Early Embryonic Development and Gastrulation

Focus on early embryo development, specifically preimplantation development and gastrulation, a foundational aspect of developmental biology.
Covers preimplantation development, gastrulation, modeling of gastrulation in model organisms, and body patterning.
Cartoon overview: Fertilization to zygote development (2, 4, 8 cell stage), morula, blastocyst, gastrulation, and germ layer formation (endoderm, mesoderm, ectoderm).

Identify anatomical structures of embryos during cleavage, delamination, implantation, and gastrulation.
Recognize the origin and derivatives of the three germ layers (ectoderm, mesoderm, and endoderm).
Understand axis formation, left-right patterning, and the role of the node in early embryogenesis.

Fertilization occurs in the fallopian tubes, near the ovary.
Microcelia in the fallopian tubes direct the fertilized egg (zygote) towards the uterus.
Disruption of ciliary function can lead to ectopic pregnancy, where the zygote implants in the fallopian tubes, a life-threatening condition.
The zygote must move through the fallopian tubes, undergo early cleavage stages, and reach the uterine cavity.
By days 6-7, the embryo develops into a blastocyst, characterized by trifectoderm cells (outer layer) and an inner cell mass.
Implantation into the uterine wall occurs between days 6 and 9.

Fertilized zygote with two peronuclei about to undergo syngamy.
Early cleavage stages.
Day 5 blastocyst showing inner cell mass and trifectoderm cells.
Hatching blastocyst: the embryo hatches out of the hard shell (zona pellucida) to interact with the uterine wall for implantation.
The zona pellucida was generated with the slow block to polyspermy. Cells are dividing, compacting, and increasing cell-cell contact, differentiating to produce the early structure of the embryo.

Early stage polar body marks the animal pole/anterior pole of the embryo and determines the axis of first cleavage.
The second polar body marks the anterior pole or animal pole of the embryo.
Embryonic development initially relies on maternal proteins and RNAs within the oocyte.
The human fetal genome switches on around this stage, leading to transcriptional differences.
Cleavage division occurs approximately once per day for the first 3-4 days.
The blastocyst at five days contains just over 100 cells, with inner cell mass, trophoblast, and zona pellucida.
Anterior-posterior axis forms first, with the dorsal-ventral axis forming at right angles to the AP axis. Left-right axis forms after gastrulation, defined by the first two axes.

The embryo hatches from the zona pellucida around day 6.
The inner cell mass delaminates, forming layers:
- Epiblast: will develop into the embryo.
- Hypoblast: forms extraembryonic tissues.
From day 6 to 8, inner cell mass cells become epithelial-like and delaminate to form the epiblast and hypoblast.
By days 8-9, the blastocyst implants in the uterine wall and interacts with the maternal environment.

The zygote is totipotent, capable of becoming all cells of the embryo and extraembryonic tissues.
The inner cell mass cells are pluripotent; they can become many, but not all, cell types (excluding extraembryonic tissues).
Gastrulation leads to the formation of three germ layers: ectoderm, mesoderm, and endoderm.
Cells become progressively more specified and differentiated, losing potency as development progresses.
Cells going through gastrulation are considered multipotent.

Consolidate knowledge of preimplantation development by reviewing related questions.