Introduction

The skin is the largest organ in the body, coveringits entire external surface.The skinhas3 layersthe epidermis, dermis, and hypodermis,which have different anatomical structures and functions (seeImage.Cross Section, Layers of the Skin). The skin's structure comprises an intricate network that serves as the body's initial barrier against pathogens, ultraviolet (UV) light, chemicals, and mechanical injury.This organ also regulates temperature and the amount of water released into the environment.

Skin thicknessvariesby body region and isinfluenced by the thickness of the epidermal and dermal layers. Hairless skin in the palms of the hands and soles of the feet is the thickest due to the presence ofthe stratum lucidum, an extra layer in the epidermis.Regions lacking this extra layer are considered thin skin. Of these regions, the back has the thickest skin because it has a thick epidermis.[1][2][3]The skin's barrier function makes it susceptible to various inflammatory and infectious conditions. In addition, wound healing, sensory changes, and cosmesis are significant surgical concerns. Understanding the skin's anatomy and function is crucial for managing conditions across all medical fields.

Epidermis

The epidermis, the skin's outermost layer, is composed ofseveral strata and various cell types crucial for its function.

Layers of the epidermis:From the deepest to the most superficial, the epidermal layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale,also known asstratum germinativum, is separated from the dermis by the basement membrane (basal lamina) and attached to it by hemidesmosomes. The cells in this layer are cuboidal to columnar, mitotically active stem cells that constantly produce keratinocytes. This layer also contains melanocytes. The stratum spinosum, comprising 8to 10 cell layers, isalsocalled the prickle cell layer. This layer contains irregular, polyhedral cells with cytoplasmic processes, sometimes called spines,that extend outward and contact neighboring cells by desmosomes. Dendritic cells can be found in this layer.[4][5]

The stratum granulosum has 3to 5 cell layers and containsdiamond-shaped cells with keratohyalin and lamellar granules. Keratohyalin granules contain keratin precursors that aggregate, cross-link, and form bundles. The lamellar granules contain the glycolipids secreted to the cell surfaces, functioning as anadhesiveto maintain cellular cohesion. The stratum lucidumcomprises 2 to 3 cell layers and ispresent in thicker skin on the palms and soles. This thin and clear layer consists of eleidin, a transformation product of keratohyalin. The stratum corneum has 20to 30 cell layers and occupies the uppermost epidermal layer. The stratum corneumis composed of keratin and dead keratinocytes (anucleate squamous cells) that form horny scales. This layer has the most variable thickness, especially in callused skin. Dead keratinocytesrelease defensins within this layer, which are part of our first line of immune defense mechanisms.[6][7]

Cells of the epidermis:The epidermal cells include keratinocytes, melanocytes, and Langerhans and Merkel cells(seeImage.Cells of the Epidermis). Keratinocytes are the predominant cells of the epidermis,originating from the basal layer. These cells produce keratin and lipids essentialforformingthe epidermal water barrier. Keratinocytes also contribute to calcium regulation by enablingUVB light absorption in the skin,which iscriticalfor vitamin D activation. Melanocytes derive from neural crest cells and primarilysynthesize melanin, the main skin pigment component. These cells are found between stratum basale cells. UVB light stimulates melanin secretion, protecting against further UV radiation exposure and acting as a built-in sunscreen. Melaninforms during the conversion of tyrosine to dihydroxyphenylalanine by the enzyme tyrosinase. Melanin then travels from cell to cell, relying on the long processesconnecting the melanocytes to the neighboring epidermal cells. Melanin granules from melanocytestransit through the lengthy processes to the cytoplasm of basal keratinocytes. This transfer occurs through cytocrine secretion, where keratinocytes phagocytose the tips of melanocyte processes.

Langerhanscellsare dendritic cells that act as the skin's first-line cellular immune defenders andare crucialfor antigen presentation. Special stains allow visualization of these cells in the stratum spinosum. Langerhans cells are of mesenchymal origin, derived from CD34-positive bone marrow stem cells, and are part of the mononuclear phagocytic system.These cells contain Birbeck granules and tennis racket-shaped cytoplasmic organelles. Langerhans cells express major histocompatibility complex (MHC) I and MHC II molecules, uptake antigens in the skin, and transport them to the lymph nodes. Merkel cells are oval-shaped modified epidermal cells found in the stratum basale, directly above the basement membrane. These cells serve as mechanoreceptors for light touch and are found in the palms, soles, and oral and genital mucosa, with the highest concentration in the fingertips. Merkel cells bind toadjoining keratinocytes through desmosomes and contain intermediate keratin filaments. The cell membranes of Merkel cells interact with free nerve endings in the skin.

