Skin, and How the Body Forms

What you will be able to do

Part one: skin

Skin is the largest organ in the body, and the only one you assess on every single patient whether you intend to or not. You see its colour, feel its temperature and texture, notice its scars, and press on it to reach everything beneath.

Figure 1 · What skin does

The layers

Figure 2 · The structure of the skin

Illustration to be added

A three-dimensional block of skin, cut away. Epidermis at the top with its layers named from the base upwards: stratum basale, spinosum, granulosum, lucidum (in thick skin) and corneum, with a melanocyte at the base. Dermis beneath, divided into papillary and reticular layers, with dermal papillae interlocking with the epidermis. Show within the dermis: a hair follicle with its arrector pili muscle and sebaceous gland, an eccrine sweat gland with its duct reaching the surface, a capillary loop, and the sensory receptors labelled - free nerve endings in the epidermis, Meissner corpuscle in a dermal papilla, Pacinian corpuscle deep in the hypodermis. Hypodermis at the bottom with fat lobules and a larger vessel. Warm skin tones, navy line work, gold leader lines.

The epidermis renews itself continuously. Cells are produced at its base, move upwards, flatten and fill with keratin as they go, and are shed from the surface. The whole journey takes roughly a month. This is why the skin can repair itself endlessly, and why a superficial injury leaves nothing behind.

What skin tells you on examination

Pressure injury deserves particular attention, because preventing it is squarely a physiotherapist's business. Sustained pressure over a bony prominence blocks the small vessels in the dermis, and tissue that cannot be perfused dies. The sacrum, heels, greater trochanters and ischial tuberosities are the classic sites, because at each the skin is thin and the bone is close.

Part two: how the body forms

You are not asked to become an embryologist. You are asked to understand three things: where tissues come from, how the segmental pattern of the body arises, and why limbs are supplied the way they are. All three explain something you will use clinically.

The three germ layers

Very early in development the embryo organises itself into three layers, and every tissue in the adult body traces back to one of them.

Figure 3 · The three germ layers and what each becomes

Two entries in that list are worth dwelling on.

Mesoderm gives rise to bone, cartilage, all connective tissue and all muscle — which is to say, almost everything a physiotherapist treats. If you remember only one germ layer, remember that one.

Neural crest is a population of cells that leaves the edges of the forming neural tube and migrates widely. It becomes the sensory ganglia, the autonomic ganglia, the Schwann cells that myelinate peripheral nerves, the adrenal medulla, the pigment cells of the skin and much of the face. It is an unusually versatile group of cells and it explains why some congenital conditions affect an odd-looking combination of structures at once.

Neurulation

Figure 4 · Neurulation and the neural crest

The nervous system begins as a thickened plate of ectoderm along the back of the embryo. Its edges rise into folds, the folds meet in the midline, and the whole thing rolls up into a tube that sinks beneath the surface. That tube becomes the brain and spinal cord.

The tube closes from the middle outwards, which means the two ends close last. Failure at either end produces a recognisable problem.

Adequate folate before and during early pregnancy substantially reduces the risk of these defects, which is why supplementation is recommended before conception rather than after a pregnancy is confirmed — the tube has closed before most people know.

Somites, and the segmental body

This is the section that pays off every day of your working life.

Alongside the neural tube, the mesoderm divides into paired blocks called somites, arranged one after another down the length of the embryo. Each somite then splits into three parts.

Figure 5 · What a somite becomes

Because one somite produces the bone, the muscle and the skin of one segment, and because one spinal nerve grows out to serve that somite, the adult body keeps a segmental pattern. A single nerve root therefore supplies:

• a group of muscles — its myotome
• a strip of skin — its dermatome
• and a part of the skeleton and its joints

Dermatomes overlap considerably in the adult, which is why damage to a single root often reduces sensation rather than abolishing it. Muscles are usually supplied by more than one root for the same reason, so a single root lesion typically weakens rather than paralyses.

How limbs form, and why they end up facing different ways

Figure 6 · Limb development and rotation

Illustration to be added

Top row: the sequence of upper limb development from a small bud on the side of the embryo, through paddle stage, digital rays appearing, tissue between the rays being removed, to separated fingers. Bottom section: a front-facing figure at two stages. Stage one, both limb pairs projecting outwards with palms and soles facing forwards and elbows and knees pointing the same way. Stage two, after rotation, with curved arrows showing the upper limb rotating laterally and the lower limb rotating medially, and the resulting adult arrangement labelled: elbow pointing backwards, knee pointing forwards, extensors of the arm posterior and extensors of the thigh anterior. Include a small panel showing how the dermatome bands are carried into the limb and become spiral as a result of the rotation.

Limbs begin as small buds on the side of the embryo, at the levels their nerve supply comes from. The bud flattens into a paddle, ridges appear within it, and the tissue between the ridges is removed to separate the digits.

At first both pairs of limbs point outwards with their future soles and palms facing forwards. Then they rotate — and this is the part worth remembering, because it explains a great deal of confusing adult anatomy.

This is why the elbow and knee bend in opposite directions, and why the extensors of the thigh sit at the front while the extensors of the arm sit at the back. It also explains why dermatomes spiral down a limb in the pattern they do rather than running in neat horizontal bands: the segments were laid down first, and then the limb twisted.

Where students get this wrong

Thinking the epidermis is where sensation happens

The epidermis has no vessels and no receptors. The receptors sit in the dermis, which is also why damage that reaches sensation is damage that will scar.

Judging a wound by its width

Depth decides scarring. A long superficial graze may heal invisibly; a small deep cut will not.

Waiting for a pressure area to break down

Non-blanching redness over a bony prominence is already an injury. The time to act is then, not when the skin has broken.

Learning dermatomes as a picture to memorise

They are a segmental sequence carried into a limb that then rotated. Understand the rotation and the map becomes logical.

Assuming one root lesion abolishes sensation

Dermatomes overlap. A single root lesion usually reduces sensation rather than removing it, and usually weakens a muscle rather than paralysing it.

Forgetting where mesoderm goes

Bone, cartilage, connective tissue and every kind of muscle. Almost the entire subject matter of your profession comes from one germ layer.

Check yourself

Ten questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Reviewed by the Physiotherapist India Team. · Human Anatomy contents