparations. Immediately beneath the epidermis you should see a layer which at low magnification appears rather evenly stained. At high magnification the stain should resolve into a fine network of collagen fibres, which blend with equally fine elastic fibres. Cells are more numerous in the papillary layer and you should see more nuclei in this area than in the deeper reticular layer. Also, the papillary layer contains the capillary network which supplies the epidermis, The reticular layer contains coarse collagen and elastic fibres and the larger vessels which feed into the capillary network of the papillary layer..
Draw part of the epidermis and the underlying dermis. Label the layers of the dermis and structures contained within them.
A characteristic feature of the human skin is the apparent lack of hair (pili) on most of the body surface. This is actually not quite true. Most of the skin is haired although the hair in most areas is short, fine and only lightly pigmented. This type of hair is called vellus hair.
Truly hairless are only the palms of hands and soles of feet, the distal phalanges and sides of fingers and toes and parts of the external genitalia.
In those parts of the skin which we perceive as "hairy" we find terminal hairs. The free part of each hair is called the shaft. The root of each hair is anchored in a tubular invagination of the epidermis, the hair follicle, which extends down into the dermis and, usually, a short distance into the hypodermis. The deepest end of the hair follicle forms an enlargement, the bulb. Cells in the bulb are mitotically active. Their progeny differentiates into the cell types which form the hair and the cells that surround its root, the root sheath. Hair cells keratinise within the lower one-third of the hair follicle. Above this level it is not possible to identify individual cells within the hair. Each hair follicle has an associated bundle of smooth muscle, the arrector pili muscle. This muscle inserts with one end to the papillary layer of the dermis and with the other end to the dermal sheath of the hair follicle.
Hair growth is discontinuous. Hairs are lost and replaced by new ones. The hair follicle goes through different stages that reflect the discontinuous hair growth. Anagen is the phase of growth. The resting stage is called telogen. The length of the anagen is variable in different regions of the body - lasting only a few months for hair of the eyebrows and eyelashes but 2 to 5 years for hair of the scalp. Hair growth is controlled by a number of hormonal and hereditary factors and their interactions.
Skin, hairy - H&E
Hair follicles of terminal hair span the entire dermis and usually extend deep into the hypodermis. Most of them will be cut at odd angles and only a few good longitudinally or transversely cut profiles are visible. The hair may have been lost during the preparation of the specimen and not all hair follicles will contain hairs. Although it is often possible to see the attachment of the arrector pili muscle into the hair follicle or the papillary layer of the dermis, both attachments are hardly ever visible in the same section.
Draw a hair follicle at low magnification. Try to draw a composite from several hair follicles and associated structures, which captures their appearance from the bulb to the epidermis.
Sebaceous glands empty their secretory product into the upper parts of the hair follicles. They are therefore found in parts of the skin where hair is present. The hair follicle and its associated sebaceous gland form a pilosebaceous unit.
Sebaceous glands are also found in some of the areas where no hair is present, for example, lips, oral surfaces of the cheeks and external genitalia.
Sebaceous glands are as a rule simple and branched (Remember the nomenclature of glands!). The secretory portion consists of alveoli. Basal cells in the outermost layer of the alveolus are flattened. Basal cells are mitotically active. Some of the new cells will replenish the pool of basal cells, while the remaining cells are displaced towards the centre of the alveolus as more cells are generated by the basal cells. The secretory cells will gradullay accumulate lipids and grow in size. Finally their nuclei disintegrate, and the cells rupture. The resulting secretory product of lipids and the constituents of the disintegrating cell is a holocrine secretion.
The lipid secretion of the sebaceous glands has no softening effect on the skin, and it has only very limited antibacterial and antifungoid activity. Its importance in humans is unclear. Clinically the sebaceous glands are important in that they are liable to infections (e.g. with the development of acne).
Skin, hairy - trichrome, H&E
Sebaceous glands will be present in all types of skin other than thick skin. Their numbers should correlate with the number of hair follicles. If your section does not contain hair follicles you are unlikely to see a good sebaceous gland. Sebaceous glands are usually embedded in the dermis. Although they empty into the hair canal of the hair follicle, this point will only be visible for a few of them because of the thinness of the sections. It should however be possible to follow the fate of the secretory cells. Deep in the sebaceous glands cells are smaller with intact nuclei. Cell size increases with the accumulation of sebum as the cells are gradually displaced towards the opening of the gland into the hair follicle. The nuclei condense, become darker and irregularly shaped.
Draw a sebaceous gland. Emphasise the appearance of the secretory cells in different parts of the gland. If possible include part of the associated hair follicle.
Two types of sweat glands are present in humans. They are distinguished by their secretory mechanism into merocrine (~eccrine) sweat glands and apocrine sweat glands. In addition, they differ in their detailed histological appearance and in the composition of the sweat they secrete.
Merocrine sweat glands are the only glands of the skin with a clearly defined biological function. They are of critical importance for the regulation of body temperature. The skin contains ~3,000,000 sweat gland which are found all over the body - with the exception of, once again, parts of the external genitalia.
The excretory ducts of merocrine sweat glands empty directly onto the surface of the skin.
|unusually thick, haematoxylin stained section of the skin|
Apocrine sweat glands occur in, for example, the axilla. They are stimulated by sexual hormones and are not fully developed or functional before puberty. Apocrine sweat is a milky, proteinaceous and odourless secretion. The odour is a result of bacterial decomposition and is, at least in mammals other than humans, of importance for sexual attraction.
The histological structure of apocrine sweat glands is similar to that of merocrine sweat glands, but the lumen of the secretory tubulus is much larger and the secretory epithelium consists of only one major cell type, which looks cuboidal or low columnar. Apocrine sweat glands as such are also much larger than merocrine sweat glands.
The excretory duct of apocrine sweat glands does not open directly onto the surface of the skin. Instead, the excretory duct empties the sweat into the upper part of the hair follicle. Apocrine sweat glands are therefore part of the pilosebaceous unit.
Some texts argue that the apocrine sweat glands use
a merocrine or a combined merocrine / apocrine secretory mechanism.
Skin, thick - H&E
Scan along the border between dermis and hypodermis and locate a sweat gland. The secretory tubulus and the initial segment of the duct usually form a cluster of round or irregularly shaped profiles, which stain darker than the surrounding connective tissue. The structural preservation of the sweat glands may vary quite a bit in the different preparations. The different cell types in the secretory epithelium of merocrine sweat glands are only visible in well preserved glands. The red rim around the secretory tubulus is formed by the cytoplasm of myoepithelial cells. Their small, dark nuclei may be visible close to the periphery of the tubulus.
Draw a small schematic illustrating the relative position of the sweat gland in the skin. Identify and draw the secretory tubulus and excretory duct. Label as many features as can be identified.
Nipple - H&E
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last updated: 6/08/09