Dermis

The dermis is connected to the epidermisby the basement membrane.The dermis consists of 2 connective tissue layers, papillary and reticular, which merge without clear demarcation. Thepapillary layeris the upper dermal layer,which isthinner and composed of loose connective tissue that contacts the epidermis. Thereticular layeris the deeper layer, which is thicker and less cellular. This layer consists of dense connective tissuecomposedof collagen fiber bundles. The dermis houses the sweat glands, hair, hair follicles, muscles, sensory neurons, and blood vessels.

Hypodermis

The hypodermis, also known as the subcutaneous fascia, is located beneath the dermis.This layer is the deepest skin layer and contains adipose lobules,sensory neurons, blood vessels, and scanty skin appendages, such as hair follicles.

Functions

The skin's comprehensive roles highlight its complexity and importance in maintaining overall health and well-being.These roles are discussed below.[8][9]

Barrier function:The skin has multiple protective roles, acting as a barrier against various external threats. The skinshieldsthe body fromexcessive water loss or absorption, invasion by microorganisms, mechanical and chemical trauma, and UV light damage. The cell envelope establishes the epidermal water barrier, a layer of insoluble proteins on the inner surface of the plasma membrane. This barrier is formed through thecross-linkingof small proline-rich proteins. Larger proteins such as cystatin, desmoplakin, and filaggrincontribute to the barrier's robust mechanics. The lipid envelope is a hydrophobic layer attached to the outer surface of the plasma membrane. Keratinocytes in the stratum spinosum produce keratohyalin granules and lamellar bodies containing a mixture of glycosphingolipids, phospholipids, and ceramides assembled within Golgi bodies. The contents of lamellar bodies are then secreted through exocytosis into the extracellular spaces between the stratum granulosum and corneum.

Immunological defense:The skin plays a crucial role in both adaptive and innate immunity. In adaptive immunity, antigen-presenting cells initiate T-cell responses, leading to increased levels of helper T cells, such as TH1, TH2, or TH17. In innate immunity, the skin produces various peptides with antibacterial and antifungal properties. The skin-associated lymphoid tissue is a significant component of the immune system,aiding in preventing infections, as even minor skin breaks can lead to infection. Langerhans cellsare part of the adaptive immune system, presenting foreign antigens encountered in the skin to T cells.

Regulation of homeostasis:The skin plays a vital role in maintaining body temperature and water balance. This organregulates heat exchange with the environment, particularly through the blood vessels and sweat glands. The skin managesthe rate and amount of water evaporation and absorption.

Endocrine and exocrine functions:Keratinocytes produce vitamin D by converting 7-dehydrocholesterol under UV light exposure. These cells also express the vitamin D receptor and contain enzymes that activate vitamin D, essential for the proliferation and differentiation of keratinocytes. The skin's exocrine functions include temperature control by perspiration and skin protection by sebum production. Sweat and sebaceous glands are crucial to these functions.

Sensory functions:The skin is equipped with nociceptors that allow for the sensation of touch, heat, cold, and pain, facilitating interaction with the environment. The skin's sensory roles are essential for an individual's movement, protection, andinteraction with the environment.

Diagnostic indicator:Skin characteristics such aspigmentation, smoothness, elasticity, and turgor provide insights into an individual's overall health status. Skin assessment is often a crucialpartof a person's physical examination.[10][11]

Cell division, desquamation, and sheddingin the skin:Cell division occurs in the stratum basale. Basal cells (young keratinocytes)begin the synthesis of keratinous tonofilaments, whichare grouped into bundles called tonofibrils. Older keratinocytes are then pushed into the stratum spinosum after mitosis.Skincellsbegin toproduce keratohyalin granules with intermediate-associated proteins, filaggrin, and trichohyalin in the upper part of the spinous layer. Thisprocess helps aggregate keratin filaments and convert granular cells into cornified cells, known as keratinization. Cells also produce lamellar bodies during this stage.

Keratinocytescontinue to move into the stratum granulosum afterward, where they become flattened and diamond-shaped. The cells accumulate keratohyalin granules mixed between tonofibrils.Keratinocytesthen continue to the stratum corneum, flattening and losing organelles and nuclei. The keratohyalin granules turn tonofibrils into a homogenous keratin matrix.Cornified cells reach the surface and are desquamated when desmosomes disintegrate. The proteinase activity of kallikrein-related serine peptidase is triggered by lowered pH near the surface. The processes ofskinshedding and desquamation vary slightly by body region.Hairless skincomprisesmore layers, withtheadditionof thestratum lucidum. Thus, keratinocytes in body regions with hairless skin go through more layers before reaching the surface.[12][13]

The epidermisis derived from ectodermal tissue. The dermis and hypodermisare derived from mesodermal tissue from somites. The mesoderm is also responsible for the formation of Langerhans cells. Neural crest cells, responsible for specialized sensory nerve endings and melanocyte formation, migrate into the epidermis during epidermal development.[14][15]

