Integumentary SystemSKIN

Dr. eman khammas al-sadipediatrician

Dr: Eman khammas alsadiEmbryology lecturer

• The skin is the largest organ in the body and has a dual origin: (a) A superficial layer, the epidermis, develops from the surface ectoderm. (b) A deep layer, the dermis, develops from the underlying mesenchyme

EpidermisInitially1. the embryo is covered by a single

layer of ectodermal cells,

2. In the beginning of the second month, this epithelium divides, and a layer of flattened cells, the periderm, is laid down on the surface.

3. With further proliferation of cells in the basal layer, a third, intermediate zone is formed.

4. at the end of the fourth month, the epidermis acquires its definitive arrangement, and four layers can be distinguished.

1. 4th layer composed of “ 1)The basal layer, or germinative layer,is responsible for production of new cells. This layer later forms ridges and hollows, which are reflected on the surface of the skin in the fingerprint.

2) A thick spinous layer consists of large polyhedral cells containing fine tonofibrils.

3) The granular layer contains small keratohyalin granules in its cells

4) The horny layer, forming the tough scale-like surface of the epidermis, is made up of closely packed dead cells containing keratin.

Cells of the periderm are usually cast off during the second part of intrauterine life and can be found in the amniotic fluid.

During the first 3 months of development, the epidermis is invaded by cells arising from the neural crest. These cells synthesize melanin pigment in melanosomes.

As melanosomes accumulate, they are transported down dendritic processes of melanocytes and are transferred intercellularly to keratinocytes of the skin and hair bulb.

In this manner, pigmentation of the skin and hair is acquired.

Cells of the periderm are usually cast off during the second part of intrauterine life and can be found in the amniotic fluid.

During the first 3 months:1. the epidermis is invaded by cells

arising from the neural crest.

2. These cells synthesize melanin pigment in melanosomes.

3. As melanosomes accumulate, they are transported down dendritic processes of melanocytes and are transferred intercellularly to keratinocytes of the skin and hair bulb.

In this manner, pigmentation of the skin and hair is acquired.

Clinical Correlates • Pigmentary Disorders• A large number of pigmentary disorders occur, and

these can be classified as diseases of melanocyte development, function, and survival.

• Examples of abnormalities of melanocyte function include

• piebaldism (patchy absence of hair pigment) and Waardenburg syndrome (WS), which feature patches of white skin and hair.

• There are several types of Waardenburg syndrome, but they share some common characteristics,

1. including patches of white hair (usually a forelock),

2. heterochromia irides (eyes of different colors),3. white patches of skin4. and deafness.•

• The defects arise because of faulty migration or proliferation of neural crest cells (absence of melanocytes derived from these cells in the stria vascularis in the cochlea accounts for deafness in these diseases).

• Diseases of melanocyte function• albinism characterized by globally reduced

or absent pigmentation in the skin, hair, and eyes.

1. oculocutaneous albinism (OCA).: In most cases, abnormalities of melanin synthesis or processing produce the abnormalities.

2. Vitiligo :results from a loss of melanocytes due to an autoimmune disorder.,There is patchy loss of pigment from affected areas, including the skin and overlying hair and the oral mucosa. Vitiligo is also associated with other autoimmune diseases, particularly of the thyroid.

• Fingerprints1) The epidermal ridges that produce

typical patterns on the surface of the fingertips, palms of the hand, and soles of the feet are genetically determined

2) They form the basis for many studies in medical genetics and criminal investigations (dermatoglyphics).

3) In children with chromosomal abnormalities, the epidermal pattern on the hand and fingers is sometimes used as a diagnostic tool

Dermis: is derived from mesenchyme that has three sources:

( a ) lateral plate mesoderm supplying cells for dermis in the limbs and body wall,

( b )paraxial mesoderm supplying cells for dermis in the back, and

( c ) neural crest cells supplying cells for dermis in the face and neck.

• During the third and fourth months

• the corium ,forms many irregular papillary structures, the dermal papillae, which project upward into the epidermis.

• Most of these papillae contain a small capillary or sensory nerve end organ.

• The deeper layer of the dermis, the subcorium, contains large amounts of fatty tissue

• At birth,• the skin is covered by a

whitish paste, the vernix caseosa, formed by secretions from sebaceous glands and degenerated epidermal cells and hairs.

