Ultrastructure of the male genital ducts of the clearnose skate Raja eglanteria

Tissues from the male genital ducts of six specimens of the clearnose skate Raja eglanteria, comprising the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle, were fixed and embedded for ultrastructural examination. In the Leydig gland, two types of columnar cells were identified, one bearing microvilli, a basal nucleus and evidence of active secretion with plentiful endoplasmic reticulum and numerous secretory droplets, and the other pyriform with cilia, and swathes of cytofilaments emanating from prominent desmosomes. Occasional crystalloid intramitochondrial inclusions were seen in the first type, with a periodicity of 24 nm. The upper epididymis was composed of cuboidal cells with microvilli and cilia and irregular electron dense granules, some of which were basally situated and extremely large, often within cells resembling intraepithelial leucocytes; such cells were also seen in the stroma underlying the epithelium. The lower epididymis cells also bore microvilli and cilia and were heavily vacuolated with fatty inclusions as well as the granule-laden leucocytes seen previously. In the ductus deferens, cells had masses of long cilia with occasional microvilli; endoplasmic reticulum was well developed, forming complex arrays with sparse secretory droplets and basal mitochondria. In the seminal vesicle there were two cell types, the most common having long cilia and short microvilli and an occasional, paler cell with supranuclear accumulations of small, round mitochondria. These ultrastructural appearances have been related to cell glycosylation and functions including protein secretion, water absorption and waste removal, and illustrate how structure and function vary down the length of the genital tract.

Ultrastructure of the maternal-fetal interface of the yolk sac placenta in sharks

The blacknose shark, Carcharhinus acronotus, is a viviparous anamniote that develops an epitheliochorial yolk sac placenta. The fetal portion of the uteroplacental complex consists of a proximal portion that forms saccular evaginations. The cells are bilayered stratified squamous with surface microvilli and a high concentration of cytoplasmic filaments. The tertiary egg envelope intervenes between the distal portion of the placenta and uterus. It has delaminations on the uterine surface and is compacted on the placental surface. The uterine epithelium is cuboidal to columnar and is characterized by prominent RER, Golgi, and secretion vesicles. The capillary endothelium is continuous. Nutrient and respiratory exchange is effected between the uterus and distal portion of the yolk sac. The distal portion of the placenta is a bilayer. An elaborate array, of microvilli forms an interface with the egg envelope. Dense non-membrane bound granules occur in the interspace between the egg envelope and the distal placenta. This material, presumably of uterine origin, is endocytosed in smooth-walled vesicles of the placental cells. The endothelium of the capillaries is fenestrated.

Structure and glycosylation of the term yolk sac placenta and uterine attachment site in the viviparous shark Mustelus canis

The viviparous shark Mustelus canis nurtures its young within the uterus by means of a modified yolk sac which functions as a placenta. Two term specimens have been examined with a panel of 21 biotinylated lectins to assess whether glycans form a prominent interface between fetal and maternal tissues as in their therian counterparts. The yolk sac placenta was lined by a thin egg envelope which apposed though did not make contact with the uterine epithelium, and expressed fucosyl, N-acetyl glucosamine/lactosamine residues and some complex N-glycan, while the attached, thin ectoderm cells stained selectively with lectins from Anguilla anguilla and Arachis hypogaea indicating fucosyl and beta-galactosyl residues; other lectins bound to a variable degree. Large yolk sac endoderm cells were heavily glycosylated and expressed a wide range of glycans. The apposing uterine epithelium had two epithelial layers with distinctive patterns of glycosylation, the apical layer stained strongly with Anguilla anguilla lectin and the basal cells with lectins from Wisteria floribunda and Helix pomatia, the latter indicating plentiful N-acetyl galactosamine though both layers stained variably with other lectins also. A population of sparse, large, globular cells expressed alpha2,3-linked sialic acid and N-acetyl glucosamine oligomers. Fetal and maternal vessels were heavily glycosylated as in their therian counterparts. These data indicate a prominent role for glycans at the fetomaternal interface of these chondrichthyan fishes.

