REPRODUCTIVE SYSTEM: Female

Description
Figure 1. Ovary (a)
Figure 2. Ovary (b)
Figure 3. Stage III oocytes
Figure 4. Stage V vitellogenic oocyte
Figure 5. Stage V oocytes, detail of follicular wall

DESCRIPTION

Fathead minnow ovaries are bilateral elongate lobes oriented longitudinally within the abdominal cavity and containing ova at varying stages of maturity (Fig. 1). They suspend from the ventral surface of the gas bladder by mesenteries termed mesovaria (Harder, 1975). Arteries, veins, lymph vessels, and nerves that supply the ovaries enter via the mesovaria. The ovaries themselves are enclosed in a fibrous connective tissue tunica albuginea which is contiguous with the mesovaria (Fig. 2). The lumenal surface of the tunica albuginea folds into ovigerous lamellae oriented perpendicular to the long axes of each ovarian lobe. Lamellar walls are composed of germinal and follicular epithelia supported by a connective tissue stroma.

Development of ova from germinal cell to mature egg is divided into six stages defined according to morphological characteristics of nucleus, oviplasm, and follicular wall. Oocytes increase in size as they proceed through the developmental stages. Mitotic proliferation of germinal cells produces clustered of Stage I oocytes (oogonia) nested within the germinal epithelium. These cells have large nuclei and lightly eosinophilic cytoplasm. Stage II oocytes have basophilic cytoplasm and a conspicuous central nucleus with diffuse chromatin. Stage III oocytes break from the germinal epithelium continuing maturation within the folds of the ovigerous lamellae. At this stage a simple squamous follicular epithelium envelopes the ova and provitelline nucleoli become evident in the karyoplasm (Fig. 3). Stage IV is defined by the appearance of yolk granules and fat vacuoles in the oviplasm. Euvitelline nucleoli are evident along the nuclear membrane and a distinct vitelline envelope (chorion) appears beneath the follicular epithelium. Yolk vesicles increase significantly during stage V filling the entire oviplasm except narrow basophilic bands around the nucleus and beneath the vitelline envelope (Figs. 4 & 5). The nucleus contains fewer uvitelline nucleoli and the nuclear membrane begins to degenerate. Prior to final maturation the nucleus migrates peripherally

REPRODUCTIVE SYSTEM: Ovary (a)

Figure 1. Ovary (a) (Formalin, H&E, Bar = 412 µm). 1. connective tissue of the
tunica albuginea; 2. stage II oocyte; 3. stage III oocyte; 4. stage IV oocyte;
5. stage V oocyte; 6. atretic oocyte.

REPRODUCTIVE SYSTEM: Ovary (b)

Figure 2. Ovary (b) (Formalin, H&E, Bar = 186 µm). 1. connective tissue of the
tunica albuginea; 2. stage III oocyte; 3. stage IV oocyte; 4. late stage V oocyte;
5. early stage V oocyte; 6. atretic oocyte; 7. ovarian vein.

REPRODUCTIVE SYSTEM: Stage III oocytes

Figure 3. Stage III oocytes (Formalin, H&E, Bar = 36.4 µm). 1. nucleus;
2. cytoplasm; 3. provitelline nucleoli; 4. fibrous connective tissue stroma;
5. follicular cell layer; 6. vitelline envelope (chorion); 7. yolk vacuoles of stage
V oocyte; 8. yolk granules of stage V oocyte.

REPRODUCTIVE SYSTEM: Stage V vitellogenic oocyte

Figure 4. Stage V vitellogenic oocyte (Formalin, H&E, Bar = 36.6 µm).
1. germinal vesicle; 2. yolk vacuoles; 3. yolk granules; 5. vitelline envelope
(chorion); 6. follicular cell layer.

REPRODUCTIVE SYSTEM: Stage V oocytes, detail of the follicular wall

Figure 5. Stage V oocytes, detail of the follicular wall (Formalin, H&E,
Bar = 21.8 µm). 1. follicular cell layer; 2. vitelline envelope (chorion);
3. oviplasm; 4. yolk vacuoles; 5. yolk granules.