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Dual Origins of the Skin

Epidermis(superficial layer) –develops from surface ectoderm

Dermis(deep layer) –develops from the underlying mesenchyme


Initially, the embryo is covered by a single layer of ectodermal cells. In the beginning of the second month, this epithelium divides, and a layer of flattened cells,__, is laid down on the surface.

the periderm, or epitrichium


With further proliferation of cells in the basal layer during the second month, a third, ___ is formed

intermediate zone


Finally, at the end of the fourth month, the epidermis acquires its definitive arrangement, and four layers can be distinguished, namely:

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.

SPINOUS LAYER - consists of large polyhedral cells containing fine tonofibrils.

GRANULAR LAYER - contains small keratohyalin granules in its cells.

HORNY LAYER - forming the tough scalelike surface of the epidermis, is made up of closely packed dead cells containing keratin.


True or False

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 __



Dermis is derived from mesenchyme that has three sources:

1. Lateral plate mesoderm - supplying cells for dermis in the limbs and body wall
2. Paraxial mesoderm - supplying cells for dermis in the back
3. 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 called __ which contain a capillary and sensory nerve organ

dermal papillae


- a deeper layer of the dermis which contains a large amount of fatty tissue



- a whitish paste formed by secretion from sebaceous glands and degenerated epidermal cells and hairs
- it protects the skin against the macerating action of amniotic fluid

vernix caseosa


Hairs begin development as solid epidermal proliferations from the germinative layer that penetrates the underlying dermis. At their terminal ends, ___ invaginate.

hair buds


- are rapidly filled with mesoderm in which vessels and nerve endings develop

hair papillae


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 ___ is formed by the surrounding mesenchyme. A small smooth muscle (arrector pili), also derived from mesenchyme, is usually attached to the dermal root sheath.

dermal root sheath


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, __, is shed at about the time of birth and is later replaced by coarser hairs arising from new hair follicles.

lanugo hair


Cells from small bud at epithelial wall of hair follicle form the __

sebaceous glands


Cells from the central region of the gland degenerate, forming a fat-like substance __ secreted into the hair
follicle, and from there, it reaches the skin.



Two types of Sweat Glands

1. Eccrine - Buds from germinative layer of the epidermis
2. Apocrine - From same epidermal buds that produce hair follicles


Sweat glands and mammary glands develop from __

epidermal proliferations


Mammary glands are modified sweat glands and first appear as bilateral bands of thickened epidermis called __

mammary lines or mammary ridges


Development of Nails

- Ectodermal thickening in the dorsum of digits
- Nail fields, surrounded by nail folds


- Largest organ
- 15-20%
- Integument, cutaneous covering



Functions of the Skin

- Protection
- Excretion
- Sensory
- Thermoregulation
- Metabolic
- Sexual signaling


the skin is composed of:

epidermis - an epithelial layer of ectodermal origin
dermis - a layer of mesodermal connective tissue


At the irregular junction between the dermis and epidermis, projections called __ interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers.

dermal papillae


Beneath the dermis lies the ___ a loose connective tissue layer usually containing pads of adipocytes.

subcutaneous tissue or hypodermis


- consists mainly of a stratified squamous keratinized
epithelium composed of cells called keratinocytes.
- forms the major distinction between thick skin and thin skin
- Germinal basal layer mitosis



There are also three much less abundant epidermal cell

pigment-producing melanocytes
antigen-presenting Langerhans cells
tactile epithelial cells called Merkel cells


Rate of mitosis/ Rate of desquamation

15-30 days, 25 to 50 days