Category: Normal Skin

Layers and Sub-Layers of the Skin

May 24, 2019

The skin is composed of two main layers. The epidermis is the outer layer which comes into contact with the external environment. It is considerably thinner than the dermis, approximately 10% of total skin thickness. The dermis is the powerhouse of the skin, providing the supportive structures to allow the epidermis to function.

Ninety percent of epidermal cells are keratinocytes which are thought of as the building blocks of the epidermis. shown here are the three layers of the skin.

Epidermis

The epidermis, which forms the top layer of skin, is constantly shedding millions of dead cells. It is estimated that normal skin sheds at a rate of a million cells every 40 minutes (Hinchcliffe et al., 1999) which equates to around 18 kg over a lifetime (Marieb and Hoehn, 2007). This process of skin cell shedding is known as desquamation. As skin cells are shed, new cells are constantly pushing up from underneath to replace them. If cells develop too quickly, the skin becomes piled up and thickened (as in skin diseases such as psoriasis) and if too slowly, the skin will be thin and atrophied (as occurs in old age). The normal transit time for epidermal cells (i.e. the time they take to move from the bottom layer of the epidermis to the top layer) is around 35 days. Epidermal thickness varies over the surface of the skin and can be thought of as either ‘thick’ skin or ‘thin’ skin. Thick skin occurs on the palms and soles and has neither hair follicles nor sebaceous glands but does have sweat glands. In these areas, the epidermis is between 400 and 600 μm. Thin skin, which covers the rest of the body, has hair follicles, sebaceous glands and sweat glands and is between 75 and 150 μm thick.

Ninety percent of epidermal cells are keratinocytes which are thought of as the building blocks of the epidermis. They start off as actively dividing cells and by the time they reach the skin surface they are anucleate bundles of keratin known as corneocytes. Keratin is synthesised within the keratinocytes from amino acids, particularly cysteine which allows for disulphide bond cross-linking which gives added strength to the skin. This is particularly predominant in hair and nails. Keratin is the same basic building block that is found in hair and nails in humans and horns, claws, hoofs and feathers in animals and birds.

Other epidermal cells include melanocytes (around 8% of total number of cells) and Langerhans cells.

Sub-layers of our Epidermis

Basal layer

Also known as the stratum basale or stratum germinativum, this is a single layer of columnar-shaped keratinocytes, some of which are stem cells undergoing constant cell division to produce new keratinocytes (Tortora and Derrickson, 2006). Each active basal cell divides every 4 days to produce daughter cells which then go on to differentiate and mature. These basal cells are ‘power-houses’ of activity containing various cellular structures which allow them to replicate effectively. Each cell has a large nucleus made up of cytoplasm containing ribosomes, which are attached to rough endoplasmic reticulum, a small Golgi complex and a few mitochondria The basal layer also includes the melanocytes, responsible for producing melanin which gives colour to skin and hair and protection from ultraviolet (UV) radiation. The production of melanin is under genetic control and is regulated by melanocyte stimulating hormone (MSH) secreted from the anterior lobe of the pituitary gland. Clinically, it is interesting to note that MSH is very similar in structure to adrenocorticotrophic hormone (ACTH). People with increased ACTH secretion, for example in Addison’s disease, show increased pigmentation in sun-exposed sites and where they experience mild trauma (Hinchcliffe et al., 1999) because ACTH acts as MSH.

The production of melanin occurs within organelles known as melanosomes, in the cytoplasm of the melanocytes. Within these melanosomes, the amino acid tyrosine is converted into melanin in the presence of the enzyme tyrosinase. From here it is transferred into the cytoplasm of the surrounding keratinocytes. Variations in hair pigment are caused by biochemical differences in the melanin produced in blondes, brunettes and redheads. The racial differences in skin pigment can be explained by the fact that in Caucasians, melanosomes are grouped in complexes which degenerate as the keratinocytes move towards the surface of the skin. In darker-skinned people the skin contains the same number of melanocytes, but the melanosomes are larger, remain separate and persist throughout the thickness of the epidermis (Graham-Brown and Burns, 1996). The quantity of melanin found in keratinocytes depends to a large extent on genetic make-up and the environment, that is how much UV exposure someone is subjected to.

