Embryonic Tissue Development and the Integumentary System

Vocabulary practice flashcards covering Lecture 6: Embryonic Tissue Development and the Integumentary System, including embryogenesis, gastrulation, skin layers, accessory appendages, pigmentation, and wound repair.

Development

The gradual modification of anatomy and physiology from fertilization to maturity, progressing from a single cell to an individual with trillions of cells.

Prenatal Development

The stage of development that occurs before birth, comprising both embryological and fetal developmental phases.

Postnatal Development

The stage of development that begins at birth and continues until physical maturity.

First Trimester

The initial three months of prenatal development characterized by embryological and early fetal development.

Second Trimester

The middle three months of prenatal development during which organs and organ systems develop and body proportions change significantly.

Third Trimester

The final three months of prenatal development marked by rapid fetal weight gain and functional maturation of organ systems.

Fertilization

The fusion of two haploid gametes ($23$ chromosomes each) into a single diploid zygote ($46$ chromosomes), typically occurring in the ampulla or isthmus of the uterine tube approximately $24\text{ hours}$ post-ovulation.

Secondary Oocyte at Ovulation

The female gamete released during ovulation surrounded by the corona radiata and first polar body, suspended in metaphase of meiosis II until sperm penetration.

Amphimixis

The fusion of the male pronucleus and female pronucleus, which completes fertilization and initiates the first cleavage division.

Cleavage

A series of rapid mitotic cell divisions without intervening cellular growth, producing increasingly smaller blastomeres beginning roughly $30\text{ hours}$ after fertilization.

Morula

A solid ball of blastomeres formed by day $3$ of embryonic development.

Blastocyst

A hollow sphere of cells formed by day $5$ containing an inner cell mass (embryoblast), an outer trophoblast layer, and a fluid-filled blastocoele cavity.

Trophoblast

The outer epithelial cell layer of the blastocyst that adheres to and invades the endometrium around day $7$ to initiate implantation and nourishment.

Invagination

A morphogenetic movement during gastrulation wherein cells from the outer layer of the blastula fold inward, forming a pocket or infolding.

Involution

The rolling or turning inward of an outer sheet of cells over the interior surface of the blastula, contributing to the formation of the archenteron (primitive gut).

Ingression

The inward migration of individual cells from the surface layer of the embryo into the interior, helping to form the mesoderm and other tissues.

Delamination

The cellular process in which a single sheet of embryonic cells splits into two parallel layers, contributing to the formation of the mesoderm and endoderm.

Epiboly

The expansion and flattening of epiblast cells (outer layer) as they migrate over the underlying hypoblast (inner layer) to establish the ectoderm.

Ectoderm

The superficial primary germ layer that gives rise to the nervous system and the epidermis of the skin.

Mesoderm

The middle primary germ layer that gives rise to muscle tissue, bone, connective tissues, and the cardiovascular system.

Endoderm

The innermost primary germ layer that forms the epithelial lining of the gut, liver, pancreas, and respiratory tract.

Cutaneous Membrane

The skin, consisting of a superficial stratified squamous epithelium (epidermis) and an underlying connective tissue layer (dermis), making up roughly $16\%$ of total body weight.

Hypodermis (Subcutaneous Layer)

The adipose-rich layer deep to the dermis that anchors the skin to underlying fascia and allows independent movement; not considered part of the integument itself.

Keratinocytes

The most abundant epithelial cells found in the epidermis, responsible for producing keratin and organized into multiple strata.

Stratum Corneum

The exposed surface layer of the epidermis consisting of $15\text{--}30$ layers of dead, flattened, keratinized cells interconnected by desmosomes that shed after approximately $2\text{ weeks}$.

Stratum Lucidum

A glassy, clear epidermal layer found exclusively in thick skin (palms and soles) located directly beneath the stratum corneum, composed of flattened dead cells filled with keratin and keratohyalin.

Stratum Granulosum

An epidermal layer comprising $3\text{--}5$ layers of keratinocytes that have ceased dividing and actively synthesize high amounts of keratin and keratohyalin.

Stratum Spinosum

An epidermal layer consisting of $8\text{--}10$ layers of living keratinocytes bound by desmosomes, containing dendritic cells that participate in immune defense against pathogens and superficial cancers.

Stratum Basale

The deepest layer of the epidermis, formed by a single row of basal stem cells attached to the basement membrane via hemidesmosomes that divide continuously to replace lost epidermal cells.

