Page 330 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

312 Veterinary Histology of Domestic Mammals and Birds

the ovary increases to a size of 110 × 70 mm and reaches may occur but only one spermatozoon fuses with the
VetBooks.ir a weight of approximately 60 g. nucleus of the ovum.
In the newly hatched chick, the ovary consists of a
In birds, the female is heterogametic. The oocyte con-
cortex (cortex ovarii) and an inner medulla (medulla tains either a Z or a W chromosome, whereas only the Z
ovarii). The cortex contains the ovarian follicles, compris- chromosome is present in avian spermatozoa. Sex is thus
ing oocytes surrounded by follicular epithelial cells. At determined prior to fertilisation.
sexual maturity, the macroscopic distinction between the Each subsequent ovulation takes place about half an
cortex and medulla is lost. hour after an egg is laid. Not all oocytes enter the infun-
The mature oocyte of birds is the largest female dibulum. At the beginning and end of the laying period,
gamete in the animal kingdom (Figure 14.15). At the some ovulated oocytes pass into the coelomic cavity where
preovulatory stage, the surrounding follicular wall consists they are quickly resorbed. Occasionally these may undergo
of several cell layers. The follicle is connected to the ovary concretion and persist for some time.
by a peduncle containing blood vessels, nerves and smooth Immediately after ovulation, the wall of the follicle
muscle. The richly vascularised and innervated follicle wall regresses. Only an insignificant vestige remains after 6
also contains smooth muscle cells. Positioned meridionally days and this disappears altogether after 1 month. A cor-
in the follicle is a pale and relatively poorly vascularised pus luteum resembling that of mammals is not formed.
region, the stigma. Endoscopic visualisation of ovarian fol- Oestrogens are produced by endocrine cells of the theca,
licles can be used for sex determination in monomorphic while androgens are synthesised by interstitial cells. It is
avian species. unclear whether the postovulatory follicle produces pro-
gesterone. Oestrogens are responsible for stimulating
OOGENESIS production of the yolk by the liver. The yolk is transported
Development and maturation of the avian oocyte begins by the blood stream to the ovarian follicles.
early in embryogenesis. Primordial germ cells migrate
from the embryonic yolk sac to the gonadal area and dif- Uterine tube (tuba uterina)
ferentiate into oogonia. Mitotic division of oogonia gives The uterine tube is a paired tubular organ that extends
rise to primary oocytes. These remain in the diplotene from its opening into the abdomen near the ovary (ostium
state of prophase until shortly before ovulation. abdominale) to the horn of the uterus. The internal lining
During this phase, the oocyte increases in size through of the uterine tube undergoes cyclic changes that ensure
the uptake of large amounts of yolk into its cytoplasm optimal metabolic conditions for the oocyte, spermato-
(vitellogenesis). The gel-like yolk (vitellus) consists of zoa, zygote and early stages of embryonic development
lipids and soluble proteins. (blastomeres). The uterine tubes contribute to:
Yolk formation takes place in three phases of differ-
ent length. During the first phase, which may take several · transport of the gametes,
years, the oocytes undergo a modest increase in size. In the · final maturation of the female oocyte (completion
subsequent phase, the volume of the oocyte grows mark- of meiosis II upon fertilisation) and
edly over a period of 8–10 months (oocyte reaches 4 mm). · final maturation of the spermatozoa (capacitation).
The third phase is associated with a substantial accumula-
tion of yolk, with the oocyte reaching its typical final size After fertilisation in the ampulla tubae uterinae (see below),
of around 40 mm in the chicken and 20 mm in the pigeon. the zygote and developing blastomeres are conveyed by the
This stage takes approximately 14 days. uterine tubes to the uterus. Throughout this passage, the
Completion of the first meiotic division occurs just mucosa of the uterine tube provides nutrition for, and
a few hours before ovulation, resulting in a secondary facilitates forward progression of, the embryo. The uterine
oocyte and production of the first polar body. An addi- tube thus plays a crucial role in the reproductive process.
tional division of this polar body has not been observed The initial portion of the uterine tube (infundibulum
in birds. tubae uterinae) gathers up the oocyte with its coating of
Ovulation occurs under the influence of luteinising zona pellucida and (in some species) follicular cells.
hormone. The secondary oocyte is released by rupture Fertilisation takes place in the subsequent segment,
of the follicular wall along the stigma. Spermatozoa, if the expanded ampulla tubae uterinae (Figures 14.16 to
present, penetrate the oocyte around 15 minutes after 14.18). Folds of varying height and density (longitudinal,
ovulation. This is followed, within the infundibulum of secondary and tertiary folds) increase the metabolically
the oviduct, by the second meiotic division (resulting active surface area of the mucosa, facilitating the provi-
in a mature female gamete and a second polar body) and sion of a suitable environment for the gametes. The oocyte
fertilisation. In contrast to mammals, polyspermy (pen- inserts itself within mucosal niches where it is nourished
etration of the oocyte by more than one spermatozoon) by mucosal secretions (see below) for up to 24 hours.

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