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This X-linked condition accounts for more cases of mental retardation in males than any condition except Down’s syndrome; about one in 4000 to 6000 males is affected; the condition also affects intellectual function in females about 50% less frequently than in males. The first marker for this condition was a small gap, or fragile site, evident near the tip of the long arm of the X chromosome. Subsequently, the condition was found to be due to expansion of a trinucleotide repeat (CGG) near a gene called FMR1. All individuals have some CGG repeats in this location, but as the number increases beyond 52, the chances of further expansion during spermatogenesis or oogenesis increase. Being born with one FMR1 allele with 200 or more repeats results in mental retardation in most men and in about 60% of women. The more repeats, the greater the likelihood that further expansion will occur during gametogenesis; this results in anticipation, in which the disorder can worsen from one generation to the next. Affected (heterozygous) women show no physical signs other than early menopause, but they may have learning difficulties or frank retardation. Affected males show macroorchidism (enlarged testes) after puberty, large ears and a prominent jaw, a high-pitched voice, and mental retardation. Some show evidence of a mild connective tissue defect, with joint hypermobility and mitral valve prolapse.

Men who are not retarded but carry an increased number of CGG repeats in the FMR1 locus (premutation carriers) are at increased risk for developing intention tremor, ataxia, or both. Likewise, women who are premutation carriers (55–200 CGG repeats) are at increased risk for premature ovarian failure and mild cognitive or behavioral abnormalities. Male and female premutation carriers are at risk for developing tremor and ataxia beyond middle age. Because of the relatively high prevalence of premutation carriers in the general population, older people in whom any of these problems develop should undergo testing of the FMR1 locus.

DNA diagnosis for the number of repeats has supplanted cytogenetic analysis for both clinical and prenatal diagnosis. This should be done on any male or female who has unexplained mental retardation.

Hagerman PJ et al: The fragile-X premutation: a maturing perspective. Am J Hum Genet 2004;74:805. [PMID: 15052536]

Hatton DD et al: Problem behavior in boys with fragile X syndrome. Am J Med Genet 2002;108:105. [PMID: 11857559]

Jacquemont S et al: Penetrance of the fragile X-associated tremor/ataxia syndrome in a premutation carrier population. JAMA 2004;291:460. [PMID: 14747503]

Kenneson A et al: The female and the fragile X reviewed. Semin Reprod Med 2001;19:159. [PMID: 11480913]

Sutherland GR et al: Fragile X syndrome and other causes of X-linked mental handicap. In: Emery and Rimoin’s Principles and Practice of Medical Genetics, 5th ed. Rimoin DL et al (editors). Churchill Livingstone, 2006.

Terracciano A et al: Fragile X syndrome. Am J Med Genet C Semin Med Genet 2005;137:32. [PMID: 16010677]

Histology is that branch of anatomy that studies tissues of animals and plants. This textbook, however, discusses only animal, and more specifically human, tissues. In its broader aspect, the word histology is used as if it were a synonym for microscopic anatomy, because its subject matter encompasses not only the microscopic structure of tissues but also that of the cell, organs, and organ systems.

The body is composed of cells, intercellular matrix, and a fluid substance, extracellular fluid (tissue fluid), which bathes these components. Extracellular fluid, which is derived from plasma of blood, carries nutrients, oxygen, and signaling molecules to cells of the body. Conversely, signaling molecules, waste products, and carbon dioxide released by cells of the body reach blood and lymph vessels by way of the extracellular fluid. Extracellular fluid and much of the intercellular matrix are not visible in routine histological preparations, yet their invisible presence must be appreciated by the student of histology.

The subject of histology no longer merely deals with the structure of the body; it also concerns itself with the body’s function. In fact, histology has a direct relationship to other disciplines and is essential for their understanding. This textbook, therefore, intertwines the disciplines of cell biology, biochemistry, physiology, embryology, gross anatomy, and, as appropriate, pathology. Students will recognize the importance of this subject as they refer to the text later in their careers. An excellent example of this relationship will be evident when the reader learns about the histology of the kidney and realizes it is the intricate and almost sublime structure of that organ (down to the molecular level) that is responsible for the kidney’s ability to perform its function. Alterations of the kidney’s structure are responsible for a great number of life-threatening conditions.