Since the classical studies of Pío del Río-Hortega, microglia research has come a long way. In particular, recent advances in bulk and single-cell (sc) transcriptomics have yielded many fascinating new insights into these intriguing immune cells at the interface with the central nervous system (CNS), both in small animal models and human samples. In parallel, tools developed by advanced mouse genetics have revealed the unique ontogeny of microglia and their striking dynamic interactions with other cells in the brain parenchyma. In this chapter, we will discuss various applications of the Cre/loxP-based approach that have enabled the study of microglia in their physiological context of the mouse brain. We will highlight selected key findings that have shaped our current understanding of these cells and discuss the technical intricacies of the Cre/loxP approach and some remaining challenges.

Spexin (SPX1) is a neuropeptide of 14 amino acids (aa), originally identified by bioinformatics, which has been implicated in various physiological functions in vertebrates via galanin receptors 2 and 3 (GALR2/3). To clarify the biological role of SPX1 in the control of reproduction in yellowtail kingfish, which is regarded as a promising species for offshore aquaculture worldwide, cDNA sequences of spx1 and six potential receptors were identified in the current study. The open reading frame of yellowtail kingfish spx1 was 363 nucleotides in size that encoded a 120-aa preprohormone, and its mature peptide was highly conserved among other species. The cDNA sequences of six GALRs (galr1a, galr1b, galr2a, galr2b, galr type 1, and galr type 2) were 1053 base pairs (bp), 1068 bp, 981 bp, 1137 bp, 1038 bp, 924 bp, which encoded G protein-coupled receptors of 350 aa, 355 aa, 326 aa, 378 aa, 345 aa, 307 aa, respectively. Tissue distribution analysis showed that spx1, galr1b, and galr2b transcripts were mainly detected in the brain. The highest mRNA levels of galr1a and galr2a were observed in the pituitary, followed by the brain and ovary. Both galr type 1 and galr type 2 were widely expressed in various tissues, with a peak level in the kidney. Moreover, all spx1 and galr genes significantly fluctuated during early ontogeny, exhibiting different expression patterns. Intraperitoneal injection of SPX1 significantly increased brain gnrh1, gnih, spx1, gal, and tac3 expression, while it inhibited gnrh2, kiss1r, and kiss2r mRNA levels. In the pituitary, SPX1 injection reduced transcript levels of gh, lhβ, and fshβ. Overall, our results have revealed the involvement of SPX1 in the reproductive functions in yellowtail kingfish.

Chirostoma estor (Jordan, 1879) is an endemic freshwater species with a high potential for aquaculture; however, as in many other fish, larviculture of this species is the most critical stage, in which the higher mortality rates. Therefore, it is necessary to fully describe the development of the digestive system to establish better feeding protocols in the larval culture of C. estor, both for aquaculture and restoration purposes. In the present study, larviculture was carried out from hatching to 20 days after hatching (DAH). The organisms were fed with the rotifer Brachionus plicatilis from 2 to 14 DAH, and nauplii of Artemia sp. from 15 to 20 DAH. A total of 12 organisms (0, 3, 5, 10, 15, and 20 DAH) were taken for size and weight growth and histological and histochemical analysis. The histological analysis indicated that after 3 DAH, the opening of the mouth and anus was observed, coinciding with the beginning of exogenous feeding. In addition, the digestive system developed, with differentiation of the oropharyngeal sections, esophagus, and intestine, folding of the intestinal mucosa, as well as associated organs (liver and pancreas) that reach their maximum development at 20 DAH. Thus, C. estor at this stage of development can digest and absorb nutrients despite being an agastric fish. The results obtained in this study will facilitate a better understanding of the ontogenetic morphophysiological development processes, associated with the transition of larvae to exogenous feeding, which ensures a higher percentage of survival during larval development and of course, adds to the diversity ontogenetics of teleostean.

