Early Embryology Week 1 (DLA)

Cytokines

Small proteins produced by cells that act as messengers for other cells.

Coelom

Space/cavity.

Syncytium

Multinucleated cell mass.

Chemotaxis

Movement of cells toward or away from a chemical signal.

Blast

Cell (usually immature form).

Cleavage

Rapid mitotic division of cells.

Differentiation

Process whereby cells acquire their mature morphological and biochemical characteristics. Often considered a 'final step' of development in which cells take on their mature function.

Induction

Alteration in cell fate resulting from an external signal (inducer), a form of cell-cell signaling.

Ingression

Migration of individual cells from surface layers into the interior of the embryo.

Involution

A type of cell movement during gastrulation involving the inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.

Mesenchyme

Mesodermal cells in a developing embryo with the ability to move freely and individually.

Embryogenesis

The development of the embryo from male and female gametes to a fully formed embryo.

Fertilization

The process that occurs on Day 1 of embryogenesis, where male and female gametes unite.

Cleavage

A process that starts on Day 1 and ends with the formation of the blastocyst.

Zygote

The initial cell formed when a sperm cell fertilizes an egg cell.

Morula

A solid ball of cells that forms after the zygote undergoes cleavage.

Blastocyst

The structure formed when the morula develops a fluid-filled cavity and enters the uterine cavity on Day 5.

Zona Pellucida

The glycoprotein layer surrounding the blastocyst that it hatches from during implantation.

Endometrium

The lining of the uterus where the blastocyst attaches during implantation.

Implantation

The process that begins on Day 6 when the blastocyst attaches to the endometrium.

Fertilization

The process that occurs on Day 1 of embryogenesis, where male and female gametes unite.

Fertilization Location

Ampulla of the uterine tube.

Fertilization Timing

Occurs 12 to 24 hours after ovulation.

How does fertilization occur?

Through the process of chemotaxis.

Sequence of Events in Fertilization

1. Capacitation

2. Acrosome reaction

3. Fusion of the plasma membranes of sperm and oocyte

4. Cortical reaction – prevents other sperm from entering.

Sperm Chromatin Decondensation

Leads to the formation of the male pronucleus.

Oocyte Nucleus

Completes the second meiosis during fertilization.

Cleavage

Continues from Day 1 to approximately 24 hours after fertilization, involving repeated mitotic division of blastomeres.

Zona Pellucida

The blastomeres remain surrounded by this glycoprotein layer during cleavage.

Movement through the Uterine Tube

The developing embryo moves toward the uterine cavity during cleavage.

Compaction

At the approximately 10 cell stage, cells form a compact ball, resulting in segregation of cells forming the embryo and the membranes.

Rostro-Caudal and Lateral Axes

Establishment of these axes occurs during cleavage, giving orientation to the embryo.

Morula

A solid ball of cells consisting of 16 to 32 blastomeres, forming after cleavage.

Inner Mass vs. Trophoblast

In the morula, there is a distinction between the inner mass (which develops into the embryo) and the trophoblast (which forms the membrane).

Blastocyst Formation

The morula (30 cell stage) starts absorbing fluid from the uterine cavity, leading to the formation of the blastocystic cavity and the blastocyst.

Separation into Components

The blastomeres further separate into:

• Embryoblast (future embryo)

• Trophoblast (fetal component of the placenta).

Movement into Uterine Cavity

The blastomere enters the uterine cavity, which leads to enzymatic degeneration of the zona pellucida.

Hatching from Zona Pellucida

The blastocyst hatches from the zona pellucida and attaches to the uterine wall.

Implantation

Starts at Day 6 and continues until Day 10 when the blastocyst attaches to the epithelium of the endometrium (inner lining of the uterus).

Implantation Location

Occurs on the posterior wall of the body of the uterus close to the fundus.

Trophoblast Differentiation

The trophoblast proliferates rapidly and differentiates into:

• Outer layer of syncytiotrophoblast

• Inner layer of cytotrophoblast.

Outer Syncytiotrophoblast

Secretes and invades, forming a multinucleated mass with fingerlike projections that invade into the connective tissue, secreting enzymes that erode maternal tissue and human chorionic gonadotrophin (HCG).

Inner Cytotrophoblast

Mitotically active, forming new cytotrophoblast cells.

Day 7 Changes

Formation of hypoblast on the surface of the embryonic pole that faces the blastocystic cavity, marking the start of changes in the embryoblast.