Development of esophageal epithelium in the fetal and neonatal mouse

Development of the fetal mouse esophageal epithelium was followed using light microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and radioautography. At 15 days of gestation in the cervical (C), mediastinal (M), and abdominal (A) segments of the esophagus, the epithelium was two or three cells thick, and only cells located in the basal (germinal) layer incorporated tritiated thymidine. Ciliated cells were sparse in all three segments. At 17 days of gestation, longitudinal mesenchymal ridges became more differentiated in the distal segment. Labeling indices were lower than at preceding stages in each segment. Ciliated cells had increased in number and appeared to be evenly distributed along the whole esophagus. In periodic acid-Schiff (PAS)-stained sections, an increasing proximodistal distribution of glycogen stores was observed, with greatest concentrations found in segment A. At 18 days of gestation, labeling indices were comparable in segments C and M (11.7% +/- 2.9% and 12.8% +/- 1.9%, respectively) but remained higher in segment A (17.9% +/- 2.0%). Ciliated cells were still present. At this stage, transverse circular furrows and ridges started to appear. They increased in number at 4 days after birth and were very closely distributed in the adult. In longitudinal sections, these ridges corresponded to projections of stratum granulosum and of the overlying stratum corneum. After birth, ciliated cells desquamated rapidly but some patches were still present at 4 days. At 8 days, the esophageal epithelium was not yet keratinized.     

Expression of SARS-CoV-2 entry factors in human oral tissue

The distribution of cells expressing SARS-CoV-2 entry factor angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) in human oral tissues were tested. The investigation was conducted with normal flesh tissue and paraffin-embedded specimens. The ACE2 and TMPRSS2 expression was detected with all subjects in the normal mucosa of the keratinized stratified squamous epithelia of the tongue and non-keratinized stratified squamous epithelia of the lip and cheek. It was found that ACE2 is expressed in the cytoplasm and on the cell membrane mainly in the stratum granulosum of the epithelia while the TMPRSS2 is strongly expressed on the cell membrane mainly in the stratum granulosum and stratum spinosum, but not in the stratum basale. Antibodies’ reactions for ACE2 and TMPRSS2 were not observed in the nuclei or keratin layer. The expression of ACE2 and TMPRSS2 in the oral epithelia appears to be general, and the expression was also observed in the mucous and serous acini of the labial glands. The SARS-CoV-2 may transiently attach to the oral mucosa and the minor salivary glands which are present under all of the oral mucosa. The oral cavity can be considered an important organ for SARS-CoV-2 attachment and may provide a preventive medical avenue to guard against COVID-19 by preventing saliva from scattering.     

Mucous metaplasia of the hamster cheek pouch epithelium under hypervitaminosis A.

Mucous metaplasia of the hamster cheek pouch epithelium induced by implantation of vitamin A pellet was studied electron-microscopically. After 5–10 days of vitamin A treatment, the epithelium was devoid of stratum corneum, and often infiltrated with inflammatory cells. The intercellular spaces were wide, and membrane-coating granules underwent modifications believed to be the earliest signs of mucous production. After 15 days of vitamin A treatment the inflammation had largely gone, and the epithelium was made up of several layers of cuboidal cells. The intercellular spaces and keratin fibrils had greatly decreased. Some mucous granules appeared for the first time in the cuboidal superficial cells. Various other changes in the cheek pouch epithelium were observed in animals treated with vitamin A for 20–25 days. In some regions hyperplasia or inflammatory reactions were observed, whereas in other regions complete mucous metaplasia was found. In completely metaplastic regions a surface layer of typical mucous cells was seen lying atop two or three layers of cuboidal cells.     

