How BIG are the CELLS?

 

Figure 1: Dividing HeLa cells as seen by a scanning electron micrograph (colored). The image is taken during cell division (cytokinesis). The transient connecting midbody formed by microtubules can be seen. Credit: Steve Gschmeissner / Photo Researchers, Inc

Microorganisms vary greatly in size, ranging from tiny viruses to larger eukaryotic cells. Here’s a general size comparison:

Viruses: These are the smallest microorganisms, typically measuring between 20 to 300 nanometers (nm) in diameter. Some viruses, like the influenza virus, are around 100 nm, while others, such as the poliovirus, are about 25-30 nm.

Bacteria: Most bacteria are around 0.5 to 5 micrometers (μm) in size. For example, Escherichia coli (E. coli), a commonly studied bacterium, is about 1–2 μm in length. Larger bacteria, like Bacillus species, can grow up to 10 μm.

Fungi (Yeasts and Molds): Yeast cells are typically around 3–4 μm in diameter, while mold hyphae (the branching structures of molds) can be much larger, ranging from 10 to 50 μm.

Protozoa: These single-celled eukaryotic organisms can range in size from about 2 μm to several millimeters. Larger protozoans, like Amoeba proteus, can be up to 1 millimeter in diameter.

Algae: Microscopic algae can range from 1–10 μm for the smallest types, like diatoms, to larger forms that can be visible to the naked eye.

To visualize these, here’s a comparison:

A human hair is roughly 70-100 μm wide.

Bacteria are typically about 10-100 times smaller than a human hair.

Viruses are around 1000 times smaller than a typical bacterium.

This scale highlights just how minuscule many microorganisms are, yet they have profound impacts on health, ecology, and various natural processes.

Video credit 📸- All credit Goes to "Metaball Studios" Official Youtube Channel

How big is a human cell?

A human is, according to the most recent estimates, an assortment of 3.7±0.8×1013 cells (BNID 109716), plus a similar complement of allied microbes. The identities of the human cells are distributed amongst more than 200 different cell types (BNID 103626, 106155) which perform a staggering variety of functions. The shapes and sizes of cells span a large range as shown in Table 1. Size and shape, in turn, are intimately tied to the function of each type of cell. Red blood cells need to squeeze through narrow capillaries and their small size and biconcave disk shape achieve that while also maximizing the surface area to volume ratio. Neurons need to transport signals and when connecting our brains to our legs can reach lengths of over a meter (BNID 104901) but with a width of only about 10 µm. Cells that serve for storage, like fat cells and oocytes have very large volumes.

Table 1: Characteristic average volumes of human cells of different types. Large cell-cell variation of up to an order of magnitude or more can exist for some cell types such as neurons or fat cells whereas for others the volume varies by much less, for example red blood cells. The value for beta cell comes from a rat but we still present it because average cell sizes usually changes relatively little among mammals.

The different shapes also enable us to recognize the cell types. For example, the leukocytes of the immune system are approximately spherical in shape while adherent tissue cells on a microscope slide resemble a fried egg with the nucleus analogous to the yolk. In some cases, such as the different types of white blood cells, the distinctions are much more subtle and only reflected in molecular signatures.

Mature female egg cells are among the largest cell types with a ≈120µm diameter. Other large cell types include muscle fiber cells that merge together to form syncytia where multiple nuclei reside in one cell and megakaryocytes, bone marrow cells responsible for the production of blood platelets. Both of these cell types can reach 100 µm in diameter (BNID 106130). Red blood cells, also known as erythrocytes, are some of the smallest and most abundant of human cells.  These cells have a characteristic biconcave disk shape with a depression where the nucleus was lost in maturation and have a corresponding diameter of 7-8 µm (BNID 100509) and a volume of ≈100 µm3 (BNID 101711, 101713). Sperm cells are even smaller with volume of about 20-40 µm3 (BNID 109892, 109891).

Certain human cell lines have been domesticated as laboratory workhorses. Perhaps the most familiar of all are the so-called HeLa cells, an example of which is shown dividing in Figure 1. Such immortal cancer cell lines divide indefinitely, alleviating the need to sacrifice primary animal tissue for experiments. These cell lines have been used for studies such as the molecular basis of signal transduction and the cell cycle. In these cell types, the cell volumes are captured by a rule of thumb value of 2000 µm3 with a range of 500-4000 µm3 (BNID 100434). HeLa cells adhere to the extracellular matrix and like many other cell types on a microscope slide spread thinly to a diameter of ≈40 µm (BNID 103718, 105877, 105878) but only a few µm in height. When grown to confluence they press on each other to compact the diameter to ≈20 µm such that in one of the wells of a 96 multiwell plate they create a monolayer of ≈100,000 cells. One should note that as in bacteria and yeast, average cell size can change with growth conditions. In the case of HeLa cells a >2 fold decrease in volume was observed when comparing cells 3 days and 7 days after splitting and re-plating (BNID 108870, 108872). 

Figure 2: Distribution of cell sizes for L1210, a mouse lymphoblast cell line. The cell volumes are reported in units of fL (1 fL = 1 µm3). (Adapted from A. Tzur et al. Science 325:167, 2010)

A snapshot of the variability of mammalian cells was achieved by a careful microscopic analysis of a mouse lymphocyte cell line as shown in Figure 2. The distribution is centered at about 1000 µm3 with a variance of about 300 µm3. To put these cellular sizes in perspective, if we think of E. coli as having the size of a human being, then a HeLa cell is about the size of a blue whale.

