136. Healy, J.M. and Jamieson, B.G.M. 1994f.
The
ultrastructure of spermatogenesis and epididymal spermatozoa of the
tuatara Sphenodon
punctatus (Sphenodontida, Amniota). Philosophical Transactions
of the
Royal Society London, B 344, 187-199.
Using transmission electron microscopy (TEM)
the events
ofspermatogenesis are described for the first time in the tuatara Sphenodon
punctatus punctatus(Gray), a representative of the 'reptilian'
order
Sphenodontida. Secondary spermatocytes contain two greatly elongate
(8.0 um), rod-shaped centrioles which lie parallel
to one another and are each associated with a small deposit of dense
material
and a short centriole. Spermatids contain only one rod-shaped centriole
(associated with a short centriole) which gives rise to the flagellar
axoneme
thereby becoming the distal centriole. Four stages of spermatid
development can
be distinguished: (i) the early stage (nucleus round; nuclear contents
granular
with a thin, condensed periphery; mitochondria scattered; acrosomal
vesicle
spheroidal, slightly depressed onto nuclear surface); (ii) the middle
stage
(nucleus pyriform with two endonuclear canals are formed; nuclear
contents
fibro-granular with thick periphery: mitochondria chiefly posterior;
acrosomal
vesicle flattened; centriolar complex attached to nucleus); (iii) the
advanced
stage (nucleus elongate and rod shaped; nuclear contents coarsely
granular;
mitochondria (containing linear cristae) clustered around the distal
centriole;
acrosomal vesicle conical; centriolar complex attached to posterior
fossa of
nucleus); (iv) the late stage (nucleus very elongate and associated
with a
longitudinally arranged microtubular sheath; nuclear contents very
condensed
midpiece fully formed and featuring mitochondria with concentric
cristae and a
dense intramitochondrial body; centrioles associated with dense,
lateral body).
Testicular sperm have a conical acrosomal vesicle (length 4 um)
and subacrosomal cone, an elongate
(length 54-56 um) helical
nucleus, a midpiece (length 8 um, featuring spheroidal mitochondria
containing
concentric cristae and a dense body), an annulus, an elongate principal
piece
(length 74-78 um, featuring a
dense, fibrous sheath) and a short end piece (length 2-4 um.
Epididymal sperm differ from those in the
testis by having a more developed lateral body in the midpiece and a
sheath of
flocculent material surrounding the fibrous sheath in the principal
piece. The
relatively large number of epididymal sperm still associated with a
cytoplasmic
droplet suggests that sperm spend a significant period maturing within
the
epididymis. The features of spermatogenesis and mature sperm suggest
that the
Sphenodontida are primitive amniotes, with only chelonians having fewer
spermatozoal apomorphies while the crocodilians are little more
advanced.