| Prolactin cell | |
|---|---|
| Details | |
| System | Endocrine system |
| Location | Anterior pituitary gland |
| Function | Prolactin production |
| Identifiers | |
| MeSH | D052682 |
| TH | H3.08.02.2.00022 |
| FMA | 83096 |
| Anatomical terms of microanatomy | |
Aprolactin cell (also known as alactotroph,mammotroph, orlactotrope) is a specializedendocrine cell located in theanterior pituitary gland. Its primary role is to secrete the peptide hormoneprolactin. Prolactin serves multiple reproductive and homeostatic roles within an organism, including stimulation oflactation,mammary tissue development, regulation of theimmune response, and activity of thecentral nervous system. Prolactin cells constitute approximately 20-55% of the population of cells within the anterior pituitary gland, relative to the sex, physiological status, and species of the animal[1]. Women characteristically exhibit higher levels of prolactin activity as compared to men. Males and non-pregnant, non-lactating females typically have low levels of prolactin. The number for prolactin cells in a pregnant female will increase to allow for breast tissue development.
Prolactin cells are regulated bydopamine,estrogen, andthyrotropin-releasing hormone. The monitoring of the activity and secretion of prolactin cells provides clinical significance for reproductive andendocrine disorders within the body[2].
The location of prolactin cells within the pituitary gland is regulated largely by thehypothalamus. The pituitary gland is divided into posterior and anterior regions[2]. Within the anterior pituitary gland are the prolactin cells, where they secrete the hormone prolactin. Prolactin cells vary in number, size, and appearance depending on female reproductive status. Prolactin cells specifically increase in response to the physiological state ofpregnancy, in particular, the need for the development of breast tissues and milk production. During pregnancy, prolactin cells will undergohypertrophy (enlarging to support increased prolactin production) as well ashyperplasia (an increase in cell number). The pituitary gland increases in size due to the amount of prolactin cells. The secretory granules of prolactin cells fluctuate from sparsely granulated (during periods of low prolactin production) to densely granulated (during periods of high prolactin production). Prolactin cells contain a large amount ofrough endoplasmic reticulum, where prolactin synthesis occurs. The trans-Golgi layer is responsible for storing the prolactin hormone into secretory granules, which are dissolved upon secretion out of the cell.Lysosomal enzymes are involved in the degradation of the secretory granules. The prolactin hormone is a singlepolypeptide chain protein composed of 199amino acids in humans. It consists of “three intramoleculardisulfide bonds located between sixcysteine residues (Cys4-Cys11, Cys58-Cys174, and Cys191-Cys199”[1].
Prolactin cells are best known for their role infemale reproduction, particularly in stimulating the growth of mammary tissue and promoting lactation (milk production). Beyond female reproduction and common to both sexes, the prolactin hormone released by prolactin cells contribute to other physiological processes such as the regulation of theimmune system, thestress response, and mood.
Prolactin binds to receptors located on alveolarepithelial cells, stimulating the synthesis of the milk components includinglactose,casein, andlipids. Lactose is the carbohydrate of milk, and casein is the protein of milk. While a mother is nursing and receiving nipple stimulation, prolactin levels spike and milk production occurs. “Prolactin levels fall to non-pregnant levels after 1 to 2 weeks” when the mother is no longer nursing the child[1] . Prolactin, therefore, can be considered a short-termpositive feedback mechanism, as high levels of prolactin stimulate more prolactin secretion from the prolactin cells of the pituitary gland[2].
During pregnancy, prolactin influences the body metabolically, increasing appetite, fat storage, and the transfer ofglucose to the fetus[3]. Prolactin regulates both bone and calcium homeostasis, acts to suppressovulation, and stimulates secretion ofoxytocin.
In addition to the pituitary gland, prolactin is produced byT cells,B cells (lymphocytes of the immune system), andmacrophages. Within the immune system, prolactin acts to promote “lymphocyte differentiation, proliferation, and function” through the activation of enzyme protein kinase C signaling pathway[4]. Prolactin has immunomodulatory and anti-inflammatory effects within the immune system, contributing to immune system maintenance.