Glycosylation of the male genital ducts and spermatozeugmata formation in the clearnose skate Raja eglanteria

Genital ducts of three male Raja eglanteria were fixed and embedded in epoxy and methacrylate resin. Epoxy resin sections from the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle were stained with 20 labelled lectins to examine their glycosylation. The Leydig gland consisted of columnar epithelial cells expressing N-linked glycans, N-acetyl galactosamine, glucosamine and lactosamine residues and sialic acid. Interspersed were ciliated cells of a different glycotype. The upper epididymis of cuboidal epithelium had a strongly glycosylated, ciliated apical surface and cytoplasmic granules that stained heavily with many lectins, with increased glycosylation compared to the Leydig gland. In the lower epididymis, tall, vacuolated cells showed some differences and a slight reduction in lectin staining. The ductus deferens contained two cell types and showed increased terminal N-acetyl galactosamine. The ciliated cuboidal epithelium of the seminal vesicle had marked differences from the ductus epithelium, with decreased N-acetyl galactosamine and lactosamine expression but increased subterminal N-acetyl lactosamine and galactosamine expression and sialylation. Spermatozoa were suspended in a glycosylated matrix and, in the seminal vesicle, were embedded in solid masses of matrix forming spermatozeugmata. These data show changes in glycan expression along the male genital tract, probably related to the nurture and maturation of the spermatozoa as they travel towards the seminal vesicle.

The interleukin 1 (IL-1) system in the uteroplacental complex of a cartilaginous fish, the smoothhound shark, Mustelus canis

Cartilaginous fish are the oldest extant jawed vertebrates and the oldest line to have placentae. Their pivotal evolutionary position makes them attractive models to investigate the mechanisms involved in the maternal-fetal interaction. This study describes the tissue expression of the cytokine interleukin-1 (IL-1) alpha, IL-1 beta and its specific membrane receptor, IL-1 receptor type I (IL-1R tI) in a placental cartilaginous fish, the smoothhound shark, Mustelus canis. The presence of this cytokine has been reported in many mammalian placentae, as well as in the placenta of a squamate reptile and this study extends these observations to the cartilaginous fishes. The uteroplacental complex in M. canis consists of a yolk sac modified into a functional yolk sac placenta and complimentary uterine attachment sites. Immunohistochemistry for IL-1 alpha, IL-1 beta and the receptor reveals leucocytes of both the mother and fetus to be positive, as well as the apical aspect of paraplacental cells and the apical vesicles in the umbilical cord epithelium. Yolk sac endoderm is also positive with all the stains while the ectoderm is positive only for IL-1 alpha. Immunoreactivity in the uterine epithelium was obtained for IL-1 alpha and the receptor. The egg envelope is always negative. In light of the recent finding of IL-1 beta gene in a cartilaginous fish and of the high level of conservation of proteins implicated in IL-1 action, our data suggest that IL-1 system is a key mediator of the materno-fetal interaction since the oldest extant placental vertebrates.

Ultrastructure of the reproductive system of the black swamp snake (Seminatrix pygaea). III. Sexual segment of the male kidney

In mature male snakes and lizards, a distal portion of the nephron is hypertrophied in relation to its appearance in females and immature males. This sexual segment of the male kidney apparently provides seminal fluid that is mixed with sperm and released into the female cloaca during copulation. In this article, we provide the first study at the ultrastructural level of seasonal variation in the sexual segment of the kidney of a squamate, the natricine snake Seminatrix pygaea. Previous workers have indicated that the sexual segment is secretory only when the testes are spermatogenically active. The sexual segment of the kidney in S. pygaea does not go through an extended period of inactivity but does show a cycle of synthesis and secretion that can be related to the spermatogenic cycle and mating activity. We show that synthesis of secretory product is initiated with the onset of spermatogenic activity in the spring and culminates with completion of spermiation in the fall. Secretion of the product, however, occurs in a premating period in March when the testes are inactive. Secretion during this premating period is probably necessary to provide time for the passage of the products down the ureter in order to mix with sperm during mating later in spring.