Our examination of the sizes of different cell types will serve as a jumping off point for developing intuition for a variety of other biological numbers we will encounter throughout the book.  For example, when thinking about diffusion we will interest ourselves in the time scale for particular molecules to traverse a given cell type and this result depends in turn upon the size of those cells.

https://tinyurl.com/mpe43jzn

Scripps Research scientists develop new technique for studying mitochondria

 

Advance offers a new way of investigating diseases—including Alzheimer’s, Parkinson’s and different cancers—where mitochondria are disrupted.

LA JOLLA, CA—An advanced imaging-based method from scientists at Scripps Research offers a new way of studying mitochondria, which are best known as the “powerhouses” of cells.

In their report on February 14, 2023, in the Journal of Cell Biology, the scientists described a set of techniques that enables the imaging and quantification of even subtle structural changes inside mitochondria, and the correlation of those changes with other processes ongoing in cells.

Mitochondria are involved not only in energy production, but also in several other critical cellular functions, including cell division and cell-preserving responses to various types of stress. Mitochondrial dysfunctions have been observed in a host of diseases including Alzheimer’s, Parkinson’s disease and different cancers, and researchers are eager to develop treatments that can reverse these dysfunctions. But the scientific tools for studying the fine details of mitochondria structure have been limited.

“We now have a powerful new toolkit for detecting and quantifying structural, and thus functional, differences in mitochondria—for example, in diseased versus healthy states,” says study senior author Danielle Grotjahn, PhD, assistant professor in the Department of Integrative Structural and Computational Biology at Scripps Research.

The co-first authors of the study were Grotjahn lab members Benjamin Barad, PhD, a postdoctoral research associate, and Michaela Medina, a PhD candidate.

Mitochondria are one of the many membrane-bound molecular machines, or “organelles,” that dwell within the cells of plants and animals. Typically numbering in the hundreds to thousands per cell, mitochondria have their own small genomes, and have a distinctive structure with an outer membrane and a wavy inner membrane where key biochemical reactions occur. Scientists know that the appearances of mitochondrial structures can change dramatically depending on what the mitochondrion is doing, or what stresses are present in the cell. These structural changes therefore can be highly useful markers of cell conditions, though until now there hasn’t been a good method for detecting and quantifying them.

In the study, Grotjahn’s team put together a computational toolkit to process imaging data from a microscopy technique called cryo-electron tomography (cryo-ET)—which essentially images biological samples in three dimensions, using electrons instead of light. The researchers’ “surface morphometrics toolkit,” as they call it, enables the detailed mapping and measurement of the structural elements of individual mitochondria. This includes the bends of the inner membrane and the gaps between membranes—all potentially useful markers of important mitochondrial and cellular events.

“It allows us essentially to turn the beautiful 3-D pictures of mitochondria we can get from cryo-ET into sensitive, quantitative measurements—which we can potentially use to help identify the detailed mechanisms of diseases, for example,” Barad says.

The team demonstrated the toolkit by using it to map structural details on mitochondria when their cells are subjected to endoplasmic reticulum stress—a type of cell stress that is seen often in neurodegenerative diseases. They observed that key structural features such as the curvature of the inner membrane, or the minimum distance between inner and outer membranes, changed measurably when under this stress.

With their successful, proof-of-principle demonstrations of the new toolkit, the Grotjahn lab will now use it for studying in more detail how mitochondria respond to cellular stresses or other changes induced by diseases, toxins, infections and even pharmaceuticals.

“We can compare the effects on mitochondria in cells treated with a drug versus the effects on untreated mitochondria, for example,” Medina says. “And this approach is not limited to mitochondria—we can also use it to study other organelles within cells.”

“Quantifying organellar ultrastructure in cryo-electron tomography using a surface morphometrics pipeline,” was co-authored by Benjamin Barad, Michaela Medina, Daniel Fuentes, Luke Wiseman, and Danielle Grotjahn, all of Scripps Research.

The research was funded in part by the National Institutes of Health (R01NS095892, RF1NS125674) and the American Cancer Society.

https://cutt.ly/B7UiMp7

Plant Cell V/S Animal Cell : Difference between Plant Cell and Animal Cell

 Plant and animal cells have several differences and similarities. For example, animal cells do not have a cell wall or chloroplasts but plant cells do. Animal cells are mostly round and irregular in shape while plant cells have fixed, rectangular shapes.

Plant and animal cells are both eukaryotic cells, so they have several features in common, such as the presence of a cell membrane, and cell organelles, like the nucleus, mitochondria and endoplasmic reticulum.

Contents: Plant Cell vs Animal Cell

Cell Wall

A difference between plant cells and animal cells is that most animal cells are round whereas most plant cells are rectangular.Plant cells have a rigid cell wall that surrounds the cell membrane. Animal cells do not have a cell wall. When looking under a microscope, the cell wall is an easy way to distinguish plant cells.

Chloroplasts

Plants are autotrophs; they produce energy from sunlight through the process of photosynthesis, for which they use cell organelles called chloroplasts. Animal cells do not have chloroplasts. In animal cells, energy is produced from food (glucose) via the process of cellular respiration. Cellular respiration occurs in mitochondria on animal cells, which are structurally somewhat analogous to chloroplasts, and also perform the function of producing energy. However, plant cells also contain mitochondria.

Centriole

All animal cells have centrioles whereas only some lower plant forms have centrioles in their cells (e.g. the male gametes of charophytes, bryophytes, seedless vascular plants, cycads, and ginkgo).

Vacuoles

Animal cells have one or more small vacuoles whereas plant cells have one large central vacuole that can take upto 90% of cell volume. In plant cells, the function of vacuoles is to store water and maintain turgidity of the cell. Vacuoles in animal cells store water, ions and waste.

