embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

, Rathke's pouch is an evagination at the roof of the developing mouth in front of the buccopharyngeal membrane. It gives rise to the

anterior pituitary A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis or pars anterior) is the glandular, anterior lobe that together with the posterior lobe ( posterior pituitary, or the neurohypophysis) makes up the ...

(adenohypophysis), a part of the

endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...

Development

Rathke's pouch, and therefore the anterior pituitary, is derived from

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

. The pouch eventually loses its connection with the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its ...

giving rise to the

. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form '' pars distalis'' and ''

pars tuberalis The pars tuberalis is part of the anterior lobe of the pituitary gland, and wraps the pituitary stalk in a highly vascularized sheath. Histology Principal cells of the pars tuberalis are low columnar in form, with the cytoplasm containing nume ...

''. The posterior wall forms '' pars intermedia''. In some organisms, the proliferating anterior w