Barr Body

Definition and Discovery

Left panels: Barr bodies (arrows) in female epidermal spinous cells; right panels: nuclear "drumstick" appendage (arrow) in female neutrophils. Males show neither.

Mechanism: X-Chromosome Inactivation (Lyonization)

• Soon after the blastocyst stage, one X chromosome in each somatic cell of a genetic female is randomly inactivated.
• The choice (maternal vs. paternal X) is random and then permanent - all daughter cells carry the same inactivated X.
• Inactivation is initiated at the X-inactivation center on the chromosome, partly via the transactivating factor CTCF (CCCTC-binding factor).
• The inactivated X condenses into facultative heterochromatin - this is what appears as the Barr body.
• Key rule: Number of Barr bodies = (Number of X chromosomes) - 1

Where It Is Found (Sites of Demonstration)

1. Buccal Smear (Buccal Mucosa Scraping) - Most Common

• The buccal smear test (Barr body test) is the standard method.
• Barr body appears as a small planoconvex mass, ~1 micron in diameter, lying near/against the nuclear membrane (Dixon and Tarr).
• Females: 20-80% of nuclei contain chromatin bodies.
• Males: Only 0-4% of nuclei show chromatin bodies (considered negative).

2. Polymorphonuclear Leukocytes (Neutrophils) - Davidson Body

• In females, the inactive X forms a small drumstick-shaped nuclear appendage called the Davidson body, seen in 1-15% (Ganong) or ~3% (Essentials of FMT) of neutrophil nuclei.
• Completely absent in males.

3. Peripheral Blood Smear

• Can be assessed from blood samples when buccal smear is unavailable.

4. Hair Root Sheath Cells

• Root sheath cells are particularly useful because they resist putrefaction.
• Both the Barr body and Y chromosome can be demonstrated in hair root cells, making them valuable in decomposed/skeletal forensic samples.

5. Epidermal Cells (Spinous Layer)

• Demonstrated in skin biopsies (as shown in the textbook image above).

6. Amniotic Fluid Cells (Antenatal Sex Determination)

• Used in amniocentesis for fetal sex determination.

Clinical Significance

1. Sex Determination (Nuclear Sexing)

• Presence of a Barr body indicates at least two X chromosomes - suggesting a genetic female (or an XXY male).
• Used in forensic medicine for sex identification from buccal smears, blood stains, or hair in living persons, decomposed remains, and skeletal material.
• Stained with orcein reagent (for Barr body) or quinacrine (for Y body fluorescence in males).

2. Klinefelter Syndrome (47,XXY)

• Barr body is found in 80% of cases (Langman's).
• Clinical features: sterility, testicular atrophy, hyalinization of seminiferous tubules, gynecomastia.
• Incidence ~1 in 500 males; most common cause is nondisjunction of XX homologues.
• More X chromosomes (e.g., 48,XXXY) = more likely cognitive impairment.

3. Turner Syndrome (45,X)

• Barr body absent (chromatin body negative) because only one X is present.
• ~55% of affected females are monosomic 45,X; the absence of Barr body was noted in 1954.
• Clinical features: gonadal dysgenesis, short stature, webbed neck, lymphedema, broad chest with widely spaced nipples.

4. Triple X Syndrome (47,XXX)

• Two Barr bodies present.
• Often undiagnosed due to mild features; affected females may have speech problems and self-esteem issues.

5. Antenatal Sex Determination

• Important for X-linked conditions (hemophilia, Duchenne MD, etc.) where knowing fetal sex guides genetic counseling.

6. Forensic Medicine / Medicolegal Importance

• Sex determination from evidence (crime scene stains, unidentified remains).
• Can be performed on advanced decomposed or skeletal remains using hair root sheath cells.
• Y chromosome can be demonstrated with quinacrine fluorescence (at least 50 positive nuclei required for a positive result).

Y Body (for Comparison)

Barr Body

1. What Is a Barr Body?

Core concept: It is not a separate structure - it is simply one of the X chromosomes that has been permanently switched off and compacted.

2. How Does a Barr Body Form? (Mechanism)

Steps of Lyonization:

Why Does This Happen?

Important caveat: Not all genes on the inactivated X are silenced. Genes in the pseudoautosomal regions (PAR) escape inactivation - which is why 45,X (Turner) females are abnormal despite having one active X, because these escaping genes are absent.

