The inheritance of growth and form in the mouse. IV. Changes in the variance components of weight, tail length and tail width during growth.

A complete diallel cross, including inbreds and reciprocals, was made among six inbred lines of mice. Body weight, tail length and tail width were measured at ages of 1 through 12 weeks. The analysis described by Griffing (1956a, 1956b, 1958) as the modified diallel, method 3, model II was made for each trait at each age, a separate analysis being made for each sex. Inbreds did not contribute to estimates of the effects in the diallel model, but were used to estimate heterosis. Positive heterosis was observed for all three traits. Heritability increased with age for all three traits, although it remained small for tail width; for body weight it was larger in females than for males, while for tail length and width the opposite was true. Non-additive genetic variance was observed for all three traits. Maternal effects variance was virtually non-existent for tail length, but for body weight and tail width exhibited a marked peak around weaning, followed by a gradual decline for body weight and a rapid decline for tail width. Environmental variance exhibited a marked peak at weaning for all three traits and was larger for male body weight and tail length from four weeks onward. Residual reciprocal effects were important for tail length at all ages, but were small or negligible for body weight and tail width. It is concluded that the relative importance of the individual's genotype in determining size increases with age, while that of non-genetic factors declines.

The inheritance of growth and form in the mouse. V. Allometric growth.

A complete diallel cross, including inbreds and reciprocals, was made among six inbred lines of mice. Body weight, tail length and tail width were measured at ages of 1 through 12 weeks. The parameters of 4 longitudinal allometric relations, y = bxa, were determined for each mouse: tail length vs. body weight for weight for weeks 1 through 9 and for weeks 1 through 12, and tail width vs. body weight for the same periods. The estimated value of a was taken as one variable and the value of log y corresponding to a weight of 12 g or 15 g for 9 or 12 pairs of measurements was taken as the other. The allometric equation appears to provide a satisfactory empirical description of the relative growth of tail length and width vs. body weight. The diallel analysis suggests a low heritability for a for both sexes and both periods and an intermediate heritability for the location parameters.

The fine structure of the melanocytes of the adult mouse hair follicle during their amelanotic phase (telogen and early anagen).

Melanocyte-precursor cells have been described previously in telogen and early anagen hair germs by observation of Epon sections using light microscopy. The existence of these precursor cells has not been generally accepted due to lack of ultrastructural evidence. It is our purpose to describe these cells using transmission electron microscopy. We studied hair germs of the dorsal skin of adult mice in the appropriate stages of the hair cycle prior to melanogenesis. Telogen melanocytes are distinguishable from adjacent keratocytes by their lack of desmosomes, lack of coarse clumps of microfilaments, sparsity of ribosomes, unusually few nuclear pores and presence of dendrites. Melanocytes in early anagen show extensive growth with abundant polysomes, elaborate RER and Golgi zones. They still differ from keratocytes in their lack of desmosomes and clumps of microfilament, and also in the occasional presence of premelanosomes.

The incorporation of tritiated uridine in hair germ and dermal papilla during dormancy (telogen) and activation (early anagen).

During dormancy there is very little incorporation of [3H]uridine in cells of hair germ and dermal papilla. During activation (both spontaneous and experimental) there is a marked increase in this parameter, expecially in the cells of the germ. After experimental activation by plucking, the increase in label in the germ is evident by 6 hr, reaching an initial maximum by 24 hr. The initial increase in [3H]uridine incorporation after activation precedes the increase in [3H]thymidine incorporation found previously, by about 6 hr. No evidence was found that the increase in [3H]uridine of papilla cells precedes the increase in [3H]uridine of germ cells. This finding militates against the possibility that the dermal papilla is triggered into an inductive role (vis-á-vis the germ cells) by means of circulating steroid hormones.

Wound healing in X-irradiated mammalian skin.

Certain parameters of wound healing were investigated in mouse skin given 950 rads and 3325 rads of X-irradiation at various times relative to wounding. Increased inflammation, delayed dermal regeneration, and delayed contraction were noted in all irradiated groups. The activation of surrounding hair follicles, a process that usually accompanies wounding in mouse skin, occurred earlier, over a shorter elapsed time, and over a greater area of skin in animals irradiated prior to wounding than in the controls or in those irradiated and wounded simultaneously. Epidermal mitotic activity in wounds made at the time of irradiation was initially depressed but recovered by the second postoperative day. Wounds in pre-irradiated animals gave an unexpected result. They responded with an immediate burst of mitotic activity without the usual 24-hr lag that was seen in controls. In the pre-irradiated specimens a substantial number of cells appeared to die after dividing.