Immunohistochemical detection of epidermal growth factor and epidermal growth factor receptor in the lingual mucosa of rats during the morphogenesis of filiform papillae

We examined the immunofluorescence labelling epidermal growth factor (EGF) and epidermal growth factor receptor (EGFR), as well as differential interference contrast (DIC) images, during the morphogenesis of filiform papillae and the keratinization of the lingual epithelium of rats on semi-ultrathin sections of epoxy resin-embedded samples using laser-scanning microscopy. We also examined semi-ultrathin sections of epoxy resin-embedded, toluidine blue-stained samples by light microscopy to obtain details of cell histology and morphology. No immunoreactivity specific for EGF and EGFR was detected on the lingual epithelium of fetuses on days 12 and 16 after conception (E12 and E16), during which time the number of layers of cuboidal cells in the lingual epithelium increased from one to several. Immunoreactivity specific for EGF and EGFR was first detected on the lingual epithelium of fetuses at birth or on postnatal day 0 (P0). Immunoreactivity specific both for EGF and EGFR appeared in the connective tissue and the basal cells of the papillary and interpapillary cell columns. The lingual epithelium was composed of stratified squamous cells. The rudiments of filiform papillae were compactly arranged and interpapillary cell columns were very narrow. Immunoreactivity specific for EGF and EGFR was distinct on the cell membrane of basal cells of the papillary cell column and weakly positive on the cell membrane of basal cells of the interpapillary cell column on postnatal day 21 (P21). Thus, the patterns of immunoreactivity of EGF and EGFR differed as the filiform papillae developed. Filiform papillae developed gradually from P0 to P21. The width of interpapillary spaces also increased during this period. These observations indicate a possibility that EGF might affect the expression of keratins in the lingual epithelium via epithelium-mesenchymal interactions.     

Interrelationships between cornification and cell migration of fetal rat epidermis in vitro

Samples of dorsal skin were obtained from 18-, 19- and 20-day rat fetuses. Comparative developmental studies were carried out from a portion of the samples, while the remaining samples were cultured for four days in a medium containing tritiated thymidine and studied autoradiographically. At 18 days the epidermis contained a periderm, a stratum intermedium and a stratum basale. By 19 days the strata granulosum and spinosum had developed. At 20 days, a stratum corneum was present. Cultured samples of 18-day fetal rat skin resembled those that developed in utero with respect to time, whereas, the samples of 19- and 20-day fetal rat skin developed a more extensive stratum corneum, fewer keratohyalin granules, and a more condensed stratum Malpighii. Autoradiographic studies were made to determine the time taken for cells to migrate from the stratum basale to the outermost layer of the stratum granulosum (transit time). A similar “transit time” was noted for the three fetal ages studied. The rates of cell migration varied. The results of this study suggest that the extent of cornification impedes cell migration by providing a physical resistance or barrier against the outward forces presumably exerted by the division of basal cells.     

Ultrastructure of the amnion and amniotic plaques of the white-tailed deer

The ultrastructure of the aminon and aminotic plaques of the white-tailed deer was studied throughout pregnancy. The single layered amniotic epithelium had short, blunt, club-shaped microvilli, tortuously folded lateral plasma membranes enclosing an intercellular space, and basal foot-procsses. Wherever lateral plasma membrane processes abutted, desmosomes occurred. Hemidesmosomes were observed only in association with the foot-processes. The epithelium contained granular endoplasmic reticulum, rod-shaped mitochondria, free ribosomes, Golgi complexes, tonofibrils, tonofilaments and glycogen granules. The epithelium was supporte by a basal lamina. The subjacent connective tissue layer contained mesenchymal cells, fibroblasts and collagenous fibers. The amniotic plaques varied in size from microscopic to 4 mm in diameter. The small plaques were formed by mitotic activity of the aminion cells. their sites of formation appeared randomly distributed. Each microscopic plaque had one to three layers of basal columnar to cuboidal cells, and a single layer of covering cells next to the amniotic cavity. Further mitotic activity of basal columnar cells led to the formation and differentiation of large plaques which contained over 15 cell layers. The main mass of a large plaque showed four zones: stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Cytoplasmic organelles were plentiful in the basal and spinous cells, but were sparse in the granular and cornified layers. Keratohyalin granules and dense tonofibrils were usually found in the granular cells, but only occasionally in cornified cells. Each plaque contained varying amounts of PAS-positive glyucogen granules. The tortuously folded lateral plasma membranes enclosed intercellular spaces which extended from basal to cornified layers. Small portions of amniotic cells and layers of cornified cells sloughed throughout pregnancy and thus contributed to the composition of amniotic fluid. The homology of the plaques with mammalian skin and their distribution in the eutherian mammals is discussed.     

