Allantois: (allantoic sac) designation of one type of placenta, because of its roots in other mammals; thin membrane between amnion and chorion [Gk. allantos = sausage]. The allantoic sac connects to the fetal bladder by the allantoic duct or urachus through the umbilical cord.

Amnion: thin membrane surrounding the fetus; lamb's caul [Gk. amnos = lamb] Note: Because we use chorionic and not choriotic, it is here preferred to speak of amnionic, rather than amniotic. Hyrtl (1880) explored the various terminology used to describe placental structures. He concluded that amnios and amnion were both correct. Galen, referring to skin, first used it. The reference to lamb comes from Vesalius.

Aromatase: enzyme of many placentas, best explored in primates, that "aromatizes" and thus changes, androgens into estrogens.

Blood chimerism: the composite of several genotypes [Gk. chimaira = a monstrous beast. In Greek mythology a monster made of the head of a lion, body of a goat, and tail of a dragon.], confined to the blood.

Chorion: outer membrane around embryo [Gk. chorion = "Little gut." According to
Hyrtl (1880) the term was also used by Galen as the outer shell of the membranes.].

Chorion laeve: the membranous portion of the chorionic sac [L. levis = smooth, without villi].

Cytotrophoblast: cellular type of the trophoblast [Gk. kytos = cell + trephein = to nourish + blaste = germ].

Decidua: the endometrium at end of the luteal phase [L. decidere = fall, die].

Decidualization: the hormonal alteration of the endometrium in pregnancy.
Fetal/maternal ratio: the weight relationship of fetus to placenta.
HCG: human chorionic gonadotropin, glycoprotein secretion of syncytiotrophoblast.

Hippomanes: debris found in the allantoic sac of many species, containing cellular debris and crystals, often oxalates.

HLA antigens: surface cellular recognition sites that specify individuals and thus prevent inter-individual transplantation, unless they are identical twins.

Imprinting: the usage of genes from one parent over the other, specially used in
embryo/placenta formation.

Langhans' cells (layer): this is cellular trophoblast, so named for the human placenta, of trophoblast cells whose main or only function is the production of cytoplasm and nuclei that are then incorporated into the syncytiotrophoblast.

Monochorial: single layer of trophoblast separating mother and fetus.
Omphalopleure: this is usually a bilaminar membrane ("pleure"), lined on one side with trophoblast, on the other with endoderm (yolk sac-derived). Reichert's membrane makes up the connective tissue upon which the epithelia rest.

Placenta: [L. flat cake] According to Hyrtl (1880) this term, with an originally Greek root, was introduced in 1559 by Realdus Columbus. Others referred to it as "secundines."

Placentome: It is customary to speak of a placentome when discussing the artiodactyl placentas. A placentome is the combination of cotyledon and caruncle, not a neoplasm.

Somatopleure: the membrane formed of ectoderm and mesoderm.
Splanchnopleure: visceral layer composed of mesoderm and entoderm. It relates to "somatopleure".

Sub-placenta: The sub-placenta is a structure that is especially prominent in rodent placentas and is the site of implantational modification of the endometrium by specialized trophoblastic cells.

Syncytin: this is a newly discovered protein derived from a retroviral envelope gene that expresses in (or leads to the formation of) the syncytiotrophoblast.
Syncytium: multinuclear mass, derived from cell fusion [Gk. syn = together + kytos = cell].

Syncytiotrophoblast: syncytial type of trophoblast.

Total body chimerism: the composite of several genotypes [Gk. chimaira = a monstrous beast. In Greek mythology a monster made of the head of a lion, body of a goat, and tail of a dragon.], expressed in all tissues.

Trophoblast: the epithelium that covers the placenta [Gk. trophe = nourishment + blastos = germ].

Vitelline: belonging to the yolk sac [L. vitellus = yolk of an egg].

X-cells (Intermediate Trophoblast): specialized cell type of human placentas, also called "intermediate" or extravillous trophoblast that secretes "MBP" (major basic protein).

Yolk sac: extraembryonic tissues surrounding the yolk. In mammals it is connected through the omphalomesenteric duct (vitelline duct) to the ileum (where remains may become Meckel's diverticulum).
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