A study published in Nature Biotechnology describes the development of an in vivo model of functional and vascularized human brain organoids. The researchers transplanted human brain organoids into the adult mouse brain, where they integrated, matured, and formed functional neuronal networks and blood vessels. The organoids exhibited progressive neuronal differentiation, gliogenesis, and axonal outgrowth, with functional synaptic connectivity between the grafts and the host brain. In vivo two-photon imaging and optogenetics confirmed the presence of functional neuronal activity and graft-to-host connectivity. The study highlights the potential of this in vivo model for disease modeling under physiological conditions. The development of adequate model systems to study the human brain in health and disease has been challenging. While animal models and cell culture systems have provided insights into brain development and dysfunction, they lack the complexity of the human brain. Three-dimensional brain organoids derived from human pluripotent stem cells (hPSCs) offer a promising alternative, as they contain multiple cell types and more closely resemble in vivo physiology than two-dimensional cultures. However, current brain organoid systems lack the microenvironment, neuronal circuits, vascular circulation, and immune system present in vivo. Vascularization is particularly important for oxygen and nutrient delivery, and the lack of it can lead to necrosis in the center of the organoid. To address these limitations, the researchers developed an in vivo engraftment model of hPSC-derived brain organoids. The transplanted organoids integrated into the mouse brain, exhibited progressive neuronal differentiation, developed a functional vasculature system, and displayed axonal outgrowth to generate mature and functional human brain tissues. The organoids survived for up to 233 days and showed persistent expression of neuronal markers. In vivo two-photon imaging and optogenetics confirmed the presence of functional neuronal activity and synaptic connectivity between the grafts and the host brain. The study also demonstrated extensive axonal growth from the grafts into the host brain, with axons growing through the cortical layers and into various brain regions. The grafts showed synaptic connectivity with the host brain, indicating functional integration. The organoids were vascularized, with blood vessels invading the graft area by 7–10 days post-transplantation and becoming extensively vascularized by 14 days. Vascularization was essential for graft survival, as organoids that failed to vascularize showed little or no fluorescent signal. The study also demonstrated the viability of the organoids in the in vivo environment, with lower rates of cell death compared to in vitro cultures. In vivo two-photon calcium imaging showed synchronized neural activity in the grafts, indicating functional maturation. Electrophysiological recordings confirmed the presence of neuronal activity and state-dependent firing changes in the organoids. Optogenetics revealed functional connectivity between the grafts and the host brain, with laser-evoked responses and local field potential changes indicating excitatory inputs from the graft. The study also tested the spatialA study published in Nature Biotechnology describes the development of an in vivo model of functional and vascularized human brain organoids. The researchers transplanted human brain organoids into the adult mouse brain, where they integrated, matured, and formed functional neuronal networks and blood vessels. The organoids exhibited progressive neuronal differentiation, gliogenesis, and axonal outgrowth, with functional synaptic connectivity between the grafts and the host brain. In vivo two-photon imaging and optogenetics confirmed the presence of functional neuronal activity and graft-to-host connectivity. The study highlights the potential of this in vivo model for disease modeling under physiological conditions. The development of adequate model systems to study the human brain in health and disease has been challenging. While animal models and cell culture systems have provided insights into brain development and dysfunction, they lack the complexity of the human brain. Three-dimensional brain organoids derived from human pluripotent stem cells (hPSCs) offer a promising alternative, as they contain multiple cell types and more closely resemble in vivo physiology than two-dimensional cultures. However, current brain organoid systems lack the microenvironment, neuronal circuits, vascular circulation, and immune system present in vivo. Vascularization is particularly important for oxygen and nutrient delivery, and the lack of it can lead to necrosis in the center of the organoid. To address these limitations, the researchers developed an in vivo engraftment model of hPSC-derived brain organoids. The transplanted organoids integrated into the mouse brain, exhibited progressive neuronal differentiation, developed a functional vasculature system, and displayed axonal outgrowth to generate mature and functional human brain tissues. The organoids survived for up to 233 days and showed persistent expression of neuronal markers. In vivo two-photon imaging and optogenetics confirmed the presence of functional neuronal activity and synaptic connectivity between the grafts and the host brain. The study also demonstrated extensive axonal growth from the grafts into the host brain, with axons growing through the cortical layers and into various brain regions. The grafts showed synaptic connectivity with the host brain, indicating functional integration. The organoids were vascularized, with blood vessels invading the graft area by 7–10 days post-transplantation and becoming extensively vascularized by 14 days. Vascularization was essential for graft survival, as organoids that failed to vascularize showed little or no fluorescent signal. The study also demonstrated the viability of the organoids in the in vivo environment, with lower rates of cell death compared to in vitro cultures. In vivo two-photon calcium imaging showed synchronized neural activity in the grafts, indicating functional maturation. Electrophysiological recordings confirmed the presence of neuronal activity and state-dependent firing changes in the organoids. Optogenetics revealed functional connectivity between the grafts and the host brain, with laser-evoked responses and local field potential changes indicating excitatory inputs from the graft. The study also tested the spatial