The quantitative morphological characteristics of the spinal neurons in the kitten developing under conditions of limited afferent input]

Golgi preparations of cervical part of the spinal cord were used to study sparely and densely branching neurons of lamina VII, sparely and densely branching neurons of lamina VIII and large densely branching motor neurons of lamina IX in 30-day kittens developing under the conditions of limited reticulospinal input. Limitation was induced by electrocoagulation of bulbar magnocellular nucleus and caudal reticular pontine nucleus performed at the 7th day. Computerized morphometry revealed that partial deafferentation affects the geometry in all studied cell types except densely branching neurons of lamina VIII. Adaptive nature of structural reorganization of spinal neurons and possible relationship between morphological properties and functional profile of cells are discussed.     

Fine structure of the first optic ganglion (lamina) of the cockroach, Periplaneta americana

The structural organization of the first optic ganglion (lamina) of the cockroach (Periplaneta americana) was investigated by the use of light and electron microscopy. Each compound eye of the cockroach is composed of up to 2000 visual units (ommatidia) of the fused rhabdom type. The ommatidia themselves consist of eight receptor cells which terminate as axons in either the first or second optic ganglion. Three different short visual fibre types end in two separate strata in the lamina, and one long fibre type ends in the second optic ganglion. Monopolar second-order neurons with wide field branching patterns in the middle stratum of the first synaptic region have postsynaptic contacts with short visual fibres. Horizontal fibre elements with branching patterns at different levels of the lamina apparently form three horizontal plexuses with presynaptic and/or postsynaptic connections to first- and secondorder neurons. The lack of well-organized fibre cartridges containing a constant number of first and second order neurons in each fascicle and the presence of only unistratified wide field monopolar cells could represent, as compared to other insect orders, a primitive stage in the development of the first optic ganglion.     

Morphometric parameters of neurons in reticular nuclei of the brain stem in kittens

Three types of Golgi-stained neurons were discovered in brain stem reticular nuclei of 30-day-old kittens: sparsely branching reticular neurons, those with densely branching dendritic trees, and giant multipolar neurons (Leontovich's classification). Adopting computerized morphometric techniques enabled 23 different parameters to be measured in cells of these types. The measurements taken from the neuronal groups investigated revealed statistically significant differences between them for most parameters. It was concluded from this that each of the neuronal types distinguished has its own morphological identity (or stability). Characteristics of structural differences and properties of differing cell types in reticular nuclei are discussed in relation to their functional properties.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 399–409, July–August, 1991.     

Synaptic connections of the neurokinin 1 receptor-like immunoreactive neurons in the rat medullary dorsal horn

The synaptic connections between neurokinin 1 (NK1) receptor-like immunoreactive (LI) neurons and γ-aminobutyric acid (GABA)-, glycine (Gly)-, serotonin (5-HT)- or dopamine-β-hydroxylase (DBH, a specific marker for norepinephrinergic neuronal structures)-LI axon terminals in the rat medullary dorsal horn (MDH) were examined under electron microscope by using a pre-embedding immunohistochemical double-staining technique. NK1 receptor-LI neurons were observed principally in laminae I and III, only a few of them were found in lamina II of the MDH. GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were densely encountered in laminae I and II, and sparsely in lamina III of the MDH. Some of these GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were observed to make principally symmetric synapses with NK1 receptor-LI neuronal cell bodies and dendritic processes in laminae I, II and III of the MDH. The present results suggest that neurons expressing NK1 receptor within the MDH might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons located in the MDH and 5-HT- or norepinephrine (NE)-containing descending fibers originated from structures in the brainstem.     

Myoinhibitory peptide (MIP) immunoreactivity in the visual system of the blowfly <Emphasis Type="Italic">Calliphora vomitoria</Emphasis> in relation to putative clock neurons and serotonergic neurons

