RESPIRATORY SYSTEM

Description
Figure 1. Buccal cavity, parasagittal section
Figure 2. Gill arch, sagittal section
Figure 3. Ceratobranchial bone of the gill arch, sagittal section

Figure 4. Gill filament, sagittal section
Figure 5. Gill filament, sagittal section through venous sinus
Figure 6. Gill filament, sagittal section through cartilaginous support
Figure 7. Pseudobranch, sagittal section


DESCRIPTION

Gills

Fathead minnow gills are comprised of four gill arches on either side and extending from floor to roof of the buccal cavity (Fig. 1). Two holobranchs (rows of filaments called primary lamellae) project from the posterior edge of each gill arch (Fig. 2). Anterior edges of the arches contain gill rakers which protect the fragile filaments and assist in food acquisition. Bony opercula protect the outside of the gills and regulate water movement across the filaments. Arches are supported by mixed bone (acellular and spongy) and cartilage with associated striated abductor and adductor muscles facilitating movement of gills to favorable respiratory positions (Fig. 3).

Filaments have a central cartilaginous support (Fig. 6), afferent and efferent arterioles (Fig. 5), and a thin epithelial covering. This epithelium is contiguous with the covering of gill arches and the oral mucosa of the buccal cavity. Secondary lamellae originate on the superior and inferior surfaces of primary lamellae and are oriented perpendicular to the filaments. The thin epithelial covering of the secondary lamellae lies on a basement membrane supported by pillar cells. Spaces between pillar cells, called lacunae, connect afferent and efferent arterioles. The contractile pillar cells control the lacunar diameter thus regulating blood flow. Thin lamellar walls allow close proximity of blood within lacunae to the external environment. The direction of blood flow from afferent to efferent arterioles is opposite the direction of water flow over the lamellae. Short diffusion distances and counter-current blood flow promotes efficient exchange of oxygen and soluble metabolic wastes (e.g. carbon dioxide and ammonia).

Other cell types found on primary and secondary lamellae include, melanocytes, lymphocytes, macrophages, endothelial cells, mucous cells, rodlet cells, and chloride cells. Vacuolated mucous cells, found primarily at the base of secondary lamellae, produce a thin mucus coating which protects against abrasion and bacterial infection and helps reduce drag. Due to their hyperosmotic state freshwater fish tend to passively lose ions and gain water through diffusion across the gills. Copious release of dilute urine, required to expel excess water, increases the loss of ions. Chloride cells, located between secondary lamellae on gill filaments (Fig. 4), provide the primary means for maintaining internal ionic homeostasis. Richly endowed with mitochondria and intricately laced with a tubular network of smooth endoplasmic reticulum, these cells actively transporting ions (e.g. Na+ and Cl-) against concentration gradients into the fish.

Pseudobranch

In teleosts pseudobranchiae occur bilaterally along the interior of the opercula anterior to the first pair of gill arches (Fig. 1). While morphologically similar to the gills, pseudobranchiae have a single row of filaments and receive oxygen-rich blood from the first efferent arteries (and are therefore unlikely to serve a respiratory function). In Cyprinidae, including Pimephales promelas, the pseudobranch is completely covered by the opercular epithelium. Lamellae, lacking any contact with the external medium, are fused to each other forming a "glandular pseudobranch" (Laurent and Dunel-Erb, 1984). The epithelium is comprised predominantly of pseudobranchial cells which are morphologically similar to chloride cells (but unique to the pseudobranch) (Fig. 7). Functions attributed to the pseudobranch include; larval respiration before maturation of gill arches, regulation of oxygen to the eyes, enzyme production for use in the gas bladder, osmoregulation, and many others. In a revue of pseudobranch morphology and function, Laurent and Dunel-Erb (1984) found none of these explanations completely stisfactory, concluding that a likely function "should be at least partly sensory." These authors believe the pseudobranch, which is richly innervated, functions primarily in maintaining blood pressure.


RESPIRATORY SYSTEM: Buccal cavity, parasagittal section

Figure 1. Parasagittal section of the buccal cavity through the gill arches
(Bouins, H&E, Bar = 440 µm). 1. gill arches; 2. pseudobranch; 3. thymus; 4. operculum;
5. semicircular canals containing ampullae; 6. cranium; 7. retina; 8. lens of the eye.

 

RESPIRATORY SYSTEM: Gill arch, sagittal section

Figure 2. Gill arch: sagittal section
(Bouins, H&E, Bar = 90.2 µm). 1. gill raker;
2. mucosal epithelium; 3. basement membrane; 4. submucosa; 5. bone;
6. adipose tissue; 7. efferent branchial arterioles; 8. afferent branchial artery;
9. primary lamellae; 10. secondary lamellae.

 

RESPIRATORY SYSTEM: Ceratobranchial bone of the gill arch,
sagittal section

Figure 3. Ceratobranchial bone of the gill arch, sagittal section
(Formalin, H&E, Bar = 72 µm). 1. acellular bone; 2. hypertrophic zone;
3. growth zone; 4. apical zone; 5. abductor muscle; 6. efferent branchial
artery; 7. mucosal epithelium; 8. primary lamella; 9. secondary lamellae.

 

RESPIRATORY CHAPTER: Gill filament, sagittal section

Figure 4.
Gill filament, sagittal section (Formalin, H&E, Bar = 16.7 µm).
1. primary lamella; 2. secondary lamella; 3. epithelial cell; 4. mucous cell;
5. pillar cell; 6. lacuna (capillary lumen); 7. erythrocyte within capillary lumen;
8. chloride cell; 9. rodlet cell; 10. undifferentiated basal cell.

 

RESPIRATORY CHAPTER: Gill filament, sagittal section through
venous sinus

Figure 5.
Gill filament, sagittal section through venous sinus (Formalin, H&E,
Bar = 16.7 µm). 1. primary lamella; 2. secondary lamella; 3. epithelial cell;
4. mucous cell; 5. pillar cell; 6. lacuna (capillary lumen); 7. erythrocyte
within capillary lumen; 8. undifferentiated basal cell; 9. central venous sinus.

 

RESPIRATORY CHAPTER: Gill filament, sagittal section through
cartilaginous support

Figure 6. Gill filament, sagittal section through cartilaginous support

(Formalin, H&E, Bar = 15.6 µm). 1. primary lamella; 2. extracellular
cartilaginous matrix; 3. chondrocytes; 4. secondary lamella; 5. epithelial
cell; 6. mucous cell; 7. chloride cell; 8. pillar cell; 9. lacuna (capillary
lumen); 10. red blood cells within lacuna.

 

RESPIRATORY SYSTEM: Pseudobranch, sagittal section

Figure 7. Pseudobranch, sagittal section (Formalin, H&E, Bar = 18.0 µm).
1. afferent pseudobranchial artery containing red blood cells; 2. secondary
pseudobranchial lamella; 3. glandular pseudobranch cell; 4. epithelial cell.

 

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