Page 323 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

290 SECTION | II Organ Toxicity

VetBooks.ir leukocytosis, mucosal secretion and myometrial tone; vagina, cervix or uterine body of the female reproductive
tract, depending on the species. Spermatozoa can be lost
altered tissue electrical conductivity; and initiation of the
from the female reproductive tract by retrograde loss and
growth of endometrial and mammary glands (Senger,
2003). Like estrogens, progesterone also has several phagocytosis by leukocytes (Senger, 2003). Contractions
effects on the reproductive tract of the female, but the of the smooth muscle within the tubular genitalia (muscu-
effects of progesterone generally oppose those of estro- laris), as well as interactions involving components of the
gens, favoring pregnancy maintenance and sexual non- seminal fluid and luminal secretions of the female repro-
receptivity over ovulation and appropriately timed sexual ductive tract, facilitate the transport of sperm to the ovi-
receptivity associated with estrogenic stimulation ducts (uterine tubes) where, depending on the species,
(Ginther, 1992; Senger, 2003; Evans et al., 2007). fertilization takes place in the ampulla or at the junction
Progesterone is generally associated with negative feed- of the ampulla and the isthmus (ampullary isthmic junc-
back to the hypothalamus and anterior pituitary gland tion) (Senger, 2003; Genuth, 2004b). While sperm can be
which limits GnRH and gonadotropin secretion (Senger, rapidly transported to the ampullary isthmic junction or
2003; Evans et al., 2007; Evans and Ganjam, 2017). ampullae of the oviducts within minutes of natural or
Sexual receptivity and myometrial contractility and tone artificial semination, the relatively slow, sustained trans-
are diminished in an endocrine environment dominated port of motile sperm from reservoirs of spermatozoa in
by progesterone, while mammary and endometrial gland the cervix and uterotubal junctions is the primary mecha-
development and secretion are promoted (Senger, 2003). nism by which the viable sperm that can participate in fer-
Toxicants which disrupt the communication and coordina- tilization actually enter the oviducts (Senger, 2003).
tion between the ovary and the other parts of the repro- Xenobiotics which interfere with the endocrine milieu
ductive tract (e.g., xenoestrogens, xenoandrogens and required for appropriate muscularis contractility and the
antiestrogens) will alter the appearance and function of cervical and uterine mucosal secretions which facilitate
the reproductive organs and can interfere with survival of sperm transport (e.g., phytoestrogens) can prevent sper-
the oocyte, embryo and/or fetus. matozoa from getting to the site of fertilization in a timely
manner.
Oocyte/Sperm Transport, Normal
Capacitation of Sperm and Fertilization Capacitation of Spermatozoa
Spermatozoa can generally survive in the oviducts for
Transport of the Ovulated Oocyte several days following insemination. Ejaculated sperm are
The primary reproductive organs involved in the transport not competent to either bind to the zona pellucida or to
of ovulated secondary oocytes (primary oocytes in the undergo the acrosomal (acrosome) reaction, both of which
bitch) are the oviducts or uterine tubes. Each oviduct con- are required for fertilization of ova by mature spermato-
sists of an infundibulum, isthmus and ampulla, which zoa. Sperm must be capacitated in order to interact with
have some distinct differences in structure, as well as the ovum. The capacitation process involves calcium
function (Evans et al., 2007; Evans and Ganjam, 2017). influx and biochemical changes to the sperm plasma
The ovulated ovum enters the funnel-like opening to the membrane which result in the “removal” or modification
infundibulum and is transported through the isthmus to of epididymal and seminal plasma proteins and the expo-
the ampulla or ampullary isthmic junction for fertiliza- sure of the surface molecules required for spermatozoal
tion. Unlike spermatozoa which can generally survive for binding to the zona pellucida of the ovulated secondary
several days in the oviduct, secondary oocytes usually, oocyte (Senger, 2003; Genuth, 2004b). Depending on the
depending on the species, are viable for 12 24 h species and, to some extent, the site of their deposition,
(Genuth, 2004b). The appropriate endocrine environment spermatozoa become capacitated within the cervix, uterus
is required for adequate oviductal entry and transport of and/or the oviduct (Senger, 2003).
ovulated oocytes to the site of fertilization. Delayed trans-
port of oocytes within the uterine tubes can result in the Fertilization
death of ova before contact can be made with fertile
Fertilization of secondary oocytes by capacitated sperm is a
spermatozoa.
complex process involving a cascade of events which pre-
vents fertilization of an ovum by more than one sperm
Transport and Capacitation of Spermatozoa
(polyspermy) and ends in the fusion of the male and female
Transport of Spermatozoa pronuclei (syngamy) (Senger, 2003; Evans and Ganjam,
During mammalian copulation, mature sperm stored in 2017). In the oviductal ampulla or at the ampullary isthmic
the caudae epididymidies travel through the vas deferens junction, the motility of capacitated sperm becomes
and penile urethra to be ejaculated into the anterior hyperactive, facilitating the precise sequence of events