Uterine epithelial-sperm interaction, endometrial cycle and sperm storage in the terminal zone of the oviducal gland in the placental smoothhound, Mustelus canis

The fate of spermatozoa deposited within the female reproductive tract has been described in the smoothhound, Mustelus canis. Evidence of uterine epithelial-sperm interaction is presented, as well as documentation of sperm storage specifically in the terminal zone of the oviducal gland. Sperm fate is correlated with morphology of the endometrial cycle and specificity of storage in the oviducal gland. The endometrium of M. canis undergoes dramatic tissue remodeling associated with gestation. In females harboring fertilized ova or preimplantation yolk-reliant embryos, the uterine epithelium is simple cuboidal with mucous droplets for lubrication. The presence of the embryo elicits a response from the uterus, which becomes modified for nutrient and respiratory exchange into vascular uterine attachment sites that abut the distal aspect of the yolk sac. Areas of the uterus adjacent to the uterine attachment sites are termed paraplacental sites. Uterine attachment sites are simple squamous while the paraplacental epithelium is simple columnar. Paraplacental cells have basal metachromatic vesicles and a dense array of apical cytoplasmic filaments. Immediately postpartum the uterine attachment sites, now termed uterine or placental scars, begin to remodel to a mucous epithelium for the next gestational cycle. Paraplacental cells slough off the apical filamentous portion, and sperm become embedded in the epithelium. Bundled sperm occur throughout gestation in the terminal zone of the oviducal gland. Sperm are not embedded in the terminal zone epithelium as in the uterus. Following sperm release from the uterus, the paraplacental epithelium reverts to a mucous epithelium for the next reproductive cycle. Fertilization is presumed to occur in the anterior oviduct above the oviducal gland. The physiological mechanisms that mediate sperm-uterus attachment, release, and storage in the terminal zone of the oviducal gland are currently under investigation.

Ultrastructure of sperm storage and male genital ducts in a male holocephalan, the elephant fish, Callorhynchus milii

In chondrichthyes, the process of spermatogenesis produces a spermatocyst composed of Sertoli cells and their cohort of associated spermatozoa linearly arrayed and embedded in the apical end of the Sertoli cell. The extratesticular ducts consist of paired epididymis, ductus deferens, isthmus, and seminal vesicles. In transit through the ducts, spermatozoa undergo modification by secretions of the extratesticular ducts and associated glands, i.e., Leydig gland. In mature animals, the anterior portion of the mesonephros is specialized as the Leydig gland that connects to both the epididymis and ductus deferens and elaborates seminal fluid and matrix that contribute to the spermatophore or spermatozeugmata, depending on the species. Leydig gland epithelium is simple columnar with secretory and ciliated cells. Secretory cells have periodic acid-Schiff positive (PAS+) apical secretory granules. In the holocephalan elephant fish, Callorhynchus milii, sperm and Sertoli cell fragments enter the first major extratesticular duct, the epididymis. In the epididymis, spermatozoa are initially present as individual sperm but soon begin to laterally associate so that they are aligned head-to-head. The epididymis is a highly convoluted tubule with a small bore lumen and an epithelium consisting of scant ciliated and relatively more secretory cells. Secretory activity of both the Leydig gland and epididymis contribute to the nascent spermatophores, which begin as gel-like aggregations of secretory product in which sperm are embedded. Fully formed spermatophores occur in the ductus. The simple columnar epithelium has both ciliated and secretory cells. The spermatophore is regionalized into a PAS+ and Alcian-blue-positive (AB+) cortex and a distinctively PAS+, and less AB+ medulla. Laterally aligned sperm occupy the medulla and are surrounded by a clear zone separate from the spermatophore matrix. Grossly, the seminal vesicles are characterized by spiral partitions of the epithelium that project into the lumen, much like a spiral staircase. Each partition is staggered with respect to adjacent partitions while the aperture is eccentric. The generally nonsecretory epithelium of the seminal vesicle is simple columnar with both microvillar and ciliated cells.

