Embryology: Fertilization and Early Development

Comprehensive practice flashcards covering human embryological development from fertilization through the eighth week of gestation.

Macula Pellucida (Stigma)

An avascular area that develops on the ovary due to progressive restriction of blood flow as the ovarian follicle enlarges.

Luteinizing hormone (LH)

The hormone that serves as the trigger for ovulation, resulting in the release of a secondary oocyte.

Ampulla

The usual site of fertilization located within the uterine tube.

Corona Radiata

The outer layer of follicular cells that envelops the secondary oocyte during ovulation.

Zona Pellucida

The inner glycoprotein layer surrounding the oocyte membrane that sperm must penetrate for fertilization.

Vesiculase

An enzyme produced by the prostate gland that assists with the liquefaction of the seminal fluid coagulum.

Capacitation

A $7$-hour period of conditioning in the female genital tract that freshly ejaculated sperm must undergo to prepare for the acrosome reaction.

Acrosome Reaction

A reaction occurring when sperm comes into contact with the zona pellucida, involving the release of enzymes like hyaluronidase and acrosin.

ZP3

The specific glycoprotein on the zona pellucida to which the sperm binds to trigger the acrosome reaction.

Polyspermy

A condition where an egg is fertilized by more than one sperm; it is prevented by the zona reaction which hardens the zona pellucida.

Ootid

The oocyte containing two haploid pronuclei before they fuse to form a zygote.

Intracytoplasmic Sperm Injection (ICSI)

A variation of IVF used for male infertility where each egg is injected with a single sperm via a microscopic needle.

Morula

A solid ball of $12$ to $32$ blastomeres that forms approximately $3$ days after fertilization as the embryo enters the uterus.

Blastocyst

The stage of the embryo forming approximately $4$ days after fertilization, characterized by the presence of a fluid-filled cavity called the blastocoel.

Trophoblast

The thin outer layer of the blastocyst that gives rise to the embryonic part of the placenta.

Embryoblast (Inner Cell Mass)

A group of centrally located blastomeres in the blastocyst that gives rise to the embryo.

Syncytiotrophoblast

The outer, rapidly expanding multinucleated mass of the trophoblast that produces enzymes to erode maternal tissues and secretes hCG.

Human Chorionic Gonadotrophin (hCG)

A hormone secreted by the syncytiotrophoblast starting as early as $8$ days postovulation that maintains the corpus luteum.

Epiblast

The thicker layer of high columnar cells in the bilaminar embryonic disc that gives rise to the amniotic cavity and all three germ layers.

Hypoblast

The layer of cuboidal cells facing the blastocoel in the bilaminar embryonic disc, also known as the primary endoderm.

Chorion

The outermost fetal membrane composed of extraembryonic mesoderm, cytotrophoblast, and syncytiotrophoblast.

Gastrulation

The process occurring during the third week by which the bilaminar embryonic disc is converted into a trilaminar embryo.

Primitive Streak

The first sign of gastrulation, resulting from the proliferation of epiblast cells at the caudal end of the embryonic disc.

Sacrococcygeal Teratoma

The most common tumor in newborns, derived from remnants of the primitive streak and containing tissue from all three germ layers.

Embryonic Ectoderm

The germ layer that gives rise to the epidermis, nervous systems (CNS and PNS), eyes, and internal ears.

Embryonic Mesoderm

The germ layer that gives rise to muscles, blood cells, the cardiovascular system, and most of the skeletal system.

Embryonic Endoderm

The germ layer that provides the epithelial linings for the respiratory and digestive tracts, as well as glands like the liver and pancreas.

Prechordal Plate

A small circular area of columnar endodermal cells that serves as the head organizer and primordium of the oropharyngeal membrane.

Notochord

A cellular rod that defines the primordial longitudinal axis, gives rigidity to the embryo, and persists as the nucleus pulposus in adults.

Allantois

A small outpouching from the caudal wall of the yolk sac involved in early blood formation and development of the urinary bladder (urachus).

Meningocele

A type of spina bifida where the meninges and CSF project through a vertebral defect, while the spinal cord remains in its normal position.

Somites

Blocks of paraxial mesoderm located on either side of the neural tube used for determining fetal age and giving rise to the axial skeleton.

Cleft Sternum

A body wall defect resulting from incomplete closure of the chest wall, which may lead to ectopia cordis (heart outside the body).

Gastroschisis

A defect lateral to the umbilicus caused by non-closure of the abdominal wall, resulting in herniation of abdominal contents directly into the amniotic cavity.

Foregut

The division of the primitive gut supplied by the celiac trunk, extending from the oropharyngeal membrane to the liver outgrowth.

Midgut

The division of the primitive gut supplied by the superior mesenteric artery (SMA), including most of the duodenum and the ascending colon.

Hindgut

The division of the primitive gut supplied by the inferior mesenteric artery (IMA), including the sigmoid colon and rectum.

Mesonephric Ridge

A structure appearing in the fifth week that serves as the site of the primordium of the kidneys.

Physiologic Herniation

A normal developmental event in the sixth and seventh weeks where the rapidly growing intestines temporarily enter the extraembryonic coelom.

Auricular Hillocks

Small swellings around the first pharyngeal groove that eventually form the auricle (pinna) of the external ear.