Umbilicus

Primary umbilical endometriosis is a rare disorder and is defined as the presence of ectopic endometrial tissue within the umbilicus.
Open references
https://www.hindawi.com/journals/criog/2016/9302376/

umbilical primary tumors

Umbilical granulomas show variable inflammatory changes ranging from abscess formation to granulomatous areas.
Sometimes hair shafts or debris are present with associated foreign body giant cells. There is variable fibrosis.
See also
human development digestive development
omphalomesenteric duct remnant
enteric fistula
umbilical sinus
subcutaneous cyst
umbilical polyps
umbilical (...)

The fibrous umbilical polyp appears to be a distinctive lesion of early childhood with an uncertain pathogenesis.
It has a marked predilection for boys. A series of 19 cases from one institution suggests that it is not rare. They ranged in size from 0.4 to 1.2 cm in diameter.
Microscopy
The fibrous umbilical polyp is a dome-shaped lesion with a stromal proliferation of moderately cellular fibrous tissue without significant inflammation.
Fibroblastic cells are plump to elongate with (...)

The umbilical polyp presents as a bright red polyp or fleshy nodule, 0.5–2 cm in diameter; it may discharge a mucoid secretion.
A distinctive umbilical polyp, devoid of any epithelial component is termed a fibrous umbilical polyp.
Umbilical polyps (omphalomesenteric duct polyps) are covered by epithelium which is usually of small bowel or colonic type, but occasionally of gastric type. Ectopic pancreatic tissue has also been described.
There is usually an abrupt transition from epidermis (...)

vitelline cyst

vestiges of omphalomesenteric duct; Omphalomesenteric duct remnant
Images
Omphalomesenteric duct remnants https://twitter.com/haephyo/status/866899949304270849
Types
omphalomesenteric duct cyst
umbilical mucosal polyp
umbilical sincus
umbilical fistula
Meckel diverticula
umbilical cyst
See also
Umbilicus (navel)
urachus remnants

vitelline duct

navel

References
Leung AK, McLeod DR. Autosomal dominant flat umbilicus. Am J Med Genet. 2004 Dec 15;131A(3):307-9. PMID: #15523632#