Embryonic Development Terms


Allantois: An extraembryonic membrane at the caudal end of the yolk sac that extends into the connecting (body) stalk about 16 days after conception; structural base for the umbilical cord, and it forms part of the urinary bladder.


Amnion: The innermost extraembryonic membrane that contains amniotic fluid and grows to envelop the embryo; it is derived from epiblast cells.


Blastocyst: The conceptus in the postmorula stage, about 5 to 7 days after conception; it has a single layer of outer cells (trophoblast) which encloses an inner cell mass (embryoblast) and a fluid-filled blastocyst cavity (blastocele).


Blastomeres The cells formed by the early cleavage divisions of the zygote, about 1 to 4 days after conception.


Chorion: The outermost extraembryonic membrane that contributes to the formation of the placenta, and grows to enclose the embryo and the other extraembryonic membranes.


Chorionic villi: Vascular processes of the chorion of the embryo that becomes the fetal part of the placenta.


Cleavage: Rapid mitotic division of the zygote with decrease in size of individual blastomeres to form the morula


Conception: Fertilization of an oocyte by a sperm.


Conceptus: The products of conception, including all structures that develop from the zygote.


Cytotrophoblast: Inner cellular layer of the trophoblast of a blastocyst; forms part of the wall of the yolk sac.


Decidua: The endometrium of the uterus in a pregnant woman; the decidua basalis is the maternal part of the placenta; the decidua capsularis is part of the endometrium at the uterine cavity face of the implanted embryo; the decidua parietalis is the altered endometrium other than at the site of the chorion.


Ductus arteriosus: A shunt that allows fetal blood to be pumped from the right ventricle into the aorta to bypass the lungs; it is closed after birth, and remnants are called the ligamentum arteriosum.


Ductus venosus: A small vessel in the fetus that helps the blood to bypass the liver; it degenerates after birth, and remnants are called the ligamentum venosum.


Ectoderm: The outermost layer of the three primary germ layers of the embryo that forms the nervous system, the epidermis and glands of the skin, and the external sense organs such as the ear and eye.


Embryo: The developing organism from conception until the end of the eighth week (56 days).


Embryonic disc: Flat bilaminar plate of cells derived from the inner cell mass (embryoblast) of the blastocyst soon after implantation, about 8 to 14 days after conception; it contains epiblast and hypoblast layers.


Endoderm: The innermost layer of the three primary germ layers of the embryo that forms epithelial linings of the digestive, respiratory, and urogenital systems, and most glands.


Epiblast: The upper layer of the bilaminar embryonic disc present during the second week of development that gives rise to the amnion; during gastrulation, the epiblast cells form the ectoderm, mesoderm and endoderm.


Estrogens: Female sex hormones produced by the ovaries; governs appearance of secondary sex characteristics and maintenance of female reproductive structures, regulates the menstrual cycle, stimulates development of oocytes, and reduces bone resorption.


Extraembryonic membranes: Tissue that originates from the zygote, but develops outside the embryonic body and does not form part of the embryo proper; consists of the allantois, amnion, chorion, and yolk sac.


Fertilization: The process beginning with penetration of the secondary oocyte by the sperm and completed by fusion of the male and female pronuclei. Also called conception.


Fetal viability: The capability of a fetus to survive after birth; most die if born lighter than 500 g or before 21 weeks after conception; with expert postnatal care, half of the babies may survive if born 23 weeks after conception, and most of the babies may survive if born over 25 weeks after conception.


Fetus: The product of conception from the beginning of the ninth week (57 days) of gestation to birth.


Follicle-stimulating hormone (FSH): Hormone produced by the anterior pituitary gland; in females it stimulates the maturation of ovarian follicles and secretion of estrogen; in males it stimulates spermatogenesis and enhances production of androgen-binding protein (ABP) by sustentacular (Sertoli) cells. Also called follitropin.


Foramen ovale: An opening in the interatrial septum of the fetal heart that allows blood entering the right atrium to flow directly into the left atrium; it is closed after birth, and remnants are called the fossa ovalis.


Gastrulation: The process that occurs about 15 to 17 days after conception, when the bilaminar embryonic disk of the blastocyst becomes trilaminar by forming the ectoderm, mesoderm, and endoderm.


Gestation period: The duration of pregnancy; in human embryology, it is the time from conception to birth and lasts for around 38 weeks (266 days); in obstetrics (OB), it is assumed to begin on the first day of the woman’s menstrual period and lasts for around 40 weeks (280 days).


Human chorionic gonadotropin (hCG): A hormone secreted by the chorion during the first trimester that stimulates the corpus luteum to secrete progesterone and estrogen.


Human chorionic thyrotropin (hCT): A hormone secreted by the chorion that increases maternal metabolism.


Human placental lactogen (hPL): A hormone secreted by the placenta that causes maturation of the breasts, glucose sparing in the mother, and stimulates fetal growth. Also called human chorionic somatomammotropin.


Hypoblast: The lower layer of the bilaminar embryonic disc present during the second week of development that forms part of the wall of the yolk sac.


Implantation: Attachment of a blastocyst to the epithelial lining of the uterus, and its embedding in the compact layer of the endometrium, which usually occurs 6 to 7 days after conception.


