First and Second Week of Development

Week 1 of human development

Fertilization, cleavage, hatching, and implantation

Cleavage

Cell divisions that start about 24 hours after fertilization begins; each division takes 12–24 hours

Rotational cleavage

The first and second cleavage divisions occur at perpendicular angles

Blastomere

Each individual cell in a cleavage-stage embryo

Asynchronous cleavage

Not all blastomeres divide at the same time, so the embryo often has an odd number of cells

Embryo size during cleavage

Cleavage does not increase the overall size of the embryo; the fertilized egg is divided into progressively smaller cells

Compaction

At approximately the 8–16 cell stage, blastomeres flatten and adhere tightly to one another

Inside-outside polarity

Blastomeres develop different positions and fates depending on whether they are located inside or outside the embryo

Inside-outside hypothesis

Inner blastomeres become the inner cell mass while outer cells become the trophoblast

Inner blastomeres

Become the inner cell mass and eventually form the embryo

Outer blastomeres

Become the trophoblast and eventually form the placenta

First differentiation event

Compaction, when cells on the inside and outside of the embryo develop different fates

Totipotent

Describes cells before compaction that have the potential to form all embryonic and extraembryonic tissues

Cell potential after compaction

After compaction, cell potential becomes limited to two pathways: inner cell mass or trophoblast

Morula

A compacted embryo composed of approximately 16–32 cells

Morula timing

The morula forms within about 4 days and reaches the uterus around day 5

Blastocoel

Fluid-filled cavity that begins forming in the morula around day 5

Blastocyst

Embryo containing a blastocoel, inner cell mass, and surrounding trophoblast

Inner cell mass (ICM)

A cluster of cells located on one side of the blastocoel; also called the embryoblast

Embryoblast

Another name for the inner cell mass

ICM fate

The inner cell mass becomes the embryo

ICM cell potential

Cells of the inner cell mass are pluripotent but no longer totipotent

Trophoblast

A single layer of epithelial cells surrounding the blastocyst

Trophoblast fate

The trophoblast forms extraembryonic tissues, including the placenta

Trophoblast cell potential

Trophoblast cells are bipotential and can become two types of trophoblast cells

Hatching

The process in which the blastocyst exits the zona pellucida around day 5–6

Zona pellucida

A protective layer surrounding the embryo that prevents premature attachment and implantation

Purpose of hatching

Hatching allows the blastocyst to attach to the uterine endometrium and begin implantation

Implantation

Attachment of the embryo to the uterine endometrium

Timing of implantation

Implantation in the endometrium begins around day 6–7

Ectopic pregnancy

Pregnancy in which the embryo implants somewhere other than the uterus

Common location of ectopic pregnancy

The oviduct; approximately 90% of ectopic pregnancies occur there

Ectopic pregnancy symptoms

The pregnancy can appear normal until approximately 6–8 weeks

Ectopic pregnancy emergency

The growing embryo can rupture the oviduct, causing severe hemorrhaging

Ectopic pregnancy viability

The embryo is almost always incapable of survival and cannot be relocated to the uterus

Risk factors for ectopic pregnancy

Prior STDs, smoking, pelvic inflammatory disease, premature hatching from the zona pellucida, and ciliary defects in uterine tubes

Week 2 of human development

A period in which the embryo changes rapidly and the inner cell mass and trophoblast undergo further differentiation

Bilaminar germ disc

The two-layered embryoblast consisting of the epiblast and hypoblast

Epiblast

The external layer of the bilaminar germ disc; also called the primary ectoderm

Hypoblast

The internal layer of the bilaminar germ disc; also called the primary endoderm

Syncytiotrophoblast

Trophoblast cells that actively invade the endometrium

Cytotrophoblast

Trophoblast cells that divide to create more syncytiotrophoblast cells

Amniotic cavity

A cavity that forms between epiblast cells

Amnioblasts

New epiblast-derived cells that separate the amniotic cavity from the cytotrophoblast and become the amniotic membrane

Amnion

Another name for the amniotic cavity

Amniotic membrane

Membrane formed by amnioblasts

Extraembryonic endoderm

Tissue formed when hypoblast cells migrate to cover the inner surface of the cytotrophoblast and line the blastocoel cavity

Heuser's membrane

Another name for the extraembryonic endoderm lining the blastocoel cavity

Primary yolk sac

The blastocoel cavity after it becomes lined by extraembryonic endoderm

Extraembryonic mesoderm

Cells that migrate from the epiblast between Heuser's membrane and the cytotrophoblast

Extraembryonic mesoderm function

Surrounds the embryo and separates the amnion from the cytotrophoblast

Chorionic cavity

A cavity that forms when the extraembryonic mesoderm splits into two layers; also called the extraembryonic coelom

Extraembryonic coelom

Another name for the chorionic cavity

Connecting stalk

The portion of extraembryonic mesoderm that remains after the chorionic cavity expands around the embryo

Definitive yolk sac

The secondary yolk sac formed when hypoblast cells expand and push the primary yolk sac away from the embryo

Secondary yolk sac

Another name for the definitive yolk sac

Primary yolk sac fate

The primary yolk sac becomes connected by a thin stalk and eventually degenerates

First site of hematopoiesis

The extraembryonic mesoderm surrounding the definitive yolk sac

Hematopoiesis

Blood cell formation

Blood islands

Structures in the yolk sac formed from primitive endothelial and blood cells

Primitive endothelial and blood cells

Migrate into the yolk sac and form blood islands

End of week 2

The embryo is suspended in the chorionic cavity by the connecting stalk

Future umbilical cord

The connecting stalk develops into the future umbilical cord