UNSW Embryology

Beginnings, Growth and Development

© Dr Mark Hill (2008)

Acknowledgements

Laboratory 3 - Fertilization and Blastocyst

The movie below shows the process of in vitro fertilization (IVF).

The movie below summarizes the events occuring after fertilization during the first week of blastocyst (1-chambered conceptus) development development.

Movie (click image to play) showing process of fertilization.

(Note Movie requires Quicktime to be installed on your computer)

Overview of development of the 1-chambered conceptus (blastocyst)

Ovulation is the initial release of oocyte.

Follicular fluid and fimbriae together aid oocyte movement into infundibulum then the ampulla of the uterine tube.

Sperm deposited in the vagina then enter the uterus, mature (capacitation), then actively migrate along the uterine tube.

Fertilization generally occurs in the ampulla region of the tube.

Following fertilization, repeated rounds of cell division occur without growth forming initially a solid ball of cells (morula), which cavitates to form the 1-chambered conceptus (blastocyst). Liberation of the blastocyst from the zona pellucida the allows attachment (adplantation) to the uterine wall.

Note also the events occurring in the ovary following ovulation leading to formation of the corpus luteum from the ovulating follicle.

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Glossary

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Terms

• adplantation - Initial adhesion of blastocyst (released from zona pellucida) to uterine wall. Adplantation is followed by implantation.
• ampulla - longest segment (approximately 2/3 of overall length) of uterine tube (oviduct or Fallopian tube). Medial segment forming the remainder of the tube is called the isthmus.
• antrum- (L. a cave), cavity; a nearly-closed cavity or bulge. In the ovary this refers to the follicular fluid-filled space within the follicle.
• blastocyst - the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the coceptus to adplant and then implant into the uterine wall.
• capacitation &endash; the process of activation of sperm, requires removal of surface glycoproteins and increased motility. The sperm now become capable of fertilizing an egg.
• cavitates - to form a space within a solid object.

• conceptus the product of conception, that is all the structures derived from the zygote. This includes not only the embryo, but also the placental and membrane components formed from the conceptus. (More? Week 1 Notes)

• fertilization The penetration of the egg by the sperm and the resulting combining of genetic material that develops into an embryo. The union of two haploid gametes to form a diploid cell or zygote. (More? Week 1 Notes | Fertilization)

• fimbriae (Latin, fimbria = a fringe) finger-like projections at the ovarian end of uterine tube. At ovulation they sit over the ovary to aid egg movement into the uterine tube. (More? Week 1 Notes)

• follicular fluid - the fluid found in the antrum of an antral follicle (secondary follicle). Secreted by cells in the wall of the follicle, this fluid is released along with the oocyte at ovulation.
• infundibulum - funnel-shaped initial segment of uterine tube (oviduct or Fallopian tube) opening into peritoneal cavity and connected to the ampulla. The peritoneal opening sitting over the ovary.
• morula (Latin, morula = mulberry) early stage in development (week 1) where the cells have divided to produce a solid mass of cells (12-15 cells) with a "mulberry" appearance. This stage is followed by formation of a cavity in the mass (blastocyst stage). (More? Week 1 Notes | Week 1 Early Cell Division)
• oocyte &endash; (egg or ovum) female germ cell.
• ovulation- release of the oocyte from the mature follicle.
• triploidy - in humans, three sets of 23 chromosomes instead of 2 (diploid) combine to form the embryo. This occurs mainly by fertilization of a single egg by two sperm and less frequently by a diploid egg or sperm. Most human triploids abort spontaneously, with very rare survival to term.
• tetraploidy - in humans, four sets of 23 chromosomes instead of 2 (diploid) due to a failure of the first mitotic division after fertilization, these fertilization events do not development.
• uterine tube - (also called oviduct or Fallopian tube) the laterally paired tubes that connect the ovary to the uterus. Is the site for oocyte fertilization and initial development of the conceptus.
• uterine wall - the site of normal blastocyst implantation.
• zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development

uterus- site of embryo implantation and development. Uterine wall has 3 layers; endometrium, myometrium, and perimetrium.

zona pellucida- extracellular layer lying directly around the oocyte underneath follicular cells. Consists of glcosaminoglycans and glycoproteins (ZP1, ZP2, ZP3).

Online Resources

UNSW Embryology

Human Menstrual Cycle

Week 1 Development

Week 2 Development

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UNSW Embryology ISBN: 978 0 7334 2609 4

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