Blood vessels and lymphatic vessels are found in the skin's dermal layer.Blood supply to the skin comprises 2 plexusesonebetween the papillary and reticular dermal layers and another between the dermis and subcutaneous tissues.Blood supply to the epidermis is through the superficial arteriovenous plexus (subepidermal/papillary plexus). These vessels are important for temperature regulation. The body regulates temperature by increasing blood flowto the skin, transferring heatfrom the bodyto the environment.The autonomic nervous system controls the changes in blood flow.Sympathetic stimulation results in vasoconstriction, resulting in heat retention.Conversely, vasodilationleads to heat loss. Vasodilation is the body's response to increased body temperature,resulting from inhibiting the sympathetic centers in the posterior hypothalamus. In contrast, decreased body temperature causes vasoconstriction.[16][17]

Nerves of the skin include both somatic and autonomic nerves. The somatic sensory systemtransmitspain (nociception), temperature, light touch, discriminative touch, vibration, pressure, and proprioception sensations to the central nervous system. Specialized cutaneous receptors and end organs mediate perception, including Merkel disks and Pacinian, Meissner, and Ruffini corpuscles. Autonomic innervation controls vasculature tone, hair root pilomotor stimulation, and sweating. The free nerve endings extend into the epidermis and are responsible for sensing pain, heat, and cold. These sensory structures are most numerous in the stratum granulosum layer andaround most hair follicles. Merkel disks sense light touch andreaches the stratum basale layer. The other nerve endings are found in the deeper portions of the skin and include the Pacinian, Meissner, and Ruffini corpuscles. The Paciniancorpusclessense deep pressure. The Meissner corpuscles sense low-frequency stimulation at the level of the dermal papillae.TheRuffini corpuscles sense pressure.[18][19][20]

The arrector pili muscles are bundles of smooth muscle fibers attached to the connective tissue sheath of hair follicles. Contraction of these muscles pulls the hair follicle outward, erecting the hair. The arrector pili also compress the sebaceous glands, facilitating sebum secretion. Hair does not exit perpendicularly but at an angle. The erection of hair, known as piloerection, produces goosebumps, giving the skin a bumpy appearance when exposed to cold temperatures.[21]Studies show that piloerection contributes to thermoregulation and stem cell growth.[22]

Langer lines, also known as cleavage lines, are topological lines used to defineskin tension.Theselines correspond to the alignment of collagen and elastic fibers in the reticular dermis. Less scarring occurs ifsurgical incisions are made along these lines.[23]

The skin's clinical significance spans all medical disciplines. A few are discussed below.

Dermatomes

Dermatomes areskin segments divided based on afferent nerve distribution, numbered according to spinal vertebral levels. Spinal nerves comprise8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygealnerve. Diseasessuch as shingles caused by varicella-zoster infection manifest pain and rashes in dermatomal patterns. Dermatomes also aid inlocalizing spinal injuries.

Squamous Cell Carcinoma

Squamous cell carcinoma is amalignancy arising from mutated keratinocytes,typically due to UV damage in individuals with type I or II skin types. These individuals typically have light skin, blue or green eyes, and red or blonde hair and burn without tanning.The lesions oftenappearasscaly, flaky, thick red patches that may bleed.Somesquamous cell carcinoma tumors resemble warts.This type of skin cancer can metastasize. Squamous cell carcinoma often arises from actinic keratosespremalignant lesions with cutaneous hornsdeveloping fromchronicUV damage.[24]

Basal Cell Carcinoma

Basal cell carcinoma is a malignant neoplasm of the basal layers of the epidermis. Unlikesquamous cell carcinoma, itis much less likely to metastasize.This type of skin cancer is more common in sun-exposed areas, often appearing as pearly papules on the face, with telangiectasias and a great tendency to ulcerate.

Melanoma

Melanoma is a highly invasive malignant melanocyte tumor that is fatal but rarer than skinsquamous cell carcinoma and basal cell carcinoma. This neoplasm's high metastatic potential is significantly mediated bylesiondepth.Melanoma can be found anywhere on the body and is typically irregularly pigmented but can be amelanotic.[25]

Langerhans Cell Histiocytosis

Langerhans cell histiocytosis is a type of cancer in which Langerhans cells accumulate in the body andformgranulomas, often in the bones, causing bone pain.These granulomas can also appear in the skin, producing rashes, erythematous papules, or blisters (seeImage.Histology, Trichodysplasia Spinulosa). Notably, Langerhans cellhistiocytosis can affect the pituitary gland, leading to diabetes insipidus, infertility, or other endocrine disorders due to hormone deficiencies. Pancytopenia is apotentially fatal Langerhans cellhistiocytosis complication, manifesting with anemia, thrombocytopenia, and leukocytopenia, caused by overcrowding of Langerhans cells in the bone marrow.[26]

