Chapter 4

生物醫學暨環境生物學系 助理教授 張學偉

The cell cycle

Mitotic cycle time (Tc) = cell cycle time = time between successive divisions

Autoradiography 自動放射顯影

Bromodeoxyuridine(BrdUrd)

Two advantages in BrdUrd over autoradiography

1. Not involve radioactive material

2. Shorten the time for result

1. The relative length of different cell cycle phase vary in different cell types

2. Human cells of Tc > Hamster cells of Tc

3. S> G2 > M

Synchronously Dividing cell cultures

同步化

Definition:All of the cells occupy the same phase of the cell cycle at a given time.

Techniques for synchrony:

1. Mitotic harvest [shake off]

2. Drug [e.g.,hydroxyurea]<1> S phase cells are killed<2> Block at the end of G1

Most population in mitosis

Irradiated at various times afterward,Corresponding to different cell cycle phase

Cell cycle phase & radiosensitivity

shoulder

EarlyG1

laterG1

The pattern cannot be distinguished in hamster cells because G1 is too short.

1. Pattern similar2. Important difference

Summary of radiosensitivity with cell age in the mitotic cycle

1. Cells are most sensitive at or close to mitosis

2. Resistance is usually greatest in later S phase

3. Resistance at early G1 & sensitivity at later G1

4. G2 phase is usually sensitive, perhaps as M phase

Molecular checkpoint genes

Flow cytometer細胞流速分析儀

Flow cytometer & separation of cell cycle phase

DNA content

Cel

l nu

mbe

r

G0/G1

M

2N 4N

The effect of oxygen at various phases of the cell cycle

Oxygen enhancement ratio (OER) ch6The ratio of hypoxic to aerated doses needed to achieve the same biological effect is called OER.

Sur

viva

l

Dose (Gy)

aerated

hypoxic

OERS (2.3-2.4) < G1 & G2

OER varies significantly through the cycle.

The age-response function for a tissue in vivoVariation of sensitivity with cell age for neutrons

1. The variation of thymidine uptake with time.

2. First wave = DNA synthesis period

3. Correlated to max resistance to x-ray

CPM = count per min

The in vivo pattern of response as a function of cell age is similar to -ray and neutrons, with max resistance occurring in later S phase for in vitro culture cells.

Mechanism for the age-response function

1. DNA amount or form

2. Naturally occuring sulfhydryl compounds

The possible implications of the age-response function in radiotherapy

Sensitization due to reassortment

Dose at Asynchronous cell population S phase remained synchronize cell cycle move Dose later at sensitive phase sensitizing called