Lysosomes

A lysosome is a membrane-bound spherical vesicle which contains hydrolytic enzymes that can break down many kinds of biomolecules. It is involved in cell processes,like secretion, plasma membrane repair, cell signaling, and energy metabolism. Animal cells have clearly defined lysosomes. The presence of lysosomes in plant cells in under debate. A few studies have reported presence of animal lysosomes in plant vacuoles therefore suggesting plant vacuoles fulfilling the role of the animal lysosomal system.

WHAT ARE PLANT CELLS?

Plant cells are eukaryotic cells, i.e. cells that have a nucleus which is bound by a membrane. The DNA of the plant cell is enclosed within the nucleus of a cell. Generally, plant cells are rectangular or cube-shaped, and they are larger than animal cells. Do you know that plant cells have a cell membrane with an outer lining called the cell wall? The cell wall is the most prominent feature of the plant cell, thus making it an exceptional eukaryotic cell. The cell wall is made up of cellulose and enzymes. Plants cells also contain several other cellular structures within itself which carry out specific functions, necessary for a plant’s survival. They produce hormones, enzymes, and other metabolic activities in a plant cell.

• Plant cells contain structures like the cell wall, plastids, and large vacuoles other than the nucleus.
• The cell wall provides structural support and rigidity to the plant cells.
• The storage of plant products in the plant cells is carried out by plastids.
• Chloroplasts are responsible for carrying out photosynthesis in plants which helps them produce food.
• The vacuoles help in the storage of water, minerals and other useful materials.

The Function Of The Parts Of A Plant Cell – What Do The Various Cell Structures Do?

Every part of the plant cell operates in tandem to ensure the proper functioning of the cell. Here is the role that each component plays.

The Cell Wall
The cell wall surrounds the plant cells like a rigid layer. It consists of 3 layers: the primary cell wall, the secondary cell wall and the middle lamella. Located outside the cell membrane, it provides rigidity, strength, and protection against stress and infection.

The Cell Membrane
The outer boundary of the cell, the cell membrane encloses the cytoplasm and other organelles of a plant cell. It is semi-permeable and allows growth-inducing minerals to pass through while blocking other materials.

Chloroplasts
Chloroplasts have two membranes and have structures that look like stacked coins. It is an elongated organelle that contains the chemical chlorophyll. The chlorophyll is the green pigment that gives colour to the leaves. It absorbs sunlight and helps in the process of photosynthesis by converting light energy into chemical energy.

WHAT ARE ANIMAL CELLS?

Animal cells are eukaryotic cells with a nucleus in the centre and specialized organelles. Like plants, the organelles carry out different types of growth-sustaining functions. However, unlike plant cells, animal cells do not have cell walls or chloroplasts.

The Features Of An Animal Cell Are As Follows:

• The nucleus contains the genetic material or the DNA which controls all the activities of a human body. The nucleus regulates the genes, which instead controls the cell’s activity and functioning.
• The cells also contain organelles called centrosomes, which help organize DNA during cell division.
• The cells also contain ribosomes where proteins are synthesized.
• The endoplasmic reticulum is a maze of membranous sacs called cisternae which modifies and transports proteins made by ribosomes.
• Vesicles transport molecules throughout the cell from one organelle to another and are also involved in metabolism.
• The mitochondria are the powerhouse of the cell that host the process of cellular respiration.

The Function Of The Parts Of An Animal Cell – What Do The Various Parts Of The Cell Do?

The cell membrane surrounds the entire cell and is made up of phospholipids. Phospholipids are molecules with a phosphate head that are attached to glycerol and two tails of fatty acid. They form double membranes in water due to hydrophilic properties of the phosphate head and the hydrophobic properties of fatty acids. The cell membrane is selectively permeable and allows molecules of oxygen and carbon dioxide to pass through easily. It obstructs charged molecules mostly but allows some to pass through a special channel in the membrane, thus maintaining homeostasis within the cell.

Animal cells carry out all the bodily processes like production and storage of energy, creation of proteins, replication of the DNA, and transportation of molecules through the body. As discussed earlier, you already know that every cell organelle performs its particular task.Unlike in plants, the human body contains 200 different types of cells. The red blood cells contain haemoglobin, which carries oxygen.

Differences Between Animal Cells and Plant Cells

Size

Animal cells are generally smaller than plant cells. Animal cells range from 10 to 30 micrometers in length, while plant cells range from 10 and 100 micrometers in length.

Shape

Animal cells come in various sizes and tend to have round or irregular shapes. Plant cells are more similar in size and are typically rectangular or cube shaped.

Energy Storage

Animals cells store energy in the form of the complex carbohydrate glycogen. Plant cells store energy as starch.

Proteins

Of the 20 amino acids needed to produce proteins, only 10 can be produced naturally in animal cells. The other so-called essential amino acids must be acquired through diet. Plants are capable of synthesizing all 20 amino acids.

Differentiation

In animal cells, only stem cells are capable of converting to other cell types. Most plant cell types are capable of differentiation.

Growth

Animal cells increase in size by increasing in cell numbers. Plant cells mainly increase cell size by becoming larger. They grow by absorbing more water into the central vacuole.

Cell Wall

Animal cells do not have a cell wall but have a cell membrane. Plant cells have a cell wall composed of cellulose as well as a cell membrane.

Centrioles

Animal cells contain these cylindrical structures that organize the assembly of microtubules during cell division. Plant cells do not typically contain centrioles.

Cilia

Cilia are found in animal cells but not usually in plant cells. Cilia are microtubules that aid in cellular locomotion.

