Skin: Development Flashcards
In what week of development does the periderm form?
4th week
(after neurulation when the neural tube is formed)
Layer beneath the periderm
Basal layer
This layer forms the stratum germinativum and the intermediate layer
Basal layer
Post-neurulation in the 4th week, the surface epithelium proliferates and forms this, which is a new outer layer of simple squamous epithelium
Periderm
The periderm consists of this morphological cell type
Simple squamous epithelium
This constitutes the layer of stem cells (stratum basale) that will continue to replenish the epidermis throughout life
Stratum germinativum
The cells of this layer contain the keratin proteins, which is characteristic of differentiated epidermis (therefore these cells are called keratinocytes)
Intermediate layer
The periderm is sloughed off by this week of development
21st week
This layer is replaced by the 3 definitive layers of the outer epidermis: the inner stratum spinosum, the middle stratum granulosum, and the outer stratum corneum
Intermediate layer
Peridermal cells are eventually desquamated and form part of this, a greasy substance of peridermal cells and sebum from the sebaceous glands that protects the embryo’s skin
Vernix caseosa
The vernix caseosa is a greasy substance composed of these 2 things, which protects the embryo’s skin
Peridermal cells and sebum
Proliferation of cells in this layer also produces epidermal ridges, which extend into the developing dermis
Stratum germinativum
The pattern of these that develops on the surface of the palms of the hands and the soles of the feet is determined genetically and constitutes the basis for examining fingerprints (dermatoglyphics) in criminal investigation and medical genetics
Epidermal ridges
Epidermal ridges appear in embryos at this week
10 weeks
Epidermal ridges appear in embryos at 10 weeks and are permanently established by this week
17th week
These are derived from neural crest cells that migrate into the stratum basale of the epidermis
Melanoblasts
Melanoblasts are derived from these which migrate into the stratum basale of the epidermis
Neural crest cells
Melanoblasts are derived from neural crest cells that migrate into this location
Stratum basale
These differentiate into melanocytes by mid-pregnancy, when pigment granules called melanosomes are observed
Melanoblasts
When do melanoblasts differentiate into melanocytes?
By mid-pregnancy
This is when melanosomes are also observed
Merkel cells arise from this type of cell
Epidermal cells
These cells function as slow-adapting mechanoreceptors in the skin, but cytochemical evidence suggests that they may also function as neuroendocrine cells at some stage
Merkel cells
When separated, do ectoderm and underlying mesenchyme differentiate?
No
Recombinations show that the dermis (mesoderm) determines the nature of the ectodermal differentiation
Recombinations show that this determines the nature of the ectodermal differentiation
The dermis (mesoderm)
Tissue with a triple embryonic origin that underlies the epidermis and contains blood vessels, hair follicles, nerve endings, sensory receptors
Dermis
The dermis is derived from these three different sources
Lateral plate mesoderm (limbs and body wall)
Neural crest cells (head and neck)
Paraxial mesoderm (back)
Dermis derived from lateral plate mesoderm applies to these regions of the body
Limbs and body wall
Dermis derived from neural crest cells applies to these regions of the body
Head and neck
Dermis derived from paraxial mesoderm applies to these regions of the body
Back
The dermis is loose connective tissue made of these cells
Mesoderm cells (aka mesenchymal cells or mesenchyme)
The mesenchymal cells in the dermis secrete a watery-type ECM rich in these two components
Glycogen and Hyaluronic acid
These cells secrete a watery-type ECM rich in glycogen and hyaluronic acid
Mesenchymal cells of the dermis
These cells differentiate into fibroblasts, which secrete increasing amounts of collagen and elastic fibers into the ECM
Mesenchymal cells
The mesenchymal cells differentiate into fibroblasts, which secrete increasing amounts of these two components into the ECM
Collagen and elastic fibers
Where are the tactile sensory receptors (such as Meissner corpuscles) found in the skin?
In the projections of the dermis into the epidermis, called dermal papillae
Principle types of fibers in the dermis
Type I and III collagen and elastic fibers
This initially covers the entire nail plate, but it eventually degenerates, except for a thin persisting rim along the proximal end of the nail
Eponychium
The thickened epidermis underly the distal most part of the nail
Hyponychium
This marks the border between dorsal and ventral skin under the distal part of the nail
Hyponychium
In humans, mutation in a novel WNT ligand, R-SPONDIN-4, or the FRIZZLED 6 RECEPTOR, causes this
Nail defects, including anonychia (absence of nails)
In humans, mutation in this causes nail defects, including anonychia (absence of nails)
A novel WNT ligand, R-SPONDIN-4 or FRIZZLED 6 RECEPTOR
These glands form as diverticula of hair follicle shafts, budding from the root sheath about 4 weeks after the hair germ elongation begins to elongate
Sebaceous glands
Glands that initially form over most of the body but are lost in later months except in certain areas, such as the axillae, mons pubis, prepuce, scrotum, and labia minora
Begin to secrete at puberty
Apocrine sweat glands
In the development of eccrine sweat glands, the central cells degenerate to form the gland lumen, and the peripheral cells differentiate into an inner layer of secretory cells and an outer layer of this type of cell, which are innervated by sympathetic fibers and contract to expel sweat from the gland
Myoepithelial cells
During the development of ectodermal placodes, mammary glands, hair follicles, and tooth buds are also differentiating as derivatives of this
Epidermal invaginations
These first appear in the 4th week as thickened lines of epidermis that extend from the thorax to the medial thigh
Mammary ridges
Secondary mammary buds form during the 3rd month and become canalized to form these during the last 3 months of fetal life
Lactiferous ducts
When do secondary mammary buds form?
During the 3rd month
When in development do secondary mammary buds become canalized to form lactiferous ducts?
During the last 3 months of fetal life
A group of skin disorders that results from an excess of keratin formation
The skin is characterized by dryness and scaling, which may involve the entire body surface
Ichthyosis
Ichthyosis is a group of skin disorders that results from an excess of this
The skin is characterized by dryness and scaling, which may involve the entire body surface
Keratin formation
Disorder resulting from a rare keratinizing disorder that is inherited as an autosomal recessive trait and caused by a mutation in the ABCA12 gene
Harlequin ichthyosis
What is the inheritance pattern of Harlequin ichthyosis?
Autosomal recessive
What mutation causes Harlequin ichthyosis?
Mutation in the ABCA12 gene
What is the prognosis of neonates with Harlequin ichthyosis?
Poor
Most require intensive care, 70% die early
The periderm is normally shed completely by this week of development
21st week
Condition where the periderm persists until birth, forming a “cocoon” around the newborn
Is either removed by the physician or shed spontaneously during the first weeks of life
Collodion babies
In collodion babies, this persists until birth rather than being normally shed completely by the 21st week of development
Periderm
