Hematolymphopoietic and Endocrine System. The Adult Salamander

The hematolymphopoietic system of amphibians has been reviewed (Plyzycz et aI., 1995). All amphibians possess a thymus, which is one source of T-lymphocytes in amphibians. The thymus is present throughout life in amphibians, although malnutrition and stress may elicit involution and atrophy of the thymus in captive amphibians (Plyzycz et aI., 1995). Hibernation may also cause a reduction in the size of the thymus. The spleen has both red and white pulps in amphibians, which act as the respective centers of erythropoiesis and myelopoiesis. Seasonal variation has been noted in the size of the spleen (Plyzycz et ai., 1995). Gut-associated lymphoid tissue (GALT) is present in amphibians. In

general, aquatic amphibians lack functional bone marrow, but sites that serve as functional equivalents are found in the liver and kidneys.

The Adult Caecilian. Caecilians lack functional bone marrow, but lymphopoiesis occurs in various sites such as the liver, kidneys, thymus, and spleen.

The Adult Salamander. The bone marrow of terrestrial salamanders has sites of lymphomyelocytopoiesis which are lacking in aquatic salamanders. The ventral meninges of some species of primitive aquatic salamanders also act as hematolymphopoietic tissue. Lymphomyeloid organs are lacking in all caudates.

The Adult Anuran. Terrestrial anurans possess functional bone marrow, but it does not serve as a site for erythropoiesis, only lymphocytopoiesis and myelothrombocytopoiesis. Lymphomyeloid organs are sentinels that process antigens to produce a strong humoral response. The lymphomyeloid organs are not connected with the lymphatic system, rather the afferents of lymphomyeloid organs are either arteries or veins or both.

Endocrine System. The endocrine system of the amphibian is similar to that of other vertebrates, and the function of the various organs is similar, although the actual secretions produced may have significant structural differences

from their analogues in other vertebrates (e.g., calcitonin). A review of the endocrine system has been published (Gorbman, 1964). Due to the long use of amphibians as models for study in the fields of embryology and endocrinology, detailed information concerning the amphibian endocrine system may be found in many endocrinology texts. The endocrine glands (and associated secretions) include the following: adrenals (epinephrine, norepinephrine, corticosteroids), gonads (testosterone, estrogen, progesterone), pancreatic islets (insulin), parathyroids (calcitonin, parathyroid hormone), pineal body (melatonin), pituitary (adrenocorticotropin or ACTH, antidiuretic hormone or ADH, arginine vasotocin, follicle-stimulating hormone or FSH, luteinizing hormone or LH, melanophore-stimu- lating hormone or MSH, oxytocin, prolactin), thymus (thymosin), thyroid (triiodothyronine or T3, tetraiodothyronine or T4), and ultimo branchial bodies (calcitonin). The locations of the endocrine glands in am- phibians are similar to the locations in reptiles, with a few exceptions as noted below.

The Adult Caecilian. The adrenal glands of the caecilian are located on the ventrimesal surface of the kidneys.

The Adult Salamander. The adrenal glands of the salamander are located on the ventrimesal surface of the kidneys. Parathyroid glands are lacking in mudpuppies, Necturus spp. The ultimo branchial gland is singular in most species of salamanders but is paired in amphiumas, Amphiuma spp., and mudpuppies, Necturus spp.

The Adult Anuran. The adrenal glands vary in location depending on the species of anuran, but remain in close association to the kidneys. The adrenals lie lateral to the kidneys in advanced anurans, but are ventrimesal or medial in primitive anurans. It has been documented that the corpora lutea secrete pro- gesterone in some viviparous species (e.g., African tree toads, Nectophrynoides spp.). Parathyroid glands regress during the winter in some species and may not be detectable. The ultimo branchial glands are found adjacent to the larynx in most anurans but are lacking in clawed frogs, Xenopus spp.

Reproductive System. The reproductive system of the amphibian is fairly standardized considering the diversity of reproductive modes utilized by this class of vertebrates. The gonads are paired, and gametes travel to the cloaca through ducts. The cloaca also receives waste from the digestive tract and the bladder.

The Adult Caecilian. The paired testes of the caecilian are intimately connected to the kidneys by the mesorchium. Seasonal fluctuation in testicular size occurs. The testes are elongated and lobed. Depending on the species, either the anterior lobes or the posterior lobes may be larger, or all lobes may be approximately equal in size (Wake, 1968). Spermatogenesis occurs within locules of the lobe of the testis, and sperm is transported from the testis through vas efferentia to nephric collecting tubules which empty into the Wolffian duct. The Wolffian duct empties sperm into the cloaca. An intromittent organ, the phallodeum, is present, which allows for internal fertilization. Mullerian ducts are present in caecilians.

The paired ovaries are intimately connected to the kidneys by the mesovarium. The follicles are surrounded by a thin membranous ovisac. The ovisac must rupture for ovulation to occur, and the ova are released into the coelom. Cilia within the coelom direct the ova to the infundibulum, which lies near the lungs. The oviducts are straight and elongate. In vi- viparous caecilians the oviductal lining may be consumed by the embryos as a food source (Plate 3.12).

Fat bodies are associated with the gonads and probably serve as a nutrient source for the gonads as their size changes in relation to gonadal activity.

The Adult Salamander. The testes of the salamander are lobed, and additional lobes may be added with each breeding season. Sperm transport to the cloaca is similar to that described for caecilians, but in some species of salamanders the nephric collecting tubules empty sperm or ova directly into the cloaca. In female salamanders the dorsal aspect of the cloaca is modified into a spermatheca. Glandular material lines the walls of the cloaca in male salamanders, which engorge during the breeding season. No intromittent organ is present, but internal fertilization is achieved by transfer of a spermatophore.

The spermatophore is a gelatinous structure produced by the male that encapsulates the sperm to protect it from the environment before it is taken up by the female's cloaca. External fertilization occurs in two families, Hynobiidae and Cryptobranchidae. The oviducts are minimally convoluted. Ovulation is as described for caecilians. Fat bodies are associated with the gonads.

The Adult Anuran. The testes of the anuran are not lobed, and in some species of anurans the testes are pigmented. Sperm transport to the cloaca is similar to that described for caecilians. A seminal vesicle may be present. The Bidder's organ is a remnant of ovarian tissue that is found on the testes of adult bufonids.

Members of the Leiopelmatidae possess an intromittent organ, but this structure is not described for any other frogs. Fertilization is external for most species, but internal fertilization is known or suspected for some species (e.g., the viviparous toads, Nectophrynoides spp. and some species of robber frogs, Eleuthrodactylus spp.). Mullerian ducts are present in some species of anurans. The oviducts are convoluted. Ovulation is as described for caecilians. Corpora lutea develop in some viviparous species. Fat bodies are associated with the gonads.

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