Skin stem cells (SCs) play a pivotal role in supporting tissue homeostasis. Several types of SCs are responsible for maintaining and regenerating skin tissue. These include bulge SCs and others residing in the interfollicular epidermis, infundibulum, isthmus, sebaceous glands, and sweat glands. The emergence of skin SCs commences during embryogenesis, where multipotent SCs arise from various precursor populations. These early events set the foundation for the diverse pool of SCs that will reside in the adult skin, ready to respond to tissue repair and regeneration demands. A network of molecular cues regulates skin SC behavior, balancing quiescence, self-renewal, and differentiation. The disruption of this delicate equilibrium can lead to SC exhaustion, impaired wound healing, and pathological conditions such as skin cancer. The present review explores the intricate mechanisms governing the development, activation, and differentiation of skin SCs, shedding light on the molecular signaling pathways that drive their fate decisions and skin homeostasis. Unraveling the complexities of these molecular drivers not only enhances our fundamental knowledge of skin biology but also holds promise for developing novel strategies to modulate skin SC fate for regenerative medicine applications, ultimately benefiting patients with skin disorders and injuries.

Acoustic signalling, integral to intraspecific communication and reproductive behaviour, undergoes notable changes during an animal\'s ontogenetic development. The onset and progression of this maturation in fish remains poorly understood. Here, we investigated the ontogeny of acoustic communication in the miniature teleost Danionella cerebrum, one of the smallest known vertebrates and an emerging model organism. Its adult males produce audible clicks that appear in sequences with a repetition rate of ∼60 or ∼120 Hz, caused by consecutive unilateral or alternating bilateral compressions of the swim bladder. To investigate the maturation of this ability, we performed long-term sound recordings and morphological studies of the sound production apparatus in D. cerebrum throughout its ontogenetic development. We found that fish start producing clicks during the second month of their lives and continually increase their abundance and structured repetition over the course of the following 1 to 2 months. The sound production machinery, including specialised bone and cartilage structures, starts to form in males after approximately 4 weeks and prior to reaching sexual maturity. Although clicks increase in amplitude as animals mature, click repetition rates of 60 and 120 Hz are stable throughout development. This suggests fully mature pattern generation in juvenile males, yet a continued development of the drumming apparatus capable of creating louder sounds.

The helper-like ILC contains various functional subsets, such as ILC1, ILC2, ILC3 and LTi cells, mediating the immune responses against viruses, parasites, and extracellular bacteria, respectively. Among them, LTi cells are also crucial for the formation of peripheral lymphoid tissues, such as lymph nodes. Our research, along with others\', indicates a high proportion of LTi cells in the fetal ILC pool, which significantly decreases after birth. Conversely, the proportion of non-LTi ILCs increases postnatally, corresponding to the need for LTi cells to mediate lymphoid tissue formation during fetal stages and other ILC subsets to combat diverse pathogen infections postnatally. However, the regulatory mechanism for this transition remains unclear. In this study, we observed a preference for fetal ILC progenitors to differentiate into LTi cells, while postnatal bone marrow ILC progenitors preferentially differentiate into non-LTi ILCs. Particularly, this differentiation shift occurs within the first week after birth in mice. Further analysis revealed that adult ILC progenitors exhibit stronger activation of the Notch signaling pathway compared to fetal counterparts, accompanied by elevated Gata3 expression and decreased Rorc expression, leading to a transition from fetal LTi cell-dominant states to adult non-LTi ILC-dominant states. This study suggests that the body can regulate ILC development by modulating the activation level of the Notch signaling pathway, thereby acquiring different ILC subsets to accommodate the varying demands within the body at different developmental stages.

The mechanosensory lateral line (LL) system of salmonid fishes has been the focus of comparative morphological studies and behavioral and physiological analyses of flow sensing capabilities, but its morphology and development have not been studied in detail in any one species. Here, we describe the post-embryonic development of the cranial LL system in Brook Trout, Salvelinus fontinalis, using vital fluorescent staining (4-Di-2-ASP), scanning electron microscopy, µCT, and clearing and staining to visualize neuromasts and the process of cranial LL canal morphogenesis. We examined the relationship between the timing of LL development, the prolonged life history of salmonids, and potential ecological implications. The LL system is composed of seven canals containing canal neuromasts (CNs) and four lines of superficial neuromasts (SNs) on the skin. CNs and SNs increase in number and size during the alevin (larval) stage. CN number stabilizes as canal morphogenesis commences, but SN number increases well into the parr (juvenile) stage. CNs become larger and more elongated than SNs, but the relative area occupied by sensory hair cells decreases during ontogeny in both types of neuromasts. Neuromast-centered canal morphogenesis starts in alevins (yolk sac larvae), as they swim up into the water column from their gravel nests (~4 months post-fertilization), after which yolk sac absorption is completed and exogenous feeding begins. Canal morphogenesis proceeds asynchronously within and among canal series and is not complete until ~8 months post-fertilization (the parr stage). Three characters in the LL system and associated dermal bones were used to identify their homologs in other actinopterygians and to consider the evolution of LL canal reduction, thus demonstrating the value of salmonids for the study of LL evolution. The prolonged life history of Brook Trout and the onset of canal morphogenesis at swim-up are predicted to have implications for neuromast function at these critical behavioral and ecological transitions.