Cytokinesis

Cytokinesis, the division of the cytoplasm during cell division, occurs in animal cells when a cleavage furrow forms that pinches the cell membrane in half. In plant cell cytokinesis, a cell plate is constructed that divides the cell.

Glyoxysomes

These structures are not found in animal cells but are present in plant cells. Glyoxysomes help to degrade lipids, particularly in germinating seeds, for the production of sugar.

Lysosomes

Animal cells possess lysosomes which contain enzymes that digest cellular macromolecules. Plant cells rarely contain lysosomes as the plant vacuole handles molecule degradation.

Plastids

Animal cells do not have plastids. Plant cells contain plastids such as chloroplasts, which are needed for photosynthesis.

Plasmodesmata

Animal cells do not have plasmodesmata. Plant cells have plasmodesmata, which are pores between plant cell walls that allow molecules and communication signals to pass between individual plant cells.

Vacuole

Animal cells may have many small vacuoles. Plant cells have a large central vacuole that can occupy up to 90% of the cell's volume.

Prokaryotic Cells

Animal and plant eukaryotic cells are also different from prokaryotic cells like bacteria. Prokaryotes are usually single-celled organisms, while animal and plant cells are generally multicellular. Eukaryotic cells are more complex and larger than prokaryotic cells. Animal and plant cells contain many organelles not found in prokaryotic cells. Prokaryotes have no true nucleus as the DNA is not contained within a membrane, but is coiled up in a region of the cytoplasm called the nucleoid. While animal and plant cells reproduce by mitosis or meiosis, prokaryotes propagate most commonly by binary fission.

Other Eukaryotic Organisms

Plant and animal cells are not the only types of eukaryotic cells. Protists and fungi are two other types of eukaryotic organisms. Examples of protists include algae, euglena, and amoebas. Examples of fungi include mushrooms, yeasts, and molds.

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How African turquoise killifish press the pause button on aging

 African turquoise killifish embryos put organ development on hold during a state of suspended growth called diapause. Organs like the brain, eyes and ears (pictured in a developmentally suspended embryo) enter diapause with multiple cell types, such as precursor nerve cells (pink), glial cells (blue) and mature nerve cells (green).

CHI-KUO HU

The fish can double their life span by temporarily halting cell and organ growth while embryos

By Erin Garcia de Jesus

When the ponds where one African fish lives dry up, its offspring put their lives on pause. And now researchers have a sense for how the creatures do it.  

African turquoise killifish embryos can halt their development during a state of suspended activity called diapause. Now a study shows that the embryos effectively don’t age while in that state. Genetic analyses reveal that, to stay frozen in time, the embryos put functions such as cell growth and organ development on hold, researchers report in the Feb. 21 Science.

“Nature has identified ways to pause the clock,” says Anne Brunet, a geneticist Stanford University. Knowing how killifish pause their lives could help scientists figure out how to treat aging-related diseases or learn how to preserve human organs long-term, she says.

Nematode worm larvae (Caenorhabditis elegans) can also halt development and aging when faced with a lack of food or if their environment is overcrowded. Invertebrates like nematodes, however, lack many of the features that make other animals age, such as an adaptive immune system. More than 130 species of mammals from mice to bears also have some form of diapause.

The killifish (Nothobranchius furzeri) live in ponds in Mozambique and Zimbabwe that disappear for months during the dry season, leaving the fish without a home until the rain returns (SN: 8/6/18). For adults that typically live only four to six months anyway, vanishing ponds don’t pose much of a threat. But some killifish embryos press pause on their development during dry months, until ponds fill up again. 

Killifish advance from colorful, young fish to pale, old fish within a few months, making them a good animal for scientists to use to study aging.CHI-KUO HU

Killifish embryos can put their growth on hold from five months up to two years, matching or even greatly exceeding their typical adult life span. If humans could do something similar, an 80-year-old person might instead have a life span from 160 to more than 400 years, Brunet says. But if, or how, these animals protect themselves from aging while in this limbo was unknown.

In the study, Brunet and her colleagues compared killifish embryos that halted their growth with those that bypassed diapause and hatched into adults. Diapause didn’t decrease an adult fish’s growth, life span or ability to reproduce — a sign that the animal didn’t age, even if it paused its development for longer than its typical lifetime, the researchers found.

The team then analyzed the genetic blueprint of embryos suspended in diapause to determine which genes were active. Although the young killifish had developing muscles, hearts and brains before diapause, genes involved in organ development and cell proliferation were subsequently turned off. But other genes were cranked up, such as some crucial for turning other sets of genes on or off.

Killifish embryos (one pictured) can pause their development for a few months or up to two years during a state of suspended activity called diapause.CHI-KUO HU

One gene, the chromobox 7 gene, or CBX7, repressed genes involved in metabolism, but turned on those important for maintaining muscle and staying in diapause, the researchers found. Embryos without CBX7 came out of diapause sooner, and their muscles began to deteriorate after one month.

The new study shows that the embryos aren’t passively waiting for better environmental conditions — their cells coordinate responses during diapause that protect killifish from the passage of time. “We have always looked at this diapause state as more passive — nothing happens there,” says Christoph Englert, a molecular geneticist at the Leibniz Institute on Aging in Jena, Germany, who wasn’t involved in the work. But the new research “shifts the paradigm of diapause as a passive, boring state to an active state of embryonic nondevelopment.”

Researchers aren’t sure how things like temperature might spark a developing killifish to begin or end diapause. But understanding what’s going on inside an embryo is a step toward pinpointing how external signals might control when the animals suspend time, Englert says.      

CITATIONS

C.K. Hu et al. Vertebrate diapause preserves organisms long term through Polycomb complex members. Science. Vol 367, February 21, 2020, p. 870. doi:10.1126/science.aaw2601.

