Ribosome heterogeneity is a paradigm in biology, pertaining to the existence of structurally distinct populations of ribosomes within a single organism or cell. This concept suggests that structurally distinct pools of ribosomes have different functional properties and may be used to translate specific mRNAs. However, it is unknown to what extent structural heterogeneity reflects genuine functional specialization rather than stochastic variations in ribosome assembly. Here, we address this question by combining cryo-electron microscopy and tomography to observe individual structurally heterogeneous ribosomes in bacterial cells. We show that 70% of ribosomes in Psychrobacter urativorans contain a second copy of the ribosomal protein bS20 at a previously unknown binding site on the large ribosomal subunit. We then determine that this second bS20 copy appears to be functionally neutral. This demonstrates that ribosome heterogeneity does not necessarily lead to functional specialization, even when it involves significant variations such as the presence or absence of a ribosomal protein. Instead, we show that heterogeneous ribosomes can cooperate in general protein synthesis rather than specialize in translating discrete populations of mRNA.

Drugs that target bacterial ribosomes are widely used in modern medicine and veterinary practice to treat bacterial infections and prevent the spread of antimicrobial resistance. However, most studies on targeting ribosomes with drugs are limited to just a few model organisms. Consequently, we do not know whether the ribosomal drug-binding sites observed in model bacteria are as highly conserved across bacteria as is currently implied. In this study, we address this question using a simple but powerful computational pipeline that filters out rare variants and sequencing errors to identify conserved changes at ribosomal drug-binding sites across the bacterial tree of life. This allows us to assess the conservation of 82 individual drug-binding residues of bacterial ribosomes from 8,809 bacterial species. For each of these residues, we trace its evolution throughout >4 billion years of bacterial history. Contrary to the common belief in the high conservation of ribosomal drug-binding residues, we find extensive variation among bacterial phyla at drug-binding sites. Furthermore, we show that approximately 10% of bacterial species bear ribosomal RNA (rRNA) substitutions that were previously observed only in clinical isolates of drug-resistant bacteria. Overall, our work illustrates that our traditional division of ribosomes into bacterial and eukaryotic types is oversimplistic and misleading, as it overlooks widespread lineage-specific variations that make the drug-binding sites of some bacteria more dissimilar to Escherichia coli than E. coli is to humans. These findings will have numerous implications for the lineage-specific use of ribosome-targeting antibiotics that are currently viewed as universal inhibitors of bacterial protein synthesis. ### Competing Interest Statement The authors have declared no competing interest.

Drugs that target eukaryotic ribosomes are becoming increasingly important as research tools and as potential therapies against cancer, fungi and other pathogenic eukaryotes. However, in the absence of comparative studies, we currently do not know how many eukaryotes possess ribosomal drug-binding sites identical to those in humans, and how many significantly differ from humans. Currently, this gap in knowledge is exacerbated by the presence of pseudogenes in eukaryotic genomes, making these comparative analyses challenging due to our inability to discriminate between genuine mutations, pseudogenes and sequencing artifacts. In this study, we resolve this problem by using a novel approach that leverages evolutionary relationships among species. Using this approach, we determine sequence variants for each of the 42 ribosomal drug-binding residues across 8,201 representative eukaryotes, tracing the evolutionary history of these variations from the emergence of eukaryotes 2 billion years ago to their subsequent divergence into distinct lineages. Unexpectedly, we find that yeasts and humans, commonly used as model eukaryotes to study ribosome/drug interactions, differ from most other eukaryotes due to the rRNA substitutions that mainly occur in animals and fungi but are absent in most other eukaryotes. Furthermore, we demonstrate that structural variants previously identified in pathogenic Leishmania and Plasmodium and viewed as idiosyncratic to a few eukaryotic species, are in fact shared by large clades of eukaryotes. Notably, some eukaryotic lineages exhibit ribosomal drug-binding sites that are more dissimilar to those of humans than humans are to bacteria. Overall, our study provides the most complete overview of ribosomal drug-binding sites evolution in eukaryotes, at the levels of single species, single residues, and single drugs, illuminating the eukaryotic lineages with structurally distinct ribosomal drug-binding sites compared to those of humans. These findings open new avenues for employing ribosome-targeting drugs as research tools and for the development of lineage-specific inhibitors against eukaryotic parasites. ### Competing Interest Statement The authors have declared no competing interest.