Towards economy of effort in quantitative ultrastructural pathology: efficient sampling schemes for studying experimental carcinogenesis

Hamster cheek pouch mucosa was painted with DMBA in order to compare samples of epithelium at different pathological stages of carcinogenesis. Stereological methods were applied to ultrathin sections of tissue to estimate one ultrastructural index of change, the surface ratio of lamina densa compared with the overlying plasma membrane of cells in stratum basale. Analysis of variance techniques were then employed to isolate and quantify the contributions which different levels of sampling (animals, tissue blocks, microscopic fields) made to the total observed variation in this surface ratio. Economical sampling schemes for future use were calculated from the sampling variances by taking into account the relative costs at each sampling level. Though illustrated by means of the hamster cheek pouch-DMBA model, our results are pertinent to many other experimental models for quantitative histopathology.     

Lectin binding patterns and monoclonal antibodies to epidermal antigens in tumours of the skin

Monoclonal antibodies to epidermal antigens and cell surface carbohydrate markers, as defined by lectin binding, were used to analyze the cells in squamous and basal cell carcinomas of the skin (SCC and BCC). The cells in BCC failed to stain with the lectin peanut agglutinin (PNA), which stains surface carbohydrates of cells in the stratum spinosum and stratum granulosum layers of normal epidermis, confirming histological observations that the cells in BCC are incapable of differentiation beyond the basal cell stage. Conversely, the central cells in SCC did react with PNA, suggesting that they can differentiate to a stage equivalent to the stratum spinosum of epidermis. The zone immediately surrounding BCC differed from that around SCC in lectin binding and staining with antisera to laminin and fibronectin, an observation which could be connected with the failure to metastasize. It was of interest that histologically normal skin immediately adjacent to and overlying these tumours showed marked changes in reaction with markers of normal epidermis. The outer layers of this epidermis showed aberrant retention of the lower molecular weight cytokeratins marked by the monoclonal antibodies LMM2 and LMM3, and occasional strong staining of individual cells by the stratum granulosum-reactive LMM1. These changes appear to be indicative of a 'premalignant' state in these cells and the monoclonal antibodies are thus potentially useful reagents for early detection of skin malignancies.     

Lack of insulin-like growth factor 1 (IGF1) in the basal keratinocyte layer of diabetic skin and diabetic foot ulcers

Wound healing, including re-epithelialization, is delayed in diabetes. Growth factors influence the healing process and amongst these, insulin-like growth factor (IGF) has been shown to stimulate keratinocyte proliferation in vitro. Monoclonal antibodies to insulin-like growth factors 1 and 2 (IGF1 and IGF2) were used to investigate their distribution in diabetic foot ulcers and surrounding tissues by immunohistochemistry, compared with diabetic and non-diabetic uninjured skin. IGF2 was found throughout the epidermis (stratum granulosum, spinosum, and basale) in all three groups. Staining for IGF2 was intense in both normal and diabetic skin as well as in diabetic foot ulcers, being greatest at the ulcer edge. IGF1, in comparison, was found throughout the epidermis of non-diabetic skin; expression was restricted to the stratum granulosum and spinosum of uninjured diabetic skin and was absent in the basal layer at the ulcer edge. A similar absence of IGF1 in dermal fibroblasts was found in tissue sections from diabetic patients. This lack of expression of IGF1 within the basal layer and fibroblasts may contribute to retarded wound healing in diabetes mellitus.     

