Thursday, July 2

Study of skin - Structure of skin

  ›     ›   Study of skin structure
The study of skin (dermatology) is the acquisition of knowledge of its anatomical structure, physiological and pathological characteristics. The study of skin structure includes the anatomy and histology of skin components and the constituent cells.

The study of skin structure and its physiology helps us in acquisition of the required knowledge for getting and maintaining younger, healthier looking skin. The human skin is the largest organ of the integumentary system. The skin is the covering of the body, protecting all the internal organs. The structure of skin is multilayered with individual layers having multiple functions.

Skin structure and layers

Skin contains two primary layers, namely epidermis and dermis.

Epidermis structure

The epidermis is the outer primary layer.
The epidermis structure contains strata, corneum, lucidum, granulosum, spinosum and germinativum.
The melanocytes, melanosomes, melanin pigments, Langerhans cells and Merkel cells are also present.
About 90% of the cells present in epidermal layer of skin are keratinocytes.
The epidermis lacks blood vessels, lymph vessels and capillaries.
Stratum corneum
The stratum corneum (meaning 'horned layer') is the outermost layer of the epidermis and is located above the stratum lucidum or the stratum granulosum depending upon the overall thickness of epidermis.
The stratum corneum structure contains 15 to 20 layers of flattened dead cells (corneocytes), ceramides, corneodesmosomes, dense network of keratin and glutenous protein.
Stratum lucidum
The stratum lucidum (meaning 'clear layer') is a thin, translucent, clear, flattened layer located between stratum corneum and the stratum granulosum layers in the epidermis.
The stratum lucidum structure is composed of 3-5 layers of dead eleidin-filled keratinocytes and is readily visible only in areas of thick skin like palms and soles.
Stratum granulosum
The stratum granulosum (or granular layer) is a thin layer of cells lying between the stratum lucidum and the stratum spinosum layers of the epidermal skin.
Keratinocytes, also known as granular cells, containing keratohyalin granules are found in the stratum granulosum.
As these keratinocytes migrate to stratum corneum, they secrete lamellar bodies into the extracellular space to form the hydrophobic lipid envelope.
Stratum spinosum
The stratum spinosum meaning 'spinous layer' is located between the stratum granulosum and stratum basale layers of the epidermis.
The stratum spinosum is also known as prickle-cell layer.
Keratinization, synthesis of cytokeratin and formation of tonofibrils and then the desmosomes takes place in the polyhedral keratinocytes present in the stratum spinosum of epidermal skin.
Stratum basale (germinativum)
In the structure of epidermis, the stratum basale (aka stratum germinativum) is the innermost layer of epidermis lying below the stratum spinosum layer.
The stratum basale structure is composed of a continuous layer of basal keratinocyte cells, usually with one cell thickness.
The stratum basale also contains melanocytes, Langerhans cells and Merkel cells.


Structure of dermis

The dermis is the inner primary layer of the skin.
The dermis lies between the epidermis layer and lower subcutaneous tissues.
Its structure comprises two sublayers namely, the outer papillary dermis and inner reticular dermis.
The dermis is bound to the epidermis through a basement membrane.
This layer contains glandular structures like sweat glands, sebaceous glands and apocrine glands.
Collagen, elastic fibers, and extrafibrillar matrix are some of the components of its structure.
Nerve endings (mechanoreceptors), fibroblasts, macrophages, adipocytes, lymphatic vessels and blood vessels are also found in this layer.
Papillary dermis
The papillary dermis is the outermost layer of the dermis and found above the reticular dermis.
The dermal papillae are small finger-like extensions of the papillary dermis into the epidermis.
The dermal papillae, when prominent, form the papillary ridges (fingerprints).
The papillary dermis structure is composed of collagen fibers, blood vessels, and nerve cells.
Reticular dermis
The reticular dermis is the innermost layer of the dermis and is found below the papillary dermis.
It composes of dense irregular connective tissue, collagen fibers and dermal elastic fibers.
Most of the components of the dermis are located in this layer.

Skin structure terminology

adipocytes (lipocytes)- fat cells storing and releasing lipids
basement membrane- a thin layer of connective tissue fibers binding epidermis and dermis
ceramide- waxy lipid molecule composed of sphingosine and a fatty acid
collagen- structural protein synthesized by fibroblasts
corneocyte- dead, flattened, keratin-filled, transformed keratinocyte squamous cell in the stratum corneum.
corneodesmosome- modified desmosome holding corneocytes together
cytokeratin- proteins of keratin-containing intermediate filaments
dermis- primary inner layer of skin
desmosomes- complexes of cell adhesion proteins and linking proteins
epidermis- primary outer layer of skin
fibroblast- a cell synthesizing the extracellular matrix and collagen
integumentary system- organ system consisting skin and subcutaneous tissue
keratin- a family of fibrous protein structures of human skin
keratinization- terminal differentiation and cornification
keratinocyte- predominant cell type in the epidermis
keratohyalin- a protein structure forming dense cytoplasmic granules
lamellar bodies- secretory organelles secreted from keratinocytes
Langerhans cells- dendritic cells (DCs) or antigen-presenting cells
macrophage- an immune cell that engulfs foreign proteins and dead cells
mechanoreceptor- a sensory receptor that responds to mechanical pressure or distortion
melanin pigment- a group of naturally occurring dark pigments
melanocyte- melanin producing cell
melanosome- organelle found in melanocyte and is the site of synthesis, storage and transport of melanin
Merkel cells- a mechanoreceptor in the epidermis with sensitivity to light touch
papillary dermis- outermost layer of dermis
reticular dermis- innermost layer of dermis
stratum corneum- means 'horned layer' / outermost epidermal layer of skin
stratum lucidum- means 'clear layer' / located between the stratum corneum and stratum granulosum
stratum granulosum- means 'granular layer' / located between stratum lucidum and stratum spinosum
stratum spinosum- means 'spinous layer' / located between stratum granulosum and stratum basale
stratum basale- also known as basal layer or stratum germinativum / innermost layer of epidermis
subcutaneous tissue- a layer tissue below the dermis
tonofibrils- cytoplasmic protein structures converging at desmosomes
Epidermis is the uppermost layer in skin structure which we see and this layer makes our skin waterproof. The epidermis does not have blood vessels and is nourished by the dermis by diffusion. There is continuous cell division in stratum basale layer and the cells are pushed to the outer layer of skin. As the cells move up they are filled with keratin and later they die due to non-availability of nutrients. These dead cells get squashed and compacted and the uppermost layer of the epidermis may contain up to thirty dead cell layers.

The dermis layer of the skin is the area of maximum activity. Dermis contains nerve endings and all skin sensations are felt at this region. There is connective tissue in this layer and it cushions all the strain and stress. The collagen and connective tissue present in the dermis impart stretchability and elasticity to the skin. Hypodermis is the layer below the dermis and it connects the skin to the bones and the muscles.

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