Ribosomes are often used in synthetic biology as a tool to produce desired proteins with enhanced properties or entirely new functions. However, repurposing ribosomes for producing designer proteins is challenging due to the limited number of engineering solutions available to alter the natural activity of these enzymes. In this study, we advance ribosome engineering by describing a novel strategy based on functional fusions of ribosomal RNA (rRNA) with messenger RNA (mRNA). Specifically, we create an mRNA-ribosome fusion called RiboU, where the 16S rRNA is covalently attached to selenocysteine insertion sequence (SECIS), a regulatory RNA element found in mRNAs encoding selenoproteins. When SECIS sequences are present in natural mRNAs, they instruct ribosomes to decode UGA codons as selenocysteine (Sec, U) codons instead of interpreting them as stop codons. This enables ribosomes to insert Sec into the growing polypeptide chain at the appropriate site. Our work demonstrates that the SECIS sequence maintains its functionality even when inserted into the ribosome structure. As a result, the engineered ribosomes RiboU interpret UAG codons as Sec codons, allowing easy and site- specific insertion of Sec in a protein of interest with no further modification to the natural machinery of protein synthesis. To validate this approach, we use RiboU ribosomes to produce three functional target selenoproteins in Escherichia coli by site- specifically inserting Sec into the proteins' active sites. Overall, our work demonstrates the feasibility of creating functional mRNA- rRNA fusions as a strategy for ribosome engineering, providing a novel tool for producing Sec- containing proteins in live bacterial cells.

To conserve energy during starvation and stress, many organisms use hibernation factor proteins to inhibit protein synthesis and protect their ribosomes from damage 1 , 2 . In bacteria, two families of hibernation factors have been described, but the low conservation of these proteins and the huge diversity of species, habitats and environmental stressors have confounded their discovery 3 – 6 . Here, by combining cryogenic electron microscopy, genetics and biochemistry, we identify Balon, a new hibernation factor in the cold-adapted bacterium Psychrobacter urativorans . We show that Balon is a distant homologue of the archaeo-eukaryotic translation factor aeRF1 and is found in 20% of representative bacteria. During cold shock or stationary phase, Balon occupies the ribosomal A site in both vacant and actively translating ribosomes in complex with EF-Tu, highlighting an unexpected role for EF-Tu in the cellular stress response. Unlike typical A-site substrates, Balon binds to ribosomes in an mRNA-independent manner, initiating a new mode of ribosome hibernation that can commence while ribosomes are still engaged in protein synthesis. Our work suggests that Balon–EF-Tu-regulated ribosome hibernation is a ubiquitous bacterial stress-response mechanism, and we demonstrate that putative Balon homologues in Mycobacteria bind to ribosomes in a similar fashion. This finding calls for a revision of the current model of ribosome hibernation inferred from common model organisms and holds numerous implications for how we understand and study ribosome hibernation.

When ribosome-targeting antibiotics attack actively growing bacteria, they occupy ribosomal active centers, causing the ribosomes to stall or make errors that either halt cellular growth or cause bacterial death. However, emerging research indicates that bacterial ribosomes spend a considerable amount of time in an inactive state known as ribosome hibernation, in which they dissociate from their substrates and bind to specialized proteins called ribosome hibernation factors. Since 60% of microbial biomass exists in a dormant state at any given time, these hibernation factors are likely the most common partners of ribosomes in bacterial cells. Furthermore, some hibernation factors occupy ribosomal drug-binding sites – leading to the question of how ribosome hibernation influences antibiotic efficacy, and vice versa. In this review, we summarize the current state of knowledge on physical and functional interactions between hibernation factors and ribosome-targeting antibiotics and explore the possibility of using antibiotics to target not only active but also hibernating ribosomes. Because ribosome hibernation empowers bacteria to withstand harsh conditions such as starvation, stress, and host immunity, this line of research holds promise for medicine, agriculture, and biotechnology: by learning to regulate ribosome hibernation, we could enhance our capacity to manage the survival of microorganisms in dormancy.

