Porth's Essentials of Pathophysiology, 4e

96

Cell and Tissue Function

U N I T 1

Cell Division Cells reproduce by duplicating their chromosomes and dividing in two. There are two types of cell division: mitosis and meiosis. Mitosis is the cell cycle process in which nongerm cells are replicated. It provides a way for the body to replace cells that have a limited life span, such as skin and blood cells; increase tissue mass during periods of growth; and repair tissue, such as in wound healing. Meiosis is limited to replicating germ cells and takes place only once in a cell line. It results in the formation of gametes or reproductive cells (i.e., ovum and sperm), each of which has only a single set of 23 chromosomes. Meiosis is typically divided into two distinct phases: meiosis I and meiosis II (Fig. 5-7). During meiosis I, homologous chromosomes pair up, forming a double-structured chromosome containing four chromatids (four strands) and therefore called a tetrad (two chromatids per chromosome). They are also sometimes called bivalents. The X and Y chro- mosomes are not homologs and do not form biva- lents. While in meiosis I, an interchange of chromatid segments can occur (Fig. 5-8). This process, called crossing over, allows for new combinations of genes, increasing genetic variability. After cell division I, each of the two daughter cells contains one member of each homologous pair of chro- mosomes and a sex chromosome (23 double-stranded chromosomes). No DNA synthesis occurs before mei- otic division II. During cell division II, the 23 double- stranded chromosomes (two chromatids) of each of the two daughter cells from meiosis I divide at their centro- meres (central regions where the chromatids meet). Each subsequent daughter cell receives 23 single-stranded chromatids. Thus, a total of four daughter cells are formed by a meiotic division of one cell. Chromosome Structure Cytogenetics is the study of the structure and numeric characteristics of the cell’s chromosomes. Chromosome studies can be done on any tissue or cell that grows and divides in culture. Lymphocytes from venous blood are frequently used for this purpose. After the cells have been cultured, a drug called colchicine is used to arrest mito- sis before the chromosomes separate. A chromosome spread is prepared by fixing and spreading the chro- mosomes on a slide. Subsequently, appropriate staining techniques show the chromosomal banding patterns so they can be identified. The chromosomes are imaged, and the photoimages of each of the chromosomes are cut out and arranged in 23 pairs according to a standard classification system (see Fig. 5-6). The completed pic- ture is called a karyotype, and the procedure for prepar- ing the picture is called karyotyping. While the chromosomes are aligned on the equato- rial plate of the cell, each chromosome takes the form of chromatids to form an “X” or “wishbone” pat- tern. Human chromosomes are divided into three types according to the position of the centromere (Fig. 5-9).

Meiosis I

Pulling apart of double-structured chromosomes

Pairing begins

Pairing of chromosomes

Chiasma formation

A

C

B

D

Anaphase of 1st meiotic division

E

First meiotic division (cells have 23 double-structured chromosomes)

Meiosis II

F

Second meiotic division (23 single chromosomes)

G

Meiosis I

These cells contain 46 double-structured chromosomes Primary oocyte after DNA replication Primary spermatocyte after DNA replication

First maturation division (23 double-structured chromosomes)

Secondary spermatocyte

Secondary oocyte

Second maturation division (23 single chromosomes)

Meiosis II

Polar bodies (22 + X)

Mature oocyte (22 + X)

Spermatids (22 + Y)

A

B

FIGURE 5-7. First and second meiotic divisions. ( Top ) Meiosis I, during which homologous chromosomes (A) approach each other and (B) pair; (C) intimately paired homologous chromosomes interchange chromatid fragments (crossing over) and (D) double-structured chromosomes pull apart. (E) Anaphase of first meiotic division. During meiosis II (E, F) , the double-structured chromosomes pull apart at the centromere to form four single-stranded chromosomes (reduction division). ( Bottom ) Events occurring during meiosis I and II in female and male gametes. (A) The primitive female germ cell (oocyte) produces only one mature gamete, the mature oocyte. (B) The primitive male germ cell (primary spermatocyte) produces four spermatids, all of which develop into spermatozoa. (Adapted from Sadler RW. Langman’s Medical Embryology, 9th ed. Philadelphia, PA: Lippincott Williams &Wilkins; 2003.)

Made with