Female sperm storage in reptiles

Internal fertilization and oviparity most likely are symplesiomorphies for modern reptiles, and viviparity has evolved independently numerous times in Sauria and Serpentes. Oviducal sperm storage is known in females of all taxa except Amphisbaenia. However, in Rhynchocephalia and Crocodilia, sperm storage is poorly studied, and specialized sperm storage tubules (Ssts) are unknown. We use the molecular phylogenetic hypothesis [(Chelonia+Archosauria) (Squamata)] to trace evolution of sperm storage characters. Ssts arose independently in Chelonia and Squamata. Turtles possess albumen-secreting glands in the anterior half of the oviduct (the tuba or isthmus), and the most distal of these glands also serve as Ssts; in addition, some turtles possess Ssts in the adjacent segment of the oviduct, the uterus. Squamates lack albumen-secreting glands, and the ancestral state is possession of Ssts in the posterior infundibulum (uterine tube). Secondarily, iguanids have evolved vaginal Ssts. In this paper, we present the first ultrastructural observations on vaginal Ssts in lizards, using Anolis sagrei (Polychrotidae). Proximally, the neck of these simple tubular glands continues the alternation of ciliated and secretory cells lining the lumen of the vagina. However, the epithelial cells of the distal sperm storage area are neither secretory nor ciliated. The Ssts of Anolis are more similar to those of birds more than to infundibular receptacles in snakes and lizards.

Sperm storage in the oviduct of the internal fertilizing frog Ascaphus truei

This study provides the first descriptions of sperm storage at the tissue and cellular levels in a female frog or toad. Oviducal anatomy was studied by light and electron microscopy in Ascaphus truei from north coastal California. Ascaphus truei is one of the few species of anurans in which fertilization is internal. Unlike other anurans with internal fertilization, however, mating in A. truei consists of a unique combination of amplectic and copulatory mechanisms that we term "copulexus." Posterior to a short, aglandular infundibular region, the oviduct possesses: 1) a proximal, convoluted ampullary region where intrinsic tubular glands secrete gelatinous envelopes around eggs; 2) a middle ovisac region where fertilization occurs; and 3) a distal oviducal sinus formed by medial junction of the ovisacs. Sperm storage tubules (SSTs) occur in the anterior portions of the ovisacs and consist of simple tubular glands. SSTs and the rest of the oviducal lining stain positively with the periodic acid-Schiff's procedure for neutral carbohydrates and this reaction is especially intense in reproductively active females. Sperm were found in the SSTs of gravid females as well as some nonvitellogenic females. The sperm are in orderly bundles in the SSTs, and although occasionally sperm nuclei were embedded in the epithelium, no evidence for spermiophagy was found. Oviducal sperm storage in A. truei is homoplastic, with closest structural similarities to squamate reptiles. Oviduct/sperm design constraints appear to limit the options for expression of features associated with oviducal sperm storage.

Ultrastructural analysis of folliculogenesis in the ovary of the yellow spotted stingray, Urolophus jamaicensis

The ovary of the yellow spotted ray, Urolophus jamaicensis, is embedded in the epigonal gland, a lymphomyeloid organ. The covering of the ovary is composed of a germinal epithelium that is cuboidal and dome-shaped with microvilli. Adjacent cells have elaborate intercellular folds that create dilated intercellular spaces. In previtellogenic follicles, the follicle cells are simple cuboidal and contain modest amounts of synthetic or transport organelles. As vitellogenesis proceeds, the epithelium becomes multilaminar. Follicle cells are columnar as yolk precursors are transported from the maternal circulation, through the follicle cell cytoplasm, to the oocyte. Large, round cells occur in the follicle wall that contain lipid-like substances. These cells decrease in size and number as folliculogenesis proceeds and eventually disappear prior to ovulation. Columnar follicular cells and the oocyte have cellular extensions that impinge upon the zona pellucida. Transosomes are follicle cell extensions that indent the oocyte membrane. Tips of transosmes become enclosed by a layer of oocyte plasmalemma. The tips of transosomes pinch off and become resident in the ooplasm. Dense staining material occurs on the inner surface of the transosome membrane derived from the follicle cell. In Other animals, this material has been described as ribosome-like. This study is the first to document the presence of transosomes in a group other than Aves or reptiles. Follicle cells are supported by an extremely thick basal lamina. Subjacent to the lamina is the vascularized theca with fibroblasts embedded in a collagenous network. There is no differentiation into definitive theca interna and externa. In vitellogenic eggs, extensive inward folding of the follicular epithelium occur thereby generating more surface area for the transport of yolk precursors to the oocyte. Atretic follicles are common.