Protein translation is orchestrated through tRNA aminoacylation and ribosomal elongation. Among the highly conserved structure of tRNAs, they have distinguishing features which promote interaction with their cognate aminoacyl tRNA synthetase (aaRS). These key features are referred to as identity elements. In our study, we investigated the tRNA:aaRS pair that installs the 22nd amino acid, pyrrolysine (tRNAPyl:PylRS). Pyrrolysyl-tRNA synthetases (PylRSs) are naturally encoded in some archaeal and bacterial genomes to acylate tRNAPyl with pyrrolysine. Their large amino acid binding pocket and poor recognition of the tRNA anticodon have been instrumental in incorporating >200 noncanonical amino acids. PylRS enzymes can be divided into three classes based on their genomic structure. Two classes contain both an N-terminal and C-terminal domain, however the third class (ΔpylSn) lacks the N-terminal domain. In this study we explored the tRNA identity elements for a ΔpylSn tRNAPyl from Candidatus Methanomethylophilus alvus which drives the orthogonality seen with its cognate PylRS (MaPylRS). From aminoacylation and translation assays we identified five key elements in ΔpylSn tRNAPyl necessary for MaPylRS activity. The absence of a base (position 8) and a G-U wobble pair (G28:U42) were found to affect the high-resolution structure of the tRNA, while molecular dynamic simulations led us to acknowledge the rigidity imparted from the G-C base pairs (G3:C70 and G5:C68).

Transfer RNAs (tRNAs) facilitate the fundamental process of protein translation; converting an mRNA sequence into protein. Among the highly conserved secondary and tertiary structure of tRNAs, include distinguishing features which promote interaction with their cognate aminoacyl tRNA synthetase (aaRS) for aminoacylation. These features are referred to as tRNA identity elements and usually consist of 4-7 bases. Our interest is in the tRNA:aaRS pair which installs the 22nd amino acid, pyrrolysine (tRNAPyl:PylRS).

Ribosomes from different species can markedly differ in their composition by including dozens of ribosomal proteins that are unique to specific lineages but absent in others. However, it remains unknown how ribosomes acquire new proteins throughout evolution. Here, to help answer this question, we describe the evolution of the ribosomal protein msL1/msL2 that was recently found in ribosomes from the parasitic microorganism clade, microsporidia. We show that this protein has a conserved location in the ribosome but entirely dissimilar structures in different organisms: in each of the analyzed species, msL1/msL2 exhibits an altered secondary structure, an inverted orientation of the N-termini and C-termini on the ribosomal binding surface, and a completely transformed 3D fold. We then show that this fold switching is likely caused by changes in the ribosomal msL1/msL2-binding site, specifically, by variations in rRNA. These observations allow us to infer an evolutionary scenario in which a small, positively charged, de novo-born unfolded protein was first captured by rRNA to become part of the ribosome and subsequently underwent complete fold switching to optimize its binding to its evolving ribosomal binding site. Overall, our work provides a striking example of how a protein can switch its fold in the context of a complex biological assembly, while retaining its specificity for its molecular partner. This finding will help us better understand the origin and evolution of new protein components of complex molecular assemblies-thereby enhancing our ability to engineer biological molecules, identify protein homologs, and peer into the history of life on Earth.

Throughout the tree of life, cells and organisms enter states of dormancy or hibernation as a key feature of their biology: from a bacterium arresting its growth in response to starvation, to a plant seed anticipating placement in fertile ground, to a human oocyte poised for fertilization to create a new life. Recent research shows that when cells hibernate, many of their essential enzymes hibernate too: they disengage from their substrates and associate with a specialized group of proteins known as hibernation factors. Here, we summarize how hibernation factors protect essential cellular enzymes from undesired activity or irreparable damage in hibernating cells. We show how molecular hibernation, once viewed as rare and exclusive to certain molecules like ribosomes, is in fact a widespread property of biological molecules that is required for the sustained persistence of life on Earth.

In the process of evolution, the multicameral stomach acquires a unique morphological structure, which is predetermined by the influence of various natural and environmental factors. The pre-ventricle, which consists of three chambers, serves as a reservoir for storing and preparing the resulting food coma from plant food. The aim of the study is to determine the dynamics of the increase in the absolute mass of the chambers of the multicameral stomach in sheep of the Edilbaevsky breed at different age periods of growth and development. The material for the study was corpses and organ complexes obtained from sheep of the Edilbaevsky breed from the farm “Slaughter point” IP Yusubov O.M. of the Leningrad region in the amount of 13 pieces. The age of the animals from which the biomaterial was obtained varied and ranged from 10 days to 14 months. During the study, a complex of modern measures was applied, including various research methods: fine anatomical dissection; morphometry; the absolute mass of the multicameral stomach was determined on an electronic scale “MS-K07” with an accuracy of 0.10 g. It was found that by 5-6 months of age, the absolute mass of the book increases most actively, this is due to a change in diet and the transition from the dairy period to feeding with coarse feeds. From 5-6 months of age to 12-14 months, the highest rate of increase in absolute mass is developed in the mesh, this is due to an increase in the functional load on this camera. During the same period, the absolute mass of the scar, the book and the abomasum increases relatively evenly. Over the entire period of studies of multicameral stomachs in different age groups of sheep of the Edilbaev breed, we note that the absolute mass of the abomasum increases evenly, this is due to the initial functional activity of this chamber from birth. At the same time, the greatest increase in absolute mass over the entire period of research is noted in the book.

