Integumentary System Development

Integumentary System Development

• The Integumentary system (skin) is composed of two layers:
  • Epidermis: derived from surface ectoderm
  • Dermis: formed by mesenchyme derived from dermatomes of somites
• Epidermis:
  • Initially, a single-layered structure
  • During the second month, forms two layers:
    • Periderm/epitrichium: superficial layer of flat cells
    • Basal/germinative layer: deep layer of cuboidal cells
    • Later, a third intermediate layer forms from the basal layer proliferation.
    • The basal layer is known as stratum germinativum due to its continuous proliferation
  • During the third to fifth month, the epidermis transforms into stratified squamous epithelium with five layers:
    • Stratum basale
    • Stratum spinosum
    • Stratum granulosum
    • Stratum lucidum
    • Stratum corneum
  • Keratinisation and desquamation are continuous processes, replacing peridermal cells
  • Periderm layer gradually sloughs off into the amniotic fluid.
  • Melanoblasts, McSCs and melanocytes express specific genes during development
• Other epidermal cells:
  • Merkel cells: pressure-detecting mechanoreceptors found in palms and soles, appearing in the fourth to sixth months.
  • Langerhans cells: tissue macrophages migrating into the skin from the seventh week, formed by the primitive yolk sac and fetal liver hematopoiesis.
• Clinical correlations:
  • Albinism: Reduced/absent pigmentation in skin, hair, and eyes caused by abnormal melanin synthesis.
    • May be autosomal recessive or autosomal dominant.
  • Vitiligo: Patchy loss of epidermal melanocytes resulting in depigmentation.
    • Etiology is multifactorial, possibly involving:
      • Genetic predisposition
      • Autoimmune destruction of melanocytes
      • Oxidative stress
      • Intrinsic melanocyte defects
    • May be triggered by stress or skin injury.
• Dermis (Corium):
  • By the 11th week, mesenchymal cells start producing collagen and elastic fibers.
  • The dermis-epidermis junction initially is straight but develops regular, spaced thickenings (projections) that penetrate the dermis during the third and fourth months.
  • These projections form the dermal papillae.
  • Further epidermal thickening in these areas results in surface elevations (epidermal ridges).
  • The dermis differentiates into two layers:
    • Papillary layer (superficial)
      • contains loops of capillary plexus and sensory receptors
    • Reticular layer (deep)
      • Contains collagen and adipose tissue.
• Blood vessels in the dermis:
  • Develop initially as endothelium-lined structures from mesenchyme.
  • Primordial vessels develop into capillaries (angiogenesis)
  • Angiogenesis in the dermis is complete by the end of the first trimester.
• Skin Appendage Development:
  • Nails
    • Develop at the tip of each digit from the ectoderm.
    • This ectoderm then migrates to the dorsal aspect
    • Nail field appears at the end of the 10th week as a thickened area.
    • U-shaped epidermal nail folds appear at the base of the nail field.
    • Fingernails reach the fingertips by 32 weeks; toenails reach the toe tips by 36 weeks.
    • After birth, nails grow about 0.5 mm per week.
  • Hairs
    • Develop early (7-12 weeks) from epidermal downgrowths into the dermis.
    • Cells of the stratum germinativum proliferate to form a solid epithelial cord, eventually forming the hair bud.
    • The terminal part of the hair bud becomes club-shaped and forms the hair bulb.
    • The hair bulb is invaginated by mesenchymal condensation of the dermis, forming an inverted cup-shaped structure.
    • The dermis within the cup-shaped hair bulb is called the dermal papilla.
    • Cells at the summit of the dermal papilla form the germinal matrix, producing hair.
    • Peripheral cells of the developing hair follicle form the epithelial root sheath.
    • The dermis condenses around the sheath to form the dermal root sheath.
    • The germinal matrix proliferates to form the hair root, which is pushed outside the follicle as a hair shaft.
    • Melanoblasts migrate into the hair bulbs and differentiate into melanocytes.
    • Melanin from the melanocytes is transferred to proliferating cells of the germinal matrix, determining hair color.
  • Glands
    • Sebaceous glands
      • Develop as buds from the epithelial root sheath of hair follicles.
      • Grow into the dermis and divide into branches to form alveoli and ducts.
      • Cells at the center of alveoli degenerate, producing sebum.
      • Sebum is released into the hair follicle and then to the skin surface.
    • Sweat glands
      • Develop as epidermal downgrowths, later becoming canalized.
    • Mammary glands
      • Arise from the surface ectoderm along a milk line extending from the axilla to the inguinal region.

Integumentary System Development

• The skin is the largest organ in the body and is comprised of two main layers: the epidermis and the dermis.
• The epidermis is derived from the surface ectoderm.
• The dermis is formed by mesenchyme derived from dermatomes of somites.
• The skin also includes appendages: nails, hair, and glands.

