Types of vertebrate uteri

Genetic regulation and embryonic development

The formation of internal genital organs begins with the so-called indifferent stage. At this stage, the embryo possesses two duct systems simultaneously: mesonephric (wolffian) and paramesonephric (mullerian). Further differentiation strictly depends on the genetic sex and hormonal background.

Male-type development (XY karyotype) is triggered by the SRY gene, which is localized on the Y chromosome. This gene initiates testis formation. In the resulting testes, Sertoli cells begin to secrete anti-Mullerian hormone (AMH). Under the influence of AMH, the paramesonephric (mullerian) ducts completely regress, while the wolffian ducts develop into elements of the male reproductive system.

Female-type development (XX karyotype) occurs in the absence of the SRY gene and anti-Mullerian hormone. Without the stimulating influence of male hormones, the wolffian ducts gradually degenerate. At the same time, the mullerian ducts continue to develop actively under the stimulating influence of estrogens. Specific genes, such as Wnt4 and DAX1, play a key role in the proper positioning and differentiation of these structures. As a result, the mullerian ducts form the fallopian tubes, the uterus itself, and the upper part of the vagina. It is precisely the extent to which these two initial ducts fuse with each other that determines the final anatomical type of the organ in various animal species.

Evolutionary classification of uterine types

During phylogenesis, a clear evolutionary sequence can be traced, reflecting the gradual fusion of the paired mullerian ducts into a single unpaired organ. Four main morphological variants are distinguished.

1. Double uterus (uterus duplex). This consists of two completely separate uterine bodies that do not fuse with each other. Each of these bodies opens into the common vagina via its own independent cervix. This most primitive type of structure is characteristic of elephants and certain rodent species.

1. Bipartite uterus (uterus bipartitus). In this variant, the two uterine bodies are already partially fused at their posterior (distal) parts. They open into the common vagina via a single cervix, but a distinct longitudinal septum remains within this cervix. This type of structure can be found in pigs, ruminants, as well as some rodents and carnivores.

1. Bicornuate uterus (uterus bicornis). This is the most common structural type among vertebrate animals. The organ consists of two prominent uterine horns that merge distally into a single, unpaired uterine body. This body opens via a single common cervix into the vaginal lumen. The bicornuate variant is typical for artiodactyls, perissodactyls, cetaceans, insectivores, and many carnivorous mammals.

1. Simple uterus (uterus simplex). This demonstrates the maximum degree of fusion of the embryonic ducts. The organ consists exclusively of a single robust body with a single cavity and a well-developed myometrium. Two fallopian tubes open directly into this unpaired body, which communicates with the vagina via a single cervix. This structural type is inherent to humans, primates, and most bats.

Morphological variants and developmental anomalies

Disorders during embryogenesis (dysembryogenesis) can lead to the formation of uterine structural anomalies in species that normally feature a simple uterus (e.g., humans). Many of these anomalies visually replicate the stages of vertebrate phylogenesis.

Among the main variants are:

• Normal uterus (Normal uterus): simple, with a single cavity.
• Subseptate uterus (Subseptate uterus): fusion has occurred, but a tissue septum remains inside the cavity.
• Arcuate uterus (Arcuate uterus): characterized by a small indentation in the fundus region resembling a saddle.
• Bicornuate uterus (Bicornuate uterus): incomplete fusion of the upper segments.
• Bicornuate uterus with double cervix (Bicornuate uterus, Double cervix): a deeper degree of organ splitting.
• Unicornuate uterus (Unicornuate uterus): formed due to aplasia of one of the mullerian ducts.
• Bicornuate uterus with rudimentary horn (Bicornuate uterus, Rudimentary horn): one of the ducts developed incompletely.
• Uterus didelphys (Uterus didelphys, Septate vagina): presence of two uteri, two cervices, and a longitudinal vaginal septum.
• Cervical atresia (Cervical atresia): complete fusion or absence of the cervical canal.

Class and exam questions

What else to study on this topic

• Mechanism of cellular action of anti-Mullerian hormone (AMH)
• The role of estrogens in the differentiation of paramesonephric ducts
• The process of wolffian duct regression in the XX karyotype
• Clinical manifestations and diagnostics of uterine anomalies (uterus didelphys, arcuate uterus)
• Genetic mutations of Wnt4 and DAX1 genes and their consequences

Other topics in the section "Phylogeny of the Vertebrate Urogenital System"

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