Follicular cells are cells that form or line follicles, the small sac-like or spherical structures found in several organs throughout the body. The term does not refer to a single cell type. Thyroid follicular cells produce hormones that regulate metabolism. Ovarian follicular cells nurture developing eggs. Hair follicle cells cycle through growth and rest to produce hair shafts. And in the immune system, several cell types carry “follicular” in their name because they reside in or interact with the follicles of lymph nodes and spleen. What unites them is an architectural principle: cells organized around or within a small enclosed space, working together to carry out a specialized task.
Thyroid Follicular Cells
The thyroid gland is built from millions of tiny spheres called follicles, each lined by a single layer of epithelial cells known as thyrocytes or thyroid follicular cells. These cells are polarized, meaning they have a clearly defined top and bottom. The apical surface faces inward toward the follicle’s cavity, which is filled with a thick protein-rich substance called colloid. The basolateral surface faces outward toward the blood supply.
This polarity is central to how the cells do their job. Thyroid follicular cells concentrate iodide from the bloodstream using a specialized pump on their outer surface called the sodium/iodide symporter. Once inside the cell, iodide moves to the apical surface and is incorporated into a large protein called thyroglobulin in a process known as organification.1PubMed. Structural and functional aspects of the thyroid follicular epithelium The iodinated thyroglobulin is then stored in the colloid, essentially creating a reservoir of raw material for hormone production.2Molecular, Genetic, and Nutritional Aspects of Major and Trace Minerals. Iodine and Thyroid Hormone Synthesis, Metabolism, and Action
When the body needs thyroid hormone, the follicular cells retrieve thyroglobulin from the colloid, break it down, and release the hormones T3 and T4 into the bloodstream. The stored thyroglobulin also feeds back on the follicular cells themselves, influencing how much thyroglobulin and iodide-handling machinery the cells produce. This creates a self-regulating loop that helps keep hormone output steady.3PubMed Central. Intrinsic regulation of thyroid function by thyroglobulin
The thyroid follicle itself appears to be an evolutionary novelty unique to vertebrates. The ability to store iodine in follicular colloid likely gave early vertebrates a significant advantage in freshwater environments where iodine is scarce.4PubMed Central. Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates Invertebrates like the sea squirt have a precursor organ called the endostyle that processes iodine but lacks the follicular storage design.
Parafollicular Cells Are Not Follicular Cells
A common point of confusion: the thyroid also contains parafollicular cells, or C cells, which sit between or adjacent to follicles rather than lining them. Parafollicular cells produce calcitonin, a hormone involved in calcium regulation, and they have a completely different developmental origin from true thyroid follicular cells. During fetal development, parafollicular cells appear as early as the fourteenth week of gestation and gradually migrate from scattered positions to locations alongside or between follicular cells as the follicles enlarge.5PubMed Central. Development of Parafollicular Cells and their Relationship with Developing Thyroid Follicles in Human Foetuses Despite sharing a neighborhood, these two cell types have distinct jobs and distinct disease profiles. Medullary thyroid cancer arises from parafollicular cells, while follicular thyroid cancer arises from the follicular epithelium.
Ovarian Granulosa Cells
In the ovary, the follicular cells are called granulosa cells, and their primary job is supporting the developing egg. A single oocyte sits at the center of each ovarian follicle, surrounded by layers of granulosa cells that provide metabolic support, hormonal signaling, and structural integrity.6PubMed. The role of granulosa cells in oocyte development and aging: Mechanisms and therapeutic opportunities
The relationship between the oocyte and its granulosa cells is remarkably intimate. As the egg grows, it builds up a thick outer coat called the zona pellucida. To maintain contact despite this barrier, granulosa cells extend thin cytoplasmic fingers called transzonal projections that bridge the gap and physically touch the egg’s surface. Through gap junctions at these contact points, granulosa cells pass nutrients and small signaling molecules directly into the oocyte. These deliveries sustain the egg’s metabolism and also keep it from maturing too early. The egg, in turn, secretes growth factors that influence how the granulosa cells multiply and specialize.7PubMed Central. Regulation of germ cell development by intercellular signaling in the mammalian ovarian follicle It is genuinely a two-way conversation, not a one-way supply chain.