Ribosomal genes are widely used as 'molecular clocks' to infer evolutionary relationships between species. However, their utility as 'molecular thermometers' for estimating optimal growth temperature of microorganisms remains uncertain. Previously, some estimations were made using the nucleotide composition of ribosomal RNA (rRNA), but the universal application of this approach was hindered by numerous outliers. In this study, we aimed to address this problem by identifying additional indicators of thermal adaptation within the sequences of ribosomal proteins. By comparing sequences from 2021 bacteria with known optimal growth temperature, we identified novel indicators among the metal-binding residues of ribosomal proteins. We found that these residues serve as conserved adaptive features for bacteria thriving above 40°C, but not at lower temperatures. Furthermore, the presence of these metal-binding residues exhibited a stronger correlation with the optimal growth temperature of bacteria compared to the commonly used correlation with the 16S rRNA GC content. And an even more accurate correlation was observed between the optimal growth temperature and the YVIWREL amino acid content within ribosomal proteins. Overall, our work suggests that ribosomal proteins contain a more accurate record of bacterial thermal adaptation compared to rRNA. This finding may simplify the analysis of unculturable and extinct species.

AbstractRibosomal genes are widely used as “molecular clocks” to infer the evolutionary relatedness of species. It is unclear, however, whether these genes can also serve as “molecular thermometers” to precisely estimate an organism’s optimal growth temperature. Previously, some estimations were made using the average nucleotide content in ribosomal RNA, but the universal application of this approach was prevented by numerous outliers. Here, seeking to bypass this problem, we asked whether ribosomal genes contain additional markers of thermal adaptations, aside from their nucleotide composition. To answer this, we analyzed site-specific variations in sequences of ribosomal proteins from 2,021 bacteria with known optimal growth conditions. We found that ribosomal proteins comprise a few “mutational hotspots”—residues that vary in a temperature-dependent manner and distinguish heat- and cold-adapted bacteria. Most of these residues coordinate metal ions that support protein folding at high temperatures. Using these residues, we then showed that the upper and lower limits of an organism’s optimal growth temperatures can be estimated using just 0.001% of the genome sequence or just two amino residues in the cellular proteome. This finding illustrates that laboratory-independent estimation of optimal growth temperatures can be simplified if we abandon the traditional use of rRNA and protein sequences to assess their content and instead focus on those few residues that are most critical for protein structure. This finding may simplify the analysis of unculturable and extinct species by helping bypass the need for laborious, costly, and at times impossible laboratory experiments.

