Epiphyseal Plate: Growth, Anatomy and Clinical Notes

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The epiphyseal plate (also called the physis or growth plate) is a layer of specialized hyaline cartilage between the epiphysis and metaphysis of a growing long bone that produces longitudinal bone growth through ordered chondrocyte proliferation, hypertrophy, and replacement by bone. The epiphyseal cartilage plate is the only structure in the immature skeleton capable of adding length to a bone, and it does so without thickening the cartilage layer, because production of new cartilage at the top of the plate is matched by conversion of old cartilage to bone at the bottom.

This matters to every clinician who treats young animals. A fracture, infection, nutritional deficiency, or endocrine disorder that crosses the physis can slow, stop, or distort growth in that limb for the rest of the animal's life. Understanding the zones of the plate, the cells that populate them, and the blood supply that feeds them turns a vague concept into a working diagnostic framework for lameness, angular limb deformity, and short stature in dogs, cats, horses, and ruminants.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What the Epiphyseal Plate Is and Where It Sits

A long bone in a growing animal has four regions. The epiphysis is the rounded end that forms the joint surface. The physis, or epiphyseal plate, is the cartilaginous disc just below it. The metaphysis is the flared region of the shaft immediately below the physis, where the newly formed bone is being remodeled. The diaphysis is the midshaft.

The physis is sandwiched between two bony structures: the secondary ossification center in the epiphysis above, and the primary ossification center in the diaphysis below. Growth in length happens at the physis, while growth in width and remodeling happen at the periosteum and endosteum. The plate itself is avascular in its cartilaginous layers, which is a key point for understanding why it heals poorly and why infections and fractures behave differently there than in bone.

The epiphyseal cartilage is not uniform. It is a stratified tissue with distinct functional zones arranged in a column from the epiphyseal side to the metaphyseal side. Cells move through these zones in one direction, and the position of each zone is defined by cell morphology, matrix composition, and metabolic activity rather than by a physical membrane.

The Five Zones of the Epiphyseal Plate in Order

The classical description divides the plate into five zones. Each zone has a characteristic cell type, a characteristic matrix, and a characteristic blood supply. The table below summarizes the sequence, and the sections that follow explain what each zone does and why it matters clinically.

Zone 1: Reserve Zone (Resting Zone)

The reserve zone sits at the top of the plate, immediately beneath the epiphysis. Its chondrocytes are small, round, and sparse. They do not divide rapidly and they do not produce much matrix. For decades this zone was treated as a passive reservoir, but lineage-tracing work has changed that view.

The reserve zone supplies the progenitor cells that feed the rest of the plate. Resting-zone cells that express parathyroid hormone-related protein (PTHrP) function as skeletal stem cells, and they give rise to the proliferating chondrocytes below them [1]. When this progenitor pool is depleted, the plate cannot sustain growth. Conditional deletion of the adhesion G protein-coupled receptor ADGRG6 in mice shortens the resting zone, produces cell clusters in the proliferative zone, and elongates the hypertrophic zone, which shows that maintaining slow-cycling resting cells is a distinct requirement from driving proliferation [2].

The matrix of the reserve zone is rich in aggrecan and hydrated proteoglycans. Aggrecan matters here. In a mouse model of aggrecan haploinsufficiency, growth failure was driven by decreased extracellular matrix and impaired chondrocyte hypertrophy rather than by a proliferation defect [3]. The reserve zone matrix is not inert packing material. It is a signaling niche that influences how growth factors and exosomes reach the cells below [4].

Zone 2: Proliferative Zone

The proliferative zone is where the plate gets its length. Chondrocytes here are flattened and arranged in longitudinal columns, and they divide in a plane that stacks daughter cells on top of one another along the axis of growth. Each division adds one cell to the column, and the column lengthens.

Column formation is not automatic. Analysis of multicolor clones in mouse growth plates showed that many proliferating cells do not achieve the typical stacked arrangement, and that disordered clusters promote lateral expansion while ordered columns support elongation [5]. In other words, the geometry of cell division determines whether the plate grows longer or wider, and the plate normally biases toward length.

Proliferation is controlled by a network of paracrine signals. The PTHrP and Indian hedgehog (IHH) feedback loop is central. PTHrP keeps chondrocytes in the proliferative state and delays hypertrophy, while IHH signals back to regulate PTHrP production. G-protein-coupled estrogen receptor-1 activation increases proliferative zone thickness and chondrocyte proliferation in mice and raises the PTHrP to IHH ratio [6]. Neuropeptide Y promotes proliferation through mTORC1 and PTHrP but inhibits hypertrophic differentiation, which shows that proliferation and hypertrophy can be uncoupled [7]. MicroRNAs add another layer. miR-433 is expressed in precursor and proliferating chondrocytes, and inhibiting it reduces resting zone cell number and produces abnormal hypertrophic-like cells in the proliferative zone [8]. miR-1 overexpression disorganizes the proliferative zone and decreases proliferation [9].