Granulosa cells also act as gatekeepers for follicle selection. They produce anti-Müllerian hormone (AMH), which shows up from the primary follicle stage onward and helps regulate which follicles grow and which stay dormant. AMH works partly by dampening the response to follicle-stimulating hormone and by inhibiting the recruitment of additional primordial follicles into the growth pool.8PubMed. Oocyte regulation of anti-Müllerian hormone expression in granulosa cells during ovarian follicle development in mice AMH blood levels have become a widely used clinical marker of ovarian reserve precisely because granulosa cells are its source.
How Granulosa and Theca Cells Cooperate to Make Estrogen
Estrogen production in the ovary requires teamwork between two cell populations. Theca cells, which form the outer shell of the follicle, are the only cells expressing the enzyme needed to make androgens. Granulosa cells, on the inside, are the only ones equipped with aromatase, the enzyme that converts those androgens into estrogen. Neither cell type can make estrogen alone. Theca cells respond to luteinizing hormone (LH) by producing androgens, which then pass to granulosa cells. Granulosa cells respond to follicle-stimulating hormone (FSH) by activating aromatase and converting the androgens into estradiol.9PubMed. Follicular oestrogen synthesis: the ‘two-cell, two-gonadotrophin’ model revisited
As a follicle approaches ovulation, the granulosa cells develop receptors for LH, allowing them to respond to the ovulatory hormone surge directly. They also produce inhibin, which can amplify androgen production by the neighboring theca cells, creating a local feedback loop that ramps up estrogen output in the dominant follicle.
What Happens to Granulosa Cells After Ovulation
Once an egg is released, the leftover follicle does not simply collapse and disappear. It transforms into the corpus luteum, a temporary endocrine gland that pumps out progesterone to support a potential pregnancy. The granulosa cells undergo a dramatic identity shift called luteinization: within hours, they begin turning on genes associated with progesterone production while turning off genes for estrogen production.10PubMed Central. Distinct Luteinization Profiles of Cultured Human Granulosa Cells From Small Antral and Preovulatory Follicles The resulting luteal cells are larger, lipid-rich, and structurally different from the granulosa cells they used to be.
The corpus luteum is a site of rapid remodeling where cells derived from both granulosa and theca layers mix with blood vessel cells and connective tissue.11Endocrine Reviews. The Molecular Control of Corpus Luteum Formation, Function, and Regression If pregnancy does not occur, the corpus luteum breaks down after about two weeks. The vast majority of follicles never reach ovulation at all; they instead undergo a process called atresia, in which granulosa cells die through programmed cell death and the follicle degenerates.12Frontiers in Endocrinology. Mechanisms of follicular atresia: focus on apoptosis, autophagy, and ferroptosis Over a lifetime, the overwhelming majority of a person’s hundreds of thousands of primordial follicles are lost this way.
Follicular Cells in the Hair Follicle
Hair follicles contain several distinct cell populations that cooperate to produce a hair shaft, cycle through growth and rest, and add pigment. The term “follicular cells” here is broader and less precisely defined than in the thyroid or ovary; it encompasses stem cells, keratinocytes, melanocytes, and specialized mesenchymal cells that each play a role in the follicle’s lifecycle.
