Early Development

placenta

A membranous vascular organ which develops during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord; it develops from the chorion (and allantois) of the embryo and the decidua basalis of the maternal uterus; it secretes the hormones human chorionic gonadotropin = hCG, estrogens, and progesterone to contribute to regulation of the pregnancy and provides the tissue medium through which nutrients, respiratory gases and wastes are exchanged between the maternal and fetal blood circulations, without the direct mixing of maternal and fetal blood and typically involving the interlocking of fingerlike vascular chorionic villi with corresponding modified areas of the uterine mucosa; following birth, the placenta is expelled. For additional illustrations, see Human Placenta.

amnion

A thin but tough extraembryonic serous membrane which lines the chorion and contains the embryo/fetus and the secretes amniotic fluid around the embryo/fetus it is derived from the trophoblast membrane; it is the innermost of the three mammalian extraembryonic membranes.

developmental anatomy

The branch of anatomy that studies structural changes of an individual from fertilization to maturity; in particular, the anatomy of the embryo or fetus.

amniotic fluid

The clear, slightly yellowish serous fluid in which the embryo is suspended inside the amnion within the uterus during pregnancy; it provides a protective cushion against outside trauma, allows space for growth and freedom for fetal movement, provides some thermal insulation to the fetus, and contributes to proper lung development; it is constantly circulated by the fetus which swallows and "inhales" existing fluid and replacing it through "exhaling" and urination; some intestinal discharge, bile-tinged greenish meconium, is also added to this fluid. {See figures below illustrating amniocentesis.]

blastocoele

The fluid-filled, central cavity of a blastula, surrounded by trophectoderm cells of the trophoblast membrane. aka segmentation cavity, cleavage cavity. [Note: C in the figure above right.]

chorion

The highly vascular outer extraembryonic membrane which is associated with the allantois in the formation of the placenta; its outer epithelial layer is derived from the trophoblast; develops villi (vascular fingers) through which nutrients, respiratory gases and wastes are exchanged between the maternal and fetal blood circulations, without the direct mixing of maternal and fetal blood.

cytotrophoblast

The inner cellular layer of the trophoblast of an embryo which gives rise to the plasmodial syncytiotrophoblast covering the placental villi. aka - Langhans' layer, layer of Langhans.

inner cell mass = embryoblast

The mass at the embryonic pole of the blastocyst concerned with the formation of the body of the embryo; the inner cell mass will form all of the tissues of the human body, therefore, these are the cells that develop into the fetus; these inner cell mass cells are pluripotent; this means that they are able to give rise to many, but not all cell types necessary for fetal development (e.g., they are able to give rise to fetal tissues, but not placental tissue). [Note: ICM in the figure below right.]

allantois

The middle extraembryonic membrane of the human embryo; initially it is a vascular membrane formed from the hindgut but as development continues, it will be transformed into certain of the tissues of the placenta and the umbilical blood vessels.

blastocyst

The modified blastula which is characteristic of placental mammals, including humans; further cleavage of the morula produces a thin walled hollow sphere, whose wall is the trophoblast, with the embryo proper being represented by a mass of cells at one side; this second stage of embryonic development must be completed before successful implantation into the uterine wall is possible.

syncytiotrophoblast

The outer syncytial layer of the trophoblast that actively invades the uterine wall forming the outermost fetal component of the placenta. aka - syntrophoblast [Note: A syncytial layer is an epithelium or tissue in which there is a cytoplasmic continuity between the constituent cells, i.e., the cells are not separated entirely by outer cell membranes, they have formed a giant multinucleate mass.]

trophoblast

The outermost layer of cells, membrane which forms the wall of the blastocyst and which attaches the fertilized ovum to the uterine wall and supplies nutrition to the embryo; it facilitates implantation by eroding away the tissues of the uterus with which it comes in contact allowing the blastocyst to sink into the cavity formed in the uterine wall, and differentiates into the extraembryonic membranes surrounding the embryo; after implantation of the blastocyst in the uterine wall it divides into two layers, the chorion and the placenta. aka - trophoderm.

implantation

The process by which an early embryo, in the trophoblast stage of development, attaches and becomes embedded in the uterine endometrial lining; this stimulates the adjacent endometrial cells to initiate placenta formation and, thus, a pregnancy; if this process fails, a pregnancy will not occur, even if fertization has occurred.

human chorionic gonadotropin (hCG)

The protein hormone produced only by the placenta after successful implantation of a developing embryo which signals to the hypothalamus, pituitary gland, and ovary that a pregnancy has begun; this positive feedback regulation maintains the output of estrogens and progesterone by the corpus luteum during pregnancy; this is also the hormone detected in over-the-counter urine pregnancy tests.

morula

The solid sphere or globular mass of cells (blastomeres), formed by the mitotic cleavage of the fertilized ovum = egg or zygote in the first stages of embryonic development, prior to implantation in the uterine wall.