Lecture 13: Development of Integument

Question 1

Define and distinguish b/w cytodifferentiation and histogenesis

Answer 1

cytodifferentiation: the process when cells that make up a tissue undergo increasing specialization histogenesis: the next level of complexity, when various cell types develop in concert to form specific tissues

Question 2

Describe the formation of a multiple layered epidermis, including the derivatives from the intermediate layer

Answer 2

Basal Layer Intermediate Layer (stratum spinosum and stratum granulosum) Superficial Peridermal Layer

Question 3

What role does apoptosis play in epidermal development?

Answer 3

By the sixth month, epidermis beneath periderm undergo differentiation via apoptosis and turn into layers more characteristic of postnatal epidermis

Question 4

Relate pattern of epidermal ridges to morphology of volar pads and timing of ridge formation

Answer 4

The formation of epidermal ridges is closely associated with the earlier appearance of volar pads on the ventral surfaces of the fingers and toes. The pattern of the epidermal ridges is correlated with the morphology of the volar pads when the ridges first form. If a volar pad is high and round, the epidermal ridges form a whorl; if the pad is low, an arch results. By the 5th month of preganacy, epidermal ridges become recognizable features

Question 5

What is the molecular basis for albinism?

Answer 5

• lack of skin pigmentation despite containing normal numbers of melanocytes
• melanocytes are unable to express pigmentation because they lack the enzyme tyrosinase, which is involved in the conversion of tyrosine to melanin

Question 6

List the major groups of immigrant cells

Answer 6

Melanoblasts… from neural crest

• Migrate to dermis and then into epidermis
• Stain with HMB-45
• Produce pigment by mid-pregnancy

Langerhans’ cells… from bone marrow

• Antigen-presenting cells

Merkel cells… from neural crest

• Slow-adapting mechanoreceptors

Question 7

Describe the source of dermal cells in various parts of the skin

Answer 7

Dorsal surface of body: mesenchyme from dermatome

Ventral and lateral surfaces of body: mesenchyme from lateral plate mesoderm

Cranial and anterior neck: mesenchyme from cranial neural crest

Question 8

Describe the signaling pathways leading to the differentiation of dermal cells

Answer 8

Ectodermal Wnt signaling acting through b-catenin pathway, specifies the mesenchymal cells, which are closest to the ectoderm, to become dermal cells

Question 9

Compare characteristic of dermal cells and mesenchymal cells

Answer 9

Mesenchymal cells: able to migrate as individual cells if they are provided with the proper extracellular environment

*mesenchymal cells of the ventral somatopleure become dermal cells

Dermal cells: express Dermo 1

Question 10

Describe the pathways leading to the formation of dermis on various surfaces of the body

Answer 10

Trunk dermis: from somitic dermatomes

• Ectoderm –> Wnt signaling –> somatic dermatomes secrete Dermo 1
• Aggregated mesenchymal cells develop tight junctions

Limb dermis: from lateral plate mesoderm

Face and anterior neck: neural crest

Question 11

Describe how recombination experiments between mesoderm (source of dermis) and ectoderm (source of epidermis) demonstrate instructive induction

Answer 11

• the underlying dermis determines the course of development for the epidermis
• the ectoderm determines course of development of the dermis
• ectoderm and mesenchyme can respond to eachothers signals

If the early ectodermal and mesenchymal components of the skin are enzymatically dissociated and grown separately, the ectodermal component remains simple ectoderm without differentiating into a multilayered epidermis with appropriate epidermal appendages.

If ectoderm from one part of the body is combined with dermis from another area, the ectoderm differentiates into a regional pattern characteristic of underlying dermis, rather than a pattern appropriate for the site of origin of the ectoderm.

Question 12

List commonalities in the development of epidermal derivatives

Answer 12

• Hair, Nails, Mammary Glands
• Development of these derivatives involves ectodermal-mesodermal interactions and inductions
• Hairs, nails, and mammary glands begin as epidermal downgrowth’s (ingrowths) into mesenchyme
• Ectoderm contributes to the hair follicle, hair shaft, sebaceous glands, nails, eponychium, hyponychium, and secretory and duct components of the mammary glands

Question 13

Describe the developmental pathway for hair development

Answer 13

1. Dermal induction
2. Various inducers from dermal papilla along with Ahh and cyclin D1 in epidermal placode stimulate further downgrowth of proliferation of epidermal placode
3. Final differentiation of hair primordia involves Hox genes

Question 14

Describe the adult hair cycle including the specific stages: anagen, catagen, telogen, exogen

Answer 14

Anagen: first phase, the hair is actively growing, lasts up to 5 to 6 years

Catagen: the hair follicle regresses to only a fraction of its original length, 1-2 weeks

Telogen: hair stops growing, 5-6 weeks

Exogen: when the hair is shed