A few types of peptidergic clock neurons have been identified in the fruitfly Drosophila, whereas in blowflies, only pigment-dispersing factor (PDF)-immunoreactive lateral ventral clock neurons (LN_vs) have been described. In blowflies, but not Drosophila, a subset of these PDF-expressing neurons supplies axon branches to a region outside the synaptic layer of the lamina, the most peripheral optic lobe neuropil. In Drosophila, similar lamina processes are instead supplied by non-clock neurons (LMIo) that express myoinhibitory peptide (MIP). We have investigated the distribution of MIP-immunoreactive neurons in the visual system of the blowfly Calliphora vomitoria and found neurons resembling the three LMIos, but without processes to the lamina. In Calliphora, PDF-immunoreactive processes of LN_vs in the lamina closely impinge on branching serotonin-immunoreactive axon terminations in the same region. We have also identified, in the blowfly, two types of putative clock neurons that label with an antiserum to ion-transport peptide (ITP). The presence of serotonin-immunoreactive neurons supplying processes to the lamina seems to be a conserved feature in dipteran flies. The morphology of the two types of ITP-immunoreactive clock neurons might also be conserved. However, peptidergic neurons with branches converging on the serotonin-immunoreactive neurons in the lamina are of different morphological types and express PDF in blowflies and MIP in Drosophila. The central circuitry of these PDF- and MIP-expressing neurons probably differs; consequently, whether their convergence on serotonergic neurons subserves similar functions in the two species is unclear.     

The morphology of the basal lamina and periepithelial collagen during growth and branching of the embryonic lung of mice (day 14 to 16)

In order to be able to interpret the developmental mechanism of the epithelial branching pattern, we investigated lung development of mouse embryos of gestational days 14 to 16 electron microscopically. Various fixation techniques (Karnovsky, tannic acid, ruthenium red) were employed. Four regions could be distinguished in a growing and branching epithelial bud: 1) the epithelial tube before the site of branching; 2) the actual site and gap of branching; 3) the already formed part of the new buds and 4) the actual leading, i.e., the growing tip. Regions 1 and 3 were characterized by a continuous basal lamina and a more (1) or less (3) thick sheath of accompanying collagen. The site of branching (2) showed a pronounced folding of the basal lamina to which numerous collagenous fibrils were attached. At the distal, i.e., growing site (4) the basal lamina was characterized by numerous interruptions; accompanying collagen was missing. Immunomorphological investigations of regions 1 to 3 revealed all components of a basal lamina and collagen types I, III and V and fibronectin at the border between epithelium and connective tissue. However, this amount was clearly reduced at the growing tip, and collagen type I was missing. These findings help to understand and elucidate the importance of the mesenchyme for the growth and branching process and the development of the branching pattern.     

Regional Ultrastructural Differences in Basal Laminae Isolated from the Starfish Pisaster ochraceus

The ultrastructure of different regions of the basal laminae isolated from 5-1/2-6 day-old embryos of the starfish, Pisaster ochraceus , has been described after fixation in the presence of anionic dyes. Isolated basal laminae from all regions of the embryo exhibit a lamina lucida and lamina densa. No lamina fibroreticularis is present. Instead, a coarse meshwork of thick densely stained and thinner intermediately stained fibers is embedded in the lamina densa and extends into the blastocoel forming the extracellular matrix. The coarse meshwork associated with the ectodermal basal lamina consists primarily of thick densely stained fibers with a small number of intermediate ones while that associated with the endodermal one contains much less densely stained material. These structures were morphologically identical to those found in control embryos. Examination of different regions of the endodermal basal lamina shows that the amount of dense material varies from region to region. These differences in dense material may reflect biochemical differences, particularly of proteoglycans, which could provide positional information to migrating mesenchyme cells.     

Quantitative morphological characteristics of developing neurons of the sensory trigeminal nuclei in kittens

Five types of neurons were distinguished in the sensory nuclei of the trigeminal nerve, stained by Golgi's method, in kittens aged 1–5 days and 30 days: reticular and short-dendritic cells (with few branches), and multipolar giant cells, arborescent, and bushy neurons (densely branching). Yet another special type of cell, with a few short dendrites and one long dendrite, was distinguished in preparations from the brain of newborn kittens. Analysis of the dimensions of the bodies, the number, length, and ramification of the dendrites, and the total ramification of the cell yielded quantitative morphological characteristics of these neurons at different times of development. These types of neurons differed in their qualitative and quantitative parameters and in the features of their maturation.Bushy neurons underwent regressive changes during development. Foci of maximal ramification of dendrites of densely branched neurons changed their location during the first months of life relative to the cell body, moving into the more distal regions of the dendrites. Differences in orientation of dendrites with foci of maximal ramification were found relative to neighboring brain formations, which depended on the types of cells and the animal's age. The high level of maturity of trigeminal neurons at birth was demonstrated.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 14, No. 6, pp. 592–600, November–December, 1982.     

