Effects of maturation on tissue capillarity in chickens.

Tissue capillarity, diffusion distances and myoglobin concentrations in the anterior latissimus dorsi (ALD), consisting entirely of slow tonic fibers, the posterior latissimus dorsi (PLD), consisting entirely of twitch fibers, and the gastrocnemius (GST), consisting of both twitch and tonic fibers, were determined for the domestic chicken throughout maturation. Increases in muscle mass for the three muscles are due to fiber hypertrophy and are quantified as changes in the average cross-sectional area (FCSA) of the fibers. Capillary densities were highest in the newly hatched chicks: ALD = 3967 +/- 888 caps/mm2, PLD = 8360 +/- 1078 caps/mm2, and GST = 8360 +/- 8360 +/- 1283 caps/mm2; and decreased in proportion to the increase in FCSA, to adult values of ALD = 450 +/- 50, PLD = 120 +/- 20 and GST = 267 +/- 33 caps/mm2. In contrast to mammals, the ratio of the numbers of capillaries to the numbers of muscle fibers appears to be fixed at or shortly after hatch; there is not an increase in the numbers of capillaries to offset fiber hypertrophy. Consistent with the observations on capillary densities, the diffusion distances are shortest in the newly hatched chicks where the maximal diffusion distance is 17.9 +/- 1.06 micron in the ALD, 12.5 +/- 1.08 micron in the PLD and 17.4 +/- 0.93 micron in the GST. The maximal diffusion distance increased in proportion to FCSA; to 42.4 +/- 5.75 micron in the ALD, 88.8 +/- 1.33 micron in the PLD and 64.4 micron +/- 8.59 micron in the GST. In the adult the diffusion distances varied with the fiber type of the muscle. Myoglobin concentrations were similar in the ALD and PLD and were lowest at hatch but increased to plateau values of 4.3 +/- 0.51 mg/g for the slow-tonic ALD and 2.2 +/- 0.33 mg/g for the fast-twitch PLD. In contrast, myoglobin concentrations in the GST were highest in the newly hatched chicks and decreased to a low-level plateau of 1.8 +/- 0.83 mg/g in the adult. The very high myoglobin concentrations in the GST of the newly hatched chicks may represent an adaptation to conditions of extreme hypoxia in the hind limb of the developing embryo.