Epidermis Development

• The epidermis initially forms as a single-layered structure.
• During the second month of development, the epidermis differentiates into:
  • Periderm: a superficial layer of flat cells.
  • Basal layer: a deep layer of cuboidal cells.
• The basal layer proliferates to form a third intermediate layer and is responsible for the continuous formation of the epidermis.
• From the 3rd to 5th month of development, the epidermis undergoes further stratification, resulting in five distinct layers:
  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum
  • Stratum corneum
• The periderm is gradually sloughed off into the amniotic fluid by the 21st week.

Vernix Caseosa

• The periderm and secretions from sebaceous glands mix to form a whitish, sticky substance called vernix caseosa.
• Vernix caseosa protects the fetus's skin from maceration by amniotic fluid.

Epidermal Ridges

• By the 11th week of development, cells of the stratum basale proliferate and extend into the dermis forming epidermal ridges.
• These epidermal ridges, which become permanent by the 18th week, create a pattern of fingerprints on the palms, soles, and fingers.
• The pattern is genetically determined.
• There are four basic types of dermatoglyphic patterns: arches, whorls, loops, and composites.

Melanoblasts

• Melanoblasts, derived from neural crest cells, migrate from the neural tube through the dermis to the epidermis and hair follicles.
• These melanoblasts differentiate into melanocyte precursors in the basal layer of the epidermis.
• Within the hair follicles, melanoblasts differentiate into melanocyte stem cells (McSCs).
• Melanocyte precursors and McSCs ultimately mature into melanocytes in the epidermis and hair matrix.

Somites

• The paraxial mesoderm differentiates into 40-45 pairs of somites.
• Somites are formed between the 20th and 30th day of intrauterine life.
• Each somite is a triangular structure with a cavity.

Somite Subdivision

• Somites are divided into three components:
  • Sclerotome: Forms the vertebral column and ribs.
  • Dermatome: Forms the dermis of the back of the head and trunk.
  • Myotome: Forms the striated muscles.

Dermis

• The dermis is formed by the condensation and differentiation of mesenchyme underlying the surface ectoderm.
• The mesenchyme that forms the dermis originates from three sources:
  • Dermatomes: Forms the dermis of the dorsal aspect of the head and trunk.
  • Somatopleuric Layer of Lateral Plate Mesoderm: Forms the dermis of the limbs, and the lateral and ventral aspects of the trunk.
  • Neural Crest Cells: Forms the dermis over most of the head and anterior aspect of the neck.

Dermis Development

• By the 11th week, mesenchymal cells in the dermis begin producing collagen and elastic fibers.
• The junction between the dermis and epidermis becomes wavy, forming dermal papillae and epidermal ridges.
• The dermis differentiates into two layers:
  • Papillary Layer: Superficial layer containing capillary loops and sensory receptors.
  • Reticular Layer: Deeper layer containing a high amount of collagen and adipose tissue.

Nail Development

• The nail is formed from keratinized cells and is comprised of several components:
  • Nail Plate: The body of the nail.
  • Nail Bed: Highly vascular connective tissue.
  • Hyponychium: Part of the epidermis under the free edge of the nail.
  • Germinal Matrix: Growing area of the nail.
  • Lunula: Proximal, half-moon shaped part of the nail plate overlying the nail bed.
  • Cuticle: Overlapping epidermis around the base of the nail.
  • Nail Walls: Folds of skin overlapping the sides of the nail.
  • Perionychium: Includes the nail wall and cuticle area.
  • Eponychium: Extension of the base of the nail plate.

Hair Development

• Hair follicles are formed from invaginations of the epidermis into the dermis.
• Cells of the hair bulb form a germinal matrix that produces hair.
• The growing hair root is pushed through the follicle to the surface of the skin.
• Melanoblasts migrate into the hair bulbs and differentiate into melanocytes, giving the hair its color.

Hair Types

• Lanugo: Fine, soft, lightly pigmented hairs that appear first.
• Terminal Hair: Coarser hairs that replace lanugo during the perinatal period and at puberty.

Arrector Pili muscle

• Smooth muscle fibers differentiate from the mesenchyme surrounding the hair follicle.
• These muscles connect the dermal root sheath to the papillary layer of the dermis.
• Contraction of the arrector pili muscles causes goose bumps.

Glands of the Skin

• The skin contains three types of glands:

  • Sebaceous Glands: Produce sebum, an oily secretion.
  • Sweat Glands: Produce sweat.
  • Mammary Glands: Produce milk.

• These glands are derived from the epidermis and grow into the dermis.

Sebaceous Gland Development

• Sebaceous glands develop as a bud from the epithelial root sheath of a hair follicle.
• This bud divides into branches, forming alveoli and ducts.
• Cells in the center of the alveoli degenerate, producing sebum.
• Sebum is released into the hair follicle and then to the surface of the skin.