Page 114 - Atlas of Histology with Functional Correlations

worn-out cells. Also, germ cells (spermatogonia) in the testes are continuously

dividing to produce new sperm.

CELL CYCLE: INTERPHASE AND

MITOSIS

The time interval between two successive cell divisions represents the cell cycle.

It involves cell replication by duplicating the cell’s genetic contents and
producing two identical daughter cells. The cell cycle is divided into two main
phases: interphase and mitosis. Interphase consists of a prolonged interval
comprising different phases during which time the cell size and its contents

increase. In addition, DNA, centrioles, and chromosomes replicate, and the cell
prepares for division, or mitosis, which exhibits four distinct and histologically
recognizable stages or phases.

Prophase

During this first prolonged phase of mitosis, the chromosomes condense and
become histologically visible. Each chromosome consists of two genetically
identical sister chromatids that are joined together at a pinched area called the

centromere. With the condensation of the chromosomes, the nuclear envelope
and nucleolus disappear (fragment) with only fragments visible in the cell. The
centrosome divides, and the centrioles migrate to the opposite poles of the cell
to form microtubules of the mitotic spindle (Fig. 3.1A). The microtubule

spindles continue to grow toward the chromosomes, where some of them attach
to a platelike protein complex called the kinetochore, which appears on each
side of the centromere. These kinetochore microtubules eventually align the
chromosomes in the middle of the cell. The microtubules that do not attach to the

chromosomes at the kinetochore become the polar microtubules.

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