Granule cell genesis in the hippocampus of rats treated neonatally with hydrocortisone

Rats were injected with hydrocortisone acetate on postnatal days 1–4 and the genesis of granule cells in the dentate gyrus was followed by autoradiographic techniques using tritiated thymidine. In short-survival autoradiographic experiments, the number of cells labeled by [³H] thymidine throughout the dentate hilus and stratum granulosum was decreased during and immediately after hydrocortisone treatment, but recovered to control values during the second week. Growth of this region was retarded as indicated by a 20% reduction in volume of the stratum granulosum at 7 days and 14% deficit in de-oxyribonucleic acid content in the whole hippocampus on days 4–6. At 60 days, however, the volume of the stratum granulosum in treated rats was not significantly different from that of controls.Granule cell ‘birthdays’ at dorsal and ventral levels of the hippocampus were investigated by longsurvival autoradiography in treated and control groups. Dorsally, the pattern of granule cell birthdays over the first 3 postnatal weeks was not significantly affected. Ventrally, granule cell genesis in treated rats was significantly depressed on day 5 when that in controls was maximal. This delayed the peak of granule cell birthdays at this level by several days.These experiments demonstrate that the rate and pattern of postnatal granule cell genesis in the rat hippocampus are altered by neonatal glucocorticoid treatment. Specific effects are compared to those previously reported for cerebellar neurogenesis in the glucocorticoid-treated rat.     

Early development of thymic lymphocytes in mice,studied by light and electron microscopy

Thymic rudiments from mouse embryos varying in gestational from 10–13 days were examined by light and electron microscopy in an effort to learn the origin of thymic lymphocytes. Lymphocytes were recognized and distinguished from mesenchymal cells and thymic epithelial cells by their round shape, larger nucleoli, and high concentration of cytoplasmic ribosomes and dearth of endoplasmic reticulum. No lymphocytes could be found at the earliest stage of development of the third pharyngeal pouch — at approximately ten days' gestational age. They first appeared in the mesenchyme surrounding the third pouch at a later stage, (approximately 11 days' gestation), but in most cases did not appear in the thymic epithelium until the parathyroid and thymus began to differentiate at approximately 12 days' gestation. No cells were seen that appeared to be transitional between lymphocytes and epithelial or mesenchymal cells and it was concluded that these observations support the hypothesis that lymphocytes first reach the thymic parenchyma by immigration from the surrounding mesenchyme. Most of the lymphoctyes found in the region of the thymus at these stages of development were large lymphocytes. The origin of these early lymphocytes remains unknown.     

A scanning electron microscopic study of the opossum nasal cavity prior to and shortly after birth

Summary The nasal cavities of opossums prior to and shortly after birth were examined by scanning electron microscopy. Numerous morphologically mature olfactory receptor neurons are observed in the dorso-rostralmost extent of the olfactory epithelium positioned adjacent to the opening of the nares in all prenatal stages and newborn animals examined. The remainder of the olfactory epithelium, occupying a more dorso-caudal position within the nasal cavity, is undifferentiated, and lacks morphologically mature receptor neurons. A short transition zone of stratified squamous epithelium lies between the epithelium lining the nares and olfactory epithelium. It forms an abrupt junction with the latter. The remainder of the nasal cavity in this group of animals is lined by a non-ciliated pseudostratified type (undifferentiated respiratory) of epithelium. By the end of the second postnatal week the morphologically mature olfactory epithelium is no longer observed in the vestibular area of the nasal cavity, which is lined by stratified squamous epithelium at this time. Mature receptor neurons are now observed within the olfactory epithelium lining the roof of the nasal cavity and covering the turbinates. The greater part of the nasal cavity is lined by a ciliated respiratory epithelium. It is proposed that the precocious differentiation of mature olfactory receptor neurons within the rostral-most extent of the olfactory epithelium just prior to birth is important in guiding the newborn young to the pouch.Currently Visiting Professor to The Department of Anatomy and Human Biology, The University of Western Australia, Perth, Australia     

