Injured? Maybe Antlers Could Help

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Some animals are known to regrow parts of their bodies. This includes limbs (starfish), or tails (salamanders), or heart tissue (tetra fish), or even intestines (sea cucumbers, related to starfish). Humans can regrow part of a liver, and of course we regrow on a small scale various tissues from cuts and scrapes (skin, nails, hair). Deer, on the other hand, shed whole antlers during their life and regrow  them completely again and again. This is the regeneration of an entire organ. Chinese scientists have recently investigated how this ability might be used to help healing of bone and skin tissue in humans. 

Adapted from Stem Cell Research & Therapy, 2023

First of all, what are the differences between antlers and horns? Horns are permanent unbranched attachments  to an animal's skull. They are essentially two parts: living bone surrounded by a tough layer of keratin (the stuff that makes nails and hair). These are typically found on cattle, buffaloes, goats, sheep, and antelopes. Horns of rhinoceroses, however, are solid keratin with a core of melanin and calcium unattached to the skull. The short horns on giraffes are called ossicones, made of bone covered by cartilage and fur. On the other hand, antlers are not permanent, and they are branched projections from the skull. Unlike horns, antlers are one structure made entirely of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are found on deer, caribou, elk, moose, and reindeer.

Examples of animals with horns (top) and antlers (bottom) (from animldiversity.com and Wikipedia)

In the spring, deer antlers begin to grow. The growing tip begins from a special upraised point on the skull called a pedicle. This forms around 4-5 months after the male deer is born. The pedicle itself is not antler material but a starting point. 

Pedicle under mature antlers, and pedicles beginning to form on young male deer

The growing antlers are covered by a furry velvet skin and another layer of cells underneath. Early in the growth stage, that layer (perichondrium) is made of cartilage, and later it becomes a layer (periosteum) to provide bone-growing cells. Both also supply antler cells with blood.

Cross section of growing antler from pedicle (World Journal of Stem Cells, 2021)

During summer months, the main beam of an antler can grow from the tip by as much as 3.8-5 centimeters (1.5-2 inches) per week (2 cm/0.75 inches for yearlings). The outer part of a growing antler bone will change from soft and spongy to hard compact mineralized tissue. In late winter, antlers are shed, and the cycle restarts. Many factors influence the overall size of antlers before they fall off, including age (older deer have larger antlers), genetics, health, nutrition, hormone levels, and the environment. Deer  (and their related species) are the only mammals that can regenerate such a whole organ.

After the breeding season in winter, hormones direct certain cells called osteoclasts to travel to the base of an antler. They dissolve the bone minerals there with various enzymes, loosening the antler. When the antler is shed, a wound remains for about 2 weeks as it is covered by normal scabbing from clotted tissue. The scab eventually falls off, leaving the exposed base of the antler where a new one will begin to grow. A ring of velvet-like skin around the scab location marks the beginning of a new antler's growth.

Head of a deer (eye at the bottom) showing location of shed antler during healing (Mississippi State University)

So, how does this process operate, and what cells are involved? To answer the second question, Chinese investigators in 2023 examined the growing tip of antlers to identify what types of cells were there. They found 8 types. A major one was a type of stem cell called a PMC.

Growing tips were examined (blue area in dashed boxes) (Qin et al., Science)

Stem cells in mammals are found in embryos, umbilical cord, bone marrow, and gonads. They can be considered the earliest state of mature cells, and for that reason they have the ability to develop into a variety of cell types. Finding them at the base of antlers, too, where antlers grow repeatedly every year, is no surprise. Developing into the multiple cell types of an antler is under complex control by many chemicals in the body. To investigate if those chemicals are in the surrounding skin, Chinese researcher Chunyi Li transplanted PMCs into special mice in 2021. He chose cells from a pedicle 5 days after antler shedding because they were developing rapidly and in large numbers. However, he produced only pedicle-like bumps, not antlers.

Bumps resembling pedicles grown on mice (C. Li, 2020)

But, when he transferred the cells plus deer skin, the results were better. Within 45 days, true antlers had begun to grow, including hair and then velvet on the bumps. 

Mini antlers (no branching) grown on mice using deer antler stem cells and deer skin (C. Li, 2020)

To answer the earlier question of how these stem cells work, there are two studies to summarize. The first was by yet another Chinese team in 2019. Instead of looking at bone growth, they investigated how well the stem cells could repair skin wounds.

They transferred cells from the antler pedicle to Petri dishes and grew them there. The cells released various chemicals into the liquid media, generating "conditioned media (CM)". They then tested whether the chemicals in the CM had any effects. In Petri dishes, they exposed two types of cells that are involved in wound healing with PMC-CM, and they saw that the stem cell CM stimulated both cell types more than other common cell growth factors. This same CM was also applied to rats which had circular cuts 8 mm (0.3 inches) in diameter made on their skin. 

Experiments with antler stem cell (PMC) conditioned media (Stem Cell Research & Therapy, 2019)

Again, the CM from deer antler stem cells was successful more than other conditions by shortening the healing time by 9 days (16 days vs. 25 days).

Wound healing in rat skin (Rong et al., 2019)

The CM contains biochemicals made by the stem cells which promote this healing. Chemicals are released into the growth medium like they are in the surrounding tissues to have their effects. But a new phenomenon has also been discovered recently for many types of cells. A cell may fold its membrane in on itself and pinch off inside the cell to form a bubble. That traps certain chemicals inside more protective bubbles within the bigger one, which are then released back into the environment when the main bubble fuses with the membrane. These small bubbles, only 40-160 nm in diameter are called exosomes. 

Formation of exosomes (Science, 2020)

In 2022, a fourth group of Chinese researchers injected exosomes from deer antler stem cell culture into rat knee joints after they conducted surgery to tear the ligaments. The exosome treatment was expected to stimulate growth of blood vessels and ligament material. After 9 weeks, it met with partial success.

Deer stem cell exosome repair of osteoarthritis in mice (Protein & Cell, 2022)

In a deeper study of deer PMCs, the 2023 investigators repeated Li's 2021 study of making antlers on mice, but they examined the stem cells more thoroughly. It seems there are four subtypes of stem cells in the pedicle periosteum. Each has a different function:

• one (PMC1) makes connective tissue
• another (PMC2) continues making more stem cells for self-renewal
• another (PMC3) can develop into cells required for bone development and remodeling of the bone
• another (PMC4) are the most important because they make cartilage in the antler

They monitored when all 8 cell types (including each of the stem cell subtypes) were present and what they did during antler growth. One cell type was used to repair rabbit bone tissue, a potentially important find since it worked especially well on bulges in bone like knees, elbows, jaws, and knuckles. All of their work will be useful in human studies later.

They also found that the antler stem cells shared many genes that mouse cells have in their limb development. Humans and mice are both mammals, but mice have been used extensively in research on immunity because there are some similarities in cell functions. So, this might extend to any tests of deer antler cells on mice and benefit humans in years to come. With cell culture techniques providing growth of deer antler stem cells for such animal studies, it will be easier to perform research than on live deer. And this research has shown a new source of stem cells that are not as controversial as those taken from humans or embryos.

Overall, antler stem cell research has shown promise in the following medical areas:

• wound healing
• bone repair
• osteoarthritis
• corneal injury
• liver fibrosis
• post-operative cognitive impairment