The proliferative zone is avascular. Chondrocytes here rely on diffusion from the epiphyseal vessels above and the metaphyseal vessels below. This is one reason the plate is vulnerable to anything that compromises blood flow, including sepsis, thrombosis, and elevated pressure within the joint.

Zone 3: Hypertrophic Zone

In the hypertrophic zone, chondrocytes stop dividing and enlarge dramatically. A hypertrophic chondrocyte can reach five to ten times the volume of a proliferative chondrocyte. The cells swell, their cytoplasm fills with glycogen and water, and they begin to secrete a different matrix.

Hypertrophy is the second engine of longitudinal growth. Growth in length is the product of chondrocyte proliferation plus chondrocyte hypertrophy, not interstitial expansion of the whole plate. The plate does not stretch like a rubber sheet. It adds cells at the top and enlarges them in the middle, and the combination pushes the epiphysis away from the diaphysis.

Hypertrophy is regulated separately from proliferation. Aggrecan deficiency impairs hypertrophy through suppressed Akt signaling and increased Camk1D expression while leaving proliferation largely intact [3]. miR-1 overexpression reduces type X collagen, MMP-13, and IHH in the hypertrophic zone, decreasing terminal differentiation [9]. ADGRG6 loss expands the hypertrophic zone through increased IHH signaling, and blocking hedgehog signaling restores the hypertrophic cell population [2]. IRE1α deficiency accelerates hypertrophy and mineralization by disrupting the PTHrP and IHH feedback loop [10]. miR-433 inhibition increases Sox9, IHH, PTHrP, and Bmpr1a expression and delays secondary ossification center development [8].

The matrix in the hypertrophic zone changes composition. Type II collagen gives way to type X collagen, which is a marker of hypertrophic chondrocytes. The matrix vesicles that chondrocytes release carry proteins and RNA that influence neighboring cells, making the hypertrophic zone a signaling hub rather than just a swelling compartment [11].

Zone 4: Calcification Zone

At the base of the hypertrophic zone, the cartilage matrix calcifies. Chondrocytes here are at the end of their life cycle. Some undergo apoptosis, but lineage-tracing studies show that hypertrophic chondrocytes can also transdifferentiate into osteoblasts or dedifferentiate into progenitors rather than dying [1]. This finding overturned the older assumption that every hypertrophic chondrocyte dies.

Calcification prepares the matrix for vascular invasion. The calcified cartilage provides a scaffold that osteoblasts and osteoprogenitors can use as a template. Mineralization is tightly coupled to the PTHrP and IHH loop, and disrupting that loop impairs mineralization as well as hypertrophy [10].

The calcification front is the first point in the plate where blood vessels are present within the cartilage itself. Metaphyseal vessels invade the calcified cartilage, bringing osteoprogenitor cells and hematopoietic precursors into the region. This vascular invasion is the trigger for the final zone.

Zone 5: Ossification Zone

In the ossification zone, calcified cartilage is replaced by bone. Osteoblasts deposit woven bone on the calcified cartilage scaffold, and the result is primary spongiosa, which is later remodeled into lamellar bone in the metaphysis. The transition from cartilage to bone is not a single event but a continuous front that advances at the rate the plate produces new cartilage.

The ossification zone is where the plate's output becomes visible on radiographs. The metaphyseal margin of the physis is the radiolucent line that clinicians use to judge whether the plate is open, closing, or closed. Because the ossification front advances steadily, any disruption in the zones above it shows up as an irregular or widened physeal line.

How Longitudinal Growth Actually Works

Longitudinal bone growth is the sum of two processes. Chondrocytes in the proliferative zone divide and stack, adding cells to each column. Chondrocytes in the hypertrophic zone enlarge, increasing the height of the column without adding cells. The plate maintains a constant thickness because the rate of new cartilage production at the top equals the rate of cartilage conversion to bone at the bottom.

The reserve zone supplies the progenitors that replenish the proliferative pool. Without a functional reserve zone, the plate runs out of cells and closes prematurely. This is why conditions that deplete resting-zone cells, such as ADGRG6 loss or miR-433 inhibition, cause short limbs and delayed or abnormal ossification [2][8].

Mechanical loading modulates the process. A finite element model of endochondral growth showed that the growth plate adapts its shape to the local mechanical environment, and that trabecular architecture influences load transmission and ossification bridge formation [12]. This is the biomechanical basis for the clinical observation that asymmetric loading across a physis can produce angular deformity.