Histochemical aspects concerning the synthesis and the fate of cholesterol into the epidermis

By means of a suitable histochemical method free cholesterol and its esters could be detected into the epidermis layers. The results show that in the stratum spinosum keratinocytes free cholesterol appears as an amorphous or granular structure apparently protein unbound; into the stratum granulosum keratinocytes the cholesterol becomes protein-bound and its most part undergoes esterified. The extracellular compartment nearly the stratum granulosum contains a little amount of cholesterol esters loosely bound to proteins. The results suggest that free cholesterol after being synthesized into the cytoplasm of the stratum spinosum and granulosum keratinocytes, it is partially esterified and becomes protein-bound, appearing as fine granules within the cytoplasm of the granulous cells. From this site it takes to fates:1. Its most part remains into the granulous cell cytoplasm and at the same time the granulous cell develop to the horny cell it is placed on the thick cell membrane inner surface contributing to its thickness; 2. Another part after reaching the extracellular compartment it is spread over the thick membrane out surface. Inside the thick cell membrane, into the horny layer, free cholesterol is continuously esterified since the keratinizing cell migrate to the periphery; however even at the most peripheral layers the free cholesterol predominates. Either free cholesterol or its esters, contained into the keratinizing cell thick membrane, were excreted throughout the horny layer exfoliation. The keratinizing cell cytoplasm does not contain neither free cholesterol nor its esters.     

Histochemical studies on glycosaminoglycans in Bruch's membrane of postnatal rat eyes

Localization of glycosaminoglycans (GAG) in Bruch's membrane of postnatal rat eyeballs was examined histochemically. Fixed eyeballs from postnatal rats (ages 5 days and 8 weeks) were routinely processed and embedded in paraffin wax or Quetol 651 resin. Paraffin-embedded tissue sections were stained with hematoxylin and eosin or sensitized high iron diamine procedure in combination with selective methods such as GAG-degrading enzyme digestions and/or a chemical modification, and examined by light microscopy. Quetol 651-embedded ultrathin sections were stained with heavy metals and examined by electron microscopy. In rats at postnatal day 5, Bruch's membrane contained mainly chondroitin sulfate (CS) and heparan sulfate (HS). In contrast, at 8 weeks after birth the membrane included a large amount of dermatan sulfate (DS) and HS. According to electron microscopic findings, Bruch's membrane on day 5 consisted of only 3 layers without a central elastic layer. However, at 8 weeks after birth the membrane was constructed of 5 layers. These findings suggested that the difference in GAG molecular species in the membranes at 5 days and at 8 weeks after birth could be correlated with the development and maturation of the collagenous layer in Bruch's membrane. Moreover, maturation of Bruch's membrane may contributes to the architectural stabilization of the outer portions of the photoreceptor cells.     

A histochemical study on the vitamin D synthesis into the epidermis

Using a suitable histochemical method vitamins D and 7-dehydrocholesterol could be shown into the epidermis of several mammal species. As the histochemical method used is able to discriminate vitamins D and 7-dehydrocholesterol from cholesterol and its esters, the sites where these vitamins were synthesized within the epidermis layers could be established. Vitamins D and 7-dehydrocholesterol were found into the epidermis in the same sites where cholesterol and its esters take place, such as: the keratinizing cell thick membrane and the stratum spinosum and stratum granulosum keratinocytes cytoplasm. Inside the keratinocyte cytoplasm vitamin D shows a granular pattern and appears weakly bound to proteins. The reactivity of such granules seems to be partially blocked as could be shown by an hydrolysis accomplished previously. After the hydrolysis reactive vitamin D was also found inside the epidermis intercellular space. The results suggest that vitamin D is synthesized into the cytoplasm of stratum spinosum and stratum granulosum keratinocytes, where it appears weakly bound to proteins. Afterwards it reaches the intercellular space, where its synthesis is accomplished and it becomes firmly protein-bound losing its histochemical reactivity. However, after a suitable hydrolysis the histochemical reactivity could be recovered. From the intercellular spaces vitamin D could take 2 fates: It was partially incorporated on the keratinizing cell thick membrane out surface and eliminated by means of the epidermis exfoliation. It was partially absorbed after passing across the basement membrane. On the other hand, the vitamin D placed inside the stratum spinosum and stratum granulosum keratinocytes cytoplasm become incorporated on the inner surface of the keratinizing cell thick membrane. The relationship between vitamin D biosynthesis and the epidermis lamellar bodies was discussed.     

