Classification of Germ Cells, Reproductive Cycle and Maturation of Gonads in Haliotis asinina Linnaeus

Abstract

Germ cells in the gonads of Haliotis asinina, a species of abalone found along the coast of Thailand, were classified basing on light and electron microscopies. Germ cells in oogenetic units could be classified into six stages according to their histological and ultrastructural characteristics: oogonium and five stages of oocytes, ie, Oc1 with light to intense basophilia and abundant polyribosomes, with some in large aggregates; Oc2 with intense basophilia, oil droplets, numerous well developed Golgi complexes and rough endoplasmic reticulum, but little secretory granules; Oc3 with a few yolk granules and 2 types of cortical granules; Oc4 with increasing number of yolk granules, numerous cortical granules and thin jelly coat; and Oc5 is the mature ovum with 2 types of yolk granules, numerous cortical granules and fully formed jelly coat. The cells in spermatogenetic units could be classified according to the pattern of chromatin condensation into thirteen stages: spermatogonium, five stages of primary spermatocytes, secondary spermatocyte, four stages of spermatids and two stages of spermatozoa. The gonads of adult H asinina reared in land-based culture system exhibit five phases of reproductive cycle during the year: these are proliferative, premature, mature, spawning and spent phases. Gonads in proliferative and premature phases contain primarily gonial cells, early oocytes1-3 and spermatocytes, while mature phase contains mainly late stage cells, ie, oocytes4-5 in ovary and spermatids and spermatozoa in testis. The spawning phase occurs at least twice during each year: from March to April and August to October in females, and with similar intervals but slightly prolonged duration in males. Spent phase, occurring after the period of spawning, is characterized by a complete discharge of gamete cells and the breakdown of connective tissue stroma. It takes approximately 5 to 6 months for gonads to regenerate their connective tissue stroma and germ cell population, and finally become repleted with mature cells again. In developing H asinina definitive gonads appear to be clearly separated from hepatopancreas at 2 months. Histologically, gonial cells appear at 2 months, early spermatocytes and spermatids at 4 months; early oocytes (Oc1-2) at 6 to 7 months. While completely mature spermatozoa could arise in the gonads as early as 6 to 7 months, mature oocytes (Oc4-5) occur much later at 10 to 11 months. The male animals tend to reach full sexual maturity and start normal reproductive cycle as early as 7 to 8 months, while female animals reach sexual maturity and start reproductive cycle around 11 to 12 months.