Neuronal organization of field 4 of the motor cortex of the cat brain

By means of the silver nitrate impregnation method after Golgi-Kopsch in kittens and young cats the field 4 in the cerebral motor cortex has been studied. The motor cortex of the field 4 possesses certain heteromorphism. Besides usual stellate and pyramidal neurons, that differ from real ones by some morphological signs: their body is often round, the apical dendrite is much thinner than the corresponding dendrite of a pyramidal neuron, it does not produce oblique branches along the course, never gets into the I layer, the spines arrange less densely. According to the mode of dendrites setting off, the atypical pyramidal neurons can be divided into multipolar and spindle-like with horizontal or vertical branching of the dendrites. According to the spines distribution, the multipolar atypical neurons can be divided into spinous, rare-spinous and aspinous. With respect to various cellular forms and distribution of various types of neurons in layers, every of the areas (gamma, alpha, sfu, fu) possesses specific peculiarities. The greatest variability of the neurons have the field 4 gamma and 4 alpha, where, besides stellate and pyramidal, atypical neurons can be found. The stellate neurons of the field 4 gamma are characterized with a deep arrangement, their number is essentially less, than in other areas of the field 4. In the field 4 alpha they are situated in the layers II-III. Suprafundal and fundal parts of the field do not possess pyramidal atypical neurons and are characterized with presence of large amount of the stellate neurons. In respect to the axonal branching in the suprafundal part of the field 4, 2 types of the stellate cells are distinguished.     

Morphology and physiology of multiglomerular olfactory projection neurons in the spiny lobster

Whole-cell patch-clamp recording was used to characterize olfactory projection neurons in an isolated brain preparation of the spiny lobster, Panulirus argus. Responses to electrical stimulation of the olfactory afferents were recorded from projection neuron somata using biocytin-filled electrodes. All projection neurons were multiglomerular, innervating up to 80% of all olfactory lobe glomeruli, but the innervation was heterogeneous. Most neurons densely innervated only 3–4 glomeruli; the remaining glomeruli in their dendritic arbor were sparsely innervated, thereby creating two distinct patterns of intraglomerular branching. Projection neurons responded to orthodromic stimulation with an initial depolarization and spiking followed by a 1–3 s hyperpolarization. The inhibitory phase of the response was lower in threshold and longer in latency than the excitatory phase, a response pattern also reported in olfactory projection neurons of insects and vertebrates. The somata of the projection neurons supported voltage-activated currents and TTX-sensitive action potentials, suggesting that the soma, although spatially separated from the axon and dendrites, may play a significant functional role in these cells. Dye coupling between some projection neurons correlated with the presence of multiple amplitude action potentials, suggesting that at least some projection neurons may be coupled via gap junctions.     

Golgi studies of the first optic ganglion of the ant,Cataglyphis bicolor

The neurons of the first optic ganglion (the lamina) in the desert ant, Cataglyphis bicolor, have been studied with the light microscope after Golgi silver impregnation. The different types of retinal and laminal fibres and their configuration are compared with the results obtained in the bee. The first synaptic region in the visual system of the ant lies proximally to the fenestrated layer below the basement membrane and the layer containing the monopolar cell bodies. The synaptic region can be separated into three morphologically different zones: (1) The most distal layer where the short visual fibres end at two different levels. The short visual fibres and some laminal fibres (monopolar cell fibres) also show lateral elements in this region. (2) The second layer appears almost free of branches of retinal and laminal fibres. (3) The most proximal layer, which has a characteristically dense horizontal structure resulting from the lateral elements of long visual, centrifugal, monopolar and tangential fibres. Nine cell axons arising from each ommatidium leave the retina. Six of these are short visual fibres and end at two different levels in the lamina. Three different types of short visual fibres can be distinguished by their different terminal depths and lateral branching pattern. The remaining three fibres, the long visual fibres, terminate in the medulla. They can be distinguished from each other by their lateral elements in the lamina neuropile. The five morphologically different laminal fibre types (axons of the monopolar cells in the lamina) have different shapes and different arborizations at different levels. Tangential, centrifugal and incerta sedis-fibres, which originate either from cell bodies in the cell body layer at the periphery of the outer chiasma or more centrally, terminate in the synaptic region of the lamina. Consideration is given to the clearly demarkated arrangement and length of the branching pattern of retinal and laminal fibres at different levels of the synaptic region of the lamina. In addition, a hypothetical connectivity pattern is discussed.     