Differentiation of the lingual and palatal gustatory epithelium of the rat as revealed by immunohistochemistry of alpha-gustducin.

We used alpha-gustducin, a taste-cell-specific G protein to investigate the onset of taste transduction and its relation to the development of the palatal and lingual taste buds. Frozen cryostat and paraffin sections were prepared from the palatal and lingual gustatory epithelium of the rat from birth till postnatal day 21 (PN 21d). At PN 1-7d, alpha-gustducin-immunoreactive solitary ovoid or bipolar cells were scattered among the oral epithelium either horizontally along the oral surface or vertically oriented between the basal lamina and oral surface. In the circumvallate and foliate papillae, these cells became wrapped in alpha-gustducin-immunonegative cells surrounded by an extracellular space forming a bud-like structure. Simultaneously, different stages of typical taste buds were recognized, but alpha-gustducin was only expressed in some neonatally developed pored buds. At PN 1d, alpha-gustducin was expressed in pored taste buds with a relatively higher frequency recorded in the soft palate as compared with the nasoincisor, circumvallate, and foliate papillae. The immunoreactive cells were spindle shaped with elongated processes extending from the base to the pore of the taste buds. During the second week, the solitary cells could no longer be recognized while the total counts of immunoreactive cells within the taste buds gradually increased. We argue that taste transduction is essentially required from the time of birth and can be fulfilled by both of the solitary chemosensory cells, which are immunoreactive for alpha-gustducin and scattered in the oral epithelium, and the taste cells within the mature taste buds. Moreover, the onset of taste transduction accomplished by the palatal taste buds developed earlier than that achieved by taste buds in the circumvallate and foliate papillae.     

Corneodesmosin expression in psoriasis vulgaris differs from normal skin and other inflammatory skin disorders.

SUMMARY: Corneodesmosin (Cdsn) is a late differentiation epidermal glycoprotein putatively involved in keratinocyte adhesion. The Cdsn gene lies within the susceptibility region on chromosome 6p21.3 (PSORS1) for psoriasis, a common chronic disfiguring skin disease. A particular allelic variant of Cdsn has a strong association with psoriasis. Therefore, genetically and biologically, Cdsn is a possible candidate gene for psoriasis susceptibility. To investigate a potential role for Cdsn in psoriasis pathogenesis, protein expression studies were performed by quantitative immunohistochemistry on normal skin, psoriatic skin (lesional and nonlesional), and other skin disorders using monoclonal antibodies (G36-19 and F28-27). In normal and nonlesional skin, Cdsn was expressed in stratum corneum and one or two layers of superficial stratum granulosum. In lesional psoriasis, there was a significant increase in Cdsn expression, which was observed in multiple layers of stratum spinosum and in stratum corneum. The expression pattern varied from granular, cytoplasmic immunoreactivity to cell surface labeling with weakly immunoreactive cytoplasm. In chronic atopic dermatitis, lichen planus, mycosis fungoides, and pityriasis rubra pilaris, Cdsn immunoreactivity was confined to stratum corneum and upper stratum granulosum with no stratum spinosum immunoreactivity. Immunoelectron microscopy of normal and lesional psoriatic skin demonstrated Cdsn release concomitant with involucrin incorporation into cell envelopes and completed before mature envelope formation. Extracellular release of Cdsn occurred at a lower level of the epidermis in psoriasis than normal skin. These protein expression studies provide evidence of altered Cdsn expression in psoriasis consistent with a role of Cdsn in disease pathogenesis. Further functional and genetic studies of Cdsn are justified to determine its role as a potential psoriasis-susceptibility factor.     