Melanocytes

Pigment-producing cells residing in the stratum basale that synthesize melanin from the amino acid tyrosine and package it into melanosomes for transfer to keratinocytes.

Carotene

An orange-yellow pigment derived from diet that accumulates in epidermal keratinocytes (notably the stratum corneum) and hypodermal adipose tissue.

Cyanosis

A bluish discoloration of the skin and nail beds caused by an abnormally low oxygen level in circulating hemoglobin.

Papillary Layer

The superficial layer of the dermis composed of loose areolar connective tissue containing dermal papillae, capillary loops, lymphatic vessels, and sensory nerve endings.

Reticular Layer

The deep layer of the dermis composed of dense irregular connective tissue containing abundant bundles of collagen and elastic fibers, housing glands, blood vessels, and hair follicles.

Lines of Cleavage (Tension Lines)

Patterns formed throughout the skin by the parallel alignment of collagen and elastic fiber bundles in the dermis; surgical cuts made parallel to these lines heal with minimal scarring.

Tactile Corpuscles (Meissner's Corpuscles)

Specialized sensory receptors located in dermal papillae that detect light touch and low-frequency vibrations.

Lamellated Corpuscles (Pacinian Corpuscles)

Large encapsulated sensory receptors located deep in the dermis and hypodermis that detect deep pressure and high-frequency vibration.

Terminal Hairs

Heavy, deeply pigmented, and coarse hairs found on the scalp, eyebrows, eyelashes, and elsewhere after puberty.

Vellus Hairs

Fine, short, lightly pigmented hairs covering the majority of the human body surface.

Arrector Pili Muscle

A bundle of smooth muscle cells extending from the papillary dermis to the connective tissue sheath of a hair follicle that contracts to pull the hair erect, producing goosebumps.

Anagen Phase

The active growth phase of a hair follicle lasting between $2\text{ and }5\text{ years}$, during which the hair grows at approximately $0.33\text{ mm/day}$.

Club Hair

An inactive hair whose follicle has shrunk and detached from the hair matrix during the resting (telogen) phase of the hair growth cycle.

Sebaceous Glands

Holocrine exocrine glands that secrete an oily lipid mixture called sebum into hair follicles or directly onto the epidermal surface.

Sebum

An oily lipid mixture produced by sebaceous glands that lubricates hair shafts, conditions the surrounding epidermis, and inhibits bacterial growth.

Apocrine Sweat Glands

Coiled tubular sweat glands restricted to the axillae, nipples, and pubic region that produce a viscous, cloudy secretion discharged into hair follicles, regulated by hormones.

Merocrine (Eccrine) Sweat Glands

Coiled tubular glands widespread across the integument that secrete a watery solution of electrolytes directly onto the skin surface to facilitate evaporative cooling.

Eponychium

A narrow fold of stratum corneum extending from the base of the nail wall over the proximal surface of the nail body, commonly referred to as the cuticle.

Hyponychium

A thickened area of stratum corneum located directly underneath the distal free edge of the nail plate.

Basal Cell Carcinoma

The most common form of skin cancer, originating from neoplastic cells of the stratum basale due to cumulative ultraviolet radiation exposure; typically exhibits a low metastatic rate.

Malignant Melanoma

A highly invasive cancer of melanocytes that readily metastasizes through the lymphatic system, with $5\text{-year}$ survival dropping from $99\%$ if detected early to $14\%$ following metastasis.

Epidermal Growth Factor (EGF)

A peptide hormone produced by the salivary and duodenal glands that stimulates cell division in the stratum basale and stratum spinosum, accelerates keratin production, and promotes epidermal repair.

Calcitriol

The biologically active hormonal form of vitamin $D_3$ ($1\alpha,25\text{-(OH)}_2D_3$) synthesized by the kidneys that stimulates intestinal absorption of calcium and phosphate ions.

Rickets

A childhood skeletal disorder characterized by poorly mineralized, soft, and flexible bones, caused by insufficient dietary cholecalciferol or inadequate exposure to sunlight.

Granulation Tissue

A temporary vascular healing tissue formed during skin repair consisting of a fibrin blood clot, migrating fibroblasts, and an extensive network of newly formed capillaries.

Scar Tissue

A dense, inflexible, relatively avascular fibrous tissue rich in collagen that permanently replaces damaged dermis following severe skin injury.