Reproductive Cycles of Males and Females
139
C. mydas (Owens, 1980). The bulk of their volume is from seminiferous tubules.
Within the seminiferous tubules is a population of epithelial cells, including a slowly
dividing population of stem cells. In postpubescent males the epididymis (Figure
5.1) is pendulous and distinctly enlarged (Limpus and Reed, 1985). It is a convoluted
duct extending from the ductuli efferentes, draining the testicular lobules to the
ductus deferens, which conducts spermatozoa to the urethra. Urethral tissue is the
site of spermatozoan accumulation and storage prior to ejaculation. The penis is an
intromittent organ, > 30 cm in length in C. mydas , and the hook at the end of the
penis, adjacent to the sperm duct, presumably assists in intromission and sperm
transfer (FitzSimmons, 1997; Miller, 1997).
Spermatozoa are neither motile nor capable of fertilizing ova until they have
passed through the epididymis and undergo final maturation. The ultrastructure of
spermatozoa has not been formally described in sea turtles; however, in a phylogenetic study using cladistic analysis, Jamieson and Healy (1992) found that turtles
from a range of Cryptodire and Pleurodire genera formed a single primitive clade.
Freshwater species of Cryptodire and Pleurodire turtles have spermatozoa that are
50–55 m m long and 0.9 m m wide with conspicuous spheroidal mitochondria in
the midpiece (Hess et al ., 1991; Healy and Jamieson, 1992). Several structures of
Chrysemys picta spermatozoa are unique from those seen in mammals and other
reptiles (Hess et al ., 1991). The head is curved and pointed, 11–12 m m long by
0.9 m m wide, and contains a nucleus contiguous with intranuclear tubules. The
middle section consists of proximal and distal centrioles surrounded by mitochondria. These mitochondria are speculated to maintain longevity of the sperm while
in the oviduct (Hess et al ., 1991). Sea turtle oviducts are very long (see below),
and sperm competition may occur in some females (Owens, 1980; FitzSimmons,
1998). Thus, assessing whether these unique spermatozoa structures exist in sea
turtles and developing an understanding of their function may provide a basis for
gaining further insight into the movement of spermatozoa through the oviduct,
potential longevity of turtle spermatozoa, and storage of spermatozoa within the
oviduct.
5.4.2 S PERMATOGENESIS
At puberty, the testes begin to secrete greatly increased amounts of the steroid
hormone testosterone. This hormone has a multitude of effects including stimulation
of secondary sex characteristics (such as tail elongation and softening of the plastron), the maturation of seminiferous tubules, and in adult turtles, the commencement
of spermatogenesis (Wibbels et al ., 1991; 1990; Licht et al ., 1985). During spermatogenesis, testosterone influences Sertoli cells, which differentiate into seminiferous tubules. Previously dormant primordial germ cells divide by mitosis and
differentiate into spermatogonia, eventually becoming primary spermatocytes and
migrating to the lumen of the seminiferous tubule. Primary spermatocytes then
undergo two meiotic divisions, developing first into secondary spermatocytes and
eventually into spermatids. The spermatogenic cycle for sea turtles was first
described by Wibbels et al. (1990) and has been reviewed by Owens (1997); we will
not reiterate the same points here.
1123 book.book Page 139 Thursday, November 14, 2002 11:50 AM
139
C. mydas (Owens, 1980). The bulk of their volume is from seminiferous tubules.
Within the seminiferous tubules is a population of epithelial cells, including a slowly
dividing population of stem cells. In postpubescent males the epididymis (Figure
5.1) is pendulous and distinctly enlarged (Limpus and Reed, 1985). It is a convoluted
duct extending from the ductuli efferentes, draining the testicular lobules to the
ductus deferens, which conducts spermatozoa to the urethra. Urethral tissue is the
site of spermatozoan accumulation and storage prior to ejaculation. The penis is an
intromittent organ, > 30 cm in length in C. mydas , and the hook at the end of the
penis, adjacent to the sperm duct, presumably assists in intromission and sperm
transfer (FitzSimmons, 1997; Miller, 1997).
Spermatozoa are neither motile nor capable of fertilizing ova until they have
passed through the epididymis and undergo final maturation. The ultrastructure of
spermatozoa has not been formally described in sea turtles; however, in a phylogenetic study using cladistic analysis, Jamieson and Healy (1992) found that turtles
from a range of Cryptodire and Pleurodire genera formed a single primitive clade.
Freshwater species of Cryptodire and Pleurodire turtles have spermatozoa that are
50–55 m m long and 0.9 m m wide with conspicuous spheroidal mitochondria in
the midpiece (Hess et al ., 1991; Healy and Jamieson, 1992). Several structures of
Chrysemys picta spermatozoa are unique from those seen in mammals and other
reptiles (Hess et al ., 1991). The head is curved and pointed, 11–12 m m long by
0.9 m m wide, and contains a nucleus contiguous with intranuclear tubules. The
middle section consists of proximal and distal centrioles surrounded by mitochondria. These mitochondria are speculated to maintain longevity of the sperm while
in the oviduct (Hess et al ., 1991). Sea turtle oviducts are very long (see below),
and sperm competition may occur in some females (Owens, 1980; FitzSimmons,
1998). Thus, assessing whether these unique spermatozoa structures exist in sea
turtles and developing an understanding of their function may provide a basis for
gaining further insight into the movement of spermatozoa through the oviduct,
potential longevity of turtle spermatozoa, and storage of spermatozoa within the
oviduct.
5.4.2 S PERMATOGENESIS
At puberty, the testes begin to secrete greatly increased amounts of the steroid
hormone testosterone. This hormone has a multitude of effects including stimulation
of secondary sex characteristics (such as tail elongation and softening of the plastron), the maturation of seminiferous tubules, and in adult turtles, the commencement
of spermatogenesis (Wibbels et al ., 1991; 1990; Licht et al ., 1985). During spermatogenesis, testosterone influences Sertoli cells, which differentiate into seminiferous tubules. Previously dormant primordial germ cells divide by mitosis and
differentiate into spermatogonia, eventually becoming primary spermatocytes and
migrating to the lumen of the seminiferous tubule. Primary spermatocytes then
undergo two meiotic divisions, developing first into secondary spermatocytes and
eventually into spermatids. The spermatogenic cycle for sea turtles was first
described by Wibbels et al. (1990) and has been reviewed by Owens (1997); we will
not reiterate the same points here.
1123 book.book Page 139 Thursday, November 14, 2002 11:50 AM