Finally, the epidermis contains specialised cells called Merkell cells. At the interface of the epidermis and dermis, the flattened process of a sensory neuron comes into contact with the tactile disc of the Merkell cells thus detecting certain aspects of touch and sensation.

Prickle cell layer

As keratinocytes mature and differentiate, they go through the transition to the prickle cell layer where the cells become interlocked by a network of desmosomes. Desmosomes are designed specifically to hold cells together and as such are important structures which give the resilience to the skin. They consist of a plaque on either side of the plasma membrane (where a plaque is a dense layer of protein). On one side of this plaque, extending into the intracellular space, are glycoproteins known as cadherins which attach to one another. On the other side of the plaque, filaments consisting of keratin (known as tonofilaments), stretch from one side of the cell to the other where they attach to other desmosomes. This provides the cell with structural stability. The cells in this layer are so called because when they are fixed and observed under microscope, the cells pull slightly away from each other so that the desmosomes can be seen stretching across the intracellular space giving the cells a prickle-like appearance.

The prickle cell layer of the epidermis is between 8 and 10 layers thick. Keratohyalin granules are present in the keratinocytes and they contain a substance which combines with intermediate filaments of the cytoskeleton converting them to keratin; these also contribute to the resilience of the skin. Also in this section of the skin, lipid-filled membrane coating bodies start to develop.

Langerhans cells are present in the prickle layer. These cells are highly specialised dendritic cells which are an important part of the immune system, located within the skin. They are called dendritic cells because the surface membrane is folded in a similar way to the dendrites of the nervous system. This is so that the Langerhans cells can have maximum surface area to allow interaction with other cells. As immature cells, they are highly endocytotic (i.e. their plasma membrane invaginates producing an intracellular vesicle which surrounds the ingested material). However, as the Langerhans cells differentiate they have an increased capacity to migrate to T-cell areas and to function as antigen-presenting cells (Roitt and Delves, 2001). Mature Langerhans cells are covered in molecules known as major histocompatibility complex molecules, class II. These are adept at presenting the pieces of antigen protein to T-cells, which are then destroyed (Lydyard et al., 2000).

What is the Granular layer?

Also called the stratum granulosum, this is the part of the epidermis where there is high lysosomal activity. Lysosomes are organelles which contain enzymes that digest the cell contents causing the cell nuclei to disintegrate. At this stage the keratohyalin granules become more prominent within the cell and the lipid-filled membrane coating vesicles, which have been produced in both the granular and prickle cell layers, start to undergo exocytosis extruding the glycolipid over the keratinocyte membranes, thus helping to lubricate and waterproof the skin. These lipids include 40% ceramides, the rest being comprised of fatty acids, cholesterol and cholesterol sulphate. Langerhans cells continue to be present in the granular layer.

The Skin’s Horny layer

The horny layer or stratum corneum is the outer layer that interfaces with the environment. It is vital that it is capable of keeping out unwanted allergens and pathogens and retaining moisture by preventing water loss by evaporation. The cells of this outer layer are fibrous, tough bundles of keratin known as corneocytes. Filaggrin, a protein, which is also seen in the cells at this point, binds with keratin to help provide an effective skin barrier. Recent research has shown that the correct functioning of filaggrin is essential for effective barrier function of the skin. Examining the genetic make-up of people who suffer from icthyosis and atopic eczema shows loss of function mutations for filaggrin which may explain why the barrier function is compromised in these individuals (Hoffjan and Stemmler, 2007).

The horny layer contains a number of substances known as natural moisturising factors (NMF). These substances which include lactic acid, pyrrolidonecarboxylic acid and urea are water loving. They attract and hold water thus helping to maintain the hydration of the horny layer. Around 15% of the stratum corneum is water, if this falls below 10% the skin will become dry. The lipids, which were produced in the prickle and granular layers, continue to be present in the horny layer. They form what is known as a lipid bilayer which helps to further fortify the barrier function of the skin. In order to ensure the effective barrier function of the skin, all these mechanisms need to be in place.…

Normal Skin Health Changes From Infancy to Early Childhood

May 23, 2019

Here are a number of skin changes in the early months which can be considered ‘normal’ that do not usually require any intervention except reassurance. The terms may be a little strange but please bear with science, they have a very strange way of naming things.

