Anatomy and Development of the Integumentary System: Epidermis and Dermis, Study notes of Embryology

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The Integumentary System :

Embryology & Genetic

Bases

Purnomo Soeharso Department of Medical Biology FMUI

Tissue organization of the skin (integumentum) :

• Epidermis — stratified epithelium on the outer surface
• Dermis — fibrous tissue beneath the epidermis
• Subcutaneous layer — loose connective tissue beneath the two layers, compose mostly of fatty tissues.

Germ layer of origin : Epidermis — specializes from surface ectoderm not involved in developing nervous system. Dermis — differenciate from mesenchymal cells of mesodermal somites (dermatomes).

Cross section of 24 hr chick embryo, in primitive streak area

After the 4th months the epidermis becomes stratified epithelia → consist multiple layers of cells :

• Stratum germinativum — basal cells and their immediate descendants next above.
• Stratum granulosum — layer of cells next above stratum germinativum → consist cells containing keratohyalin granules.
• Stratum lucidum — next above str. granulosum → thin & clear cells containing degenerative keratohyalin.
• Stratum corneum — flat cells on the surface → cell cytoplasm undergo cornification to become cornified dead cells with degenerate nuclei. The cornification is not intensive in some areas : red layer of lips & anus.

Immigrant cells invading epidermis Epidermis consists of cells derived from ectoderm (proliferation of str. germinativum) and foreign cells from outside that migrate to epidermis during embryonic development and remain there in adult. Melanoblast → derived from neural crest − migrate to dermis & penetrate epidermis in the 3 rd month. Melanoblast differenciate to form pigment granules (melanosomes) → to become melanocytes. The pigmentation intensity varies among races although the number of melanocytes is not significantly different from race to race. The pigmentation is dependant on the activity of tyrosinase → convert tyrosine to melanin.

Langerhans cells − morphologically indistinguishable from keratinocytes (epidermal cells); however they are recognized by histochemical methods :

• they have membrane bound ATPase
• they have specific surface antigen different from epidermal cells. Derived from precursor cells in bone marrow. Migrate to epidermis and function as antigen presenting cells that process antigen entering epidermis. The antigen is fragmented and presented it to T-lymphocytes → mediate cellular immune response to antigen.

Cross section of 48 hr chick embryo in somite area

Cross section of 48 hr chick embryo, in somite area

In some places the epidermis undergo modifications and develop into skin glands and hairs. The hair develop from epidermis that grow downwards into the dermis to become hair follicle. The messenchyme at the basal of follicle form hair papilla → sorrounded by hair follicle root sheath. The cells in the germinal matrix proliferate & develop toward the surface & keratinized → hair shaft. .

The sweat glands → located throughout the body originate as downgrowths from the epidermis into the underlying mesenchyme (dermis). As it elongates, its end coils to form the primordium of secretory part of the gland. Eccrine sweat glands begin to function shortly after birth. Large apocrine sweat glands are mostly confined to the axilla, pubic, perineal regions and areolae of the nipples. These glands open into the upper part of hair follicles superficial to the openings of the sebaceous glands. They secrete only after puberty.

The mammary glands → specialized sweat glands that develop as solid epidermal downgrowths into the underlying mesenchyme → develop from thick strips of ectoderm extending from axillary to inguinal regions − mammary ridges. Each primary bud give rise to several secondary mammary buds that develop into lactiferous ducts. Canalization of these buds is induced by placental sex hormones entering the fetal circulation. The gland is supported by fibrous connective tissues and fats that develop from the surrounding mesenchyme