The cell cycle Mitotic cycle time Tc cell

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

Autoradiography 自動放射顯影

Bromodeoxyuridine (Brd. Urd)

Two advantages in Brd. Urd 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> G 2 > 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 G 1

Most population in mitosis

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

Cell cycle phase & radiosensitivity shoulder

Early G 1 later G 1 The pattern cannot be distinguished in hamster cells because G 1 is too short.

1. Pattern similar 2. 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 G 1 & sensitivity at later G 1 4. G 2 phase is usually sensitive, perhaps as M phase

Molecular checkpoint genes

細胞流速分析儀 Flow cytometer

Cell number Flow cytometer & separation of cell cycle phase G 0 /G 1 M 2 N 4 N DNA content

The effect of oxygen at various phases of the cell cycle The ratio of hypoxic to aerated doses needed to achieve the same biological effect is called OER. Survival Oxygen enhancement ratio (OER) ch 6 OER S (2. 3 -2. 4) < G 1 & G 2 OER varies significantly through the cycle. hypoxic aerated Dose (Gy)

The age-response function for a tissue in vivo Variation of sensitivity with cell age for neutrons CPM = count per min 1. The variation of thymidine uptake with time. 2. First wave = DNA synthesis period 3. Correlated to max resistance to x-ray

The in vivo pattern of response as a function of cell age is similar to g-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 Dose at Asynchronous cell population S phase remained synchronize cell cycle move Dose later at sensitive phase sensitizing called Sensitization due to reassortment