• It protects the skin against the macerating action of amniotic fluid.

• Clinical Correlates • Keratinization of the Skin• Ichthyosis:– excessive keratinization of

the skin, is characteristic of a group of hereditary disorders that are usually inherited as an autosomal recessive trait but may also be X-linked.

– In severe cases, ichthyosis may result in a grotesque appearance, as in the case of a harlequin fetus.

HAIRHairs begin development as solid

epidermal proliferations from the germinative layer that penetrates the underlying dermis

At their terminal ends, hair buds invaginate.

The invaginations, the hair papillae, are rapidly filled with mesoderm in which vessels and nerve endings develop.

Soon, cells in the center of the hair buds become spindle-shaped and keratinized, forming the hair shaft, while peripheral cells become cuboidal, giving rise to the epithelial hair sheath

• The dermal root sheath is formed by the surrounding mesenchyme.

• A small smooth muscle, also derived from mesenchyme, is usually attached to the dermal root sheath. The muscle is the arrector pili muscle.

• Continuous proliferation of epithelial cells at the base of the shaft pushes the hair upward, and by the end of the third month, the first hairs appear on the surface in the region of the eyebrow and upper lip.

• The first hair that appears, lanugo hair, is shed at about the time of birth and is later replaced by coarser hairs arising from new hair follicles.

• The epithelial wall of the hair follicle usually shows a small bud penetrating the surrounding mesoderm (Fig. 20.3C). Cells from these buds form the sebaceous glands. Cells from the central region of the gland degenerate, forming a fat-like substance (sebum) secreted into the hair follicle, and from there, it reaches the skin.

• Clinical Correlates • Abnormalities of Hair Distribution• Hypertrichosis (excessive

hairiness) is caused by an unusual abundance of hair follicles.

• It may be localized to certain areas of the body, especially the lower lumbar region covering a spina bifida occulta defect or may cover the entire body.

• Atrichia, the congenital absence of hair, is usually associated with abnormalities of other ectodermal derivatives, such as teeth and nails.

• SWEAT GLANDS• There are two types of sweat glands:

eccrine and apocrine. • Eccrine sweat glands form in the skin

over most parts of the body beginning as buds from the germinative layer of the epidermis.

• These buds grow into the dermis, and their end coils to form the secretory parts of the glands. Smooth muscle cells associated with the glands also develop from the epidermal buds.

• These glands function by merocrine mechanisms (exocytosis) and are involved in temperature control.

• Apocrine sweat glands develop anywhere there is body hair, including the face, axillae, and pubic region.

• They begin to develop during puberty and arise from the same epidermal buds that produce hair follicles.

• Hence, these sweat glands open onto hair follicles instead of skin.

• The sweat produced by these glands contains

1. lipids2. proteins, 3. and pheromones4. , and odor originating from this

sweat is due to bacteria that break down these products.

• It should be noted that these glands classified as apocrine because a portion of the secretory cells is shed and incorporated into the secretion.

MAMMARY GLANDS• Mammary glands are modified

sweat glands and first appear as bilateral bands of thickened epidermis called the mammary lines or mammary ridges

• In a 7-week embryo, these lines extend on each side of the body from the base of the forelimb to the region of the hindlimb .

• Although the major part of each mammary line disappears shortly after it forms, a small portion in the thoracic region persists and penetrates the underlying mesenchyme

1. Here it forms 16 to 24 sprouts, which in turn give rise to small, solid buds.

2. By the end of prenatal life, the epithelial sprouts are canalized and form the lactiferous ducts.

3. Initially, the lactiferous ducts open into a small epithelial pit

4. Shortly after birth, this pit is transformed into the nipple by proliferation of the underlying mesenchyme.

5. At birth, lactiferous ducts have no alveoli and therefore no secretory apparatus.

6. At puberty, however, increased concentrations of estrogen and progesterone stimulate branching from the ducts to form alveoli and secretory cells.

• Clinical Correlates • Mammary Gland Abnormalities

• Polythelia is a condition in which accessory nipples have formed resulting from the persistence of fragments of the mammary line .

• Accessory nipples may develop anywhere along the original mammary line but usually appear in the axillary region.

Polymastia:• occurs when a remnant of

the mammary line develops into a complete breast.

Inverted nipple • is a condition in which the

lactiferous ducts open into the original epithelial pit that has failed to evert.

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