The rate of growth is not uniform across all physes. The distal femur and proximal tibia contribute most of the length of the hindlimb, and the corresponding physes near the stifle are the most active in the skeleton. This is why injuries or infections at the distal femoral physis or proximal tibial physis produce the most severe limb shortening.

Species Differences in Physeal Closure

The timing of physeal closure varies widely across species. The table below gives the clinically useful pattern for the species a veterinary reader is most likely to encounter. These are approximate ranges for the major long bone physes, and individual animals vary with breed, size, nutrition, and endocrine status.

The pattern is consistent across species: physes that contribute the most length close last, and physes near the stifle and tarsus stay open longer than physes in the distal limb. A veterinarian assessing a young animal with a suspected physeal injury should know the expected closure time for that species and that specific physis before interpreting radiographs.

Clinical Relevance: Physeal Injury and the Salter-Harris System

Physeal fractures are classified by the Salter-Harris system, which describes how the fracture line crosses the plate. The classification predicts prognosis because it reflects which layers of the plate are damaged.

Type I fractures cross the physis alone, separating the epiphysis from the metaphysis without involving bone. Type II fractures cross the physis and exit through a metaphyseal fragment. Types I and II spare the germinal layer of the plate in most cases and carry a better prognosis. Type III fractures cross the physis and exit through the epiphysis into the joint. Type IV fractures cross the metaphysis, physis, and epiphysis. Type V fractures are crush injuries of the plate without displacement.

The germinal layer, which corresponds to the reserve and proliferative zones, is the part of the plate that must survive for growth to continue. Types I and II injuries tend to spare this layer because the fracture plane runs through the hypertrophic and calcification zones, which are structurally weaker. Types III, IV, and V injuries involve the germinal layer more directly, and they carry a higher risk of growth arrest, angular deformity, and limb shortening. This is why the Salter-Harris grade is the single most useful prognostic tool at the time of presentation.

Physeal injuries are not the only threat. Any condition that disrupts the blood supply to the plate can cause growth disturbance. The epiphyseal vessels feed the reserve zone from above, and the metaphyseal vessels invade the calcification zone from below. Ischemia in either territory can slow or stop growth. Osteomyelitis, septic arthritis, and pressure from joint effusion can all compromise physeal perfusion.

Systemic disease also targets the plate. In a rat model of inflammatory bowel disease, dextran sulfate sodium colitis reduced epiphyseal growth plate height by 23 percent, with comparable reductions in the proliferative and hypertrophic zones [13]. Maternal gestational diabetes disturbs miR-322 expression in the proliferative and hypertrophic zones of offspring, and miR-322 targets Adamts5, Col12a1, and Cbx6 [14]. X-linked hypophosphatemia, caused by elevated FGF23, impairs growth plate structure and dynamics, and blocking FGF23 signaling improved bone length and growth plate architecture in a mouse model [15]. These examples show that the plate is a sensitive readout of systemic metabolic and inflammatory status.

How the Physis Is Studied and Observed

Radiography is the standard clinical tool. An open physis appears as a radiolucent band between the epiphysis and metaphysis. As the plate closes, the band narrows and becomes irregular, and eventually a thin sclerotic line marks the closed physis. Comparing the affected limb to the contralateral side is essential because normal closure times vary.

Histology remains the reference method for research. Standard hematoxylin and eosin staining shows the zonal architecture and cell morphology. Immunohistochemistry for type X collagen marks the hypertrophic zone, and markers such as MMP-13 and IHH show terminal differentiation [9]. Confocal microscopy of thick undecalcified sections has been used to define seven chondrocyte subphases, four in the pre-hypertrophic zone and three in the hypertrophic zone, which shows that the classical five-zone model is a simplification of a more continuous gradient [16].

Molecular studies use lineage tracing, conditional knockout mice, and single-cell transcriptomics. These approaches have shown that resting-zone PTHrP-positive cells act as skeletal stem cells, that hypertrophic chondrocytes can transdifferentiate into osteoblasts, and that microRNAs and exosomal cargo regulate the plate from within the matrix [1][4][11]. For the veterinary student, the practical takeaway is that the plate is a dynamic organ with its own stem cell pool, its own signaling network, and its own vascular supply, not a passive cartilage spacer.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake is treating the physis as a single uniform structure. It is not. The reserve zone, proliferative zone, and hypertrophic zone have different cells, different matrices, and different vulnerabilities. A fracture that crosses the hypertrophic zone behaves differently from one that crushes the reserve zone, and the Salter-Harris grade captures that difference.

A second mistake is assuming that a closed physis on radiographs means growth is finished everywhere. Different physes in the same limb close at different times, and a radiograph taken at one age may show some physes closed and others open. Always assess the specific physis in question.

A third mistake is underestimating the reserve zone. Because it looks inactive on histology, it is easy to dismiss. The evidence shows that it supplies the progenitors for the entire plate, and that depleting it causes premature closure and short limbs [2][1].

