Fertilization & Embryonic Development

acrosomal reaction

Release of digestive enzymes by sperm that enables them to burrow through the corona radiata and penetrate the zona pellucida of an oocyte prior to fertilization.

acrosome

Cap-like vesicle located at the anterior-most region of a sperm that is rich with lysosomal enzymes capable of digesting the protective layers surrounding the oocyte.

blastocoel

Fluid-filled cavity of the blastocyst.

blastocyst

Term for the conceptus at the developmental stage that consists of about 100 cells shaped into an inner cell mass that is fated to become the embryo and an outer trophoblast that is fated to become the associated fetal membranes and placenta.

blastomere

Daughter cell of a cleavage.

capacitation

Process that occurs in the female reproductive tract in which sperm are prepared for fertilization; leads to increased motility and changes in their outer membrane that improve their ability to release enzymes capable of digesting an oocyte’s outer layers.

cleavage

Form of mitotic cell division in which the cell divides but the total volume remains unchanged; this process serves to produce smaller and smaller cells.

conceptus

Pre-implantation stage of a fertilized egg and its associated membranes.

corona radiata

In an oocyte, a layer of granulosa cells that surrounds the oocyte and that must be penetrated by sperm before fertilization can occur.

cortical reaction

Following fertilization, the release of cortical granules from the oocyte’s plasma membrane into the zona pellucida creating a fertilization membrane that prevents any further attachment or penetration of sperm; part of the slow block to polyspermy.

ectoderm

Primary germ layer that develops into the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails.

ectopic pregnancy

Implantation of an embryo outside of the uterus.

embryo

Developing human during weeks 3–8.

embryonic folding

Process by which an embryo develops from a flat disc of cells to a three-dimensional shape resembling a cylinder.

endoderm

Primary germ layer that goes on to form the gastrointestinal tract, liver, pancreas, and lungs.

epiblast

Upper layer of cells of the embryonic disc that forms from the inner cell mass; gives rise to all three germ layers.

fertilization

Unification of genetic material from male and female haploid gametes.

fertilization membrane

Impenetrable barrier that coats a nascent zygote; part of the slow block to polyspermy.

gastrulation

Process of cell migration and differentiation into three primary germ layers following cleavage and implantation.

human chorionic gonadotropin (hCG)

Hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses and secure an environment suitable for the developing embryo.

hypoblast

Lower layer of cells of the embryonic disc that extend into the blastocoel to form the yolk sac.

implantation

Process by which a blastocyst embeds itself in the uterine endometrium.

inner cell mass

Cluster of cells within the blastocyst that is fated to become the embryo.

mesoderm

Primary germ layer that becomes the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys.

morula

Tightly packed sphere of blastomeres that has reached the uterus but has not yet implanted itself.

neural fold

Elevated edge of the neural groove.

neural plate

Thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue.

neural tube

Precursor to structures of the central nervous system, formed by the invagination and separation of neuroepithelium.

neurulation

Embryonic process that establishes the central nervous system.

notochord

Rod-shaped, mesoderm-derived structure that provides support for growing fetus.

organogenesis

Development of the rudimentary structures of all of an embryo’s organs from the germ layers.

polyspermy

Penetration of an oocyte by more than one sperm.

primitive streak

Indentation along the dorsal surface of the epiblast through which cells migrate to form the endoderm and mesoderm during gastrulation.

somite

One of the paired, repeating blocks of tissue located on either side of the notochord in the early embryo.

syncytiotrophoblast

Superficial cells of the trophoblast that fuse to form a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall.

trophoblast

Fluid-filled shell of squamous cells destined to become the chorionic villi, placenta, and associated fetal membranes.

zona pellucida

Thick, gel-like glycoprotein membrane that coats the oocyte and must be penetrated by sperm before fertilization can occur.

zygote

Fertilized egg; a diploid cell resulting from the fertilization of haploid gametes from the male and female lines.