Ultrastructure of uterine trophonemata, accommodation for uterolactation, and gas exchange in the southern stingray, Dasyatis americana

The southern stingray, Dasyatis americana, displays aplacental viviparity, embryos being retained in the maternal uterus throughout gestation and initially nourished by the yolk sac contents. During gestation the uterus develops vascularized appendages, trophonemata, that secrete viscous nutrient histotroph that is subsequently ingested by the embryo as it grows to term. There is a 3750% increase in wet mass from the egg to the term fetus. Trophonemata are 1.5 cm long, narrower at the base, and spatulate at the tip. Surface epithelial cells form a pattern of surface cables, each with a small blood vessel at its core. In females containing fertilized eggs, the epithelium is simple and cuboidal. In contrast, in uteri containing late-term fetuses, the epithelium is squamous. Epithelial cells, with periodic acid – Schiff positive cytoplasmic vesicles, form invaginated crypts. Epithelial cells produce proteinaceous, mucous, and lipid secretions, thus we have coined the term uterolactation to describe this phenomenon.

Ultrastructure of fetal alimentary organs: stomach and spiral intestine in the southern stingray, Dasyatis americana

In the fetal southern stingray, Dasyatis americana, both the stomach and spiral intestine function early in development to digest and absorb nutrient histotroph elaborated by uterine villi termed trophonemata. The gastric mucosa consists of a surface columnar mucous epithelium that is confluent with gastric pits or foveolae. Gastric glands are populated by oxynticopeptic and enteroendocrine cells. The surface mucous cells are pyramidal with apical microvilli. Oxynticopeptic cells are low columnar with a distinct and elaborate tubulovesicular system in the apical cytoplasm. Microvilli line the lumen of the gastric glands and cells have elaborate interdigitating lateral folds. Enteroendocrine cells are characterized by basal granules and a prominent rough endoplasmic reticulum. The fetal intestine is filled with bile-tinged viscous fluid. A core of submucosa supports spiral intestinal plicae that form the spiral valve from which villi project. The most prominent characteristic of the cells are enormous supranuclear vesicles formed by coalescence of smaller endocytotic vesicles. The apical cytoplasm has a profusion of smooth tubules, endoplasmic reticulum, and lysosomes. The large vesicles are interpreted as storage depots for continually ingested histotroph. Small vesicles may then bud off to be digested via the lysosomal system.

Anatomy, histology, and development of the cardiac valvular system in elasmobranchs

We report here on the anatomy, histology, and development of the three sets of cardiac valves in embryonic and adult elasmobranch fishes. The sinus venosus is the first segment of the heart to receive blood, and a pair of sinoatrial (SA) valves prevent backward flow of blood into the sinus venosus. The SA valves derive from two dorsolateral infoldings of the cardiac wall and consist of a simple endocardium covering transverse sheets rich in collagen. The SA valves are simple flaps of tissue without papillary muscles or chordae tendineae. Blood from the atrium passes the atrioventricular (AV; semilunar) valves, which are attached to papillary muscles in the ventricle by way of the chordae tendineae. A series of rows of conal or pocket valves (CV) in the conus arteriosus, equipped with chordae tendineae but no papillary muscles, prevent blood from reentering the ventricle. Chordae tendineae form in a similar fashion in both chambers. Elevations from the chamber wall emerge as a sheet covered on both surfaces with endocardium and separated by a core of connective tissue. Endocardial cells extend basal projections toward the opposing epithelium through their basal laminae. Basal cell projections make contact to create perforations that enlarge to produce spaces between the nascent chordae. Fibroblasts in the core of the chordae enlarge and strengthen the chordae by producing linear arrays of collagen fibers.

