Centrosomes of human gametes need to be characterised in more detail, since
it was recently shown in 1991 that the human embryo inherits the dominant
paternal centrosome at fertilization. Thus there has been a resurgence of c
entrosomal research in the last decade. The sperm centrosome, containing a
single centriole, is preserved and dominant while the egg centrosome is red
uced and inactivated during oogenesis, since there should be only one funct
ional centrosome to ensure normal development.
This presentation reviews the ultrastructure of gamete centrosomes and atte
mpts to define their respective roles in assisted fertilization. Human test
icular and ovarian tissues, sperm, eggs and zygotes were examined by routin
e TEM.
Developing sperm cells and oogonia have two functional centrioles (diplosom
es) in their centrosomes, showing the typical "9+0" organisation of microtu
bule triplets - common to somatic cells. The diplosomes are surrounded by p
ericentriolar material (PCM), which nucleate microtubules (MT) that organiz
e the cytoskeleton and mitotic spindles. During spermiogenesis, when the sp
ermatids transform into mature sperm, there is a partial reduction of the m
ale centrosome, during which the proximal centriole (PC) is retained intact
in the sperm neck, proximal to the nucleus, while the distal centriole (DC
) which gave rise to the sperm flagellum, is partially reduced and merges d
istally with the sperm axoneme in the midpiece and tail. The proximal funct
ional centriole is located in a "black box" in the neck, composed of the ca
pitulum beneath the basal plate and flanked laterally by 9 segmented column
s and shows the typical "9+0" organisation of triplets of MT. The PC contai
ns dense material both within and outside the triplets, and the central dou
blet of axonemal MT terminates in a clump of dense material below its lower
vault. The DC is disorganised proximally showing loss of triplets and cann
ot function as a typical centriole, since the central doublets of axonemal
MT traverse through it.
Oogonia present a pair of well-defined centrioles, which are involved in ce
ll division. These are lost during oogenesis and the mature oocyte is devoi
d of centrioles, as in most mammals. Neither does the human oocyte have gra
nular centrosomal material at meiotic spindle poles, in contrast to mouse o
ocytes which have a dominant maternal centrosome. Thus the oocyte centrosom
e is greatly reduced and inactivated. Functional centrosomal structure is,
however, restored after fertilization in the zygote with some maternal inpu
t around the sperm centriole, which duplicates at the pronuclear stage, for
ms a sperm aster and proceeds to form the first mitotic spindle. This is th
e ancestor of centrosomes in embryonic, foetal and adult somatic cells.
In 1991 (Sathananthan, 1991), we postulated that sperm centrosomal dysfunct
ion could lead to aberrant embryonic development based on centriolar defect
s in sperm with impaired motility. This hypothesis has now gained acceptanc
e and further evidence to support this theory of infertility is presented.