Inhibin: A hormone that inhibits the release of follicle-stimulating hormone by the anterior pituitary; in females, it is secreted by granulosa cells in the ovary; in males, it is secreted by sustentacular (Sertoli) cells in the testis.


Inner cell mass: An aggregation of cells at the embryonic pole of the blastocyst about 5 to 7 days after conception; it forms the embryo and some extraembryonic membranes. Also called embryoblast.


Intermediate mesoderm: Differentiated cells of the mesoderm that forms the gonads and kidneys.


Labor: The process of giving birth in which a fetus is expelled from the uterus through the vagina.


Lateral plate mesoderm: Differentiated cells of the mesoderm that consists of two parts separated by a celom (cavity); the somatic mesoderm forms the bones, ligaments, dermis of the skin, the limbs and the body wall; the splanchnic mesoderm forms the heart, blood vessels, and most connective tissues of the body


Luteinizing hormone (LH): Hormone produced by the anterior pituitary gland. In females, it stimulates ovulation, formation of the corpus luteum, and secretion of progesterone. In males, it stimulates interstitial cells to secrete testosterone and is also called interstitial cell-stimulating hormone (ICSH).


Mesoderm: The middle layer of the three primary germ layers of the embryo that forms the connective tissue, bone, cartilage, muscle, blood, blood vessels, lymphatics, notochord, pleura, pericardium, peritoneum, kidney, and gonads.


Morula: The solid spherical mass of 16 to 100 blastomeres formed by the early cleavage divisions of the zygote, about 3 to 4 days after conception.


Neurulation: The formation of the brain and spinal cord in the embryo; it is the first event of organogenesis that begins about 17 to 22 days after conception, when the ectoderm over the notochord forms the neural plate, which folds inward to form neural folds and a neural groove that fuse into a neural tube, then the cephalic end will form the brain, and the the caudal end will form the spinal cord.


Neonatal period: The first 4 weeks, or 28 days after birth.


Notochord: A rod of mesoderm that forms in the embryo about 16 days after conception; it defines the primitive axis of the embryo and acts as the center of development of the axial skeleton; it will be replaced by the vertebral column and notochord remnants become the nucleus pulposus of the intervertebral discs.


Organogenesis: Differentiation of the embryonic germ layers to form the organs and organ systems.


Oxytocin (OT): A hormone secreted by neurosecretory cells in the hypothalamus and stored in the posterior pituitary gland; it stimulates uterine muscle contraction during labor, and promotes milk release by mammary glands during lactation.


Placenta: A fetomaternal organ that exchanges nutrients and wastes between the fetus and mother; the chorion is of embryonic origin that initially receives blood vessels from the yolk sac and the allantois, which become the umbilical arteries and veins that vascularize the chorionic villi; the decidua basalis is the maternal portion of the placenta that develops blood-filled lacunae which surround the chorionic villi.


Pregnancy: The state of a female after conception to the moment of birth, usually 38 weeks, or about 266 days. Also called gestation.


Primitive gut: Embryonic structure formed from the dorsal part of the yolk sac that gives rise to most of the gastrointestinal tract.


Primitive streak: A raised groove that runs from the center to the caudal end of the embryonic disc; it establishes the longitudinal axis of the embryo, about 15 days after conception,


Progesterone: A female sex hormone produced by the ovaries that helps prepare the endometrium of the uterus for implantation of a fertilized ovum, and helps prepare the mammary glands for milk secretion. Also called pregnancy hormone.


Prolactin (PRL): A hormone produced by the anterior pituitary gland that stimulates milk production (lactation) by mammary glands in females.


Relaxin: A hormone secreted by the corpus luteum and placenta during pregnancy; it facilitates the birth process by softening and expanding the pubic symphysis and cervix, and it inhibits uterine contractions that could cause spontaneous abortion (miscarriage).


Somite: The 40 pairs of cell masses of mesoderm that form in the embryo 3 to 4 weeks after conception; they are arranged segmentally on each side the notochord; each somite has three functional parts; the sclerotome cells produce the vertebra and rib at each level; the dermatome cells form the dermis of the skin on the dorsal part of the body; the myotome cells form the skeletal muscles of the neck, trunk, and limbs (via limb buds).


Syncytiotrophoblast: The outer layer of the developing trophoblast of an implanted blastocyst, which consists of a multinucleated protoplasmic mass (syncytium); it initially digests the endometrium of the uterus to supply nutrition to the embryo, it secretes hCG (human chorionic gonadotropin), and it forms the chorion.


Trophoblast: A layer of extraembryonic ectodermal tissue covering the blastocyst that attaches the blastocyst to the endometrium of the uterus; it gives rise to the chorion.


Umbilical cord: Long ropelike structure that contains the umbilical arteries and vein; it connects the fetus to the placenta.


Yolk sac: An extraembryonic membrane that hangs from the ventral surface of the embryo; it initially provides nutrients to the embryonic disc, then produces the earliest blood cells and blood vessels, forms the primitive gut, and is the source of primordial germ cells that seed the gonads.


Zygote: A fertilized egg; the diploid cell resulting from union of a sperm and a secondary oocyte.