202
The Biology of Sea Turtles, Vol. II
7.2.1 D EVELOPMENT OF K IDNEYS AND G ONADAL R IDGE
Within the abdominal cavity, the pronephros and mesonephros are first indicated in
the anterior epithelium of the intermediary mesoderm as longitudinal ridges located
on either side of the dorsal mesentery (Collins, 1990). In early development the
nephrogenic cord, which is derived from intermediary mesoderm, segments to form
nephrotomes . The functional pronephros in embryonic Chelonia mydas (12–13 mm,
approximately stage 21–22) contains ciliated tubes that open into the coelom (Wiedersheim, 1890b). Large glomerulae develop external to the pronephros in Dermochelys and Lepidochely s (Fraser, 1950). The pronephros develops over a 13-day
period during stages 12–24 in Caretta caretta incubated at 33 ∞ C. The pronephros
and mesonephros are continuous in 8–13 mm embryos of C. mydas (Burland, 1912;
Wiedersheim, 1890b). As each mesonephric vesicle develops and elongates, the
nephrostomal canal degenerates. The distal end of the mesonephric vesicle inserts
into the wolffian duct, and its proximal extension forms Bowman’s capsule (Raynaud
and Pieau, 1985). The kidneys of hatchling marine turtles are dorsoventrally flattened
and lobate, and have many surface convolutions ( C. mydas , DeRyke, 1926; D.
coriacea , Burne, 1905).
The gonadal ridge develops from epithelial, mesothelial, and mesenchymal cells
located between the base of the forming mesonephric tubules and the base of the
dorsal mesentery ventral to the subcardinal veins during stage 17. Blood vessels
invade the genital ridge together with a perforation of mesenchymal cells. The
deposition of collagen fibers and the formation of the basal lamina separate the
epithelial medullary cords from the loose mesenchymal cells of the stroma.
7.2.2 O RIGIN AND M IGRATION OF G ERM C ELLS
In concert with the development of the gonadal ridge, the primordial germ cells
develop in the extraembryonic hypoblast at the edge of the zona pellucida at the
caudal end of the embryo (Allen, 1906; 1907; Jordan, 1917; Risley, 1933; Milaire,
1957). Primordial germ cells are distinguished from surrounding somatic cells
because they are large cells with large, round nuclei (>16 m m) containing distinct
nucleoli and large numbers of lipid droplets, yolk platelets, and glycogen particles
in the cytoplasm; they stain positive with periodic acid-Schiff reagent (Fujimoto
et al., 1979). They migrate by amoeboid action between the cells of the splanchnopleure to beneath the notochord and then enter the mesentery to reach the gonadal
anlagen (Allen, 1906; Risley, 1933; Fujimoto et al., 1979). Primordial germ cells
accumulate at the base of the gonadal ridge as the epithelium extends into the
coelomic cavity over a period of approximately 10 days (Table 7.1).
Merchant-Larios et al. (1989) reported that primordial germ cells in Lepidochelys
were first detected among the endodermal cells of the yolk sac as reported for Caretta
(Fujimoto et al., 1979); this occurred at stages 15 and 16, about the tenth day of
incubation, which was a bit later than reported for Caretta , but the conditions of
incubation were not the same. By stage 18, primordial germ cells were in the hindgut
epithelium and mesentery, and the genital ridges. The primordial germ cells were
four to five times larger than surrounding somatic cells.
1123 book.book Page 202 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
7.2.1 D EVELOPMENT OF K IDNEYS AND G ONADAL R IDGE
Within the abdominal cavity, the pronephros and mesonephros are first indicated in
the anterior epithelium of the intermediary mesoderm as longitudinal ridges located
on either side of the dorsal mesentery (Collins, 1990). In early development the
nephrogenic cord, which is derived from intermediary mesoderm, segments to form
nephrotomes . The functional pronephros in embryonic Chelonia mydas (12–13 mm,
approximately stage 21–22) contains ciliated tubes that open into the coelom (Wiedersheim, 1890b). Large glomerulae develop external to the pronephros in Dermochelys and Lepidochely s (Fraser, 1950). The pronephros develops over a 13-day
period during stages 12–24 in Caretta caretta incubated at 33 ∞ C. The pronephros
and mesonephros are continuous in 8–13 mm embryos of C. mydas (Burland, 1912;
Wiedersheim, 1890b). As each mesonephric vesicle develops and elongates, the
nephrostomal canal degenerates. The distal end of the mesonephric vesicle inserts
into the wolffian duct, and its proximal extension forms Bowman’s capsule (Raynaud
and Pieau, 1985). The kidneys of hatchling marine turtles are dorsoventrally flattened
and lobate, and have many surface convolutions ( C. mydas , DeRyke, 1926; D.
coriacea , Burne, 1905).
The gonadal ridge develops from epithelial, mesothelial, and mesenchymal cells
located between the base of the forming mesonephric tubules and the base of the
dorsal mesentery ventral to the subcardinal veins during stage 17. Blood vessels
invade the genital ridge together with a perforation of mesenchymal cells. The
deposition of collagen fibers and the formation of the basal lamina separate the
epithelial medullary cords from the loose mesenchymal cells of the stroma.
7.2.2 O RIGIN AND M IGRATION OF G ERM C ELLS
In concert with the development of the gonadal ridge, the primordial germ cells
develop in the extraembryonic hypoblast at the edge of the zona pellucida at the
caudal end of the embryo (Allen, 1906; 1907; Jordan, 1917; Risley, 1933; Milaire,
1957). Primordial germ cells are distinguished from surrounding somatic cells
because they are large cells with large, round nuclei (>16 m m) containing distinct
nucleoli and large numbers of lipid droplets, yolk platelets, and glycogen particles
in the cytoplasm; they stain positive with periodic acid-Schiff reagent (Fujimoto
et al., 1979). They migrate by amoeboid action between the cells of the splanchnopleure to beneath the notochord and then enter the mesentery to reach the gonadal
anlagen (Allen, 1906; Risley, 1933; Fujimoto et al., 1979). Primordial germ cells
accumulate at the base of the gonadal ridge as the epithelium extends into the
coelomic cavity over a period of approximately 10 days (Table 7.1).
Merchant-Larios et al. (1989) reported that primordial germ cells in Lepidochelys
were first detected among the endodermal cells of the yolk sac as reported for Caretta
(Fujimoto et al., 1979); this occurred at stages 15 and 16, about the tenth day of
incubation, which was a bit later than reported for Caretta , but the conditions of
incubation were not the same. By stage 18, primordial germ cells were in the hindgut
epithelium and mesentery, and the genital ridges. The primordial germ cells were
four to five times larger than surrounding somatic cells.
1123 book.book Page 202 Monday, November 11, 2002 11:11 AM