Hair follicle stem cells reside in a region called the bulge, located in the outer root sheath. These cells spend most of their time in a quiescent state, and the transcription factor FOXC1 plays a key role in keeping them there. Without FOXC1, stem cells exit quiescence sooner, shortening the resting phase between hair cycles. Over time, this accelerated cycling depletes the stem cell pool and impairs hair regeneration in aging.13PubMed Central. FOXC1 maintains the hair follicle stem cell niche and governs stem cell quiescence to preserve long-term tissue-regenerating potential A secondary population of primed stem cells sits just below the bulge. These cells activate first when a new growth phase begins, essentially serving as the ignition system for each hair cycle, while the deeper bulge cells are recruited later to sustain growth.14JCI Insight. Dissecting the bulge in hair regeneration
At the base of the follicle sits the dermal papilla, a cluster of mesenchymal cells that acts as a command center. The dermal papilla sends signals that instruct the overlying stem cells when to start and stop growing. Closely related dermal sheath cells wrap around the follicle and can repopulate the dermal papilla when needed, demonstrating a degree of flexibility between these two compartments.15Developmental Cell. Hair Follicle Dermal Stem Cells Regenerate the Dermal Sheath and Repopulate the Dermal Papilla to Dictate Hair Type
Hair color comes from melanocytes residing in the follicle. During each growth cycle, melanocyte precursors activate and produce melanin pigment, which they transfer into the keratinocytes that will form the hair shaft.16PubMed Central. Hair follicle pigmentation This process resets with each cycle: melanocyte stem cells must be replenished and activated anew. When those stem cells run out, the hair grows in without pigment, which is why graying tends to be progressive and irreversible. The migration and differentiation of melanocyte precursors during follicle development establishes the pigmentary system that will operate throughout a person’s life.17Journal of Investigative Dermatology Symposium Proceedings. Fate of Melanocytes During Development of the Hair Follicle Pigmentary Unit
Follicular Cells in the Immune System
Lymph nodes and the spleen contain organized clusters of immune cells called follicles, and several cell types within them carry the “follicular” label. These are structurally and functionally unrelated to thyroid, ovarian, or hair follicular cells. They share only the architectural concept of being organized around a follicular space.
Follicular dendritic cells (FDCs) are stromal cells that form a dense three-dimensional meshwork within lymphoid follicles. Their defining trick is holding onto antigens for long periods without destroying them. By trapping immune complexes on their surface, FDCs essentially put foreign proteins on display for B cells to sample. This prolonged antigen display is critical for B cells to undergo affinity maturation, the iterative process that fine-tunes antibodies to bind their targets more precisely.18Frontiers in Immunology. How Follicular Dendritic Cells Shape the B-Cell Antigenome FDCs also rescue B cells from programmed cell death and promote their differentiation into long-lived memory cells or antibody-secreting plasma cells.19Human Pathology. Follicular dendritic cells: origin, function, and different disease-associated patterns
T follicular helper cells (Tfh cells) are a specialized subset of T cells that provide essential help to B cells within germinal centers, the zones of intense immune activity that form inside follicles during an immune response.20PubMed Central. T follicular helper cell dynamics in germinal centers Tfh cells deliver signals that B cells need to survive, divide, and refine their antibodies. Without Tfh support, germinal centers do not form properly, and the body cannot produce high-quality, long-lasting antibody responses.21PubMed. Follicular Helper T Cells
Follicular B cells (FOB cells) make up the majority of B cells in the blood and secondary lymphoid tissues. They develop from transitional B cells and home to the follicular zones of the spleen and lymph nodes, where they specialize in responding to protein antigens with the help of T cells. This T cell-dependent pathway is what produces germinal center reactions and ultimately yields high-affinity memory B cells and antibody-producing plasma cells.22Journal of Immunological Methods. Overview of human B-cell development and antibody deficiencies – Section: 4.3 Follicular B cells Follicular B cells are distinct from marginal zone B cells, which sit at the border of the spleen’s follicles and tend to respond faster but with lower-specificity antibodies.