There are many techniques for studying the vascular bed in anatomy, but angiocardiography is one of the most informative. Bright and clear X‐ray images allow researchers to trace the course and branching of the vessels of the studied area and carry out their morphometry. When examining the vascular bed of voluminous organs and structures, it is difficult to “read” angio‐roentgenograms. This is due to paired vessels of the same name and large vascular collectors, the radiographic shadows of which can overlap. Given the above, we have revealed the applied methods of bilateral angiography of volumetric organs and structures. When carrying out angiocardiography, we used a mass for injections prepared according to the recipe as a radio‐opaque mass: 45% ‐ lead white, 45% ‐ gum turpentine, 10% ‐ medical gypsum powder (M.V. Shchipakina, A.V. Prusakova, D. S. Bylinskaya, S. A. Kuga, 2013). The object of the study was the vascular bed of the head of animals. Before injecting the radiopaque mass, the cadaver material was prepared as follows: the head was separated from the body at the level of the sixth cervical vertebra and heated in a water bath (temperature 45°C, time 5 hours). For the injection of the head vessels, access was made through the common carotid arteries. After their catheterization, the large arteries of the neck were ligated. Also, the spinal canal was tamponed. The infusion of a radiopaque mass was performed alternately into each of the common carotid arteries. The quality of the infusion was assessed by the degree of filling of the arteries of the oral mucosa and conjunctiva. When filling small vessels with a white mass, the infusion was considered sufficient. Then the objects of study were placed in a chamber with a temperature regime of 0°C for two days to coagulate the X‐ray contrast mass. One of the important stages of the method of bilateral angiography of volumetric structures, the preparation stage, was started. The whole point of this stage is the sequential division in the median plane of the object under study. So, initially, the skin of the dorsal surface of the head and neck was dissected. The tissues were divided along the internasal and frontal sutures along the median line of the occipital bone. At the same time, the nasal cavity and the brain were divided in the median plane, reaching the base of the skull. In the neck area, muscles, vertebral arches, spinal cord, and vertebral bodies were dissected. Only the ventral muscles of the neck were left, not dissected. In the median plane, the hard and soft palate and the skull base bones were dissected. The lower jaws were divided by fusion, while the tongue and organs of the intermandibular space were left unchanged. During X‐ray examination, the two halves of the head were spread apart, pressing the cut point to the surface. Thus, when using our technique of infusion and preparation, it is possible to perform angiography of volumetric organs and structures without imposing “shadows” of symmetric vessels of the same name and their bilateral visualization.

The evolution of microbial parasites involves the counterplay between natural selection forcing parasites to improve and genetic drifts forcing parasites to lose genes and accumulate deleterious mutations. Here, to understand how this counterplay occurs at the scale of individual macromolecules, we describe cryo-EM structure of ribosomes from Encephalitozoon cuniculi , a eukaryote with one of the smallest genomes in nature. The extreme rRNA reduction in E. cuniculi ribosomes is accompanied with unparalleled structural changes, such as the evolution of previously unknown molten rRNA linkers and bulgeless rRNA. Furthermore, E. cuniculi ribosomes withstand the loss of rRNA and protein segments by evolving an ability to use small molecules as structural mimics of degenerated rRNA and protein segments. Overall, we show that the molecular structures long viewed as reduced, degenerated, and suffering from debilitating mutations possess an array of compensatory mechanisms that allow them to remain active despite the extreme molecular reduction.

During starvation and stress, virtually all organisms arrest protein synthesis to conserve energy. Inactive ribosomes are converted into a dormant state, in which they are protected from damage by hibernation factor proteins. In bacteria, two major families of hibernation factors have been described, but the low conservation of these proteins and the huge diversity of species, habitats, and environmental stressors has confounded their discovery. In this study, using proteomics and cryo-EM, we identify a new dormancy factor from the psychrophilic bacterium Psychrobacter urativorans . By isolating ribosomes under cold-shock conditions, we observe a previously unknown protein bound to the ribosomal A site, protecting critical elements of both the decoding and peptidyl transferase centers. We show that this new factor, which we term Balon, is a homolog of the archaeo-eukaryotic translation factor aeRF1, providing a long-predicted evolutionary “missing link” between the eukaryotic and bacterial translation machinery. Our structures reveal that Balon is delivered to both vacant and actively translating ribosomes by EF-Tu, highlighting an unexpected and previously unknown role for this elongation factor in the bacterial stress response. We describe several unique structural motifs that allow Balon to bind ribosomes in an mRNA-independent manner, initiating a new mode of ribosome dormancy that can commence while ribosomes are still engaged in protein synthesis. Our bioinformatic analysis shows that putative Balon-encoding genes can be found within stress-response operons in nearly 20 % of all known bacterial species, including many human pathogens. Taken together, our work suggests that Balon/EF-Tu regulated ribosome dormancy is likely to be a ubiquitous stress-response mechanism throughout the bacterial kingdom. These findings call for a revision of our model of bacterial translation inferred from common model organisms and hold numerous implications for how we understand and study ribosome dormancy.

