CYTOGENETIC PRINCIPLES

Part of “CHAPTER 90 – NORMAL AND ABNORMAL SEXUAL DIFFERENTIATION AND DEVELOPMENT“

NOMENCLATURE AND IDENTIFICATION OF CHROMOSOMES

Normal humans have 46 chromosomes—44 autosomes and two sex chromosomes. In females, the two sex chromosomes are X chromosomes; in males, one is an X and one a Y chromosome. By convention, chromosomes are numbered according to size and the position of the centromere, the primary constriction around which cell division occurs. As with other chromosomes, the centromere divides the X and the Y chromosomes into a short arm, designated p, and a long arm, designated q. Accepted nomenclature dictates that specific chromosome bands are designated by the chromosome, the arm (p or q), the region, and the specific band; bands are numbered consecutively from the centromere distally. Complements containing structurally abnormal X or Y chromosomes also require the symbol for the aberration present (e.g., i for isochromosome, del for deletion).1,2 Beginning in 1969, the development of banding techniques that produce horizontal “stripes” on the chromosome permitted the routine identification of individual chromosomes1,2a (Fig. 90-1D). Although the mechanism of chromosome banding is not completely understood, the various bands are known to reflect variations in DNA base sequences.

FIGURE 90-1. A, Nucleus of normal female cell from orcein-stained smear of buccal mucosa showing one X-chromatin mass, or Barr body (arrow). B, The nucleus of a peripheral leukocyte from a normal male, stained with quinacrine mustard and showing the fluorescent Y-chromatin (arrow). C, Quinacrine-stained metaphase chromosomes of a normal male from leukocyte culture: arrow indicates fluorescent Y chromosome. D, Giemsa-banded karyotype of normal female from leukocyte culture. Each human chromosome can be identified by its banding pattern. Giemsa banding (G-banding) is the most widely used procedure. The number of bands identified with this technique is ˜500, which permits the detection of subtle chromosome alterations. Additional techniques include quinacrine banding (Q-banding), constitutive heterochromatin banding (C-banding), and reverse banding (R-banding). (Courtesy of Dr. Beverly White; reproduced from Rebar RW. Practical evaluation of hormonal status. In: Yen SSC, Jaffe RB, eds. Reproductive endocrinology: physiology, pathophysiology, and clinical management, 2nd ed. Philadelphia: WB Saunders, 1986:683.)

CYTOLOGIC PROPERTIES OF THE X CHROMOSOME

One of the two X chromosomes in normal females is the last chromosome to complete DNA synthesis, a fact believed to reflect tighter condensation during interphase of this chromosome than of others. The late-replicating X is said to represent the heterochromatic X and appears as a planoconvex body, now called X-chromatin body and formerly known as sex chromatin or the Barr body, near the nuclear membrane3 (see Fig. 90-1A). These bodies are observed in 30% or more of the cell nuclei from normal females. X-chromatin is the more nonfunctioning X chromosome and is present in the resting nuclei of somatic cells in mammals in which the cell has more than one X chromosome.4 In interphase, only one X chromosome usually functions in a cell at a given time; all other X chromosomes are present in the form of X-chromatin bodies.

The time of X inactivation is uncertain but is probably around the time of implantation. The choice of which X chromosome (i.e., maternal or paternal) is inactivated in a given cell is random if both X chromosomes are normal. After a given X is inactivated, however, the descendants of that cell retain the identical pattern. Because X inactivation occurs only after the embryo
contains hundreds of cells, normal females have two populations of cells: in one, the maternal X is active; in the other, the paternal X is active. The biochemical basis of X inactivation is not well understood, but the inactivated DNA appears to be more heavily methylated than in the active X. (See also ref. 4a.)

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