Milia

These are tiny white spots which appear over the nose and face of babies; they are common. Their formation is probably related to the stimulus of the sebaceous glands which become temporarily blocked. There is no need to squeeze them as they will resolve of their own accord. The sebaceous glands become small and inactive soon after birth and as they do the milia resolve. The sebaceous glands remain inactive until puberty.

Mongolian blue spot

These are also relatively common in babies of Indo-Asian or Afro-Caribbean origin and occur in over 90% of children of Mongolian extraction. They consist of a blue grey patch on the skin which often occurs on the sacrum but can occur anywhere on the body. The skin surface is normal. The cause is thought to be elongated melanocyte precursor cells in the dermis. They can be mistaken as trauma from non-accidental injury, so should be documented in the notes. For most children these patches will fade as they get older, some however will persist into adulthood.

Benign acquired melanocytic lesions

Both freckles and lentigo can be described as benign acquired melanocytic lesions. Freckles are areas of skin where melanocytes are seen to be more active than in neighbouring areas. As a result, small (less than 5 mm in diameter), flat areas of pigmentation appear, generally scattered over the face, neck and arms, appearing in a variety of shades depending on the individual and the time of the year (darker in summer). Lentigo (plural being lentigenes) are also flat and a similar variety of sizes as the freckles, but they do not vary with sun exposure. Unlike freckles where there is no increase in the number of melanocytes, in lentigo there are.

Congenital melanocytic naevi

These lesions may be small or giant and occur in approximately 1% of births. The surface of the lesion may be smooth or rough and warty; there may be one or more hair follicles in the lesion. Giant congenital melanocytic naevi (those that cover a large area of the body and may be accompanied by thousands of smaller lesions) are associated with malignant melanomas and parents will need careful counselling about what action to take. Sometimes, the lesions are too large to consider surgical excision and grafting.

Vascular naevi

Vascular naevi are caused by dilated and tortuous, but otherwise normal blood vessels. Where capillary vessels are involved, a superficial or deep type may be described.

The superficial capillary naevi are caused by abnormal dilated vessels in the superficial dermis leading to salmon-coloured patches often on the face that will fade quite quickly. They are relatively common, occurring in approximately 50% of all neonates. The deeper capillary naevi are known as ‘port wine stains’, and because the vascular abnormality extends deeper into the dermis, these do not resolve and may even extend throughout life. The colour of the patches varies from pale pinkish red to dark purple; the colours will deepen with age. These changes can be associated with intracranial vascular changes and neurological pathology, so any child with a facial port wine stain should be investigated.

Arterial naevi

Otherwise known as superficial angiomatous naevi or strawberry birth marks, these occur in around 10% of children by the age of 1. Commonly, they start growing within a few days to a few weeks of birth and are usually relatively soft and irregular in outline. Sometimes there is a deeper component to these naevi where the subcutis is involved, in these instances the changes may lead to a distortion of normal anatomy. Growth of the lesion usually stops at around 6 months and resolution is usually spontaneous and complete, although if the lesion was particularly large, lose skin or atrophy may be left. The following rule of thumb is usually quite accurate:

Forty percent are gone by the age of 4 years; 50% by 5 years; 60% by 6 years; 70% by 7 years; 80% by 8 years and 90% by 9 years. (Graham-Brown and Bourke, 1998).

If the lesion interferes with feeding, breathing or sight, treatment may be recommended. For smaller areas, this is likely to be a steroid injection, but other options may be necessary including laser therapy. These types of naevi usually occur on the head, neck, buttocks or perineal areas. If they are associated with the lower back, sacrum or buttocks, a scan is usually recommended to exclude problems of tethering of the spinal cord.

Physiological jaundice (icterus neonatorum)

At about 2 days of age, parents may notice that their newborn is a yellowish colour. This is quite normal and results from the breakdown of the excess red blood cells that the child needed when they were in utero. As the child breaths following delivery, it no longer has any need of these red blood cells, so they break down leading to high serum bilirubin levels and the consequent yellow colour. This type of jaundice should not be confused with pathologic jaundice which occurs within 24 hours of birth and may be indicative of ABO or rhesus incompatibilities.…