A fourth mistake is attributing all growth disturbance to trauma. Nutritional deficiencies, endocrine disease, inflammatory conditions, and genetic mutations all act on the plate. Aggrecan deficiency, FGF23 excess, and disrupted microRNA networks are examples of molecular causes of growth failure that present clinically as short stature or limb deformity [3][15][17].

Individual cases require a veterinarian who can integrate history, physical examination, imaging, and laboratory findings. The information here supports that assessment but does not replace it.

Quick Review

1. The epiphyseal plate is a stratified hyaline cartilage organ with five zones: reserve, proliferative, hypertrophic, calcification, and ossification.
2. Longitudinal growth occurs by chondrocyte proliferation plus chondrocyte hypertrophy, not by interstitial expansion of the whole plate.
3. The reserve zone supplies the progenitor cells that sustain the plate, and depleting it causes premature closure.
4. The proliferative zone is avascular and relies on diffusion, which makes it vulnerable to ischemia.
5. The hypertrophic zone is where cells enlarge and the matrix switches to type X collagen, and it is regulated separately from proliferation.
6. Salter-Harris type I and II injuries carry a better prognosis than types III to V because they spare the germinal layer.
7. The distal femur and proximal tibia contribute most limb length, and their physes are the last to close in dogs, cats, and horses.

Frequently Asked Questions

What is the epiphyseal plate made of?

The epiphyseal plate is made of hyaline cartilage organized into zones. The matrix is rich in aggrecan and type II collagen in the upper zones and switches to type X collagen in the hypertrophic zone.

Does the epiphyseal plate grow by stretching?

No. The plate grows by chondrocyte proliferation in the proliferative zone and chondrocyte enlargement in the hypertrophic zone. The plate does not stretch as a whole.

Why do some physeal fractures cause growth arrest?

Growth arrest happens when the injury damages the reserve or proliferative zone, which contains the progenitor cells and dividing chondrocytes needed for continued growth. Salter-Harris types III, IV, and V are more likely to involve these layers.

Which physes contribute the most to limb length?

The distal femoral physis and the proximal tibial physis contribute the most length to the hindlimb. They are also among the last physes to close in dogs, cats, and horses.

When do growth plates close in dogs and cats?

Most dog and cat physes close between 6 and 12 months, with small breeds closing earlier and giant breeds later. Some physes in large dogs remain open past 18 months.

Can nutrition affect the growth plate?

Yes. Nutritional and metabolic status affects the plate. Deficiencies, excesses, and systemic inflammatory conditions can alter chondrocyte proliferation and hypertrophy and change the rate of growth.

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Sources

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2. The G protein-coupled receptor ADGRG6 maintains mouse growth plate homeostasis through IHH signaling.
3. Growth failure in aggrecan deficiency is due to decreased extracellular matrix and impaired growth plate chondrocyte hypertrophy.
4. Emerging molecular mechanisms of the ECM-exosome growth-plate axis in idiopathic short stature.
5. Limited column formation in the embryonic growth plate implies divergent growth mechanisms during pre- and postnatal bone development.
6. G-protein-coupled estrogen receptor-1 facilitates chondrocyte proliferation in pubertal epiphyseal growth plate via PTHrP/Ihh regulation.
7. Neuropeptide Y Promotes mTORC1 to Regulate Chondrocyte Proliferation and Hypertrophy.
8. miR-433 targets BMP and Indian hedgehog signaling to coordinate murine postnatal growth plate dynamics.
9. MiR-1 is a critical regulator of chondrocyte proliferation and hypertrophy by inhibiting Indian hedgehog pathway during postnatal endochondral ossification in miR-1 overexpression transgenic mice.
10. IRE1α regulates the PTHrP-IHH feedback loop to orchestrate chondrocyte hypertrophy and cartilage mineralization.
11. miRNA-based regulation in growth plate cartilage: mechanisms, targets, and therapeutic potential.
12. Exploring the impact of mechanical stimuli on growth plate morphology and trabecular adaptation: A finite element approach.
13. Neutrophil elastase (Elane) may serve as a potential therapeutic target in inflammatory bowel disease-associated growth attenuation: focus on the epiphyseal growth plate in young male rats.
14. MiR-322-5p is involved in regulating chondrocyte proliferation and differentiation in offspring's growth plate of maternal gestational diabetes.
15. Blocking FGF23 signaling improves the growth plate of mice with X-linked hypophosphatemia.
16. A simple method based on confocal microscopy and thick sections recognizes seven subphases in growth plate chondrocytes.
17. Hsa_circ_0008870 suppresses bone formation of growth plate through inhibition of miR-185-3p/ MAPK1 axis in idiopathic short stature.