Primates show large interindividual variability in the character and quantity of interactions between mothers and their immature offspring. Multiple studies have documented associations between maternal behavior and the occurrence or frequency of certain behaviors among offspring, but it remains unclear whether and how early maternal interactions generally affect behavioral development in offspring. We followed two wild groups of Japanese macaques on Yakushima island and investigated the relationship between maternal behavior and several types of behavior performed by 35 juvenile offspring. We further asked if the impact of maternal behavior on juvenile behavior persists regardless of the distance between mother and offspring, testing whether the influence extends beyond cases when the mother is nearby. We found that juveniles whose mothers frequently rejected them approached and played with others more often, independent of their mother\'s presence. Juveniles of more protective mothers were in proximity to fewer other individuals and played less, but only if their mothers were nearby. Maternal rejection appears to exert a generalized effect on offspring behavior that endures when mothers are absent. In contrast, effects of maternal protectiveness may be temporary and/or reflect direct maternal influences, such as active intervention in offspring interactions, or effects of the mother\'s own social relationships on offspring interactions. Our results suggest that understanding how maternal behavior affects offspring development requires paying attention to the context of juvenile behavior, including the mother\'s distance from her offspring.

Study of morphogenesis and its regulation requires analytical tools that enable simultaneous assessment of processes operating at cellular level, such as synthesis of transcription factors (TF), with their effects at the tissue scale. Most current studies conduct histological, cellular and immunochemical (IHC) analyses in separate steps, introducing inevitable biases in finding and alignment of areas of interest at vastly distinct scales of organization, as well as image distortion associated with image repositioning or file modifications. These problems are particularly severe for longitudinal analyses of growing structures that change size and shape. Here we introduce a python-based application for automated and complete whole-slide measurement of expression of multiple TFs and associated cellular morphology. The plugin collects data at customizable scale from the cell-level to the entire structure, records each data point with positional information, accounts for ontogenetic transformation of structures and variation in slide positioning with scalable grid, and includes a customizable file manager that outputs collected data in association with full details of image classification (e.g., ontogenetic stage, population, IHC assay). We demonstrate the utility and accuracy of this application by automated measurement of morphology and associated expression of eight TFs for more than six million cells recorded with full positional information in beak tissues across 12 developmental stages and 25 study populations of a wild passerine bird. Our script is freely available as an open-source Fiji plugin and can be applied to IHC slides from any imaging platforms and transcriptional factors.

The Karoo Basin of South Africa is renowned for its abundance and diversity of therapsid fossils. Among the most ubiquitous and persistent of the Permian fauna is the small herbivorous dicynodont Diictodon feliceps. Intraspecific variation in Diictodon is historically confounding, and while ontogeny is frequently cited as a potential source of variation, observable developmental changes have never been calibrated. The present study revisits this issue, comparing three-dimensional landmark configurations of 82 Diictodon crania to investigate the association between shape, size and dimorphism. Beyond the statistically significant relationship between shape and allometry, our results determine the shape differences between juvenile and adult skulls of Diictodon, aligned with common craniofacial features documented in other tetrapod taxa. Functionally, these changes are attributed to development of the jaw musculature for feeding on larger, tougher plant matter during later ontogeny. Cranial morphological variation owing to sexual dimorphism is negligible, but distinct differences are noted in the allometric trajectories of each morphotype. A component of non-allometric variation cannot be accounted for, and we propose that this represents natural variation, rather than an artefact of taphonomic deformation.