Ontogeny of enkephalin-like immunoreactivity in the rat hippocampus

The postnatal development of leucine5-enkephalin-like immunoreactivity within the hippocampal formation of the rat has been analyzed using immunocytochemical techniques. From the day of birth to postnatal day three, no intrinsic hippocampal elements exhibit immunoreactivity although labeled axons are found within the fimbria, within the alveus, and in the vicinity of the angular bundle. On postnatal day 4, a few immunoreactive hippocampal neurons can be seen in stratum radiatum of the region CA3 and by postnatal day 8, within the hilus, strata pyramidale and oriens of regio superior, and the subiculum. There is a dramatic increase in the incidence of immunoreactive perikarya between postnatal days 8 and 10 in all fields as well as the appearance of labeled neurons in CA1 stratum pyramidale and stratum granulosum of the dentate gyrus. Two days after the first appearance of immunoreactive perikarya, intensely immunoreactive neurons, labeled much more extensively than is ever seen in the adult, are encountered in each subfield of the hippocampus. The spatio-temporal order in both the emergence of perikaryal immunoreactivity and the transient appearance of intensely immunoreactive neurons follows that of neurogenesis, with immunoreactivity developing 12-14 days after the peak period of last cell division for a given hippocampal region. The incidence of immunoreactive perikarya in the dentate gyrus was quantified in rat pups ranging from postnatal days 8 to 19. The appearance of labeled neurons followed the spatio-temporal gradients that have been described for neurogenesis in this region as well. Immunoreactive perikarya emerged in the suprapyramidal stratum granulosum prior to their emergence in the infrapyramidal zone and in the temporal pole of the dentate earlier than in the mid-dorsoventral dentate. The lateral perforant path and mossy fiber axons, seen to exhibit enkephalin-like immunoreactivity in the adult hippocampal formation, differ in their relative maturity at the age immunoreactivity first appears. Immunoreactivity appears as early as postnatal day 4 in the lateral perforant path, an age at which these axons are just growing into their target field while it is not found within the mossy fibers until after postnatal day 10, an age at which mossy fiber bouton elaboration is well advanced and physiologically competent mossy fiber synapses with the regio inferior pyramidal cells have been established. The latter observation indicates that enkephalin is not necessary for synaptic transmission at the mossy fiber synapse.     

The role of desmosomes in the ear plug formation in the bowhead whale (Balaena mysticetus)

The external acoustic meatus (EAM) of most baleen whales accumulates cellular debris annually in the lumen as whales age, forming a lamellated ear plug. The bowhead whale ear plug is formed from annually molting lining of the EAM as the entire epithelium releases at the level of the stratum basale during the spring migration. Epithelial regeneration is mostly completed by the fall migration, remaining intact for 6–7 months before being torn off the following spring. Desmosomes are integral to cell–cell adhesion with connecting desmosomal cadherins desmoglein (dsg) and desmocollin (dsc). Paraffin sections of the oral cavity and EAM lining of spring and fall adult bowhead whales, as well as the EAM of spring-caught juvenile, were immunohistochemically examined for the presence of these cadherins. In all fall specimens, both cadherins occurred in all layers except the superficial keratinous layer of the oral cavity. In spring, three different conditions existed: (a) oral cavity of spring-caught adults had reduced cadherins, with superficial fissuring in its keratinized layer and vacuolation in the upper stratum spinosum; (b) EAM of juvenile spring-caught whales displayed fissuring with accompanying reduction of both cadherins in its superficial lining; and (c) EAM lining of spring-caught adults displayed deep fissures, reduced cadherins, and absence of dsc1 in the fissuring zone. These results suggest that shedding of skin layers in mammals, whether normal molting, pathological, or the result of injury and wound repair all revolve around desmosome function. The specific role, structure, and location of these two cadherins need to be further addressed.