Comparative anatomy of pigment-dispersing hormone-immunoreactive neurons in the brain of orthopteroid insects

Summary In a comparative study, the anatomy of neurons immunoreactive with an antiserum against the crustacean -pigment-dispersing hormone was investigated in the brain of several orthopteroid insects including locusts, crickets, a cockroach, and a phasmid. In all species studied, three groups of neurons with somata in the optic lobes show pigment-dispersing hormone-like immunoreactivity. Additionally, in most species, the tritocerebrum exhibits weak immunoreactive staining originating from ascending fibers, tritocerebral cells, or neurons in the inferior protocerebrum. Two of the three cell groups in the optic lobe have somata at the dorsal and ventral posterior edge of the lamina. These neurons have dense ramifications in the lamina with processes extending into the first optic chiasma and into distal layers of the medulla. Pigment-dispersing hormone-immunoreactive neurons of the third group have somata near the anterior proximal margin of the medulla. These neurons were reconstructed in Schistocerca gregaria, Locusta migratoria, Teleogryllus commodus, Periplaneta americana, and Extatosoma tiaratum. The neurons have wide and divergent arborizations in the medulla, in the lamina, and in several regions of the midbrain, including the superior and inferior lateral protocerebrum and areas between the pedunculi and -lobes of the mushroom bodies. Species-specific differences were found in this third cell group with regard to the number of immunoreactive cells, midbrain arborizations, and contralateral projections, which are especially prominent in the cockroach and virtually absent in crickets. The unusual branching patterns and the special neurochemical phenotype suggest a particular physiological role of these neurons. Their possible function as circadian pacemakers is discussed.     

Distribution of serotonin-immunoreactive nerve cells and fibers in the rat gastrointestinal tract

 The distribution of serotonin-immunoreactive (5HT-IR) nerve cells and fibers was thoroughly investigated immunohistochemically in the rat stomach, duodenum, jejunum, ileum, and colon. The immunoreactivity of the 5HT neurons was compared between non-treated controls and animals treated with colchicine, colchicine plus 5-hydroxytryptophan (5HTP), colchicine plus pargyline, and reserpine. The intensity of immunoreactivity in nerve fibers as well as nerve cell bodies was enhanced mostly in colchicine plus pargyline treated animals, therefore these animals were used for an observation of precise localization of 5HT in the rat gastrointestinal (GI) tract. Immunoreactivity in the nerve cell bodies and fibers was completely abolished in the GI tract of reserpine treated animals. The pattern of localization and projection of 5HT-IR neurons was similar in all segments of the rat GI tract. 5HT-IR nerve cell bodies were located in the myenteric plexus and showed the distinctive features of Dogiel type I neurons. Prominent bundles of varicose fibers traversed the myenteric ganglia and some of them surrounded the cell bodies of immunopositive and immunonegative neurons. 5HT-IR nerve fibers were located in the submucous plexus, densely entwined about the submucosal blood vessels. Most characteristically, 5HT-IR nerve fibers invaded the lamina propria of mucosa where they underlay the crypt epithelium. In conclusion, the present study showed that 5HT-IR neurons located in the myenteric plexus projected fibers widely in the rat GI tract. The localization of fibers in the lamina propria of mucosa implies that this neuron may exert an important role in the epithelial function of the GI tract. Accepted: 8 October 1996     

Neurons of the human basal ganglia (striatum and basolateral amygdala) expressing the enzyme NADPH-d

In human striatum and basolateral amygdala NADPH-d+ neurons were revealed (after Vincent et al., 1983); and in striatum strio-cortical neurons were also revealed using DiI marker (after Dahtstrom and Belichenko, 1995). The NADPH-d+ neurons were numerous in both formations. Staining of NADPH-d+ neurons with their processes, and our previous study of striatal and amygdalar human neurons by Golgi method made it possible to identify the species of neurons with their assessment as sparsely or densely branched. The main efferent neurons of striatum and basolateral amygdala (densely branched medium spiny and bushy spiny, respectively) and their densely branched interneurons were not marked. Efferent NADPH-d+ neurons included the most numerous ones in both formations. A projection of reticular striatal neurons to cortex was also shown. The NADPH-d+ interneurons belonged to sparsely branched forms. In striatum they included slender-dendritic and long-dendritic bipolars (numerous), ordinary bipolars, twisted and large poor-dendritic cells; in amygdala--the same bipolars and radial cells. Thus, the NADPH-d positive cells in the formations under study were represented by more "ancient" or less structurally complex cell forms.     