The mitral valve, also known as the left atrioventricular valve, is located in the left side of the heart between the left atrium and the left ventricle. The valve structures are involved in the completion of the pulmonary circulation and contribute to the continuous circulation of blood in the animal's body. Knowledge of the valve leaflets' histological features in the breed aspect allows the most accurate and individual approach to diagnosing, preventing, and treating cardiac pathologies. Our study aims to research the histological features of the left atrioventricular valve structure in the Anglo-Nubian goats. The material for the study was ten corpses of Anglo-Nubian goats aged from one to two years. The material was fixed with a 10% formalin solution for 24 hours, and then it was paraffin-embedded. The prepared sections were stained with hematoxylin and eosin. The mitral valve of the Anglo-Nubian goats has anterior and posterior leaflets in its composition, also labeled septal and mural. Each leaflet consists of fibrous plates formed from dense connective tissue with a high amount of cells. The leaflets have three layers in their structure: the atrialis, which faces the left atrial cavity; the fibrosa, which faces the left ventricular cavity; and the spongiosa, which is located between the previous two. The thickness of the atrialis layer of mitral valve leaflets in examined Anglo-Nubian goats is 175.42±19.0 μm. This layer is characterized by a smooth surface, which is covered with one layer of endothelial cells. Some areas of the histological section are superimposed like scales on top of each other. In contrast to the atrialis, the fibrosa layer has a rough surface due to high outgrowths on it, from which the chordae tendineae begin. The thickness of the layer in the Anglo-Nubian goats averages 126, 35±16.3 μm. As well as atrialis, fibrosa is covered with one layer of endothelial cells and has a similar histological picture. The spongiosa layer of the mitral valve leaflets is consisting of loose connective tissue. Large amount of glycosaminoglycan is observed on a histological section of this layer. The thickness the spongiosa is 286±46.8 μm. Based on the results of our research, the following conclusions can be drawn: The leaflets of Anglo-Nubian goats' mitral valve consist of three layers, of which the spongiosa layer is most developed. A large amount of glycosaminoglycan was found on the histological section of the spongiosa layer, which confirms the development of valve leaflets from the cardiac skeleton.

The ductus venosus is a physiological vascular shunt to the umbilical and caudal vena cava in the fetus. The umbilical vein (vena umbilicalis) leaves the placenta, enters the fetus through the foramen umbilical, and goes to the liver hilum. It carries oxygenated and nutrient-rich blood from the placenta to the fetus. Part of the blood enters the liver through the umbilical vein, through the shunt - the ductus vein enters the central venous line - the caudal vena cava. This mechanism of blood redistribution is necessary for feeding the fetal brain with blood that is more saturated with oxygen. Given the scarce data on the topography, shape, and morphometric parameters of the venous duct in animals, we set the goal of revealing the methods used for its postmortem study. For a complete and comprehensive study of the topography of the venous duct, it is necessary to use a set of anatomical methods: thin anatomical preparation, vasoradiography, production of corrosive preparations, and morphometry. When conducting vasoradiography, a mass for injection prepared according to the recipe was used as a radio-opaque mass: 45% - lead white, 45% - gum turpentine, 10% - medical gypsum powder (M.V. Shchipakina, A.V. Prusakova, D.S. Bylinskaya, S.A. Kuga, 2013). In the manufacture of corrosive preparations, self-hardening dental plastic was used. It belongs to the acrylic group of cold curing plastics and consists of powder and solvent components. In both cases, the injection of these substances was carried out through the umbilical vein, without opening the abdominal cavity, by accessing through the umbilical cord. After introducing the X-ray contrast mass, the objects of study were placed in a fixing solution (10% formalin solution) and left in it for up to 5 days. Subsequently, thin anatomical preparation and photography, as well as radiography, were performed. After introducing a plastic solution, the objects were placed in chambers with a temperature of +4°C for two days. Subsequently, corrosion treatment was carried out in an aqueous potassium hydroxide solution for 4 - 5 days. The morphometry of the venous duct in the study of vaso-radiographs was carried out in the RadiAnt computer program, in the study of a corrosive preparation - using an electronic caliper (Stainless hardened). On vaso-radiographs, the ductus venous is identified as a sizeable cone-shaped vessel located in the area of the liver hilum. The ductus venosus is identified as a craniodorsal blood vessel that flows into the caudal vena cava on the casts of the vascular bed. The proposed methods for studying the ductus venosus make it possible to accurately anatomically describe the position, shape, and size of the ductus venosus in fetuses.