Merkel Cell Carcinoma

Merkel cell carcinoma is an uncommon cancer of the Merkel cells. This tumor is categorized as a neuroendocrine small cell carcinoma. Clinically,Merkelcell carcinoma often presents as a painless, solitary cutaneous or subcutaneous nodule, sometimes with a cystic appearance. The nodule can be red, pink, violet, blue, or skin-colored. Lesions may ulcerate or have satellite lesions.Merkel cell carcinoma is typically smaller than 20 mm at diagnosis but shows rapid tumor growth over a few months.[27]

Pemphigus Vulgaris

Pemphigus vulgaris is an autoimmune disease that targets the desmosomes, the intercellular proteins connecting keratinocytes. Desmosome degradation results in acantholysis and the formation ofeasily ruptured blisters within the epidermis. The disease is characterized by a positive Nikolsky sign, where the epidermis peels away upon rubbing.

Bullous Pemphigoid

Bullous pemphigoid is a blistering disease that affects older adults, causing tense subepidermal blisters.The conditionis causedby antibodies targeting hemidesmosomes, which connect the epidermis to the dermis at the basement membrane. This condition is not acantholytic and does not show a positive Nikolsky sign.[28]

ScaldedSkinSyndrome

Scalded skin syndromearises fromthe effects of the exfoliative toxin released byStaphylococcal aureus. The condition manifests as generalized skin exfoliationwith apositiveNikolsky sign, a severely burned (intensely red) appearance, and fever.[29][30]

Drug Reactions

Various drug reactions manifest in the skin, including erythema multiforme and the syndromes of drug reaction with eosinophilia and systemic symptoms, Stevens-Johnson, and toxic epidermal necrolysis. These conditions are often associated with certain medications, including sulfa-containing drugs,nonsteroidal anti-inflammatory drugs, and antiepileptics.[31][32]

The epidermis contains much of our normal flora,with the microbiome varying by body region. The microorganisms inhabiting our skin surfaces are nonpathogenic and can be commensal or mutualistic. The bacteria that tend to predominate are Staphylococcusepidermidis andS aureus, Cutibacterium acnes, Corynebacterium, Streptococcus, Candida, and Clostridium perfringens. However, infections may occur when the protective skin barrier is altered or breached.[33]

Histology, Trichodysplasia Spinulosa. The left column shows hematoxylin and eosin staining of healthy skin (A1) and trichodysplasia spinulosa lesional skin (B1) at low power. At high power, healthy (A2) and trichodysplasia spinulosa (B2) epidermis and (more...)

Cross Section, Layers of the Skin. This is a cross-section view of the hair follicles, hair roots and shafts, sweat glands, pores, epidermis, dermis, and hypodermis. The papillary and reticular layers are also included. The eccrine sweat gland is located (more...)

Cells of the Epidermis. The image shows stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale, and dermis. Contributed by C Rowe

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The weather is getting cooler and many of us are turning to hot showers and baths to warm up and wind down.

But what actually happens to your skin when you have really hot showers or baths?

Your skin is your largest organ, and has two distinct parts: the epidermis on the outside, and the dermis on the inside.

The epidermis is made up of billions of cells that lay in four layers in thin skin (such as on your eyelids) and five layers in thick skin (such as the on sole of your foot).

The cells (keratinocytes) in the deeper layers are held together by tight junctions. These cellular bridges make waterproof joins between neighbouring cells.

The cells on the outside of the epidermis have lost these cellular bridges and slough off at a rate of about 1,000 cells per one centimetre squared of skin per hour. For an average adult, thats 17 million cells per hour, every day.

Under the epidermis is the dermis, where we have blood vessels, nerves, hair follicles, pain receptors, pressure receptors and sweat glands.

Together, the epidermis and dermis (the skin):

So, your skin is important and worth looking after.

Washing daily can help prevent disease, and really hot baths often feel lovely and can help you relax. That said, there are some potential downsides.

Normally we have lots of healthy organisms called Staphyloccocus epidermis on the skin. These help increase the integrity of our skin layers (they make the bonds between cells stronger) and stimulate production of anti-microbial proteins.

These little critters like an acidic environment, such as the skins normal pH of between 4-6.

If the skin pH increases to around 7 (neutral), Staphyloccocus epidermis nasty cousin Staphyloccocus aureus also known as golden staph will try to take over and cause infections.

Having a hot shower or bath can increase your skins pH, which may ultimately benefit golden staph.

Being immersed in really hot water also pulls a lot of moisture from your dermis, and makes you lose water via sweat.

This makes your skin drier, and causes your kidneys to excrete more water, making more urine.

Staying in a hot bath for a long time can reduce your blood pressure, but increase your heart rate. People with low blood pressure or heart problems should speak to their doctor before having a long hot shower or bath.

