The structure of male reproductive organs in Lake Magadi Tilapia (Alcolapia grahami)
Alcolapia grahami (formerly Oreochromis alcalicus graham i) is a small cichlid fish inhabiting the highly alkaline (PH~ 10) and hot (temp 25-42ﾰC) lagoons of Lake. Magadi, Kenya. Despite the growing number of studies that are being conducted on this fish, there remains paucity of information on their reproductive structure, associated adaptations and functions. In this study, the morphology of the mature male reproductive organs was studied using macroscopic, histological and ultrastructural techniques. The aim was to establish the structural organization of the reproductive organs in this fish and their adaptive features, if any, in relation to their survival in the severe Lake Magadi habitat. Twenty four mature fish specimens, obtained using seine nets from the Fish Springs Lagoon of Lake Magadi, were sexed, weighed, dissected and testes fixed and processed for routine histological and transmission electron microscopy. The male reproductive system comprised creamy white bilobed testis, oriented craniocaudally within the coelomic cavity and caudally connected to the spermatic duct which exited through the genital papilla. The coelomic cavity was lined by a parietal peritoneum containing a thin pigmented membrane laden with a highly opaque substance on the medial aspect, thought to protect the testis against UV radiation. The testicular parenchyma was bound by a capsule and comprised the germinal and interstitial compartments. The two compartments were separated by a lamina propria formed primarily by myoid cells. The germinal compartment comprised seminiferous lobules ending blindly at the testicular capsule. Several spermatocysts containing germ cells at the same stage of development occupied each seminiferous lobule. The spermatids underwent three developmental stages exhibiting type-I spermiogenesis and culminating in the formation of typical primitive spermatozoa (aquasperm). At the end of spermatogenesis, mature cysts opened to release spermatozoa into the lobule lumen. The organization of the germinal compartment, therefore, presented an unrestricted lobular testicular type exhibiting complete cystic spermatogenesis. Sertoli cells rested on the basal lamina, appearing generally flat and elongated although a few appeared cuboidal in shape. They contained numerous membrane bound vesicles, phagolysosomes and cellular debris; structural characteristics that seemed to suggest their involvement in resorption and phagocytosis of damaged cells and residual bodies during spermiogenesis. They produced cytoplasmic extensions that formed the walls of cysts found in the lobular testes. The interstitial compartment contained Leydig cells, fibroblasts and blood vessels. Leydig cells were spheroidal in shape with abundant lipid droplets, smooth endoplasmic reticulum and mitochondria in their cytoplasm, indicating a role in testicular steroidogenesis. Fibroblasts occurred both in the interstitial and boundary tissues, and were considered to be the precursor of some of the connective tissue components. The interstitium between the efferent ducts in some fish exhibited nodular structures containing spheroidal cellular aggregates thought to arise following stressful conditions in the lake. The excurrent duct system consisted of collecting, efferent as well as main spermatic ducts intratesticularly while the common spermatic duct and genital papilla were located extratesticularly. Spawning appeared to initiate increased folding and apocrine secretion by the epithelium of the common spermatic duct leading to increased luminal surface area and seminal fluid production. These findings show that this fish exhibits regular spermatogenesis irrespective of the harsh environmental conditions in which they live. The role of other adaptive mechanisms employed by this fish in ensuring normal body functions including spermatogenesis in the face of the extreme environmental conditions in the lake cannot be overruled.