The clock input to the first optic neuropil of Drosophila melanogaster expressing neuronal circadian plasticity

In the first optic neuropil (lamina) of the fly's visual system, two interneurons, L1 and L2 monopolar cells, and epithelial glial cells show circadian rhythms in morphological plasticity. These rhythms depend on clock gene period (per) and cryptochrome (cry) expression. In the present study, we found that rhythms in the lamina of Drosophila melanogaster may be regulated by circadian clock neurons in the brain since the lamina is invaded by one neurite extending from ventral lateral neurons; the so-called pacemaker neurons. These neurons and the projection to the lamina were visualized by green fluorescent protein (GFP). GFP reporter gene expression was driven by the cry promotor in cry-GAL4/UAS-GFP transgenic lines. We observed that the neuron projecting to the lamina forms arborizations of varicose fibers in the distal lamina. These varicose fibers do not form synaptic contacts with the lamina cells and are immunoreactive to the antisera raised against a specific region of Schistocerca gregaria ion transport peptide (ITP). ITP released in a paracrine way in the lamina cortex, may regulate the swelling and shrinking rhythms of the lamina monopolar cells and the glia by controlling the transport of ions and fluids across cell membranes at particular times of the day.     

Neural organisation in the first optic ganglion of the nocturnal bee <Emphasis Type="Italic">Megalopta genalis</Emphasis>

Each neural unit (cartridge) in the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis contains nine receptor cell axons (6 short and 3 long visual fibres), and four different types of first-order interneurons, also known as L-fibres (L1 to L4) or lamina monopolar cells. The short visual fibres terminate within the lamina as three different types (svf 1, 2, 3). The three long visual fibres pass through the lamina without forming characteristic branching patterns and terminate in the second optic ganglion, the medulla. The lateral branching pattern of svf 2 into adjacent cartridges is unique for hymenopterans. In addition, all four types of L-fibres show dorso-ventrally arranged, wide, lateral branching in this nocturnal bee. This is in contrast to the diurnal bees Apis mellifera and Lasioglossum leucozonium, where only two out of four L-fibre types (L2 and L4) reach neighbouring cartridges. In M. genalis, L1 forms two sub-types, viz. L1-a and L1-b; L1-b in particular has the potential to contact several neighbouring cartridges. L2 and L4 in the nocturnal bee are similar to L2 and L4 in the diurnal bees but have dorso-ventral arborisations that are twice as wide. A new type of laterally spreading L3 has been discovered in the nocturnal bee. The extensive neural branching pattern of L-fibres in M. genalis indicates a potential role for these neurons in the spatial summation of photons from large groups of ommatidia. This specific adaptation in the nocturnal bee could significantly improve reliability of vision in dim light. B.G. is grateful for travel awards from the Royal Physiographic Society, the Per Westlings Fond, the Foundation of Dagny and Eilert Ekvall and the Royal Swedish Academy of Sciences. E.J.W. acknowledges the receipt of a Smithsonian Short-Term Research Fellowship and thanks the Swedish Research Council, the Crafoord Foundation, the Wenner–Gren Foundation and the Royal Physiographic Society of Lund for their ongoing support. W.T.W. was supported by general research funds from the Smithonian Tropical Research Institute     

Patch-clamp analysis of voltage- and ligand-activated whole-cell currents in freshly dissociated neurons of the locust optic lobe

