Module 1

Structural and Functional Classification of Tissues

• Neural Tissue:

  • Composed primarily of specialized cells termed neurons.

  • Function: Rapidly receives and transmits electrical impulses across intercellular communication junctions called synapsis.

• Epithelial Tissue:

  • Covers the majority of internal and external body surfaces.

  • Primary physiological functions include:

    • Protection of underlying structures.

    • Absorption of nutrients and substances.

    • Secretion of biochemical substances.

    • Excretion of waste products.

• Muscular Tissue:

  • Composed of elongated, thin cellular units known as myocytes.

  • Functional divisions based on anatomical attachment:

    • Attached to skeletal bones: Mediates voluntary movement.

    • Attached to internal visceral organs: Mediates involuntary movement.

• Connective Tissue:

  • Physically binds and anchors tissues and organs together throughout the body.

  • Acts as a physiological reservoir and storage site for excess nutrients.

Cellular Adaptations and Altered Tissue Biology

• Dysplasia:

  • Defined as an abnormal alteration in the size, shape, and architectural organization of mature cells.

  • Also clinically designated as atypical hyperplasia.

  • Is not classified as cancer and does not universally or inevitably progress to malignancy.

• Metaplasia:

  • Defined as the reversible substitution of one mature, differentiated cell type by another mature cell type.

  • Represents an adaptive cellular response to chronic environmental stress or irritation.

  • Retains the potential to progress pathologically to dysplasia and invasive cancer.

• Hyperplasia:

  • Defined as an increase in the absolute number of cells within an organ or tissue.

Standard Pedigree Chart Nomenclature and Symbols

• Individual Designations:

  • Square: Male.

  • Circle: Female.

  • Diamond: Sex undesignated.

  • Double bracket enclosing shape: Adopted individual.

  • Diamond enclosing the letter "P": Pregnancy.

  • Diagonal strike through a symbol: Deceased individual.

• Phenotypic and Carrier Status:

  • Solid filled square or circle: Individual affected with trait.

  • Half-filled square or circle: Carrier for autosomal trait.

  • Circle containing a central dot: Carrier for X-linked trait.

• Relationship and Kinship Connectors:

  • Horizontal line between a square and circle: Mating.

  • Double horizontal line between a square and circle: Consanguineous mating.

  • Horizontal line interrupted by two diagonal slashes: Divorced or separated relationship.

  • Square connected to two separate female symbols: Two matings.

  • Overhead horizontal bar with branching vertical descent lines: Siblings.

  • Numbered shape: Indicates the precise number of children (e.g., square containing $3$, circle containing $2$).

• Reproductive Outcomes and Twins:

  • Small unfilled diamond: Miscarriage, spontaneous abortion (SAB).

  • Forked diagonal descent lines originating from a single point: Dizygotic twins (fraternal twins).

  • Forked descent lines joined by a horizontal bridging bar: Monozygotic twins (identical twins).

  • Vertical descent line terminating in an anchor-like horizontal bar: No offspring.

• Clinical Genetics Proband:

  • Diagonal arrow pointing toward a symbol: Patient initiating genetic workup (designated as proband, index case, or consultand).

Autosomal and Sex Chromosome Aneuploidies

• Down Syndrome (Trisomy 21):

  • Chromosomal Etiology: Presence of an extra copy of chromosome $21$.

  • Risk Factors: Advanced maternal age greater than $35\text{ years}$ during gestation.

  • Phenotypic Features: Typically present with low IQ, poor muscle tone (hypotonia), short stature, a low nasal bridge, epicanthial folds, a protruding tongue, and low-set ears.

  • Associated Clinical Pathologies: Significantly higher risk for atlantoaxial instability, congenital heart defects, and early-onset Alzheimer’s disease.

  • Activity Clearances: Children participating in athletic or sports activities must undergo strict clinical screening for atlantoaxial instability and cardiovascular defects prior to participation to prevent serious injury.

  • Epidemiology and Mortality: Approximately $75\%$ of affected fetuses are spontaneously aborted or stillborn. Roughly $20\%$ of liveborn infants die prior to age $10$. For individuals surviving past age $10$, average life expectancy is approximately $60\text{ years}$.

• Turner Syndrome ($45,X$):

  • Chromosomal Etiology: Complete or partial monosomy of the X chromosome ($45,X$ karyotype).

  • Sex Specificity: Manifests exclusively in phenotypic females.

  • Cognitive Profile: Individuals are generally not intellectually disabled.

  • Physical Presentation: Short stature, webbed neck, widely spaced nipples, and a reduced carrying angle at the elbow (cubitus valgus).

  • Systemic Complications: Elevated risk for coarctation of the aorta and peripheral lymphedema/edema of the feet in neonates.

  • Reproductive Status: Patients are usually sterile and typically possess fibrous gonadal streaks in place of functional ovaries; these gonadal streaks carry an increased susceptibility to malignant transformation.

  • Prenatal Survival: Extremely high in utero lethality; only $0.5\%$ of $45,X$ conceptuses survive to full term.

• Klinefelter’s Syndrome ($XXY$ or $XXX$):

  • Chromosomal Etiology: Sex chromosome aneuploidy characterized by $XXY$ or $XXX$ genetic constitution.

  • Cognitive Profile: Exhibits a moderate degree of mental impairment.

  • Phenotype and Secondary Characteristics: General male phenotypic appearance; characteristically tall stature with long limbs, sparse body hair, and failure of normal secondary sexual characteristic development.

  • Endocrine and Reproductive Findings: Testes are typically small; patients are usually sterile. Gynecomastia occurs in approximately $50\%$ of affected individuals.

  • Therapeutic Window: Early medical intervention and hormonal management can potentially allow patients to develop a normal sexual and reproductive system in adulthood.

Epigenetic and Imprinting Disorders

• Prader-Willi Syndrome:

  • Etiology and Imprinting: Genetic imprinting disorder resulting from a paternal deletion of approximately $4\text{ million}$ base pairs on the long arm of chromosome $15$.

  • Clinical Characteristics: Short stature, pronounced obesity, mild to moderate intellectual disability, microacria (small hands and feet), hypogonadism, and a characteristic inverted "V" contour of the upper lip.

• Angelman Syndrome:

  • Etiology and Imprinting: Genetic imprinting disorder resulting from a maternal deletion of approximately $4\text{ million}$ base pairs on the long arm of chromosome $15$.

  • Clinical Characteristics: Severe intellectual disability, marked ataxia, recurrent seizures, and episodes of paroxysmal, uncontrollable laughter.

• Beckwidth-Wiedemann Syndrome:

  • Neonatal Manifestations: Newborns are typically large for gestational age (macrosomia) and develop significant neonatal hypoglycemia.

  • Structural Abnormalities: Macroglossia (large tongue), prominent linear creases on the earlobes, and abdominal wall defects including omphalocele.

  • Oncologic Predisposition: Substantially increased risk for developing embryonal malignancies, specifically Wilms tumor (nephroblastoma) and hepatoblastoma.