3. The Golden Rule: Barr Body Count

Memory trick: However many X chromosomes you have, one always stays active. The rest get inactivated = become Barr bodies.

4. Where Is the Barr Body Found? (Sites of Demonstration)

A. Buccal Mucosa Smear - The Standard Test

• Most commonly used site in clinical and forensic practice.
• Barr body appears as a small planoconvex mass (~1 micron in diameter) lying against the inner nuclear membrane (described by Dixon and Tarr).
• Stained with cresyl violet or orcein and examined under high-power microscopy.

B. Polymorphonuclear Neutrophils - The Davidson Body

• In females, the inactive X forms a drumstick-shaped nuclear appendage attached to the nucleus of neutrophils.
• This is called the Davidson body (or drumstick appendage).
• Seen in ~3% of neutrophil nuclei in females; completely absent in males.
• Identified on a peripheral blood smear.

Distinction: Barr body = in any nucleated somatic cell; Davidson body = specifically in neutrophils.

C. Hair Root Sheath Cells

• Particularly valuable in forensic cases involving putrefied or decomposed remains.
• Hair root sheath cells resist putrefaction and can survive long after death.
• Both the Barr body (X) and Y body (fluorescent Y chromosome) can be demonstrated from a single hair root.

D. Other Sites

5. Appearance and Staining

Left: Barr bodies visible at nuclear periphery in female cells. Right: Davidson body drumstick in a female neutrophil. Both are absent in males.

6. Clinical Significance

A. Sex Determination (Nuclear Sexing)

• A positive Barr body test (female pattern) = at least two X chromosomes present.
• A negative test = only one X chromosome (male, or Turner syndrome).
• Used clinically to determine chromosomal sex when external sex is ambiguous.
• Used forensically to identify sex from crime scene materials, remains, or decomposed bodies.

B. Klinefelter Syndrome (47,XXY)

• Phenotype: Male (Y chromosome present)
• Barr body: Present (1 Barr body) - found in 80% of cases
• Key features: Sterility, testicular atrophy, hyalinization of seminiferous tubules, gynecomastia
• Incidence: ~1 in 500 males
• Cause: Nondisjunction of XX homologues (most commonly during maternal meiosis)
• More X chromosomes (48,XXXY, 49,XXXXY) = greater likelihood of intellectual disability

C. Turner Syndrome (45,X)

• Phenotype: Female (no Y chromosome)
• Barr body: Absent (chromatin body negative)
• Key features: Gonadal dysgenesis (streak ovaries), short stature, webbed neck, lymphedema, widely spaced nipples, broad chest
• Only monosomy compatible with life; 98% of affected fetuses are spontaneously aborted
• Barr body absence was first noted in 1954; cytogenetic confirmation in 1959
• Cause: Nondisjunction in the paternal gamete in ~80% of monosomic cases

D. Triple X Syndrome (47,XXX)

• Phenotype: Female
• Barr body: 2 Barr bodies
• Often undiagnosed due to mild or absent features
• May present with speech delay, learning difficulties, and self-esteem issues

E. Antenatal Diagnosis

• Barr body status in amniotic fluid cells helps determine fetal sex.
• Critical for counseling in X-linked recessive disorders (hemophilia A/B, Duchenne muscular dystrophy, etc.) where affected offspring are predominantly male.

F. Forensic Medicine (Medicolegal Importance)

• Sex determination from evidence at crime scenes - blood stains, hair, saliva, buccal smear.
• Applicable to living persons, decomposed bodies, and skeletal remains.
• Combined with Y body demonstration (quinacrine fluorescence): At least 50 Y-positive nuclei are required to report a positive male result.
• Hair root sheath cells provide sex chromatin evidence even after significant decomposition.

7. Y Body - The Male Counterpart

8. Summary Concept Map

X-chromosome inactivation (Lyonization)
            ↓
  One X stays ACTIVE → gene expression
  Other X(s) INACTIVATED → condense
            ↓
     Forms BARR BODY
  (facultative heterochromatin)
            ↓
  Visible at nuclear periphery in interphase
            ↓
  No. of Barr bodies = (No. of X) - 1
            ↓
  Clinical use:
  ├── Normal female → 1 Barr body
  ├── Klinefelter (XXY) → 1 Barr body (present in a male!)
  ├── Turner (45,X) → 0 Barr bodies
  └── Triple X (XXX) → 2 Barr bodies