Heat from the shower or bath can activate the release of cytokines (inflammatory molecules), histamines (which are involved in allergic reactions), and increase the number of sensory nerves. All of this can lead to itchiness after a very hot shower or bath.

Some people can get hives (itchy raised bumps that look red on lighter skin and brown or purple on darker skin) after hot showers or baths, which is a form of chronic inducible urticaria. Its fairly rare and is usually managed with antihistamines.

People with sensitive skin or chronic skin conditions such as urticaria, dermatitis, eczema, rosacea, psoriasis or acne should avoid really hot showers or baths. They dry out the skin and leave these people more prone to flare ups.

The skin on your hands or feet is least sensitive to hot and cold, so always use your wrist, not your hands, to test water temperature if youre bathing a child, older person, or a disabled person.

The skin on your buttocks is the most sensitive to hot and cold. This is why sometimes you think the bath is OK when you first step in, but once you sit down it burns your bum.

You might have heard women like hotter water temperature than men but thats not really supported by the research evidence. However, across your own body you have highly variable areas of thermal sensitivity, and everyone is highly variable, regardless of sex.

Moisturising after a hot bath or shower can help, but check if your moisturiser is up to the task.

To improve the skin barrier, your moisturiser needs to contain a mix of:

Not all moisturisers are actually good at reducing the moisture loss from your skin. You still might experience dryness and itchiness as your skin recovers if youve been having a lot of really hot showers and baths.

If youre itching after a hot shower or bath, try taking cooler, shorter showers and avoid reusing sponges, loofahs, or washcloths (which may harbour bacteria).

You can also try patting your skin dry, instead of rubbing it with a towel. Applying a hypoallergenic moisturising cream, like sorbolene, to damp skin can also help.

If your symptoms dont improve, see your doctor.

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Bones make up the skeletal system of the human body. The adult human has two hundred and six bones. There are several types of bones that are grouped together due to their general features, such as shape, placement and additional properties. They are usually classified into five types of bones that include the flat, long, short, irregular, and sesamoid bones.

The human bones have a number of important functions in the body. Most importantly, they are responsible for somatic rigidity,structural outline, erect posture and movement (e.g. bipedal gait). Due to their rigidity, bones are the main 'protectors' of the internal organs and other structures found in the body.

This article will describe all theanatomical and important histological facts about the bones.

A bone is a somatic structure that is composed of calcified connective tissue. Ground substance and collagen fibers create a matrix that contains osteocytes. These cells are the most common cell found in mature bone and responsible for maintaining bone growth and density. Within the bone matrix both calcium and phosphate are abundantly stored, strengthening and densifying the structure.

Each bone is connected with one or more bones and are united via a joint (only exception: hyoid bone). With the attached tendons and musculature, the skeleton acts as a lever that drives the force of movement. The inner core of bones (medulla) contains either red bone marrow (primary site of hematopoiesis) or is filled with yellow bone marrow filled with adipose tissue.

The main outcomes of bone development (e.g. skull bones development)are endochondral and membranous forms. This particular characteristic along with the general shape of the bone are used to classify the skeletal system. The bones are mainly classified into five types that include:

These bones develop via endochondral ossification, a process in which the hyaline cartilage plate is slowly replaced. A shaft, or diaphysis, connects the two ends known as the epiphyses (plural for epiphysis). The marrow cavity is enclosed by the diaphysis which is thick, compact bone. The epiphysis is mainly spongy bone and is covered by a thin layer of compact bone; the articular ends participate in the joints.

The metaphysis is situated on the border of the diaphysis and the epiphysis at the neck of the bone and is the place of growth during development.

Some examples of this type of bones include:

The short bones are usually as long as they are wide. They are usually found in the carpus of the hand and tarsus of the foot.

In the short bones, a thin external layer of compact bone covers vast spongy bone and marrow, making a shape that is more or less cuboid.

The main function of the short bones is to provide stability and some degree of movement.

Some examples of these bones are:

In flat bones, the two layers of compact bone cover both spongy bone and bone marrow space. They grow by replacing connective tissue. Fibrocartilage covers their articular surfaces. This group includes the following bones:

The prime function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Also, due to their flat shape, these bones provide large areas for muscle attachments.

Due to their variable and irregular shape and structure, the irregular bones do not fit into any other category. In irregular bones, the thin layer of compact bone covers a mass of mostly spongy bone.

The complex shape of these bones help them to protect internal structures. For example, the irregular pelvic bones protect the contents of the pelvis.

Some examples of these types of bones include:

Sesamoid bones are embedded within tendons. These bones are usually small and oval-shaped.

The sesamoid bones are found at the end of long bones in the upper and lower limbs, where the tendons cross.

Some examples of the sesamoid bones are the patella bone in the kneeor the pisiform bone of the carpus.

