Dermatomes

A dermatome is defined as ‘a strip of skin that is innervated by a single spinal nerve‘. They are of great diagnostic importance, as they allow the clinician to determine whether there is damage to the spinal cord, and to estimate the extent of a spinal injury if there is one present.

In this article, we shall look at the embryonic origins of dermatomes, and explore their clinical uses

Fig 1.0 - Somites adjacent to the neural tube.

We can trace back the origins of dermatomes to the 3rd week of embryogenesis. 

At around day 20, the tri-laminar disc has been established and the middle layer (mesoderm) has differentiated into its different types. The portion that is directly adjacent to the neural tube is called paraxial mesoderm.

From day 20 onwards the paraxial mesoderm differentiates into segments called somites. 44 pairs of somites are formed, but 13 of these break down leaving 31 somites. This corresponds to the 31 sets of spinal nerves in the body.

The somites themselves are comprised of a ventral and a dorsal portion. The ventral portion consists of sclerotome, the precursor to the ribs and vertebral column.

The dorsal portion consists of dermomyotome. Over time, the myotome proliferates and the dermatome disperses to form dermis. As the limbs grow, the dermis associated with the precursor of the limbs is stretched and moved down the limb, creating the segmental innervation that is associated with the Keegan and Garrett dermatome map of 1948.

Dermatome Maps

There are two main maps that are accepted by the medical profession. 

The first is the Keegan and Garret map of 1948. This depicts dermatomes in a way that correlates with the segmental progression of limb development. 

The second is the Foerster map of 1933 which depicts the medial part of the upper limb as being innervated by T1-T3 which follows the distribution of pain from angina or an MI. 

This is the most commonly used map, and features in the ASIA scale of assessing spinal injury.

Both maps depict progression of limb growth around an axial line. Across this line there is no overlap between dermatomes, but often those adjacent each other have a slight overlap.

Fig 1.1 - The Keegan and Garrett dermatomal distribution.

Clinical Relevance: Assessing Spinal Cord Lesions

Following a traumatic injury that may involve the spinal cord, the clinician can test dermatomes to determine the presence and the extent of a spinal cord lesion.

Firstly, the clinician uses cotton wool to test for light touch sensation along the limbs and torso, touching areas which correspond to the different dermatomes. Secondly the clinician uses a small pin to test for responsiveness to pain. The patient is instructed to close their eyes and say when they feel contact with their skin (Light touch and pain are tested separately as their fibres travel in different parts of the spinal cord – see here).

By using their knowledge of dermatomal and peripheral cutaneous innervation, and noting any regions of paresthesia, the clinician is able to ascertain whether there is any nerve involvement. Also, they can determine whether this is at the spinal root or peripheral nerve level.

Fig 1.2 - The ASIA scale. It used to assess patients with a potential spinal nerve lesion

Embryology: Weeks 1-3

Embryology is the branch of medicine concerned with the study of embryos and their development.

In this article, we outline the processes that take place within weeks 1-3 of embryonic development – cellular division, differentiation and gastrulation. We shall also look at the clinical applications of this knowledge.

Week 1 and 2

Initial Cellular Divisions

The first stage in the development of a fetus is fertilisation – the process by which the male sperm and female egg join together.

Approximately thirty hours after fertilisation, the fertilised oocyte (egg) splits into two cells of equal size; called blastomeres. After three more divisions, there are 16 cells. At this point, the group of cells is referred to as the morula.

Within the first week, the cells of the morula reorganise to form a cavity, known as the blastocyst cavity (blastocoel). From this point, the morula is known as the blastocyst. It is comprised of two different cell types:

Outer cell mass (trophoblast) – contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta.

Inner cell mass (embryoblast) – responsible for the formation of the embryo itself.

During the second week, the trophoblast and embryoblast divide into increasingly specialised cell types. The trophoblast divides into the syncytiotrophoblast and cytotrophoblast. The embryoblast divides into the epiblast and hypoblast, forming a two-layered structure; the bilaminar disk. The amniotic cavity forms within the epiblast.

Implantation

After the initial rounds of cellular divisions, the embryo must implant into the endometrium of the uterus.

During this process, the syncytiotrophoblast becomes continuous with the uterus – such that maternal blood vessels (known as sinusoids) invade the spaces within the syncytiotrophoblast (known as lacunae). At this point, uteroplacental circulation has begun; and further embryonic development can occur.

Clinical Relevance – Placenta Praevia

In placenta praevia, implantation of the embryo occurs in the lower uterine segment (instead of in the normal position of the upper posterior uterine wall).

A low-lying placenta is more susceptible to haemorrhage, possibly due to a defective attachment to the uterine wall. Bleeding can be spontaneous, or provoked by mild trauma (e.g vaginal examination). Additionally, the placenta may be damaged as the presenting part of the fetus moves into the lower uterine segment in preparation for labour.

Week 3: Gastrulation

In the 3rd week of embryonic development, the cells of the bilaminar disk (epiblast and hypoblast) undergo a highly specialised process called gastrulation. During this process, the two cell layers become three germ cell layers, and the bodily axes observed in the mature adult are created.

Gastrulation is a process of cellular rearrangement which involves migration, invagination and differentiation of the epiblast. It is largely controlled and orchestrated by the primitive streak. The primitive streak is a groove in the midline of the epiblast which appears during the third week. 

