Figure 1.

Zebrafish development stages. Zygote (1k-cell stage) (0–0.75 hpf): the newly fertilized egg until the first cleavage occurs (after 40 min). Cleavage Period (0.75–2.25 hpf): the formation of three regular tiers of blastomeres occurs. Blastula Period (2.25–5.25 hpf): at the 128-cell stage, five blastomere tiers are formed and the cleavage planes are irregular. At the 1-k-cell stage, a yolk syncytial layer can be present, with an irregular form and asynchronous cell cycle. At 4 hpf, the egg has a spherical shape with a flat border between the blastodisc and yolk. At the dome stage, the yolk cells are bulging toward the animal pole, beginning the epiboly formation around the egg. Gastrula Period (5.25–10.33 hpf): between 50% of epiboly to the shaping stage, the blastoderm remains of uniform thickness and the embryonic shiels are visible from the animal pole. Between about 60% and 100% of epiboly continues the shape, becoming more along the animal-vegetal axis. The axis and neural plate, brain, and notochord are rudimentary. At the bud stage, the epiboly is completed and the yolk plug is closed, with the formation of a tail bud prominent. Segmentation (10.33–24 hpf): at the 1–4 somite stage, the first somitic furrow forms. During this stage, an optic primordium, otic placode, otic vesicle, weak muscular contractions, blood islands, otoliths, and midbrain-hindbrain boundary are present. Pharyngula Period (24–48 hpf): At Prim5-High pec, the zebrafish present early pigmentation, heartbeat, early touch reflex, pigmented retina, early motility, and tail pigmentation of the pectoral fins. Hatching Period (48–72 hpf): The organs are completed, and the fin, jaw, and gills develop quickly, with a 7.8 mm total body length at the end of this stage. Juvenile (30–89 days): The adult fins and pigment are present, reaching 14 mm of total body length. Adult stage (90 days–2 years): The fishes are sexually mature and ready for breeding.