The term “antrum” in anatomy refers to a generalized cavity, chamber, or hollow space. Derived from the Latin word meaning “cave,” it is applied to several distinct areas throughout the body, which can be confusing because these structures perform vastly different physiological roles. While all antra are chambers, their specific locations dictate functions ranging from aiding digestion and regulating hormones to equalizing air pressure or preparing an egg for fertilization.
The Antrum of the Stomach
The stomach’s chamber is most commonly referred to as the pyloric antrum, the lower, funnel-shaped region of the stomach. This muscular section is situated immediately before the pylorus, a narrow canal connecting the stomach to the duodenum, the first part of the small intestine. The primary function of the gastric antrum is mechanical: it acts as a powerful grinder, mixing the partially digested food, called chyme, with stomach secretions.
Muscular contractions within the antrum are strong and rhythmic, helping to filter and push small, liquefied particles of chyme through the pyloric sphincter into the small intestine. The antral lining lacks the acid-secreting parietal cells found higher up in the stomach body. Instead, it is rich in mucus-secreting cells that protect the stomach wall, along with specialized endocrine cells.
The most significant of these endocrine cells are the G cells, concentrated deep within the antral glands. These cells secrete the hormone gastrin directly into the bloodstream. Gastrin signals the parietal cells in the upper stomach to increase the production of hydrochloric acid. This regulatory loop ensures the stomach produces sufficient acid when protein fragments or stomach distention is detected in the antrum.
The Antrum of the Ovarian Follicle
In the reproductive system, the antrum is a fluid-filled cavity that develops within a maturing ovarian follicle. The presence of this follicular antrum signals that the follicle has progressed to the antral stage, a necessary step toward ovulation. Follicular fluid fills this chamber, surrounding the developing egg cell, or oocyte.
The fluid is a blend derived from blood plasma and substances secreted by the surrounding granulosa and thecal cells. Granulosa cells produce molecules like hyaluronan and versican, creating an osmotic gradient within the follicle. This gradient draws fluid from nearby blood vessels, rapidly expanding the cavity.
This follicular fluid acts as a specialized microenvironment for the oocyte, providing necessary nutrients, hormones, and growth factors for its final maturation. In fertility assessment, doctors perform an Antral Follicle Count (AFC) using ultrasound. The AFC estimates the ovarian reserve, or the number of remaining egg-containing follicles, which helps in planning assisted reproductive technologies.
The Mastoid Antrum and Sinus Cavities
In the skull, the term antrum describes two primary air-filled cavities. The mastoid antrum is a large, irregularly shaped air space located within the mastoid portion of the temporal bone, situated behind the ear. This chamber connects the middle ear cavity directly to the mastoid air cells, which are small pockets of air within the bone. This system helps maintain air pressure equilibrium within the middle ear.
The maxillary antrum, commonly called the maxillary sinus, is the largest of the paranasal sinuses. It is a paired, pyramid-shaped cavity housed within the maxillary bone of the cheek, positioned above the upper teeth. Its functions, shared with other sinuses, include reducing skull weight, contributing to voice resonance, and warming and humidifying inhaled air. The antrum is lined with a specialized mucosa that continuously produces mucus, which drains into the nasal cavity.
Conditions Affecting the Antra
Pathological states can affect any of the body’s antra, often due to their specific locations and functions.
Gastric Antrum Conditions
The most common issue in the gastric antrum is antral gastritis, inflammation of the lining frequently caused by Helicobacter pylori (H. pylori) infection. H. pylori infection can overstimulate the antral G cells, causing them to secrete excessive amounts of gastrin. This hyperfunction results in increased acid production, which contributes to the formation of peptic ulcers in the antrum or adjacent duodenum.
Ovarian Antrum Conditions
A common issue for the ovarian antrum is the formation of a follicular cyst, which occurs when a fluid-filled follicle fails to rupture and release the egg during ovulation. Conditions like polycystic ovarian syndrome (PCOS) involve abnormal follicular development. In PCOS, many small antra form but fail to mature properly, preventing successful ovulation.
Sinus and Mastoid Conditions
The mastoid antrum and its connecting air cells are susceptible to infection, a condition called mastoiditis. This usually begins as an untreated middle ear infection that spreads into the bone’s air spaces. The maxillary antrum is the most commonly affected of the paranasal sinuses, leading to maxillary sinusitis. Sinusitis, the inflammation of the sinus lining, can be caused by bacterial or viral infections or, often, by dental issues because the roots of the upper teeth are closely related to the sinus floor.