Histological structure of skin
EPIDERMIS
Epidermis is the outer
layer of skin. It is formed by stratified epithelium.
Important feature of epidermis is that, it does not have blood vessels.
Nutrition is provided to the epidermis by the capillaries of dermis.
Layers of Epidermis
Epidermis
is formed by five layers:
1. Stratum corneum 2.
Stratum lucidum 3. Stratum granulosum 4. Stratum spinosum 5. Stratum
germinativum.
1. STRATUM CORNEUM: Stratum corneum is
also known as horny
layer. It
is the outermost layer and consists of dead cells, which are called
corneocytes.
These
cells lose their nucleus due to pressure and become dead
cells. The cytoplasm is flattened with fibrous protein known as
keratin.
Apart
from this, these cells also contain phospholipids and glycogen.
2. STRATUM LUCIDUM: Stratum
lucidum is made up of flattened epithelial cells. Many cells have
degenerated nucleus and in some cells, the nucleus is absent. As these
cells exhibit shiny character, the layer looks like a homogeneous translucent zone. So, this layer is
called stratum lucidum.
3. STRATUM
GRANULOSUM: Stratum granulosum is a thin layer with two to
five rows
of flattened rhomboid
cells. Cytoplasm
contains granules of a protein called keratohyalin. Keratohyalin is
the precursor of keratin.
5. STRATUM
GERMINATIVUM: Stratum
germinativum is a thick layer made up of polygonal cells,
superficially and columnar or cuboidal epithelial cells in the deeper parts. Here,
new cells are constantly formed by mitotic division. The newly formed
cells move continuously towards the stratum corneum. The stem cells,
which give rise to new cells, are known as keratinocytes. Another type of cells
called melanocytes
are
scattered between the keratinocytes. Melanocytes produce the pigment
called melanin.
The
color of the skin depends upon melanin. From this layer, some
projections called rete
ridges
extend
down up to dermis. These projections provide anchoring and nutritional
function.
DERMIS
Dermis is the inner layer of the skin.
It is a connective tissue
layer, made up of dense and stout collagen fibers, fibroblasts and histiocytes.
Collagen fibers exhibit elastic
property and are capable of storing or holding water. Collagen
fibers contain the enzyme collagenase, which is responsible for wound
healing.
Layers of Dermis
Dermis is made up of two layers:
1. Superficial papillary layer 2. Deeper reticular layer.
1. SUPERFICIAL PAPILLARY LAYER: Superficial
papillary layer projects into the epidermis. It contains blood vessels,
lymphatics and nerve fibers. This layer also has some pigmentcontaining
cells known as chromatophores. Dermal papillae are
fingerlike projections, arising from the superficial papillary dermis. Each
papilla contains a plexus of capillaries and lymphatics, which are oriented
perpendicular to the skin surface. The papillae are surrounded by rete ridges,
extending from the epidermis.
2.RETICULAR LAYER: Reticular layer
is made up of reticular and elastic fibers. These fibers are found
around the hair bulbs, sweat glands and sebaceous glands. The reticular
layer also contains mast cells, nerve endings, lymphatics, epidermal
appendages and fibroblasts. Immediately below the dermis, subcutaneous
tissue is present. It is a loose connective tissue, which connects
the skin with the internal structures of the body. It
serves as an insulator to protect the body from excessive heat and cold of the
environment. Lot of smooth muscles called arrector pili are also found in
skin around the hair follicles
