SKIN
Essentials of Histology
The skin covers
the exterior of the body and is one of the largest organs with several
functions. The skin is composed by an epithelial portion, the epidermis
and a connective layer, called dermis. Below the dermis, is located the
hypodermis, that is also a connective tissue, but it is not, in fact part
of the skin. The skin has several functions as protection against
mechanical injury, loss of fluids, temperature regulation and receives
also information from the external environment by its receptors.
The epidermis
is composed by a squamous stratified epithelium. The cells found in this
epithelium are: the keratinocytes, responsible for the formation of the
epithelium itself; the melanocytes, cells that produce a pigment
called melanin, which helps in the protection of the body against the UV
solar radiation and works in the pigmentation of the skin; Langerhans`
cells, that are part of the organic immune system (as antigen-presenting
cells) and Merkel` cells, that may work as sensory cells, but the
real function is still unknown. In the sake of simplicity, one can study
the epidermis as composed by 5 layer or stratums. 1) Stratum basale, composed
by cuboidal cells, which are responsible for the continue renovation of
the epithelium. This layer is also called the stratum germinativum.
2) The Stratum spinosum, located above the stratum basale, and composed
by cuboidal cells, rich in desmosomes and tonofibrils. As a function of
the retraction that happens with the tissue during the preparation process,
the cells remains attached only by spikes, that in microscopical level
correspond to desmosomes, this is why this layer is called stratum spinosum
(filled with spiny). 3) The Stratum granulosum is located above the
stratum spinosum, and is characterized by flattened cells, rich in granules
of keratohyalin. These granules are expelled to the intercellular space
and help in the sealing effect of skin. 4) The stratum lucidum is composed
by flattened cells, which cytoplasm is fulfilled with filaments and electrondense
material. This stratum is not always visible in thin skin. 5) The
most external layer is called stratum corneum and is composed by flattened
dead cells, which cytoplasm was completely filled with keratin. The outermost
cells of this stratum are constantly desquamated (shed).
The dermis is
the connective tissue located just below the epidermis. The dermis create
a physical and chemical sustenance to the epidermis and can be divided
in two portions: the papillary layer, more superficial and thinner, composed
by loose connective tissue and the reticular layer, thicker and deeply
located, composed by dense irregular connective tissue.
The hypodermis
is not a part of the skin itself, and is also composed by loose connective
tissue, that connects the skin to the adjoining organs. It is in the hypodermis
that the panniculus adiposus is located. It is a fat reserve and
helps in the protection of the body against mechanical shocks and heat
loss.
The skin
presents appendages. They are in humans: the hair, nails, sweat and sebaceous
glands. The hairs are keratinized threads present in almost all surfaces
of the skin, except by the palmar and plantar surfaces, margin of the lips
and some parts of the external genitalia. The hair originates from a structure
called hair follicle, which is an infiltration of the epidermis in the
dermis. In the base of each follicle, is localized the hair bulb that surrounds
the dermal papilla. In a transverse section of the hair, it is possible
to observe from inside to outside the following organization: the medulla,
composed by weak keratinized cells; the cortex composed by keratinized
and compacted cells; the cuticle consisted of highly keratinized cells
disposed in scalelike fashion; the epithelial sheath and the connective
outer sheath. Besides the hair, the nails are also important skin appendages.
They are hard structures that covers the dorsal surfaces of the tips of
the fingers and toes. The nails are modifications from the stratum corneum,
and are also composed by flattened dead and highly keratinized cells.
The skin also
presents two types of glands: the sebaceous and the sweat glands. The sebaceous
glands are of holocrine type, where the hole cell died and is released
with the secretion. These glands are normally located near the hair follicle
and secrete in the follicle their products. The secretion is rich in lipids
and this material is commonly extracted during the preparation process.
This is why this glands never stain well with the routine histological
dyes. The sweat glands are distributed throughout the skin. They are simple
tubular glands. In fact exists two types of sweat glands: the ordinary
or eccrine gland and the apocrine gland. The first is spread in the skin,
and its secretion helps in the temperature regulation. The second one is
located in the armpit and near the genital apparatus. This glands are larger
than the ordinary ones and their secretion is modified by bacteria causing
a characteristic smell.
