The anterior pituitary complex
The pituitary gland is a pea-sized gland that sits in a protective bony enclosure called the sella turcica (Turkish chair/saddle). It is composed of three lobes: the anterior, intermediate, and posterior lobes. In many animals, these lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered as part of the anterior pituitary. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary.
The anterior pituitary is composed of three regions:
- Pars distalis
Microanatomy of the pars distalis showing chromophobes, basophils and acidophils
- The pars distalis, (distal part), comprises the majority of the anterior pituitary and is where the bulk of pituitary hormone production occurs. The pars distalis contains two types of cells including chromophobe cells and chromophil cells. The chromophils can be further divided into acidophils (alpha cells) and basophils (beta cells). These cells all together produce hormones of the anterior pituitary, and release them into the blood stream.
Nota bene: The term "Basophil" and "Acidophil" is used by some books, whereas others prefer to not use these terms. This is due to the possible confusion with white blood cells, where one may also find Basophils and Acidophils.
- Pars tuberalis
- The pars tuberalis, (tubular part), forms a part of the sheath extending up from the pars distalis which joins with the pituitary stalk (also known as the infundibular stalk or infundibulum), arising from the posterior lobe. (The pituitary stalk does not connect the hypothalamus to the posterior pituitary). The function of the pars tuberalis is poorly understood. However it has been seen to be important in receiving the endocrine signal in the form of TSHB (a β subunit of TSH) informing the pars tuberalis of the photoperiod (length of day). The expression of this subunit is regulated by the secretion of melatonin in response to light information transmitted to the pineal gland. Earlier studies have shown a localisation of melatonin receptors in this region.
- Pars intermedia
- The pars intermedia, (intermediate part), sits between the pars distalis and the posterior pituitary, forming the boundary between the anterior and posterior pituitaries. It is very small and indistinct in humans.
The anterior pituitary is derived from the ectoderm, more specifically from that of Rathke’s pouch, part of the developing hard palate in the embryo.
The pouch eventually loses its connection with the pharynx, giving rise to the anterior pituitary. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form the pars distalis and the pars tuberalis. The posterior wall of the anterior pituitary forms the pars intermedia. Its formation from the soft tissues of the upper palate contrasts with the posterior pituitary, which originates from neuroectoderm.