Primordial germ cells (PGCs) constitute an embryonic cell type that migrate to gonadal precursors and form the gametes. In many animals, PGCs are set apart from somatic cells early during embryogenesis. These cells migrate to gonadal precursors and then constitute gonads so they are useful models for cell motility studies. They have a highlighted importance for development and reproduction studies. Primordial germ cells have morphological differences from the somatic cells. Structure of these cells can be detected with light and electron microscopy in early development stages. This review describes the morphological, histological, molecular and ultrastructural features of primordial germ cells in different animals and gives an overview for simplified identification.

Plant growth regulators are the chemicals that are found in plants and produced synthetically. In agricultural applications, plant hormones are used in minor quantities for fixing the problems. In our research, we studied the effects of 2,4 diclorophenoxyacetic acid (2,4D), which is an auxin used in agricultural applications. Auxins are the group that are used most popularly in plant growth regulators. In our study, different doses of 2,4 diclorophenoxyacetic acid are given to zebrafish, and ovarium tissues are observed histomorphologically. We generated one control and three experiment groups from the stock solution. The experiment was carried out in 20 liter capacity complete glass aquarium at 24±1°C water temperature. After five days of application, fishes were dissected. Histomorphological changes of the ovarium were investigated under a light microscope. A decrease in the number of oocytes in zebrafish ovarium was observed when compared with the control group. Many deformed and underdeveloped oocytes were detected. An increase in the number of atretic oocytes was observed. It was deduced that acute doses of 2,4 diclorophenoxyacetic acid decelerates oogenesis in fishes.

The use of pesticides has been increasing day by day in order to increase productivity in agriculture. Pesticides which used in the agricultural area mix into the aquatic ecosystem through rains and groundwater etc. and it threatens the life of aquatic organisms. Nowadays, one of the most used pesticide group is neonicotinoids. Imidacloprid is the most well known in this group. In this study, it was aimed to observe the histopathological effects of imidacloprid in zebrafish intestinal tissue. Adult zebrafish individuals were administered imidacloprid in concentrations of 9.5, 19 and 38 mg/L for 5 days. When tissues are examined under light microscopy, deterioration of villi morphology, degeneration in brush border structure of epithelial cells, hyperplasia in Goblet cells and enterocytes due to inflammation, enlargement and disintegration in lamina propria were detected. In addition, hypertrophy of the smooth muscle cells and thickening in the muscularis externa layer were observed. Dysplasia was detected in the small intestine tissues in samples belonging to the highest concentration group.

Neonicotinoids are the new class of insecticides that are high target specificity to insects. Imidacloprid is a neonicotinoid that is the most widely used insecticide in the world. As a result of its widespread use in agriculture, imidacloprid interferes with the aquatic system and threatens the aquatic environment. In this study, an investigation of the histopathological effects of imidacloprid on zebrafish gonads was aimed. Zebrafish were exposed to 9.5 mg/L, 19 mg/L, and 38 mg/L of imidacloprid for 5 days, considering the 96 h LC₅₀ value. After dissecting the gonadal tissues, routine histological techniques were applied, and the tissues were stained with Periodic Acid-Schiff (PAS), Toluidine Blue (TB), and Haematoxylin and Eosin (H&E). Sections were examined under a light microscope. While normal gonad histology was observed in the control group, histopathological alternations such as degeneration and union in the seminiferous tubules, hypertrophy in spermatogenic and Leydig cells, and interstitial fibrosis were detected in testicular tissue of the experimental groups. In the ovarian tissues of the experimental groups, structural deterioration in oocytes, autolysis, increase in the number of atretic oocytes, vacuolization in cortical alveoli, thickening and curling in the zona radiata, and opening in the perifollicular layers were detected.

Triclosan, produced as a broad-spectrum antibiotic in the early 1960s, is generally used as a preservative in personal care products, fabrics, plastic products such as kitchenware and toys. As a result of the high demand for triclosan, this chemical threatens the aquatic ecosystem by contaminating wastewater sources. Environmental pollutants affect the reproductive potential of fish, one of the most critical aquatic organisms. This study aimed to investigate the histopathological and apoptotic effects of triclosan in zebrafish gonads. Fish were exposed to sublethal concentrations of triclosan for 5 days, and general histological methods were applied. Histological sections were examined under a light microscope after staining with hematoxylin and eosin and toluidine blue. Triclosan exposure caused deterioration in ovarian tissue, such as shrinkage in the ooplasm, accumulation of proteinaceous fluid in the interstitial tissue, morphological changes of oocyte and the zona radiata. In testicular tissue, triclosan exposure caused fusion in seminiferous tubules, hypertrophy in spermatogenic and Leydig cells, edema in seminiferous tubules, and karyorrhexis in spermatogenic cells. The TUNEL assay was used for the determination of apoptotic cells. Brown-colored apoptotic cells were visualized under the light microscope. TUNEL positive cells were observed in all exposure groups. Triclosan administration was found to cause apoptosis in zebrafish gonads. These findings indicate that triclosan potentially affects fish reproduction, and that its judicious disposal is essential for protecting the environment and maintaining the reproductive potential of fish.