Scanning and transmission electron microscopic observations were made on the anterior and posterior lateral line system of larvae of the blind cavefish, Phreatichthys andruzzii. The investigations were conducted on 36 specimens belonging to three different ages of juvenile: 7, 14 and 28 days post hatching. The basic sensory unit of the mechanoreceptive system is a superficial “neuromast”, which is composed of sensory hair cells (HCs) embedded in non-sensory support cells (SCs) frequently covered with a cupula. Head neuromasts had the longest cupulae. These organs presented the same structure and ultrastructure in the head and along the body of all specimens of P. andruzzii of different ages. Hair cells of neuromasts of P. andruzzii larvae were pear-shaped cells approximately 10-20 µm high and 3-5 µm in diameter. The nucleus of each hair cell was large and basally placed, the cytoplasm contained numerous mitochondria and each HC had a single long kinocilium and several short stereocilia. The majority of SCs were elongated in shape and their nuclei occupied a significant portion of the cytoplasm. The support cells at the margin of the neuromast were particularly thin. Both types of support cells possessed well developed rough endoplasmic reticulum and Golgi apparatus. The size and number of neuromasts and their component cells increased with the age of larvae (ANOVA, P< 0.05). The dimensions of the neuromasts of the anterior lateral line (head) did not differ significantly from those of the posterior lateral line (trunk) (ANOVA, P > 0.05).
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