Early fetal development of the medial canthal ligament and Horner’s muscle: a histological study

We examined horizontal and frontal sections of 15 specimens (20-30 weeks of gestation) from the large collection of human fetuses stored at the Complutense University in Madrid (Spain). Frontal sections demonstrated the orbicularis oculi muscle inserting to a raphe-like structure along the horizontal parts of the lacrimal canaliculi. In horizontal sections, the raphe-like structure corresponded to a fibrous tissue mass sandwiched by the superior and inferior lacrimal canaliculi. The tendons of Horner’s muscle were divided into 1) the so-called “reflection tendon” that included the typical myotendineous junction at the insertion into the maxilla, and 2) the so-called “direct tendon” in the roof of the lacrimal sac. However, Horner’s muscle did not insert into the canaliculi, but was simply attached to, or embedded in, the fibrous sheath around them. Notably, none of these connective tissue structures was attached to the tarsi. Horner’s muscle and its tendon might contribute to formation of the bony attachment of the future MCL, but the main part of the MCL most likely originates from the raphe-like structure. The connection between the MCL and the tarsi seems to be established after birth due to the growth of connective tissue along the lacrimal canaliculi. Although congenital entropion is a rare condition among Westerners, the present study demonstrated that the tarsus is unlikely to be fixed at a late stage in Western fetuses.