About Erin Garcia de Jesus

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Erin I. Garcia de Jesus is the Winter 2019 science writing intern at Science News. She holds a PhD in microbiology from the University of Washington and went on to earn a master’s in science communication from the University of California, Santa Cruz.

How to build a human brain

BRAIN-MAKING 101  As blobs of two types of brainlike tissue fuse, interneurons (green) migrate from the left clump to the right, linking with neurons (not stained) in the right blob. On both sides, neural support cells called glia appear in purple.

Some steps for growing mini versions of human organs are easier than others

BY 

INGFEI CHEN 

In a white lab coat and blue latex gloves, Neda Vishlaghi peers through a light microscope at six milky-white blobs. Each is about the size of a couscous grain, bathed in the pale orange broth of a petri dish. With tweezers in one hand and surgical scissors in the other, she deftly snips one tiny clump in half.

When growing human brains, sometimes you need to do some pruning.

The blobs are 8-week-old bits of brainlike tissue. While they wouldn’t be mistaken for Lilliputian-sized brains, some of their fine-grained features bear a remarkable resemblance to the human cerebral cortex, home to our memories, decision making and other high-level cognitive powers.

Vishlaghi created these “minibrains” at the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA, where she’s a research assistant. First she immersed batches of human pluripotent stem cells — which can morph into any cell type in the body — in a special mix of chemicals.

The free-floating cells multiplied and coalesced into itty-bitty balls of neural tissue. Nurtured with meticulously timed doses of growth-supporting ingredients, the cell clumps were eventually transferred to petri dishes of broth laced with Matrigel, a gelatin-like matrix of proteins.

On day 56, the blobs display shadowy clusters of neural “rosettes.” Under a laser scanning microscope, razor-thin slices of those rosettes reveal loose-knit layers of a variety of dividing neural stem cells and the nerve cells, or neurons, they give rise to. The layered structures look similar to the architecture of a human fetal brain at 14 weeks of gestation.

By eight weeks, brainlike clumps (top) show neural clusters called rosettes. Within one cluster (red box, expanded at bottom), stem cells (blue and teal) churn out layers of neural precursor cells (pink) and neurons (not stained).

Across the globe, labs such as this one, led by UCLA developmental biologist and neuroscientist Bennett Novitch, are cultivating thousands of these brainy clumps for research. Less than five years ago, a team of biologists in Austria and the United Kingdom and one in Japan wowed the world when they announced they had made rudimentary bits of 3-D human cerebral cortex in a dish. Since then, researchers have been eagerly tinkering with techniques for producing these miniature brain models, like chefs obsessively refining their favorite recipes.

“It’s like making a cake: You have many different ways in which you can do it,” says Novitch, who prefers using the Japanese method with a few tweaks. “There are all sorts of little tricks that people have come up with to overcome some of the common challenges.”

For instance, because the brain blobs lack a built-in blood supply, they must absorb enough oxygen and nutrients from the tissue-culture broth to remain healthy. To help, some labs circulate the broth around the tissue clumps. The UCLA researchers choose instead to grow theirs at higher oxygen levels and chop the blobs at the 35-day mark, when they are as wide as three millimeters, and then about every two weeks after. Sounds radical, but the slicing gives cells on the inside — some of which start dying — exposure to much-needed oxygen and nutrients. Those divided bits then continue growing separately. But cutting can be done only so many times before the expanding rosette structures inside are damaged.

With all the experimenting, researchers have cooked up a lot of innovations, including some nifty progress reported in just the last year. Scientists have concocted tiny versions of several brain regions ranging from the hypothalamus, which regulates body temperature, thirst and hunger, to the movement-controlling basal ganglia. Electrical chatter among neurons, reflecting active brain circuits, has been detected. And research groups have recently begun linking bits of specific regions like Legos. Scientists have even observed some early developmental processes as they happen within the human brain blobs.

Stem cell payoff

The work is part of a broader scientific bonanza that comes from coaxing human stem cells to self-assemble into balls of organlike tissue, known as organoids, that are usually no bigger than a lentil. Although the organoids don’t grow enough to replicate entire human organs, these mini-versions can mimic the 3-D cellular infrastructure of everything from our guts to our lungs. That’s something you can’t get from studies of rodents, which have different biology than humans do.

Mini-organ models promise enormous advantages for understanding basic human biology, teasing apart human disease processes, and offering an accurate testing ground for finding or vetting drug therapies. And by creating personalized organoids from the reprogrammed cells of patients, scientists could study disease in a very individualized way — or maybe even use organoid structures to replace certain damaged tissues, such as in the liver or spinal cord.

“Organoids offer an unprecedented level of access into the inner workings of the human brain,” Novitch says, noting that our brains are largely off-limits to poking and cutting into for research. If scientists can study accurate models of working neural circuits in these brain bits, he and others say, researchers might finally get a handle on uniquely human neurological conditions. Such disorders, which include epilepsy and, experts theorize, schizophrenia and autism (SN Online: 7/17/15), can arise when the brain’s communication networks develop off-kilter.

How to build a brain

Methods for making bits of brainlike tissue tap the innate tendency of human pluripotent stem cells to form neural tissue. Here’s one group’s process.

Day 0: Around 9,000 stem cells are transferred into V-shaped wells and suspended in a cocktail of vitamins, amino acids and (for the first six days) Y-27632, a chemical to prevent the stem cells from committing suicide. Within a few days, the multiplying cells self-aggregate into tiny balls of neural tissue. The broth is refreshed every two to three days.