When Thyroid Follicular Cells Become Tumors
Thyroid follicular cells can give rise to both benign and malignant tumors. Follicular adenomas are encapsulated growths that do not invade surrounding tissue, while follicular carcinomas breach their capsule or invade blood vessels. The clinical challenge is that these two cannot be reliably distinguished before surgery using imaging, blood tests, or even needle biopsy. A definitive diagnosis requires examining the excised tissue under a microscope for signs of capsular or vascular invasion.23PubMed Central. Follicular adenoma and carcinoma of the thyroid gland
Some follicular adenomas show nuclear abnormalities that resemble those found in carcinomas, suggesting they may be precursors to cancer rather than entirely benign endpoints. Objective analysis of nuclear patterns in these atypical tumors reveals a mixed bag: some look like typical benign adenomas, while others share molecular and structural features with follicular carcinoma or even papillary carcinoma.24European Journal of Endocrinology. Thyroid follicular adenomas may display features of follicular carcinoma and follicular variant of papillary carcinoma Advances in molecular testing for genetic mutations are gradually improving the ability to distinguish worrisome nodules before the operating room, but the field is not there yet for routine clinical use.
Granulosa Cell Dysfunction in PCOS
Polycystic ovary syndrome (PCOS) involves multiple small follicles that fail to mature and ovulate, and granulosa cell problems are central to the story. In people with PCOS, granulosa cells show significantly higher levels of oxidative stress markers and abnormal mitochondrial function compared to healthy controls.25PubMed Central. Oxidative stress and mitochondrial dysfunction of granulosa cells in polycystic ovarian syndrome The imbalance between granulosa cell growth and death is considered a key factor in the follicular arrest that characterizes PCOS, where follicles stall at small sizes rather than progressing to ovulation.26PubMed Central. The Role of Ovarian Granulosa Cells Related-ncRNAs in Ovarian Dysfunctions: Mechanism Research and Clinical Exploration Abnormal hormone production and insulin resistance in these cells compound the problem. Because granulosa cells sit at the intersection of so many follicular processes, their dysfunction ripples outward into the hormonal imbalances that define the syndrome.
Hair Follicle Stem Cell Loss and Aging
The gradual thinning and loss of hair with age is not simply a matter of follicles shutting down. Recent work in mice has shown that epithelial stem cells physically escape from the hair follicle niche, migrating out into the surrounding dermis. As stem cells leave, the follicle shrinks, a process called miniaturization. The genes responsible for cell adhesion and the structural scaffolding that anchors stem cells in place become less active in aged follicle stem cells, and loss of transcription factors like FOXC1 and NFATC1 accelerates this escape.27PubMed Central. Escape of hair follicle stem cells causes stem cell exhaustion during aging Live imaging has captured individual cells migrating away from the stem cell compartment and entire follicles disintegrating as a result. This reframes age-related hair loss not as stem cells dying in place, but as stem cells walking off the job.
Growing Follicular Cells in the Lab
The fact that follicular cells organize themselves into functional units has made them attractive targets for regenerative medicine. Researchers have generated transplantable thyroid organoids from human embryonic stem cells. When grafted into mice that lacked a thyroid, these lab-grown follicular structures restored circulating thyroid hormone levels, providing proof of concept that engineered follicular cells could one day treat hypothyroidism without lifelong medication.28Nature Communications. Transplantable human thyroid organoids generated from embryonic stem cells to rescue hypothyroidism Separately, researchers have coaxed embryonal carcinoma cells to form follicle-like structures that accumulate thyroglobulin and release it in response to thyroid-stimulating hormone, mimicking a key behavior of native thyroid tissue.29PubMed Central. Modelling Functional Thyroid Follicular Structures Using P19 Embryonal Carcinoma Cells
On the ovarian side, the tight relationship between granulosa cells and oocytes makes in vitro follicle culture an active area of fertility research. Maintaining the physical connections between granulosa cells and the egg is one of the biggest hurdles; without those transzonal projections and gap junctions, the oocyte loses its metabolic lifeline and developmental signals. Hair follicle bioengineering faces its own version of this problem: recreating the spatial relationship between dermal papilla cells, stem cells, and melanocytes in a way that produces a cycling, pigmented hair shaft remains an unsolved challenge, though dermal sheath cells’ ability to regenerate dermal papilla tissue in grafts offers a promising starting point.