The integumentary system consists of arranged connective tissue, which is a the skin and its specialized derivatives derivative of mes'pderm. called appendages. The appendages At the end of the 4th week of in-of skin include hair, nails, sweat trauterine life the embryonic skin con-glands, and sebaceous glands. De- sists of a single layer of cuboidal sur-velopmentally the mammary glands are face ectodermal cells overlying the also considered to be a part of the in- mesenchyme. tegumentary system (although functionally these glands belong to the re- cniHarmie productive system). cpiaermis
During the second month of de-
riAwAls>»w«sm+ r>* CUi« velopment the cells of the surface ecto-
ueveiopmeni ot OKin derm proliferate to form a new outer
The skin consists of two main |ayer of squamous (flat) cells; this layer
components, each of which is derived js called periderm (or epitrichium). The
from a different germ layer. The super- underlying layer of proliferating cells is
ficial component, called epidermis, is now called the basal layer.
a stratified epithelium that is derived During the 3rd month of de-
from the surface ectoderm. The deeper. velopment, proliferation of the basal
component of the skin, called dermis layer cells produces a new intermedi-
or corium, consists of dense, irregularly ate layer just deep to the periderm.
This layer is the precursor of the outer layer of the mature epidermis. Its cells contain keratin proteins and, hence, are called keratinocytes. The basal layer is now called stratum germi-nativum. This layer continues to produce new cells to' replenish the epidermis throughout life.
During the fourth month the •intermediate layer gives rise to three layers of keratinocytes: the inner stratum spinosum, the middle stratum granulo-sum, and the outer stratum corneum. Thus, by the end of the 4th month of development all the four definitive layers of epidermis (ie, stratum basal, stratum spinosum, stratum granulosum and stratum corneum) can be recognized.
By the middle of the second trimester the periderm is shed. The cast-off peridermal cells mix up with the secretion of sebaceous glands to form a whitish cheesy material called vernix caseosa. As the development proceeds, more ectodermal cells are added to the vernix caseosa from the outermost part of the stratum corneum. The vernix caseosa forms a water proof covering over the delicate fetal skin and protects it against damage by the amniotic fluid.
In addition to the keratinocytes, the epidermis also contains a relatively smaller number of non-keratinocytes, which include melanocytes, Langer-hans cells and Merkel cells. Each of these cells has a different source of origin.
The melanocytes are derived from the neural crest cells, which migrate into the developing epidermis during the 7th and 8th weeks of development. The migrating neural crest cells differentiate into melanoblasts, and settle down in the stratum basale of the epidermis. Later, the melanoblasts differentiate into melanocytes, which are dendritic cells that contain special membrane-bound structures called melanosomes. Melanin pigment is synthesized within the melanosomes and transferred to the adjacent keratinocytes through the dendritic processes of the melanocytes.
The Langerhans cells are the macrophage immune cells of the skin. They originate in the bone marrow and migrate into the epidermis during the 12th week of development. In the developing epidermis the Langerhans cells settle down mostly in stratum spinosum.
The Merkel cells are tactile mechanoreceptors, which differentiate from the epidermal ectoderm during the 12th and 13th weeks of development.They are located in the stratum basale of the epidermis