Chapter 4: The Integumentary System
Chapter 4: Development of the Integumentary Systemby John F. Neas
Development of the Layers of the Integument
All structures of the integumentary system develop from the ectodermal and mesodermal germ layers. The integument as a body covering becomes established during the early embryonic period. Other integumentary structures, such as hair, nails, and glands, develop throughout the prenatal period.
The skin consists of three structural and functional layers. The epidermis, the thinnest and outermost layer, develops from the surface ectoderm. The epithelial component of the epidermis includes the hair (shaft and follicles), nail matrix, and sebaceous, sweat, and mammary glands. The glands have secondary supporting investments from mesoderm. The epidermal layers, hair, nails, and cutaneous glands develop from ectoderm that specializes into a mitotically active germinal layer.
The dermis, which underlies the epidermis, and the hypodermis (subcutaneous layer), which attaches the dermis of the skin to underlying tissues or organs, develop from wandering mesenchymal cells (somatopleuric mesoderm of the body wall and limbs and, possibly, from the dermatome). The mesenchyme becomes arranged in a zone beneath the ectoderm.
By the fourth week following conception, a single-cell layer of ectoderm surrounds the embryo. This simple epithelium overlies a thicker, loosely organized layer of undifferentiated mesoderm called mesenchyme.
At about six weeks, the ectodermal layer differentiates into an outer flattened periderm and an inner, cuboidal germinal (basal) layer in contact with the mesenchyme. The periderm eventually sloughs off as the vernix caseosa, a cheese-like protective coat that covers the skin of the fetus. The germinal layer produces the entire epidermis, including all the glands, nails, hair, and hair follicles of the integument.
By eleven weeks, fibroblasts and other connective tissue cells develop from mesenchymal cells below the germinal cells or migrate into the area. These cells form distinct collagenous and elastic connective tissue fibers of loose connective tissue. The tensile properties of these fibers in the dermis cause a buckling of the epidermis and the formation of dermal papillae. The connective tissue contains blood vessels that bring nutrients to the region. A dense, irregular collagen fiber network dominates a deeper, less vascular region. The embryonic connective tissue below the dermis develops into the subcutaneous layer of loose connective tissue.
During early fetal development (about ten weeks), specialized neural crest cells called melanoblasts migrate into the developing dermis and differentiate into pigment cells, or melanocytes. The melanocytes soon migrate to the germinal layer of the epidermis where they produce the pigment melanin that colors the epidermis.
Over the ensuing weeks, the epithelium becomes stratified and the basement membrane is thrown into irregular folds through repeated divisions of the basal (germinative) cells. By the fourth month, all layers of the epidermis are developed and each layer assumes a structure characteristic of the adult.
Development of the Associated Structures of the Integument
Hair and Sebaceous Glands
Hair and sebaceous glands develop together during formation of a hair follicle; sweat glands and nails develop independently. All of these structures develop from the germinal layer of the epidermis and are therefore ectodermal in origin.
During the third and fourth months, small areas of epidermis divide extensively to form cords of cells, or epithelial columns, that grow into the dermis. Mesenchymal cells surround the columns as they extend ever deeper into the dermis. Hair follicles and exocrine (sebaceous and sweat) glands develop from these columns.
A hair follicle must be present before a hair can develop. Each hair follicle begins to develop between the third and fourth months as a mass of germinal cells from the stratum basale of the epidermis, called a hair bud, proliferates and grows downward into the underlying mesenchyme (dermis). The hair bud becomes a club-shaped hair bulb. The hair follicle, which physically supports and provides nourishment to the hair, develops from the hair papilla, a small mass of specialized mesenchymal connective tissue around the hair bulb, and from the germinal matrix, the epithelial cells of the hair bulb. A specialized mesenchymal dermal root sheath supports the wall of the hair follicle, or epithelial root sheath. Growth of the hair results from continuous mitotic activity in the epithelial cells of the hair bulb covering the papilla.
By the fifth or sixth month, the hair follicles produce delicate fetal hair called lanugo, first on the head and then on other parts of the body. The lanugo is usually shed before birth.
The two principal types of integumentary glands are the sebaceous (oil) and sudoriferous (sweat) glands. Both develop from the germinal layer of the epidermis. The epithelial (secretory) portions of sebaceous glands develop as outgrowths (evaginations) from the sides of the developing hair follicles and remain connected to the follicles. The oily secretions of a mature sebaceous gland empty onto and lubricate the shaft of a hair in a hair follicle.
Sweat and Mammary Glands
The epithelial portions of the sudoriferous (sweat) glands develop from downgrowths (invaginations) of the stratum basale of the epidermis into the dermis. A sweat gland develops as an epithelial column (the secretory portion of the developing gland) that proliferates into the dermal mesenchyme, coils, and becomes hollow. The connective tissue and blood vessels associated with the sweat glands develop from mesoderm.
The sweat glands appear during the fourth month on the palms and soles and a little later in other regions. At birth, the ducts of the sweat gland carry secretions of the gland cells to the skin surface.
Mammary glands are modified sweat glands that similarly develop as downward proliferations of the germinal epithelium. Unlike sweat glands, however, mammary glands only develop along a mammary ridge. Also, rather than coiling, the mammary bud branches extensively as it extends into the dermis to form the secretory and ductal portions of the mammary glands. Generally, only a single pair of pectoral (chest) mammary glands develops in human beings.
The mammary glands of newborns, male and female, are frequently swollen and, stimulated by maternal hormones that passed into the fetal circulation through the placenta; they may discharge small amounts of a secretion called "witchs milk." Further elaboration of the duct and gland system of the female mammary glands occurs at puberty, but functional maturity does not occur until late in pregnancy.
Nails begin development at about ten weeks along the dorsal aspect of each digit. Fingernails begin to develop about two weeks earlier than the toenails. The first indication of nail development is the appearance of a thickened area of epithelium called the primary nail field near the tips of the digits. These thickenings settle into the dermis, and the proximal and lateral borders of the nail field become thickened as nail folds. Continued mitotic activity at the proximal nail fold produces a toughened nail plate (nail) of keratinized epithelium that grows forward over the nail bed. Nail production at first involves all of the germinative cells of the nail field but, by birth, nail production is restricted to the nail root. The nails do not reach the tips of the digits until the ninth month.
Teeth develop from ectoderm of the oral cavity and surrounding mesoderm. Ectodermal cells called ameloblasts produce enamel. All other dental tissues develop from mesoderm (mesenchyme). Odontoblasts produce a thick layer of dentine deep to the enamel. Cementoblasts in the root of the tooth produce cement.
This photomicrograph of the scalp in a human fetus shows a developing hair (H). Compare with the figure presented above, and note the sebaceous gland (S) and arrector pili muscle (A). The large structure with the pale center below the hair and to the right of the sebaceous gland is very near the hair bulb.
This photomicrograph through the fingertip of a human fetus shows the layers of the epidermis. The stratum basale (B), or stratum germinativum, is a single layer of cuboidal cells separated from the dermis (D) by a basement membrane too thin to resolve by light microscopy. The stratum spinosum (S), or prickle cell layer, has cells that are in the process of growth and early keratin synthesis. Cells that have granules that contribute to the process of keratinization characterize the stratum granulosum (G), or granular layer. The stratum corneum (C), or cornified layer, has flattened, fused cell remnants composed mainly of the fibrous protein keratin.
This photomicrograph shows the development of human deciduous (left) and permanent teeth (right). Compare this figure with those presented earlier.
Development of the Layers of the Integument
Hair and Sebaceous Glands