Stress also acutely stimulates prolactin production, as dopamine (a prolactin inhibitor) levels are reduced under stress, the pituitary gland is in turn expressed more leading to higher prolactin levels.
Prolactin cell regulation mainly involvesdopamine, a prolactin production suppressor,estrogen, a prolactin production stimulator, andthyrotropin-releasing hormone (TRH), a prolactin release stimulator.
Dopamine, the main inhibitor, keeps prolactin levels low. Dopamine is secreted by the hypothalamus during times of high prolactin levels, which in response lowers prolactin levels, anegative feedback mechanism[2]. Estrogen, a stimulator, enhances the production of prolactin. During times of elevated estrogen levels, such as during pregnancy, prolactin production is increased. TRH stimulates the release of prolactin from the pituitary gland. TRH, also released by the hypothalamus, binds to receptors on prolactin cells, activating cellular signaling pathways that promote prolactin secretion.
Levels of the hormone prolactin within the body can indicate various conditions. Normal prolactin levels support reproductive and metabolic functions, while elevated prolactin levels (hyperprolactinemia), and low prolactin levels (hypoprolactinemia) can indicate an underlying medical disorder. Levels of prolactin within the body can also be affected by factors such as pregnancy, stress, and certain medications. Due to the widespread distribution ofprolactin receptors across the body’s organs and tissues, elevated levels of prolactin can simultaneously influence multiple organ systems of the body.
While prolactin cell levels rise during pregnancy and lactation, they can also rise due to stress, pain, exercise, sexual intercourse, and food consumption[5]. Hyperprolactinemia (elevated prolactin levels) can be caused by excessive thyrotropin-releasing hormone production or decreased dopamine levels. This may cause symptoms in both males and females such asinfertility,erectile dysfunction, andirregular periods.
Excessive release of prolactin can be due to aprolactinoma, a tumor of the pituitary gland. The tumor itself may not cause symptoms, but may cause an increased production of prolactin and decreased levels of estrogen andtestosterone. Symptoms due to a prolactinoma may include infertility, a decrease in sexual desire, andosteoporosis. In men, symptoms may include erectile dysfunction, enlarged breast tissue, and decreased body hair. In women, symptoms may include breast discharge, irregular menstrual cycles, acne, and increased body hair.
Prolactin tests measure the amount of prolactin within the blood. This test can also be used to assess the function of the pituitary gland. Dysregulation of the pituitary gland, such as due to hypopituitarism, can cause low levels of prolactin.
Normal levels of prolactin are typically lower for males in comparison to females, at 20ng/mL, compared to 25ng/mL. This level rises in women during pregnancy or breastfeeding to be between 80 and 400ng/mL[5].
Prolactin cells are classified asacidophils, staining pink/reddish due to acidic dyes such as eosin used with hematoxylin.
While the hormone prolactin was discovered in 1928, the prolactin cell itself was not identified and characterized until the 1970s through the use ofimmunohistochemistry, a technique usingantibodies to locate specific hormones within tissues. Prolactin, considered a phylogenetically old signaling molecule, can be traced back 500 million years tolampreys, a jawless vertebrate fish group[3].
The pituitary gland which prolactin cells develop from is itself unique to thevertebrate group. The emergence of the prolactin (PRL) gene in early vertebrate evolution allowed for prolactin cells to gain the regulatory and endocrine functions it maintains within the body. Over time, the role of prolactin within the body diversified as species diversified. In mammals and fish, lactation, mammary gland development, andosmoregulation are the most established functions of prolactin, whereas in birds, prolactin is important for regulating parental behavioral control[6].
The expansion of prolactin cell function over evolutionary history is responsible for the wide distribution of prolactin receptors (PRLRs) within the body, and the diverse physiological effects prolactin hormone has on multiple body systems.