Fine structure of the term umbilical cord in the Atlantic sharpnose shark, Rhizoprionodon terraenovae

The fine structure of the umbilical cord and appendiculae in the Atlantic sharpnose shark, Rhizoprionodon terraenovae, is examined by light, scanning and transmission electron microscopy. During ontogeny of placental sharks, the yolk sac and stalk become progressively modified as a functional hematrophic placenta and umbilical cord respectively. In most placental sharks the umbilical cord is smooth. In the Atlantic sharpnose shark, the epithelial ectoderm of the somatopleure forms richly vascularized extensions termed appendiculae. Scanning electron microscopy reveals that the base and shaft of appendiculae are flattened while the distal portion may be expanded to form one to three lobes. The surface of appendiculae is composed of two distinct cell types, the most plentiful are microvillar cells. The second cell type contains prominent granules. These cells are much larger than the former and are partially submerged below the surface, except for the cell apex. These cells undergo secretory cycles ending in expulsion of their contents. The possible function of the granulated cells is discussed. The umbilical cord contains an umbilical vein, umbilical artery, ductus vitellointestinalis and extraembryonic coelom. The endodermal ductus initially conveys yolk from the yolk sac to the fetal gut by activity of ciliated cells lining it. The ductus persists in the adult. Microvillar cells, also present in the ductus, may play a role in the absorption of yolk metabolites early in development, prior to yolk depletion. Enteroendocrine cells are wedged between the ciliated and microvillar cells. These cells may exert paracrine regulation of local areas of the ductus.

Subcellular organization of the placenta in the Atlantic sharpnose shark, Rhizoprionodon terraenovae

The Atlantic sharpnose shark, Rhizoprionodon terraenovae, is a viviparous anamniote that develops a yolk sac placenta composed of: a) uterine mucosa, b) egg envelope and c) fetal yolk sac mucosa. The transporting uterine mucosa is a squamous epithelial bilayer with prominent lateral and basal infoldings between contiguous cells. The surface cells have prominent secretion vesicles that empty their contents to the exterior. Immediately beneath the epithelium is a basal lamina and a profuse vascular supply with a continuous endothelium. The epithelium of paraplacental uterine sites is mucous. The tertiary egg envelope is retained throughout gestation and separates the distal part of the yolk sac from the maternal uterine mucosa. The egg envelope is compact on the yolk sac surface but displays delaminations on the uterine surface. The fetal yolk sac is composed of two portions, viz., a proximal, saccular region and a heavily vascularized, rugose, distal portion. The proximal portion has ultrastructural characteristics of a steroid hormone producing tissue, including massive smooth endoplasmic reticulum frequently forming whorled arrays. However, definitive evidence that the yolk sac is an endocrine organ is lacking. The distal portion of the fetal yolk sac is composed of a squamous epithelial bilayer that is separated from the underlying vascular network by a continuous basal lamina. The endothelium of the vessels is fenestrated. Cytoplasmic characteristics of these cells include an extensive Golgi complex, smooth walled caveolae, vesicles with electron-dense contents that are presumably endocytotic in nature and dense bodies that are suggested to be lysosomes that are involved in the digestion of material that may be yolk metabolites.

Subcellular organization of the yolk syncytial-endoderm complex in the preimplantation yolk sac of the shark, Rhizoprionodon terraenovae

The structure of the yolk syncytial-endoderm complex of the preimplantation yolk sac of the shark is examined by light- and transmission electron microscopy. The yolk syncytium is bounded by a membrane that is anchored to the plasmalemma of adjacent endoderm cells by desmosomes. Enlarged nuclei, rough endoplasmic reticulum, Golgi complexes, mitochondria, and other cellular organelles populate the syncytium. Microtubules and filamentous elements are also observed free in the syncytium. Yolk is present as pleomorphic droplets, the profiles of which are generally spherical but may be vesicular, especially at the periphery of large yolk droplets. Occasionally, large yolk droplets have a paracrystalline configuration. Small yolk droplets are modulated through the Golgi complex of the yolk syncytium, and it is suggested that acid hydrolases are added there. Small yolk droplets released from the maturing face of the Golgi complex are sequestered in membrane-limited packets. The membrane of the packets fuses with the membrane enveloping the yolk syncytium and the yolk droplets are released into the yolk syncytial-endoderm interspace. Subsequently, the yolk droplets are endocytosed by the endoderm. Yolk droplets disperse and fuse to form the large irregular yolk inclusions of the endoderm. Yolk metabolites are transported out of the endoderm through the yolk sac endothelium. The yolk sac endoderm thus mediates the transfer of metabolites from the yolk mass to the extraembryonic circulation.

