Endometrium

The endometrium (from Greek endon – inside, metra – uterus) is a highly differentiated hormone-sensitive inner mucous membrane of the uterine body.

This tissue has a unique biological potential for cyclic proliferation, secretory transformation and complete desquamation (rejection) with subsequent scarless regeneration.

Etiology and pathophysiology

The histological structure of the endometrium is a complex system consisting of a single-layer cylindrical epithelium (covering and glandular), a stroma rich in cellular elements and a specialized vascular network. Functionally and morphologically, two layers are distinguished:

1. Basal layer (stratum basale): The deep, dense, fibrous layer adjacent to the myometrium. It is supplied by short, straight arteries and has a high density of receptors for estrogen but a low density of receptors for progesterone. This layer is not rejected during menstruation and serves as a cambial (growth) reservoir for mucosal regeneration in a new cycle.
2. Functional layer (stratum functionale): The superficial layer supplied with blood by hormone-sensitive spiral arteries. In the follicular phase of the cycle, active cell mitosis (proliferation) occurs under the influence of estrogen. In the luteal phase, progesterone stops division and triggers the accumulation of glycogen, lipids and glycoproteins (secretion), preparing the “implantation window”. In the absence of pregnancy, regression of the corpus luteum causes a sharp drop in steroid levels, prolonged spasm of the spiral arteries, ischemic necrosis and rejection of this layer.

Clinical significance

The endometrium is a key tissue of the reproductive system. Its receptivity (ability to accept the blastocyst) is considered to be the leading cause of implantation failure in IVF and idiopathic infertility. Chronic endometritis (including tuberculous etiology) causes stromal fibrosis, vascular sclerosis and glandular atrophy (“thin endometrium”). Steroid hormone imbalance (absolute or relative hyperestrogenism) is a trigger for hyperplastic processes, polyps and endometrial adenocarcinoma.