Day 18: The enlarging neural balls are moved into petri dishes of broth with CDLC, a supplement that provides fats. High oxygen levels (40 percent oxygen, 5 percent carbon dioxide) help the tissue absorb enough oxygen. Well-defined clusters, or rosettes, show early layers of various types of developing neural cells by weeks four to five.

Day 35: At 2 to 3 millimeters wide, the brainlike organoids are cut in half to give cells inside more access to nutrients and oxygen. The tissue clumps are nurtured in new broth infused with growth enhancers: a gelatin-like matrix of support proteins called Matrigel, B27 vitamin supplement, heparin and the growth promoter LIF.

Day 56: Organoids are again snipped in half and transferred to new petri dishes made of oxygen-permeable plastic to optimize access to oxygen for healthy growth. From here on, the organoids are halved every two weeks or so, routinely surviving as long as 150 days.

Source: M. Watanabe et al/Cell Reports 2017

But the research is still in its early days. Although there’s been exciting headway, studies sometimes overstate the extent to which human brain organoids reproduce features of actual developing brain tissue, says stem cell biologist Arnold Kriegstein of the University of California, San Francisco. The minimodels still lack many basic components, including certain cell types, a blood-vessel network and inputs from other neural regions.

Another stumbling block is that brain organoids can vary a lot from protocol to protocol, or even batch to batch within the same lab. “The major focus now needs to be on reproducibility, and being able to get an approach that you can rely on to give you the same outcome each time,” Kriegstein says.

DIY organs

For decades, biology research has relied on cell lines grown in flat sheets in petri dishes, but those sheets lack the structural complexity of living tissue. Then came pioneering work that unveiled the do-it-yourself magic of stem cells raised free-floating in broth.

Organlike tissue bits can be generated from pluripotent stem cells that are either plucked from embryos or created by taking a person’s adult skin or blood cells and chemically inducing them to revert to an embryonic-like state. Starting in the mid-2000s, Yoshiki Sasai’s team at the RIKEN Center for Developmental Biology in Kobe, Japan, demonstrated how to grow brainlike structures using embryonic stem cells, first from mice and then humans.

In cross sections of 8-week-old brain organoids (top row), loose-knit cell layers roughly resemble the tighter bands in 14-week-old human fetal brain tissue (bottom row). The last column shows a zone of neural progenitor cells (red) and an outer layer of neurons (green).

In their groundbreaking study in 2013 in Proceedings of the National Academy of Sciences, the researchers used chemical cues to direct human embryonic stem cells to form a specific region of the human cortex. (Tragically, Sasai committed suicide the next year, after two stem cell studies that he coauthored were retracted amid scientific misconduct charges against a research colleague [SN: 12/27/14, p. 25]. Before his death, Sasai was cleared of any direct involvement. The discredited studies were not related to the organoid research.)

A few months before the 2013 Sasai team paper, Madeline Lancaster and Juergen Knoblich of the Institute of Molecular Biotechnology in Vienna and U.K. colleagues demonstrated their more freewheeling, landmark approach to growing brain organoids (SN: 9/21/13, p. 5). The recipe, described in Nature, allows human pluripotent stem cells to spontaneously attempt to assembleinto a tiny approximation of a whole brain by making whatever brain structures the stem cells choose.

Meanwhile, biologists elsewhere were whipping up other types of organoids, starting instead with adult stem cells. These rare, damage-repairing cells are found in many organs (including the brain), but the cells can transform into only a limited range of cell types. In 2009, Hans Clevers of the Hubrecht Institute in Utrecht, the Netherlands, announced that his lab unexpectedly created a miniature version of a gut while cultivating adult stem cells that the team had discovered in mouse intestinal tissue. Grown in a drop of Matrigel with a trio of growth-inducing factors, these cells coalesced into little spheres containing tiny projections that resembled the fingerlike villi that absorb nutrients in the gut.

Scientists soon were concocting tiny facsimiles of human stomachs, livers, kidneys, lungs and more (SN: 12/28/13, p. 20). “We essentially are discovering the vitality of what the stem cells actually do,” says Clevers, who is president of the International Society for Stem Cell Research. “We give [the cells] a little push, and they do whatever they’re good at.”

The trick is knowing exactly which ingredients to use to make different organs. For pluripotent stem cells, that means exposing them to just the right growth factors or inhibitors at just the right times, over about a month, says James Wells of the Center for Stem Cell and Organoid Medicine at Cincinnati Children’s Hospital Medical Center. Some of those essential instructions are well-known from decades of research on embryo development in fish, chickens and rodents; the same chemical cues generally work for all animals with spinal cords, including people.

Under a microscope, rosettes of neural cells are visible along the perimeter of an 8-week-old brain organoid. The center, where cells have died, is dark.

However, for many body parts, organoid makers must suss out recipe instructions from scratch. Working with Jorge Múnera and other colleagues, Wells recently produced a minimodel of a human colon using human induced pluripotent stem cells. But first, the team conducted months of experiments on frog and mouse embryos to identify the signals for forming a colon. “It took a while to figure out what the special sauce was,” Wells says.

Some scientists have distant dreams of using organoid methods to grow full-size livers or kidneys in the lab for transplantation. A more attainable goal may be regenerative tissue transplants, for example, replacing dying liver cells in someone with early-stage liver disease with chunks of healthy stem cells from a personalized liver organoid. Or, in patients who’ve had part of the small intestine removed, tiny pieces of gut organoid tissue could be implanted and, after growing larger, connected to the intestine.

