The Biology and Culture of Tilapias

scription are taken from noncichlids. All the available information suggests
that this is valid (Dadzie 1969; Hyder 1970a, 1970b; Polder 1971; Von Kraft
and Peters 1963; Hodgkiss and Man 1978; Babiker and Ibrahim 1979; Moreau
1979b).
In males, the testis possesses a stock of undifferentiated gonial cells
(type A) which originates from the primordial germ cells of the embryo
through mitotic divisions. Active spermatogenesis begins with the isolation
of type A spermatogonia, each one surrounded by a few somatic cells,
followed by successive synchronous mitotic divisions of spermatogonia (type
B), while surrounding somatic cells divide to form a continuous layer of
cells, called "Sertoli cells". This process results in numerous cysts of cells
throughout the testis. After an unknown number of spermatogonial mitotic
divisions, meiosis occurs synchronously for each cyst, each spermatogonium
then producing four spermatids which will differentiate into spermatozoa
(spermiogenesis). Cyst evolution occurs along testicular lobules which are
separated from each other by fibroblast cells and interstitial tissue. The latter
is particularly well developed in cichlids in comparison with some other
teleosts. Spermiation occurs when spermatozoa are released from cysts after
separation of the Sertoli cells, first into the lobules and then into the vas
deferens.
In female teleosts, the ovaries contain a stock of undifferentiated oogonia
which seems to be renewed by mitotic divisions throughout life unlike
higher vertebrates. For example, unilateral ovariectomy stimulates oogonial
mitosis in the remnant ovary of Sarotherodon aureus (Dadzie and Hyder
1976). Some oogonia begin a meiotic division but remain at arrested pro-
phase (primary oocytes). Each primary oocyte increases in size and is pro-
gressively surrounded by layers of somatic cells: inner granulosa cells which
form a monolayer directly in contact with the thickening outer oocyte
envelope (the zona radiata, i.e., the future chorion) and an outer theca, made
of several layers of fibroblasts penetrated by capillaries. The theca forms the
external cellular envelope of the ovarian follicle. The morphology of ovarian
follicles in Tilapia and Sarotherodon has been well described by Von Kraft
and Peters (1963).
Oocyte growth can be divided into two main phases: first, the previtel-
logenic phase when size increases (up to 0.6 to 0.9 mm diameter in S.
niloticus) which is considered to be the result of synthesis occurring mainly
within the oocytes (endogenous vitellogenesis) and second, the vitellogenic
phase which results from the rapid accumulation, after minor biochemical
modifications, of "vitellogenin": a lipophosphoprotein which is synthesized
in the liver, released into the blood and incorporated into the oocytes by
micropinocytosis (exogenous vitellogenesis or yolk deposition). At the end
of vitellogenesis, meiosis resumes: a general process called oocyte matura-
tion, characterized by completion of the first meiotic division, with first
polar body emission, which is accompanied by important changes in the
gross morphology of yolk and cytoplasm just before ovulation (expulsion
of mature secondary oocyte from the follicle). Oocyte maturation ends only
after sperm penetration with the second meiotic division and second polar
body emission.