Search

The adult structure of the male opossum reproductive system has been described in detail (Cowper 1704; Leydig 1850; Disselhorst 1904; van den Brock 1910; de Burlet 1921; Chase 1939; Ladman 1967; Martan et al. 1967; Martan 1983). Although generally similar to most eutherian mammals, the reproductive system of Didelphis differs in several respects, and these unusual features are briefly presented (Fig. 13). The scrotum of Didelphis is prepenial and attached to the body wall by a narrow stalk, and is separated into two distinct compartments by an internal connective tissue septum. Carbohydrate histochemistry of the scrotal skin of Didelphis demonstrated that small tubular apocrine glands associated with the epidermis of this area contain neutral, as well as acidic, mucus glycoproteins and small amounts of sialic acid (Tsukise et al. 1985). These mucus glycoproteins accumulate on the epidermal surface, and may prevent the proliferation of microorganisms and/or release odorous substances. The high content of neutral glycoproteins in the secretion also suggests that they may be important in the binding of water at the skin surface to aid in cooling of the testis. Each testis is contained within a highly pigmented tunica vaginalis. A basal lamina separates it from an underlying thick tunica albuginea, which consists of a dense fibroelastic connective tissue. Septula testes form connective tissue partitions which subdivide the testis into numerous compartments, the lobuli testis. The contained seminiferous tubules are typical and enveloped by a delicate interstitial tissue. The interstitial tissue of Didelphis is of large volume and highly cellular (Fawcett et al. 1973).

An epithelial lined vesicle results from Rathke’s pouch by the tenth day of gestation in Didelphis and lies in close association with the presumptive infundibulum of the diencephalon. At this stage of development, the newly formed multichambered pituitary vesicle is of uniform thickness. The epithelial wall of the pituitary vesicle consists of spindle-shaped cells and is pseudostratified in appearance. The dorsal wall remains uniformly thin and as development continues will form the pars intermedia. In contrast, the ventral wall of the pituitary vesicle thickens due to a proliferation of cells from within this layer and ultimately gives rise to cells of the pars distalis. Just prior to birth, the lumen of the multichambered pituitary vesicle becomes reduced to a flattened, curved cleft but continues to clearly delineate the pars intermedia from the forming pars distalis (Sherman and Krause 1990). By the end of the second postnatal week the relationships between the pars distalis, pituitary cleft, pars intermedia and pars nervosa are well established, and few changes other than size are observed during the remainder of pituitary development (Wheeler 1943).

The endoderm of the 9-day opossum forms the innermost layer of the embryo and also lines the interior of the chorion (Krause and Cutts 1985b). At this stage, the embryo appears as a flattened disk and consists of ectoderm, mesoderm, and endoderm, Cell boundaries between individual endodermal cells are distinct, and microvilli on the apical surfaces of the cells are short and scattered (Fig. 26). No obvious differences are seen between the endodermal cells that line the interior of the yolk sac chorion and those that form the third layer of the opossum embryo (Krause and Cutts 1985b,c). The endoderm consists of a single layer of cuboidal cells which are joined by junctional complexes and often bear elongated processes that unite with similar processes of adjacent endodermal cells. The intercellular space formed between the processes usually appears empty. The cytoplasm of the endodermal cells is characterized by numerous free ribosomes, occasional profiles of granular endoplasmic reticulum, and scattered mitochondria. There is little morphological evidence to suggest absorptive activity by the endoderm at this time in Didelphis. Similar observations have been reported in the Philander opossum (Enders and Enders 1969).

Various factors influence the differentiation, growth, and function of the glands associated with the gastrointestinal mucosa and also play an important role in regulating and maintaining the balance between epithelial cell proliferation and loss of cells from the gastrointestinal mucosa. Although the nature of the activities of such substances in Didelphis is unknown, there appears to be no reason to believe that they function in any way different to those in other mammals. Differentiation and growth, as well as epithelial cell proliferation and loss, are influenced by luminal, dietary, neural, and hormonal factors. Preprogrammed intrinsic factors also may play an important role in the regulation of the processes. The effect of such factors on gastrointestinal differentiation and growth have been summarized in elegant reviews by Klein and McKenzie (1983a,b), and by Johnson (1987) and Henning (1987). These factors include hypophysial hormones or tissues under hypophysial control, endogenous gastrointestinal hormones from enteroendocrine cells, neurotransmitters from the enteric nervous system, factors present in luminal content (amniotic fluid, milk, or dietary factors), and intrinsic mechanisms such as prostaglandins and chalones. Gastrin-, CCK-, somatostatin-, BPP-, glucagon-, secretin, motilin-, GIP-, neurotensin-, and 5-HT-immunoreactive cells are present in the gastrointestinal mucosa of the newborn opossum (Krause et al. 1986, 1989b). Cells with neurotensin immunoreactivity are unique in that, although present in the proximal colon of the newborn, the cells disappear and a significant population is not seen again in the intestinal mucosa until the 74th postnatal day, just prior to weaning.