Head games

The human brain, meanwhile, is vastly more complicated than any other organ. It’s unlikely that scientists will ever be able to build a full replica. While the initial brain-making recipes were stunning for what they could achieve, they left much room for improvement. In the years since the 2013 debut of human brain organoids, research groups have worked to grow bigger brain tissue clumps and more uniform structures.

The Austrian method for making whole-brain organoids, in particular, produced a random mix of neural regions laid out in a topsy-turvy manner. But bioengineering tricks may help. In a study last year, Lancaster, now at the MRC Laboratory of Molecular Biology in Cambridge, England, and Knoblich got more consistent results by adding polymer filaments as scaffolding to guide the organization of the minibrain models.

With surgical scissors, a UCLA biologist prepares to snip one of those organoids in half to give inner cells more access to oxygen and nutrients from the broth in the dish.

Other scientists, following the Japanese approach, which generally gives more predictable results, have concentrated on coaxing out specific cell types or structural features of the real brain. For instance, one constraint is that the organoids form slowly, more or less sticking to the same timeline of development as does a human brain during gestation. But without a blood supply, growth is limited; the brain bits reach only a few millimeters in size. That means organoid models are often short on cell types from later development stages, such as cells called astrocytes. These star-shaped cells are crucial for creating and curating the connections between neurons, and also may help with forming memories (SN Online: 11/15/17).

Astrocytes don’t fully mature in a baby’s brain until after birth. But Stanford University neuroscientist Sergiu Paşca has crafted a method for making and maintaining 4-millimeter-wide balls of human cortex–like tissue (he calls them spheroids) in 3-D culture for an extended time. Last August in Neuron, his team described organoids that survived for more than 20 months — long enough, analyses showed, for astrocytes to mature and function in ways that mimic their real-brain counterparts.

Of great interest, also, are the outer radial glial (oRG) cells, neural stem cells that are pivotal for constructing the unusually big cortex that’s unique to humans; oRG cells are scarce in mouse brains. When Novitch’s lab group at UCLA tried the original Japanese and Austrian organoid-making recipes, the output of oRG cells was underwhelming. So Novitch worked with Vishlaghi and postdoctoral researcher Momoko Watanabe to refine the protocol to pump up the cells’ production and reliably generate better cerebral blobs.

Among other tweaks, Novitch’s team added a dash of a molecule dubbed LIF, which recent studies by others had suggested can spur the oRGs to multiply. It worked, leading to a threefold increase in the oRG populations and enhanced growth of upper neuron layers. The researchers shared their revised protocollast October in Cell Reports.

Helping hand

Neural stem cells called outer radial glial (oRG) cells help fuel the expansion of the unusually big human brain. A growth factor called LIF tripled the number of oRG cells in growing minibrains by week 12.

Applying a growth factor to mini brain models

Source: M. Watanabe et al/Cell Reports 2017

On a different front, labs have begun assembling more complex minibrain models, like playing with self-directed Legos. For two months, Paşca’s team at Stanford grew spheroids in separate sets of dishes that mimicked either cortex tissue or an adjacent underlying region known as the subpallium. Then the researchers put the different bits side-by-side and left them overnight in a culture tube. Similar to how the two regions normally connect in the developing brain, the little pieces knew what to do. “By the next day they are essentially fused to each other,” says Paşca, who announced the results in May in Nature.

During the fusion process, the researchers took time-lapse videos of long, spaghetti-like cells called interneurons migrating from a spheroid of the subpallium into a cortexlike spheroid.

“They don’t crawl, they actually jump,” Paşca says. The images capture aspects of a hallmark phenomenon that normally unfolds during the second and third trimester of fetal gestation.

Testing ground

Once on the other side, interneurons form a circuit with — and quell the activity of — excitatory neurons in the cortexlike tissue, electrophysiological tests suggest. If not quieted, excitatory neurons will trigger neighboring cells to fire. In the real brain, maintaining a proper balance in neural network activity is important; disruptions in it appear to foster disorders such as epilepsy, and perhaps schizophrenia and autism.

Indeed, in the same paper, the Stanford team reported new discoveries using personalized brain spheroids derived from induced pluripotent stem cells of patients with Timothy syndrome — a rare condition caused by an overactive calcium channel found mainly in the brain and heart. Patients with the disorder have epilepsy, autism and heart problems. In the patients’ spheroids, interneurons migrated inefficiently but, by adding drugs that blocked the dysfunctional calcium channel, the researchers could reverse the problem. The brain organoids made these intriguing observations possible, Paşca says. “We couldn’t have done this in any other way.”

Stunted

A brain organoid infected by Zika virus at 28 days old is severely stunted two weeks later (right) compared with a healthy organoid of the same age (left).

When sliced open, at higher magnification, a healthy cortexlike organoid (left) shows normal structure: a rosette with visible cavity in the middle, neural stem cells (red) and neurons (green and blue). A Zika-infected organoid (right) shows collapse of the rosette, with fewer neural stem cells and neurons. 

Organoid experiments by others have, meanwhile, helped confirm that the Zika virus targets and kills oRG cells and other neural precursor cells, contributing to small brain size in infected infants.

In a 2016 study, Johns Hopkins University neuroscientists Guo-li Ming and Hongjun Song reported on their own techniques for creating brain bits that have a well-defined zone of oRG cells. After infecting these organoids with the Zika virus, the researchersobserved a collapse of cortexlike tissue that may partly explain the stunted brain growth (SN: 4/2/16, p. 26). 2-D cell-culture and mouse experiments also provided key evidence of the virus’s modus operandi; although the rodent brain doesn’t harbor the full contingent of human neural stem cells, it has blood vessels and immune-system components that organoids lack.