Stingray placental analogues: structure of trophonemata in Rhinoptera bonasus

The cownose ray, Rhinoptera bonasus, displays a non-placental form of viviparity since direct maternal-embryonic connections are lacking. Early stage embryos depend on yolk reserves for growth to 215 mm disc width; growth to term, 405 mm disc width, is effected by ingestion of uterine histotrophe. During late gestation, the maternal uterine epithelium possesses 2-3 cm long spatulate, villiform appendages, termed trophonemata. These secrete histotrophe, which is a viscous, nutrient fluid. Scanning electron microscopy of the trophonematal surface reveals branching ridges, each of which is supplied by a capillary. The secretory unit is composed of 8-10 cells joined by extensive junctional complexes. Characteristically, secretory cells have a rough endoplasmic reticulum whose irregular cisternae are grossly distended and filled with low density flocculent material. Uncoated vesicles are given off by an extensive juxtanuclear Golgi complex. Coated vesicles are also present, but are not directly associated with the Golgi complex. Electron dense granules, larger lysosome-like vesicles, and multivesicular bodies are in the vicinity of the Golgi.

Ultrastructure of the full-term shark yolk sac placenta. I. Morphology and cellular transport at the fetal attachment site

During ontogeny, the yolk sac of some viviparous sharks differentiates into a yolk sac placenta that persists to term. The placenta is non-invasive and non-deciduate. Hematrophic transport is the major route of nutrient transfer from mother to fetus. The placental unit consists of: (1) an umbilical stalk; (2) the smooth, proximal portion of the placenta; (3) the distal, rugose portion; (4) the egg envelope; and (5) the maternal uterine tissues. Exchange of metabolites is effected through the intervening egg envelope. The distal rugose portion of the placenta is the fetal attachment site. It consists of: (1) surface epithelial cells; (2) a collagenous stroma with vitelline capillaries; and (3) an innermost boundary cell layer. The columnar surface epithelial cells are closely apposed to the inner surface of the egg envelope. Wide spaces occur between the lateral margins of adjacent cells. Surface epithelial cells contain an extensive apical canalicular-tubular system and many whorl-like inclusions in their basal cytoplasm. Capillaries of the vitelline circulation are closely situated to these cells. A well-developed collagenous stroma separates the surface epithelium from an innermost boundary cell layer. In vitro exposure of full-term placentae to solutions of trypan blue and horseradish peroxidase (HRP) reveals little uptake by the smooth portion of the placenta but rapid absorption by the surface epithelial cells of the distal, rugose portion. HRP enters these cells by an extensive apical system of smooth-walled membranous anastomosing canaliculi and tubules. Prominent whorl-like inclusions that occupy the basal cytoplasm of the surface cells, adjacent to the pinocytotically active endothelium of the vitelline capillaries, are hypothesized to be yolk proteins that are transferred from the mother to embryo throughout gestation.

Ultrastructure of the full-term shark yolk sac placenta. III. The maternal attachment site

During mid- and late gestation, the uterus of sandbar sharks possesses specialized sites for exchange of metabolites between the mother and fetus. Attachment sites are highly vascular, rugose elevations of the maternal uterine lining that interdigitate with the fetal placenta. The maternal epithelium remains intact and there is no erosion. The attachment site consists of a simple, low columnar juxtaluminal epithelium underlain by an extensive vascular network. Juxtaluminal epithelial cells possess branched microvilli, saccular invaginations of the apical surface, and coated pits. They contain numerous coated vesicles, lipid-like inclusions, a prominent rough endoplasmic reticulum, and many free ribosomes. Tight junctions join the luminal aspect of adjacent cells. Lateral cell boundaries are highly folded and interdigitated. Capillaries are closely apposed to the basal cell surfaces. The endothelium is pinocytotically active. Comparison with the uterine epithelium of non-placental sharks, mammalian epitheliochorial placentae, and selected transporting epithelia reveals that the structure of the maternal shark placenta is consistent with its putative multiple functions, viz: (1) nutrient transfer; (2) transport of macromolecules, e.g., immunoglobulins; (3) respiration; and (4) osmotic and ionic regulation.