Whole-cell patch-clamp recordings were obtained from 116 freshly dissociated neuronal somata from the optic lobe of adult locusts (Schistocerca gregaria). Prerequisites were a papain treatment and the directed transfer of somata to the recording chamber by dabbing. Of the recorded somata, 65 were from lamina and 51 from other optic lobe neurons. All somata supported voltage-activated outward currents and some (24% of optic lobe, 3% of lamina neurons) also fast inward currents. Most lamina neurons supported an outward current that activated (V _1/2=−8.5 mV) and inactivated rapidly and a sustained outward current. Some lamina and most optic lobe neurons expressed only a sustained outward current (V _1/2=−9.4 mV). GABA and histamine elicited inward currents at negative holding potentials. Most optic lobe (95%) but only 18% of lamina neurons showed a γ-aminobutyric acid (GABA) current, whereas a similar percentage of optic lobe (50%) and lamina neurons (67%) expressed a histamine current. Both currents reversed near the chloride equilibrium potential, were reversibly reduced by picrotoxin, and did not show rundown. Thus, they likely represent chloride currents mediated by ionotropic receptors. Our data indicate that the lamina neurons recorded mainly represent monopolar cells postsynaptic to histaminergic photoreceptors. The optic lobe neurons, on which GABA and histamine apparently act as inhibitory neurotransmitters, are more heterogeneous. Accepted: 30 November 1997     

FMRFamide-like immunoreactivity in the brain and pituitary of the teleost. Eigenmannia lineata (Gymnotiformes)

The distribution of cells immunoreactive for the molluscan tetrapeptide FMRFamide in the brain and the pituitary of Eigenmannia was investigated immunohistochemically by the use of the peroxidase-antiperoxidase (PAP) technique and unlabelled antibodies. FMRFi neurons were located in the ganglion of the nervus terminalis at the rostroventral side of the bulbus olfactorius. FMRFi perikarya were also found in a dorsomedial diencephalic nucleus, in the nucleus ventromedialis, in some liquor-contacting neurons of the nucleus lateralis tuberis and of the nucleus recessus lateralis and posterior. The perikarya of the midbrain pre-pacemaker nucleus were only weakly immunoreactive for FMRFamide while large FMRFi neurons (T-cells) occurred in lamina VI of the torus semicircularis, in the brain stem, in dorsal and medial layers of the lobus lineae lateralis posterior (LLLp) and in the medullary electric organ pacemaker nucleus (pm). FMRFi fibers and nerve endings were found in the bulbus olfactorius, in medial areas of the telencephalon, and rather densely in the rostral diencephalon. Ventrocaudally to most of the hypothalamic nuclei the occurrence of immunoreactive fibres increased; many coursed to the pituitary through the pituitary stalk. FMRFi fibres also appeared in the deep layers of the tectum opticum, in the torus semicircularis, in the medial and lateral medulla and below the pacemaker nucleus. Wherever FMRFamide-immunoreactivity occurred fibres and nerve endings could be found in close contact with blood vessels.     

Ultrastructure of the basal lamina and its relationship to extracellular matrix of embryos of the starfish Pisaster ochraceus as revealed by anionic dyes

The ultrastructure of the 51/2–6-day-old embryonic asteroid basal lamina (BL) was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and after treatment with anionic dyes. Conventional fixation in glutaraldehyde and osmium reveals a BL consisting of a lamina densa separated from the basal cell surface by a lamina lucida. Little or no reticular lamina is present. Material similar in appearance to the basal lamina extends into the blastocoel, forming an extracellular matrix (ECM). Following fixation in the presence of the dye ruthenium red, proteoglycan (PG) granules are visible in the lamina lucida and immediately beneath the lamina densa. The ECM consists of granules of a similar appearance, which are associated with fibers of an intermediate electron density resembling invertebrate collagen. After fixation in the presence of alcian blue under polyanionic conditions, all aspects of the basal lamina and the ECM stain very densely. The use of alcian blue in 0.3 M MgCl₂ (monoanionic condition) or in low concentrations reveals a lamina densa consisting of a fine feltwork and tubule-like structures. A meshwork composed of thick, densely stained and thinner, intermediately stained strands is embedded in the inner aspect (that adjacent to the blastocoel) of the ectodermal lamina densa. Similar elements are present in the endodermal BL, but the dense material is represented by short regions that do not form a meshwork. The dense and intermediate strands of both basal laminae also extend into the blastocoel as ECM. The tubule-like structures extend from the dense material of the inner meshwork into the lamina densa. They also cross both the lamina densa and lucida to associatee with the basal cell membranes. The fact that the basal cell surfaces are often puckered outward at the points of contact suggests that this configuration might be providing a means whereby forces can be transferred from the ECM through the basal lamina to the cells.