IMMUNOCYTOCHEMICAL STUDIES OF CHICKEN SOMATOTROPHS AND SOMATOTROPH GRANULES BEFORE AND AFTER HATCHING

Immunocytochemical methods were used to gain information about the emb ryonic development of chicken somatotrophs before and after hatching. To localize growth hormone, anterior pituitary sections were incubated with growth-hormone antibody, and then an indirect peroxidase method was used for light microscopy and an immunogold method for electron mi croscopy. The earliest evidence of embryonic somatotrophs was seen at 12 days. At this stage somatotrophs were sparse (0.2% of parenchymal c ells) and their granules were pleomorphic with elongated ovoid and loz enge shapes predominating. Few of the immunogold-labeled somatotroph g ranules of the embryo were spherical until 15 days after fertilization . At 18 days, most of the granules were spherical (their shape in the adult chicken). During the six days between the 15-day-old embryo and the 1-day-old chick, the number of gold particles per granule section approximately doubled suggesting an increase in growth hormone content of the granules. This rise was the result of increases in the size of the granule sections and in the concentration of gold particles in th e sections. During the embryonic period of 12-20 days, somatotrophs we re not more than 3.6% of the anterior pituitary cell population. Durin g the following two days, between the 20-day-old embryo and the 1-day- old chick, the percentage of somatotrophs in the pituitary parenchymal cell population rose rapidly from 3.6% to 20.7% and then increased sl owly to 24.6% during the period of 1-5 days after hatching. Both the s harp percentage rise in somatotrophs (20-day-old embryo to 1-day-old c hick) and the rise in growth hormone content of the granules (15-day-o ld embryo to 1-day-old chick) suggested by gold-particle counts occur close to the time of hatching. These morphological changes may reflect an increased synthesis of growth hormone that is responsible for the rise in plasma growth-hormone concentration that begins about the same time and is especially abrupt two days later (1-3 days after hatching ).