The main function of the sesamoid bone is to protect the tendons from excess stress and wear byreducing friction.

Learn the basics of the skeletal system with this interactive quiz.

The bones mainly provide structural stability to the human body. Due to the development of the complex bony structures(e.g. spine) the humans are able to maintain erect posture, to walk on two feet (bipedal gait)and for all sorts of other activities not seen in animals.

Due to their rigid structure, bones are key in the protection of internal organs and other internal structures. Some bones protect other structures by reducing stress and friction (e.g. sesamoid bones) while some bones join together to form more complex structures to surround vital organs and protect them (e.g. skull, thoracic cage, pelvis).

Bones also harbor bone marrow which is crucial in production of blood cells in adults. In addition, the bone tissue can act as a storage for blood cells and minerals.

Common bone diseases often affect the bone density, e.g. in young children due to malnutrition. For example, rickets is a bone deformity seen in young children who lack vitamin D. Their legs are disfigured and they have trouble walking. The damage is irreversible though surgery may help. Osteomalacia and osteoporosis are diseases seen mainly in adulthood.

Osteomalacia is the improper mineralization of bone due to a lack of available calcium and phosphate. The bone density decreases and the bones become soft. Osteoporosis has been noted in all ages but mostly in postmenopausal and elderly women. A progressive decrease in bone density increases the risk of fracture. Patients who are on long-term steroid medication are in particular risk.

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Bone Basics and Bone Anatomy

Have you ever seen fossil remains of dinosaur and ancient human bones in textbooks, television, or in person at a museum? It's easy to look at these and think of bones as dry, dead sticks in your body, but this couldn't be further from the truth. Bones are made of active, living cells that are busy growing, repairing themselves, and communicating with other parts of the body. Lets take a closer look at what your bones do and how they do it.

The skeleton of an adult human is made up of 206 bones of many different shapes and sizes. Added together, your bonesmake up about 15% of your body weight. Newborn babies are actually born with many more bonesthan this (around 300),but many bones grow together, orfuse, as babiesbecome older. Some bones are long and thick, like your thigh bones. Others are thin, flat, and wide, like your shoulder blades.

Support: Like a house is built around a supportive frame,a strong skeleton is required to support the rest of the human body. Without bones, it would be difficult for your body to keep its shape andto stand upright.

Protection: Bones form astrong layer around some of the organs in your body, helping tokeep them safe when you fall down or get hurt. Your rib cage, for example, acts like a shield around your chest to protect important organs inside such as your lungs and heart. Your brain is another organ that needs a lot of protection. The thick bone layer of your skull protects your brain. For this purpose, being "thick-headed" is a very good thing.

Movement: Many of your bones fit togetherlike the pieces of a puzzle. Eachbone has a very specific shape which often matches up with neighboring bones. The place where two bones meet to allow your body to bend is called a joint.

How many different ways can you move your joints? Some bones, like your elbow, fit together like a hingethat lets you bend your arm in one specific direction. Other bones fit together like a ball and socket, such as the joint between your shoulder and arm. This type of jointlets you rotate your shoulder in many directions, or swing it all the way around in a circle like softball pitchersdo.

The movement of our bodies is possible because of both joints and muscles. Muscles often attach to two different bones, so that when the muscle flexes and shortens, thebones move. This allows youto bend your elbows and knees, or pick up objects. A skeleton has plenty of joints, but without muscles, there is nothing to pull the bones in different directions. More than half of the bones in your body are actually located in your hands and feet. These bones are attached to many little muscles that give you very exact control over how you move your fingers and feet.

Blood Cell Formation: Did you know that most of the red and white blood cells in your body were created inside of your bones? This is done by a special group of cells called stem cells that are found mostly in the bone marrow, which is the innermost layerof your bones.

Storage: Bones are like a warehousethat storesfat and many important minerals so they are available when your body needs them. These minerals are continuously being recycled through your bones--deposited and then taken out and moved through the bloodstream to get to other parts of your body where they are needed.

Now that you know what bones do, let's take a look at what they're made of and their anatomy.

Each bone in your body is made up of three main types of bone material: compact bone, spongy bone, and bone marrow.

Compact Bone

Compact bone is the heaviest, hardest type of bone. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day. About 80% of the bone in your body is compact. It makes up the outer layer of the bone and also helps protect the more fragile layers inside.

If you were to look at a piece of compact bone without the help of a microscope, it would seem to be completely solid all the way through. If you looked at it through a microscope, however, you would see that it's actually filled with many very tiny passages,or canals,for nerves and blood vessels. Compact bone is made of special cells called osteocytes. These cells arelined up inrings around the canals. Together, a canal and the osteocytes that surround it are called osteons. Osteons are like thick tubes all going the same direction inside the bone, similar to a bundle of straws with blood vessels, veins, and nerves in the center.

