Follicles are vesicular cavity systems, such as those found in the thyroid gland or ovary. Follicles have different functions depending on their location and organ system. Diseases such as polycystic ovary syndrome or Hashimoto’s thyroiditis are follicular diseases.
What are follicles?
Various cavity structures are present in the human body. One of these cavity structures is follicles. Vesicle-shaped conglomerations of cells are referred to as such. Accordingly, follicles are cellular clusters as observed on various structures of the human body. Ovarian follicles, for example, are cell clusters in the ovary. The hair follicles on the outer hair sheaths, the dental follicles and the lymph follicles of the lymph nodes are also such cell ballings. Other cell conglomerate cavity systems include the thyroid follicles and the lingual bellows glands. The anatomy and structure of the follicles differs with localization. The hair follicles, as cellular conglomerations, contain correspondingly different cell types than the lymphoid follicles or the ovarian follicles. The number of cells in the conglomerate also varies depending on the type of follicle. The thyroid follicles of the thyroid parenchyma are colloid filled. Ovarian follicles are divided into primordial, primary, secondary, and tertiary follicles depending on their maturation pathway, and dental follicles correspond to dental sacs in early tooth development. In turn, the lingual follicles are also grouped together as tonsilla lingualis. The only thing that human follicles have in common is their bubble shape.
Anatomy and structure
Thyroid follicles are epithelially confined, closed cavity structures of the thyroid parenchyma. Depending on their activity, the epithelial cells appear flattened or cuboidal. Their apical pole is microvillous and protrudes into the lumen. The base of the follicles is consolidated by a circumscribing basement membrane. Colloid is stored in the inner lumen of the conglomerates. Ovarian follicles consist of the oocyte and surrounding granulosa cells. In addition, the connective tissue layers theca interna and theca externa are included. Hair follicles surround the hair root. They consist of an outer epithelial hair root sheath, which appears as a funnel-like invagination of the stratum basale and envelops the hair root. The inner epithelial hair root sheath is the second layer of the hair follicle. It surrounds the hair root and is composed of the cuticle, Huxley’s layer and Henle’s layer. Into the hair follicles glands secrete their sebum and fragrances. The hair bellows muscles also attach in the hair follicle. They are controlled by fine nerve fiber endings in the hair follicles. Lymph follicles are light microscopic and spherical B-lymphocyte aggregations. They show homogeneous distribution of small lymphocytes. They contain reticulum cells, follicular dendritic cells, and T helper cells. In turn, the lingual follicles appear as mucosal bumps at the base of the tongue, and the dental follicles consist of the connective tissue surrounding the dental attachment.
Function and tasks
The individual follicles of the body have different tasks. For example, the thyroid follicles serve to synthesize and store lipophilic thyroid hormones in sealed compartments. The ovarian follicles, on the other hand, play a role in the maturation of the egg. It occurs in the ovary and is subject to control by follicle-stimulating hormone. Follicle maturation occurs in four stages. Primary follicles become secondary follicles and finally tertiary follicles, which become the so-called Graaf follicles. After completion of this development, ovulation takes place. In contrast, the hair follicles have the task of anchoring the hair in the scalp. The nerve endings in the hair follicles simultaneously serve the sense of touch and the movement of the sympathetic hair follicle muscles. Lymph follicles, on the other hand, have an immunological function. The proliferation and differentiation of B lymphocytes takes place in them. The B lymphocytes become plasma cells in the follicles. After antigen contact, B lymphocytes initially proliferate mitotically in the lymphoid follicles and eventually differentiate. Thus, the characteristic distribution of different developmental stages occurs. The lymphoid follicles are considered activated at this stage. All cells in the germinal center are activated lymphocytes, which become plasma cells via centrocytes.
Diseases
All follicles in the human body can be affected by diseases. For example, in polycystic ovary syndrome, the eggs remain in the follicle of the ovary. Therefore, the follicle enlarges like a cyst and the fertility of the affected woman is impaired. Acne and elevated blood lipid levels may develop as symptoms due to hormonal dysregulation. Menstrual irregularities are also a common symptom of this disease. Regarding the hair follicles, hair root inflammation is one of the most notable diseases. This so-called folliculitis is usually triggered by the pathogen Staphylococcus aureus. This bacterium belongs to the healthy skin flora. However, it produces toxic exotoxins. When these toxins enter the follicles, they cause inflammation there. Other bacteria and even fungi can also cause folliculitis. Hair root inflammation occurs specifically in the follicular funnel. Vesicles and nodules with yellowish-green pus form. The affected areas itch and are sensitive to pain. Later, the inflammation may develop into a foruncle. Brill-Symmer’s disease, on the other hand, is a disease of the lymph follicles. It manifests itself in hyperplasia. Glandular swelling and stenosis can be the result. In some cases, the hyperplastic follicles also migrate into organs where they are not actually found. In connection with the follicles of the thyroid gland, the disease to be mentioned above all is the autoimmune disease Hashimoto’s thyroiditis, which destroys the follicles bit by bit by immunological attacks. Hormonal disorders may develop as a consequence.
