Lecture10- The Skin and Its Derivatives

8/13/2019 Lecture10- The Skin and Its Derivatives

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InstructorDepartment of Biology

School of Science & EngineeringAteneo de Manila University

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Skin

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Figure 10 A-B. Labiodermal junction ofcanine lip. A-50X, B-200X. Theepidermis in Figure A (inset) ismagnified in Figure B. The followinglayers of the epidermis can beidentified: stratum basale (1), stratumspinosum (2), stratum granulosum (3),and stratum corneum(4). Note thepresence of pigment granules in thelower layers.

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Up to the stage of the conical feather bud, the development of the

contour feathers is morphologically similar to that of down feathers. Atthe base of the cone-shaped contour feather bud an epidermal collar produces a series of parallel barb ridges but soon the dorsal part of theepidermal collar begins to elongate to form the long shaft of the feather.

Elongation of the feather shaft occurs entirely within the cone of thefeather bud and the barb ridges bend beneath the cornified outer epidermal sheath of the feather bud begins to split, the apical part of theshaft and its associated barbs are freed from their conical restraint.

the freed barbs unroll and flatten out in a typical feather form, while atthe base the barbs are still encased in the intact epidermal hull.

erector muscle: smooth muscle that causes the contour feathers tofluff (warmth) ; depressor muscle: smooth muscle that allows thefeathers to be flattened (flying)


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Scales

The initial placodal stages of scale development differ from those of thefeather in that there is not a pronounced condensation of dermal cellsbeneath the placode; however, as the placode elevate into ridges, dermalcells aggregate and proliferate at the apical end of the scale that is

beginning to take shape.

The apical end of the scale continues to grow until it overlaps the basalportion of the next scale in line. By this point, both the epidermal and

undergone complex series of differentiative changes and have begun toform keratin specific for each surface of the scale.


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Tissue Interactions in Integumentary Development

Almost all aspects of integumentary development depend on continuingseries of reciprocal communications between the ectoderm and itsunderlying mesenchyme.

One of the simplest ways to test the importance of tissue interactions inskin development is to separate the ectodermal and mesenchymalcomponents and raise them separately. Both failed to develop intoepidermis and dermis, respectively. This emphasizes the importance of

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skin.


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Heterotropic recombination andinstructive induction


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Heterospecific recombinations

Dermal-epidermal interactionsappear to be simple one-way

avenues of communication,with the dermis sending out alocation-specific message andepidermis replying with

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(Sengels proposition) Initialmessage emanates from thepredermal mesenchyme andinstruct the overlying ectodermto form thickened placodes ina region-specific pattern.


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the ectodermal placode sendsback a message causing the

predermal mesenchyme cells tocondense beneath theplacodes.

The condensed dermis thensends a second class-specificinductive message that istransmitted back to thethickened ectodermal placode.

s secon erma n uc vemessage specifies theformation of a specific kind of appendage (contour feather,scales or hair)

second dermal inductivesignal transmits informationthat can be translated into bothspecific form of the appendage

and specific keratin proteins byepidermis


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Pattern Formation in the Integument

Feathers:

in avian embryos, feathers form as specific patches of skin called feather tracts (pterylae) while featherless areas are called apteria

Feather papilla which populate a feather tract arise in a well-defined and- . ,

nearly simultaneous appearance of feather primordia in the single row atmidline marks the start of the feather tract called primary row.

Secondary row of feather papilla arises on either side of primary row. In aregular geometric pattern, the feather germs of secondary row arise instaggered fashion laterally to those of primary row. Feather germs of thetertiary row next form laterally to those of secondary row. The sequence of additional rows continues until the feather tract is filled out.


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Epidermal Differentiation

Regardless of regional differences, the epidermis all over the body has thesame fundamental organization and function. It is a multilayered structurewhich is almost totally geared toward providing the body with animpermeable protective covering.

Cells of the basal layer contain the constellation of cytoplasmic organellesassociated with protein synthetic activity. They also contain scatteredbundles of 6- to 8- nm-thick intermediate filaments which represent

epidermal cell.

As the epidermal cells (keratinocytes) are pushed outward with the nextlayer (stratum spinosum), they develop extensive networks of keratinfilaments which converge on the desmosomes, the small patchlike

structures that bind one epidermal cell to its neighbor. The outer cells of thespinous layer begin to accumulate another maker of epidermaldifferentiation, keratohyalin granules.


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Keratohyalin granules are prominent components of the stratumgranulosum of the epidermis.

By the time keratinocytes have moved out to the granular layer, thenuclei develop the classical signs of terminal differentiation: theybecome compressed, the nuclear chromatin becomes dense, and thenuclear membrane shows signs of breaking up.

keratin subunits that are made in more mature keratinocytes havehi her molecular wei hts than do those s nthesized in the basal cells.

Keratinocytes next pass through a thin transitional layer, where thenuclei are lost and the cells become noticeably flattened. The cellsbecome little more than flattened bags of keratin filaments whichconstitute the outer stratum corneum which is held by histidine-rich

protein called filaggrin derived from the component of keratohyalingranules secreted at the intercellular spaces.


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Pigmentation Pattern in the Skin

The myriad colors and patterns one finds in animals of a typical rain forestor coral reef are attributable to a class of cells called chromatophoreswhich originate in the neural rest and from there migrate in well-defined,species-specific patterns to their final peripheral destinations.

Three major types:

1. Melano hores: ive ellowish-brown brown or black coloration

because of their melanin or a derivative of melanin2. Xanthophores: containing carotenoid or pteridine pigments whichare responsible for the yellow to red color

3. Iridophores: produce metallic effects (silvery or bluish) becausestructural elements within the cells reflect light


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Mammary Glands First indication of mammary gland development is the appearanceof bandlike ectodermal thickening (milk lines) that run from theregion of the armpit (axilla) to the groin (inguinal region).

Actual number and location of individual mammary glands varyconsiderably from one species to another, but they are all locatedsomewhere along the length of the milk lines.

In humans, the original milk line is a raised ridge of ectodermalcells underlain by a slight concentration of mesodermal cells. Asmammary gland development continues, a solid plug of ectodermalcells at the site of the future nipple pushes its way into thesubjacent mesenchyme. Soon a cluster of branching ectodermal

cords foreshadows the formation of the hollow mammary ducts.


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Initial ingrowth of mammary duct primordia into the underlyingmesenchyme is the result of an ectodermal-mesenchyme inductiveinteraction. The mammary mesoderm is the inductor and theectoderm is the responding tissue.

Two types of mammary mesoderm: fibroblastic mammarymesenchyme that closely surrounds the epithelial duct primordiaand a precursor of the mammary fat pad.

Interaction with tissue of the mammary fat pad rather than thefibroblastic mesoderm is the major factor that result in the shapingof the characteristic mammary duct system.

Although rudimentary ductal tissue persists in human male, the

mammary primordia of the male mice disappear early indevelopment as a result of testosterone-induced cell death.


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The male and female mammary epithelial primordia are equally

sensitive to the effects of testosterone and testosterone does not actdirectly upon the ductal epithelium;instead, the effect is mediatedthrough the mammary mesenchyme.

Some hormones involved in breast development and lactation:

EstrogenGrowth hormoneProlactin-inhibiting factor Prolactin releasing factor ProlactinOxytocin

Lecture10- The Skin and Its Derivatives

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