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Development of Skin and its Appendages
The Integumentary System: Consists of skin and its derivatives;
- Sweat glands
- Sebaceous glands
- Mammary glands
Development of Skin: The two major tissue organizations of epithelial (ectoderm, epidermis) and mesenchyme (mesoderm connective tissue, dermis and hypodermis) are shown within skin. In addition, extensive populating by melanocytes (neural crest) and sensory nerve endings. The first epithelial specialization from which other epithelial specializations arose are located inside the body. Ectoderm forms the surface epidermis and the associated glands. Mesoderm, from the somites, forms the underlying connective tissue of dermis and hypodermis. Neural crest cells also migrate into the forming epidermis and the skin is also populated by specialized sensory endings. Fetal skin also has the ability to heal wounds without a scar in contrast to adult skin, this may relate to differences in the fetal extracellular matrix structure. The adult epidermis contains keratinocytes, melanocytes and Langerhans cells.
Development of Epidermis: Initially the embryo's surface is covered by a single layer of ectodermal cells which, in month 2, divides to form a superficial protective layer of simple, flattened squamous epithelial cells, the periderm or epitrichium. The cells of the periderm layer continually undergo keratinization and desquamation to be replaced by cells arising from the basal layer. The basal layer of epidermis later becomes the stratum germinativum which produces new cells that are displaced into layers above. In the 4th-5th week, the skin of the embryo consists of simple cuboidal epithelium. By the 7th week, the surface ectodermal cells proliferate and form a layer of squamous epithelium, the periderm and a basal germinative layer. Replacement of peridermal cells continue until about the 21st week, thereafter the periderm disappears and the Stratum Corneum forms.
Basal Germinative Layer
Basal Germinative Layer Introduction: By week 11, the basal layer (stratum germinativum) forms an intermediate skin layer, and by the end of month 4, all the epithelial layers of the adult epidermis of skin have acquired their definitive arrangement. Four successive layers are seen (bottom to top):
- Basal (stratum germinativum) layer: responsible for continuous development of new cells. It later forms genetically determined ridges and hollows which are filled by the underlying mesoderm. The patterns so formed are reflected on the surface of the skin (palms, fingers, and soles, including toes) in the form of fingerprints (dermatoglyphics)
- Thick spinous (stratum spinosum) layer: large polyhedral cells, on top of the basal layer, connected by fine tonofibrils
- Granular (stratum granulosum) layer: cells contain small keratohyaline granules, the first signs of keratinization
- Horny (stratum corneum) layer: outermost layer which forms the scale like hard surface of the epidermis and is loaded with keratin
Melanoblasts and Melanocytes: During the first 3 months of development, neural crest migrates and invades the epidermis, to form melanoblasts and then melanocytes, which synthesize melanin pigment. In dark-skinned races, melanin granules are produced by fetal melanocytes; in white-skinned races, the fetal melanocytes contain very little to no melanin pigment. The cell bodies of melanocytes are confined to the basal layers of the epidermis, and their processes extend between the epidermal cells. The melanocytes begin producing melanin before birth and distribute it to the epidermal cells. After birth, these cells cause skin pigmentation and are found in the epidermal-dermal junction.
Layers at Birth: At birth, the skin is covered by the vernix caseosa, a whitish paste formed by sebaceous gland secretion, degenerated epidermal cells, and hairs. It protects the skin against the maceration action of the amniotic fluid.
Pigmentation Disorders: A large number of pigmentary disorders occur, and these can be classified as diseases of melanocyte development, function, and survival. Examples of abnormalities of melanocyte function include piebaldism (patchy absence of hair pigment) and waardenburg syndrome (WS), which feature patches of white skin and hair. There are several types of WS, but they share some common characteristics, include patches of white hair (usually a forelock), heterochromia irides (eyes of different colors), white patches of skin, and deafness. The defects arise because of faulty migration or proliferation of neural crest cells (absence of melanocytes derived from these cells in the stria vascularis in the cochlea accounts for deafness in these diseases). Some types of WS result from mutations in PAX3, including WSI and WS3.
Abnormal Keratinization of Skin: Ichthyosis, excessive keratinization of the skin, is characteristics of a group of hereditary disorders that are usually inherited as an autosomal recessive trait but may also be X-linked. In several cases, ichthyosis may result in a grotesque appearance, as in the case of a Harlequin Fetus.
Dermal Papillae: As the epidermal ridges are formed, the dermis projects upward into the epidermis and forms the dermal papillae. Capillary loops and sensory nerve endings develop in these papillae.
Development of Hairs
Hair: Begin to develop during the 3rd month, but they do not become visible until the 20th week. The deepest part of the hair bud becomes cup-shaped, forming a hair bulb. The hair bulb gets invaginated by mesenchymal hair papilla. The central epithelial cells of the hair bulb give rise to the shaft of the hair, that grows through the epidermis and protrudes above the surface of the skin. The peripheral cells of the hair bulb form the epithelial root sheath. The cells of the epithelial root sheath proliferate to form a sebaceous gland bud. Hairs are first recognizable in the region of eyebrows, upper lip and chin. The first set of hairs that appear are fine and colorless and are called ‘lanugo’ hair. Lanugo hair are replaced during the perinatal period by coarser hair.
Abnormalities of Hair Distribution: Hypertrichosis: Excessive Hairiness is caused by an unusual abundance of hair follicles. It may be localized to certain areas of the body, especially the lower lumbar region covering a spina bifida occulta defect or may cover the entire body. Atrichia: The congenital absence of hair, is usually associated with abnormalities of other ectodermal derivates, such as teeth and nails.
Sweat Glands and Mammary Gland
Sweat Glands: Develop at about 20 weeks as solid growth of epidermal cells into the underlying dermis. Its terminal part coils and forms the body of the gland. The central cells degenerate to form the lumen of the gland. The peripheral cells differentiate into secretory cells and contractile myoepithelial cells.
Vernix Caseosa: Vernix caseosa, is the waxy or cheesy white substance found coating the skin of the newborn. The vernix is secreted by the sebaceous glands around the 20th week of gestation. It is composed of:
- Sebum (the secretion of the sebaceous glands)
- Desquamated epithelial cells
- Fetal hair (lanugo hair)
- It protects the baby's skin from dehydration and from constant exposure to the amniotic fluid
Nails: Begin to develop at about 10th week of gestation, as thickened areas of the epidermis at the tips of the digits. Later, these nail fields extend to the dorsal surface and become surrounded by the nail folds. Cells from the proximal nail fold grow over the nail field and form keratinized nail plate, the primordium of the nail.
The Mammary Glands: Begin to develop during the 6th week as thickened strips of the ectoderm (mammary ridges) that extend from the axillary to the inguinal regions. They regress in most locations except in the area of the pectoral muscle, where they proliferate. The down growth of epithelial tissue continues to proliferate into 16 to 24 solid out buddings which give rise to the lactiferous ducts. Fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme.
Anomalies: Polythelia: A condition in which assessor nipples have formed resulting from the persistence of fragments of mammary line. Accessory nipples may develop anywhere along the original mammary line but usually appear in the axillary region. Inverted nipples: A condition in which the lactiferous ducts open into the original epithelial pit that has failed to evert.