Ultrastructure of the full-term shark yolk sac placenta. II. The smooth, proximal segment

The smooth, proximal portion of the yolk sac placenta of the sandbar shark, Carcharhinus plumbeus is comprised of: (1) An outermost epithelial ectoderm; (2) an intervening collagenous stroma; and (3) an inner mesothelium. The surface epithelium may be one to three cell layers thick. The surface epithelium comprises two cell types. A cuboidal cell that has a dome-like apical surface covered with microvilli and an ovoid nucleus predominate. These cells contain lipid inclusions, many cytoplasmic filaments, and are joined by desmosomes. The second cell type has a convoluted nucleus and a flattened cell apex with microvilli, cilia, and paddle cilia. Golgi complexes and elements of the endoplasmic reticulum are relatively uncommon in the cytoplasm of both cell types. Microplicae also occur on the surface of some cells. The smooth, proximal portion of the placenta is sparsely vascularized. The innermost cellular elements of the surface epithelium rest on a prominent basal lamina. A collagenous zone separates the epithelial basal lamina from the basal lamina of the mesothelium. The mesothelial cells are squamous with a fusiform nucleus, many pinocytotic pits and vesicles, and a large number of cytoplasmic filaments. The endoplasmic reticulum, except for occasional patches of the rough type, and the Golgi complex are poorly developed. Ultrastructural tracer studies show that this portion of the placenta does not absorb horseradish peroxidase (HRP) and trypan blue.

Morphogenesis of chordae tendineae. I: Scanning electron microscopy

The formation of the chordae tendineae of the left atrioventricular valve in the chick embryo is described using scanning electron microscopy. These supportive structures for the valve cusps develop between days 6 and 13 of incubation. Elevations which represent the primitive papillary muscles form on the ventricular wall. These elevations bifurcate into thin, web-like folds which are attached to the primitive valve cusps. The folds are the primordia of the chordae tendineae. Linear ridges develop on the web between the cusp and papillary muscle. These ridges alternate with depressions. The depressions become perforate to create the individual chorda from the linear ridges. Multiple perforations form initially but they typically consolidate to create one large aperture between two chordae. Some interchordal connections of tissue do persist throughout the period studied. During the period of perforation, prominent rounded cells are typical of the endocardium between the chordae. These cells are similar at the scanning electron microscope level to those present in the formation of the foramina secunda of the atrial septum. Primary, secondary, and tertiary chordae tendineae appear to develop in the same manner. First order chordae (those attached at the free margin of a cusp) are not found in the chick embryo. The majority of the chordae are second order, which insert into the ventricular surface of the cusp a short distance from the free edge. These chordae typically have a horizontal banding or grooving along their length. Third order chordae which extend from the papillary muscle to the ventricular wall are also present. It is suggested that chordal development is a programmed cellular and hemodynamic event.

Ultrastructure of the pre-implantation shark yolk sac placenta

During ontogeny, the yolk sac of viviparous sharks differentiates into a yolk sac placenta which functions in gas exchange and hematrophic nutrient transport. The pre-implantation yolk sac functions in respiration and yolk absorption. In a 10.0 cm embryo, the yolk sac consists of six layers, viz. (1) somatic ectoderm; (2) somatic mesoderm; (3) extraembryonic coelom; (4) capillaries; (5) endoderm; and (6) yolk syncytium. The epithelial ectoderm is a simple cuboidal epithelium possessing the normal complement of cytoplasmic organelles. The endoplasmic cisternae are dilated and vesicular. The epithelium rests upon a basal lamina below which is a collagenous stroma that contains dense bodies of varying diameter. They have a dense marginal zone, a less dense core, and a dense center. The squamous mesoderm has many pinocytotic caveolae. The capillary endothelium is adjacent to the mesoderm and is delimited by a basal lamina. The endoderm contains yolk degradation vesicles whose contents range from pale to dense. The yolk syncytium contains many morphologically diverse yolk granules in all phases of degradation. Concentric membrane lamellae form around yolk bodies as the main yolk granules begin to be degraded. During degradation, yolk platelets exhibit a vesicular configuration.