Biology Reference
In-Depth Information
3.2 Centrosomal Immaturity in Cat Testicular Spermatozoa
Spermatozoa extracted from the seminiferous tubules of testicular tissue as well as
epididymal and ejaculated cat spermatozoa have a similar-shaped head region that
contains highly compacted chromatin with a low incidence (\5 %) of DNA
damage (Comizzoli et al.
2006
). Labeling of the long midpieces with Mitotracker
Green reveals that mitochondria occupy most of the area between the head and the
flagellum (Fig.
3.1
a). The sperm centrosome is easily detectable by the presence of
centrin, a common centrosomal protein, in both centrioles located between the
head and the midpiece. Centrin appears equally abundant in testicular (Fig.
3.1
b)
as well as in ejaculated (Fig.
3.1
c) spermatozoa.
The first role of the sperm centrosome after fertilization is to organize the
formation of microtubules into a sperm aster that enables both maternal and
paternal pronuclei to migrate and undergo syngamy (Schatten and Sun
2011
). The
formation of a large sperm aster occurs about 5 h after sperm penetration into
oocytes in vitro (Comizzoli et al.
2006
; Jin et al.
2011
; Xu et al.
2011
).
As previously observed in the bovine system (Navara et al.
1996
), cat sperm aster
morphology is highly reflective of developmental potential. Specifically, pronu-
clear migration is accelerated in the presence of a larger size sperm aster that, in
turn, promotes the first cleavage division (no later than 26 h post-penetration) that
eventually encourages embryonic development to the blastocyst stage (Table
3.1
;
Comizzoli et al.
2006
). More specifically, we measured a 50 % increase in
advanced embryo formation in the presence of a large diameter sperm aster
(Comizzoli et al.
2006
). The source of spermatozoa influences the capacity to
develop asters of varying size. For example, a primary reason that testicular
spermatozoa fail to fertilize or experience delayed first cleavage and compromised
embryo development is due to inability to produce an aster or one of normal, large
size (Table
3.1
; Comizzoli et al.
2006
). Additionally, we have routinely observed
that about 25 % of all motile cat spermatozoa from ejaculates or the epididymis
consistently produced small asters. Given the clear immaturity of centrosomes in
cat testicular spermatozoa, it is expected that earlier sperm stages also contain
immature centrosomes, as has been observed in the rabbit (Tachibana et al.
2009
).
Based on studies in the porcine system, it is known that nucleation activity of
the sperm centrosome influences microtubule length by attracting c-tubulin
(Sun et al.
2001
). But this mechanism is complex and not well understood. Dys-
functional centrosomes in human and non-human primate testicular or ejaculated
spermatozoa lead to blocked pronuclear stage formation post insemination
(Hewitson et al.
1996
; Palazzo et al.
2000
; Nakamura et al.
2001
). By contrast,
centrosomal dysfunctions in cat testicular spermatozoa do not result in complete
impediments as illustrated by some minimal embryo development (Comizzoli
et al.
2006
). For the cat, this condition appears to be more of a centrosomal
immaturity (with poor nucleation capacity) rather than a true dysfunction.
According to previous studies, centrosomal maturation has been defined as the
change in microtubule nucleation potential occurring as cells generally pass