The secretory cells of the mammary gland are very diverse in their size, shape, and structure. These are highly specialized cells adapted to the synthesis, accumulation, storage, and excretion of milk. The main feature of glandular cells is the presence of secreted substances. Electron microscopy shows that substances of different secretory cells types have various electron densities. Occasionally, two or more types of secretory granules differing in size and composition can be found in one cell. The material for the morphological study was small (2-4 mm) tissue samples of goat lactating mammary glands. Pieces were taken from deeper areas of the breast parenchyma. Ultrathin sections with a 50-70 nm thickness were obtained for electron microscopic analysis using Leica UC7 ultramicrotome. Electron microscopy was performed using a JEOL JEM 1011 microscope. Micrographs were obtained using a Morada camera (Digital Imaging Solutions Inc.). The secretion of the mammary gland occurs according to the apocrine type. Picture of the lactocytes' cytoplasm apical part separation into the lumen of the alveoli is often observed on semi-thin sections. Thus, numerous droplets of material secreted by lactocytes are found in the lumen of the alveoli. The shape of lactocytes in the lactating mammary gland is close to cubic. The height of the epithelial layer, generally, is 10-12 microns (µm). The milk ducts, like the alveoli, are dilated. Most of them contain a significant amount of osmiophilic material secreted by the cells of the mammary alveoli. The cells of the epithelium of the single-layered ducts have a flattened or cuboidal shape. The diameter of the ducts varies noticeably depending on location in the gland parenchyma. The intralobular milk ducts are 10-20 µm in diameter. At the same time, the larger interlobular ducts reach a diameter of 40-50 microns and are located in groups in the connective tissue stroma of the mammary gland. Notably, an increase in the mammary alveoli mass in the lactating gland is associated with a decrease in connective tissue volume and the almost complete disappearance of fat cells groups from it. The milk ducts of the teat zone in the lactating gland are strongly expanded, round, or slightly oval-shaped in cross-section with a diameter of 30-60 microns. In contrast to the deeper zones of the mammary gland, we were unable to detect secreted substances in the dilated milk ducts near the teat area. The milk ducts we studied are located close to the most terminal sections of the mammary gland excretory system (gland cistern and teat canal). The milk is inevitably washed out from the ducts during chemical fixation and subsequent stages of objects preparation for histological analysis.

The lymphatic system of the heart is directly involved in the spread and development of many pathological processes in animals, particularly neoplasias of various etiologies. Precise knowledge of the lymphatic outflow pathways from the heart gives access to close study of pathogenesis. It allows to actively influence the elimination of the process through preventive and therapeutic measures. In this regard, our study aims to explore the anatomical and topographic features of the lymphatic outflow of the heart in an Anglo‐Nubian goat. The material for the study was fifteen corpses of Anglo‐Nubian goats aged from one to two years. The research methods were thin anatomical preparation, ink‐gelatin perfusion of lymphatic heart vessels, perfusion with Gerota's mass and yellow lead paint, and subsequent radiography of the material under study. It was established that subepicardial lymph nodes collect lymph from the endocardium, myocardium, and epicardium. In Anglo‐Nubian goats, the subepicardial lymphatic vessels of the left surface of the left ventricle and the caudal part of the left surface of the right ventricle merge in the sulcus interventricularis paraconalis and sulcus coronarius, forming lymphatic vessels of a higher order. After the fusion of the vessels, the left lymph collector of the heart is formed. It follows the ventral surface of a. pulmonalis, bends around it ventrally or dorsally, and then flows into the left tracheobronchial lymph node. Subepicardial lymphatic vessels of the cranial part of the left surface of the right ventricle and the right surfaces of the right and left ventricles merge in sulcus interventricularis subsinuosus and sulcus coronarius, also forming lymphatic vessels of higher orders named right lymph collector. The vessel passes under auricula dextra, goes to truncus brachiocephalicus, and is guided through it cranially until it flows into the cranial mediastinal lymph node, located in the Anglo‐Nubian goat at the beginning of a. subclavia sinistra. The outflowing lymphatic vessels leave the cranial mediastinal and left tracheobronchial nodes, flow either directly into the thoracic duct or into the common trunk of the mediastinal lymph nodes. The atrial lymphatic vessels of the Anglo‐Nubian goat flow either into the lymphatic vessel of sulcus coronarius or directly into the cranial mediastinal lymph nodes. Thus, according to the results of thу study, it was established that the lymphatic drainage of the heart in a goat of the Anglo‐Nubian breed is carried out in two main ways, dividing into the left and right halves of the heart. Also, according to the research data, in a goat of the Anglo‐Nubian breed, the possibility of varying the outflow of lymph from the atria in two ways was determined.