Within the primitive streak lies a primitive node at the cranial end, and within the primitive node lies the primitive pit.

Cells of the epiblast layer break off and migrate toward the primitive pit. Here, they detach and penetrate through the epiblast layer to form three new germ cell layers:

Endoderm – formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells.

Mesoderm – formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm.

Ectoderm – formed by the epiblast cells that remain in position.

These three cell layers are then responsible for forming the different tissues of the fetus.

Fig 2 – Formation of the three primary germ layers occurs during the third week of development. The embryo at this stage is only a few millimetres in length.

Cell Layer

Mesoderm

Notochord

Musculoskeletal system

Muscular layer of stomach, intestine etc

Circulatory system

Endoderm 

Epithelial lining of digestive and respiratory tracts,

Lining of urethra, bladder and reproductive System

Liver and pancreas

Ectoderm

Epidermis of skin

Cornea and lens of eye

Nervous system

Ultrastructure of skin

The skin is the largest organ in the human body and comprises approximately 8% of total body mass.

It is a versatile structure with a wide range of functions; and its exact composition varies across different regions of the body’s surface.

In this article, we will discuss the function, gross structure and ultrastructure of our skin.

Functions of Skin

The skin provides an essential barrier between the external environment and internal body contents. It protects against mechanical, chemical, osmotic, thermal, and UV damage, and microbial invasion.

Its other functions include:

A role in the synthesis of vitamin D

Regulation of body temperature

Psychosexual communication

A major sensory organ for touch, temperature, pain, and other stimuli.

Gross Structure

The composition of skin varies across the surface of the body. Skin can be thin, hairy, hirsute, or glabrous. Glabrous skin is the thick skin found over the palms, soles of the feet and flexor surfaces of the fingers that is free from hair.

Throughout the body, skin is composed of three layers; the epidermis, dermis and hypodermis. We shall now examine these layers in more detail.

Ultrastructure

Epidermis

The epidermis is the most superficial layer of the skin, and is largely formed by layers of keratinocytes undergoing terminal maturation. This involves increased keratin production and migration toward the external surface, a process termed cornification.

There are also several non-keratinocyte cells that inhabit the epidermis:

Melanocytes – responsible for melanin production and pigment formation.

Note – individuals with darker skin have increased melanin production, not an increased number of melanocytes.

Langerhans cells – antigen-presenting dendritic cells.

Merkel cells – sensory mechanoreceptors.

Layers of the Epidermis

The epidermis can be divided into layers (strata) of keratinocytes – this reflects their change in structure and properties as they migrate towards the surface. From deepest to most superficial, these layers are:

Stratum basale – mitosis of keratinocytes occurs in this layer.

Stratum spinosum – keratinocytes are joined by tight intercellular junctions called desmosomes.

Stratum granulosum – cells secrete lipids and other waterproofing molecules in this layer.

Stratum lucidum – cells lose nuclei and drastically increase keratin production.

Stratum corneum – cells lose all organelles, continue to produce keratin.

A keratinocyte typically takes between 30 – 40 days to travel from the stratum basale to the stratum corneum

Dermis

The dermis is immediately deep to the epidermis and is tightly connected to it through a highly-corrugated dermo-epidermal junction.

The dermis has only two layers, which are less clearly defined than the layers of the epidermis. They are the superficial papillary layer, and the deeper reticular layer. The reticular layer is considerably thicker, and features thicker bundles of collagen fibres that provide more durability.

The following cell types and structures can be found in the dermis:

Fibroblasts – these cells synthesise the extracellular matrix, which is predominantly composed of collagen and elastin.

Mast cells – these are histamine granule-containing cells of the innate immune system.

Blood vessels and cutaneous sensory nerves

Skin appendages – e.g. hair follicles, nails, sebaceous and sweat glands. Although present in the dermis, these structures are derived from the epidermis which descend into the dermis during development.

Hair Follicles and Sebaceous Glands

The hair follicles and sebaceous glands combine to form a pilosebaceous unit – which is only found on hirsute skin.

Sebaceous glands release their glandular secretions via a holocrine mechanism into the hair follicle shaft. The hair follicle itself is associated with an arrector pili muscle, which contracts to cause the follicle to stand upright.

Sweat Glands

There are two main types of sweat glands:

Eccrine glands – the major sweat glands of the human body. They release a clear, odourless substance, comprised mostly of sodium chloride and water – which is involved in thermoregulation.

Apocrine glands – larger sweat glands, located in the axillary and genital regions. These apocrine glandular products can be broken down by cutaneous microbes, producing body odour.

Hypodermis

The hypodermis, or subcutaneous tissue, is immediately deep to the dermis.

It is a major body store of adipose tissue, and as such can vary in size between individuals depending on the amount of fatty tissue present.

Clinical Relevance – Disorders of Skin

Alopecia Areata – alopecia is marked by autoimmune destruction of hair follicles, causing hair loss.

Vitiligo – Like alopecia, vitiligo is an autoimmune disease, where melanocytes are targeted and destroyed. Areas of symmetrical depigmentation appear, which are more apparent in darker-skinned individuals.

Psoriasis – In psoriasis, the mitosis of keratinocytes in the stratum basale is drastically increased, producing a thickened stratum spinosum. This is clinically apparent as “scaly” skin, typically on the knees and elbows.

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