Spongy Bone

Spongy bone is found mostly at the ends of bones and joints. About 20% of the bone in your body is spongy. Unlike compact bone that is mostly solid, spongy bone is full of open sections called pores. If you were to look at it in under a microscope, it would look a lot like your kitchen sponge. Pores are filled with marrow, nerves, and blood vessels that carry cells and nutrients in and out of the bone.Though spongy bone may remind you of a kitchen sponge,this bone is quite solid and hard, and is not squishy at all.

Bone Marrow

The inside of your bones are filled with a soft tissue called marrow. There are two types of bone marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets aremade. Platelets are small pieces of cells that help you stop bleeding when you get acut.Red bone marrow isfound in the center of flat bones such as your shoulder blades and ribs. Yellow marrow is made mostly of fat and is found in the hollow centers of long bones, such as the thigh bones. It does not make blood cells or platelets. Both yellow and red bone marrow have many small and large blood vessels and veins running through them to let nutrients and waste in and out of the bone.

When you were born, all of the marrow in your body was red marrow, whichmade lots and lots of blood cells and plateletsto helpyour body grow bigger. As you got older, more and more of the red marrow was replaced with yellow marrow. The bone marrow of full grown adults is about half red and half yellow.

The Inside Story

Bones are made of four main kinds of cells: osteoclasts, osteoblasts, osteocytes, and lining cells. Notice that three of these cell type names start with 'osteo.' This is the Greek word for bone. When you see 'osteo' as part of a word, it lets you know that the word has something to do with bones.

Osteoblasts are responsible for making new bone as your body grows. They also rebuild existing bones when they are broken. The second part of the word,'blast,' comes froma Greek word that means 'growth.' To make new bone, many osteoblasts come together in one spot then begin making a flexible material called osteoid. Minerals are then added to osteoid, making it strong and hard. When osteoblasts are finished making bone, they become either lining cells or osteocytes.

Osteocytes are star shaped bone cells most commonly found in compact bone. They areactually old osteoblasts that have stopped making new bone. As osteoblasts build bone, they pile it up around themselves, then get stuck in the center. At this point, they are called osteocytes.Osteocytes have long, branching arms that connect them to neighboring osteocytes. This lets them exchange minerals and communicate with other cells in the area.

Lining cells are very flat bone cells. These cover the outside surface of all bones and are also formed from osteoblasts that have finished creating bone material. These cells play an important role in controlling the movement of molecules in and out of the bone.

Osteoclasts break down and reabsorb existing bone. The second part of the word, 'clast,' comes from the Greek word for 'break,' meaning these cells break down bone material. Osteoclasts are very big and often contain more than one nucleus, which happens when two or more cells get fused together. These cells work as a team with osteoblasts to reshape bones. This might happen for a number of reasons:

It's not completely understood how bone cells in your body are able to work together and stay organized, but pressure and stress on the bone might have something to do with it.

The smallest bone in the human body is called the stirrup bone, located deep inside the ear. It's only about 3 millimeterslong in an adult.

The longest bone in the human is called the femur, or thigh bone. It's the bone in your leg that goes from your hip to your knee. In an average adult, it's about 20 inches long.

References:

Marieb. E.N. (1989) Human Anatomy and Physiology, CA: Benjamin/Cummings Publishing Company, Inc

Heller, H.C., Orians, G.H., Purves, W.K., Sadava, D. (2003) Life: The Science of Biology, 7th Edition. Sunderland, MA: Sinauer Associates, Inc. & W. H. Freeman and Company

Skeleton Image: By Lady of Hats - Mariana Ruiz Villarreal, via Wikimedia Commons.

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Before we talk about bone formation, we need to discuss how cartilage turns into bone. When you're floating around in the womb, your developing body is just beginning to take its shape, and it's creating cartilage to do so. Cartilage is a tissue that isn't as hard as bone, but much more flexible and, in some ways, more functional. Cartilage is pretty good stuff to use if you're going to mold a human good enough for the finer work, especially, such as your nose or your ear.

A large amount of that fetus cartilage begins transforming into bone, a process called ossification. When ossification occurs, the cartilage begins to calcify; that is, layers of calcium and phosphate salts begin to accumulate on the cartilage cells. These cells, surrounded by minerals, die off. This leaves small pockets of separation in the soon-to-be-bone cartilage, and tiny blood vessels grow into these cavities.

Specialized cells called osteoblasts begin traveling into the developing bone by way of these blood vessels. These cells produce a substance consisting of collagen fibers, and they also aid in the collection of calcium, which is deposited along this fibrous substance.

Eventually, the osteoblasts become part of the mix, turning into lower-functioning osteocytes. This osteocyte network helps form the spongelike lattice of cancellous bone. Cancellous bone isn't soft, but it does look spongy. Its spaces help transfer the stress of external pressures throughout the bone, and these spaces also contain marrow. Little channels called canaliculi run all throughout the calcified portions of the bone, enabling nutrients, gases and waste to make their way through.