In search of Zika-fighting treatments, Ming and Song, both now at University of Pennsylvania, and their colleagues have been screening thousands of compounds in 2-D cell cultures, and then validating the most promising candidates with tests in 3-D brain organoids. The team has found several potential antiviral and neuron-protecting agents to pursue. Novitch’s UCLA lab group has likewise used its brain organoids to pinpoint additional receptors by which the virus may gain entry into neural stem cells, and identified a few other drug leads for blocking infection.

Organoids may also prove valuable for tailoring treatments for patients, says David Panchision, chief of the developmental neurobiology program at the National Institute of Mental Health in Bethesda, Md. Researchers might generate personalized brain organoids from the reprogrammed skin cells of individuals with, say, schizophrenia and test which medications work best for patients with particular genetic profiles of the illness.

In the Netherlands, based on research reported in 2016 in Science Translational Medicine, Clevers and colleagues are already using personalized gut organoids, derived from rectal biopsies, to test whether cystic fibrosis patients will benefit from available drugs. Tailored regenerative therapies with 3-D substructures of neural tissue may also be possible, Panchision adds, for conditions like Parkinson’s disease or spinal cord injury.

Blocking infection

Images show 4-week-old brain organoids with no infection (left) and with Zika infection (middle, virus is green, dead stem cells are pink). Treatment with the drug duramycin before exposure to Zika largely staves off infection and cell death (right).

Growing pains

For now, though, scientists have hefty challenges to overcome. Much work remains in optimizing how faithfully the bits of tissue reproduce normal brain function and architecture, Panchision says. For one thing, the organoids are developmentally young and don’t reflect a mature brain. And researchers must figure out how to build in some core features: the necessary blood vessels, immune-system cells called microglia and connections from other brain regions, such as the thalamus and cerebellum. Not to mention steroid and thyroid hormones, which also shape brain growth.

However, scientists don’t necessarily need or want to create a comprehensive replica of the human brain in a dish, Panchision and others point out. Rather, the goal is to build robust and reliable models for studying specific aspects of brain function.

Thus the pressing need for standardized, reproducible organoid-making recipes. Novitch’s group and many other labs are still trying to figure out why the brain bits can vary so much in size, composition and structure. Part of the trouble is the ingredients: Subtle variations in tissue-culture chemicals and Matrigel, or in different stem cell lines and how they are grown first in 2-D culture, can have a big impact on how the organoids turn out, Novitch says.

At the same time, researchers need to do a more thorough job of analyzing brain organoids to know what’s actually in them at different developmental time points, compared with actual human fetal brain tissue, says UCSF’s Kriegstein. It’s otherwise hard to say whether a brain blob truly recapitulates the neural tissue that scientists claim it does. Labs have started tackling the problem with a tool called single-cell transcriptome analysis, which gives readouts of all the genes that are active in individual cells.

“Greater rigor is needed,” Kriegstein says. “And I am sure we will eventually get there.”

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This story appears in the March 3, 2018 issue of Science News with the headline, "Brain-making 101: Self-assembling clumps bring a dose of 3-D reality to studies of human organs."

Citations

M. Watanabe et al. Self-organized cerebral organoids with human-specific features predict effective drugs to combat Zika virus infection. Cell Reports. Vol. 21, October 10, 2017, p. 517-532. doi:10.1016/j.celrep.2017.09.047.
E. Di Lullo and A.R. Kriegstein. The use of brain organoids to investigate neural development and disease. Nature Reviews Neuroscience. Vol. 18, October 2017, p. 573. doi:10.1038/nrn.2017.107.
S. A. Sloan et al. Human astrocyte maturation captured in 3D cerebral cortical spheroids derived from pluripotent stem cells. Neuron, Vol. 95, August 16, 2017, p. 779. doi:10.1016/j.neuron.2017.07.035.
J. O. Múnera et al. Differentiation of human pluripotent stem cells into colonic organoids via transient activation of BMP signaling. Cell Stem Cell, Vol. 21, July 6, 2017, p. 51. doi:10.1016/j.stem.2017.05.020.
M. A. Lancaster et al. Guided self-organization and cortical plate formation in human brain organoids. Nature Biotechnology. Vol. 35, July 2017, p. 659. doi:10.1038/nbt.3906.
F. Birey et al. Assembly of functionally integrated human forebrain spheroids. Nature. Vol. 545, May 4, 2017, p. 54-59. doi:10.1038/nature22330.
X. Qian et al. Brain-region-specific organoids using mini-bioreactors for modeling ZIKV exposure. Cell, Vol. 165, May 19, 2016, p. 1238. doi:10.1016/j.cell.2016.04.032.
M.A. Lancaster et al. Cerebral organoids model human brain development and microcephaly. Nature. Vol. 501, September 19, 2013, p. 373. doi:10.1038/nature12517
T. Kadoshima et al. Self-organization of axial polarity, inside-out layer pattern, and species-specific progenitor dynamics in human ES cell-derived neocortex. Proceedings of the National Academy of Sciences. Vol. 110, December 10, 2013, p. 20284. doi:10.1073/pnas.1315710110.
T. Sato et al. Single Lgr5 stem cells build crypt-villus structures in vitro without a mesenchymal niche. Nature. Vol. 459, May 14, 2009, p. 262. doi:10.1038/nature07935.

Further Reading

T. Hesman Saey. Mini brains may wrinkle and fold just like ours. Science News. Vol. 193, January 20, 2018, p. 14.
M. Rosen. Year in Review: Bioengineers make headway on human body parts. Science News. Vol. 184, December 20, 2013, p. 20.
L. Sanders. Tiny human almost-brains made in lab. Science News. Vol. 184, September 21, 2013, p. 5.