Before turning into osteocytes, osteoblasts produce cortical bone. One way to imagine this process is to picture a bricklayer trapping himself inside a man-sized brick chamber of his own construction. After forming the hard shell (cortical bone), the bricklayer himself fills the chamber. Air makes its way through the brick and decays the bricklayer.

In bone, this part of the process is accomplished by osteoclasts, which make their way into the calcifying cartilage and take bone out of the middle of the shaft, leaving room for marrow to form. Osteoclasts do this by engulfing and digesting the bone matrix using acids and hydrolytic enzymes. So, our bricklayer (osteoblast) made the tomb (cortical bone), died inside the tomb (became an osteocyte), decayed over time (dissolved by osteoclasts) and left behind his remains that formed a network of mass and space inside the brick tomb.

Eventually, all the cartilage has turned to bone, except for the cartilage on the end of the bone (articular cartilage) and growth plates, which connect the bone shaft on each side to the bone ends. These cartilage layers help the bone expand and finally calcify by adulthood.

So, right now in your body, there are osteoclasts hard at work absorbing old bone cells and osteoblasts helping to build new bone in its place. This cycle is called remodeling. When you're young, your osteoblasts (the builders) are more numerous than the osteoclasts, resulting in bone gain. When you age, the osteoblasts can't keep up with the osteoclasts, which are still efficiently removing bone cells, and this leads to loss of bone mass (and a condition called osteoporosis, which we'll discuss shortly).

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How bone marrow transplants are changing the outlook for rare blood diseases?  Healthcare Radius

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TORONTO, June 13, 2025 (GLOBE NEWSWIRE) -- MediPharm Labs Corp. (TSX: LABS) (OTCQB: MEDIF) (FSE: MLZ) (“MediPharm” or the “Company”), a pharmaceutical company specialized in precision-based cannabinoids, today announced that leading independent proxy voting and corporate governance advisory firm Glass, Lewis & Co. LLC (“Glass Lewis”) published a report on June 12, 2025 recommending that MediPharm shareholders vote the GREEN Proxy or voting instruction form FOR the Company’s nominees for the Board of Directors (the “Board”) at the upcoming Annual and Special Meeting of Shareholders on June 16, 2025 (the “Meeting”).

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SOUTH SAN FRANCISCO, Calif., June 13, 2025 (GLOBE NEWSWIRE) -- RAPT Therapeutics, Inc. (Nasdaq: RAPT) (the “Company”), a clinical-stage immunology-based biopharmaceutical company focused on discovering, developing and commercializing novel therapies for patients living with inflammatory and immunological diseases, today announced that a 1-for-8 reverse stock split of its outstanding shares of common stock will be effective at 11:59 pm Eastern Time June 16, 2025.

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SAN FRANCISCO, CALIFORNIA, June 13, 2025 (GLOBE NEWSWIRE) -- GT Biopharma, Inc. (the “Company”) (NASDAQ: GTBP), a clinical stage immuno-oncology company focused on developing innovative therapeutics based on the Company's proprietary TriKE® natural killer (NK) cell engager platform, today announced the appointment of David C. Mun-Gavin to its Board of Directors.

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– Rademikibart significantly improved lung function and asthma control in patients with eosinophilic-driven type 2 asthma –

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NEW YORK, June 13, 2025 (GLOBE NEWSWIRE) -- Tiziana Life Sciences, Ltd. (Nasdaq: TLSA) (“Tiziana” or the “Company”), a biotechnology company developing breakthrough immunomodulation therapies with its lead development candidate, intranasal foralumab, a fully human, anti-CD3 monoclonal antibody, today announces that dosing has commenced at the prestigious Weill Cornell Medicine Multiple Sclerosis Center in New York City, in its ongoing Phase 2 clinical trial evaluating intranasal foralumab in patients with non-active Secondary Progressive Multiple Sclerosis (na-SPMS). This fifth site complements existing sites at Yale University, Johns Hopkins University, Brigham and Women's Hospital, and the University of Massachusetts.

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SOUTH SAN FRANCISCO, Calif., June 13, 2025 (GLOBE NEWSWIRE) -- Vaxart, Inc. (Nasdaq: VXRT) (“Vaxart” or the “Company”), a clinical-stage biotechnology company developing a range of oral recombinant pill vaccines based on its proprietary delivery platform, today held its Annual Meeting of Stockholders (the “Annual Meeting”) in a virtual-only format. Preliminary results from the Annual Meeting indicate that two proposals were approved and two proposals were rejected by Vaxart stockholders.

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11 of 12 participants (92%) enrolled in the 180mg cohort achieved a complete response

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