OSSF: Cell Fertilization/ Embryology

Embryology

Study of animal development from a single fertilized egg into a mature fetus at parturition.

Embryonic period

Time from fertilization to the earliest primordial stages of organ development.

Fetal period

Time between the end of the embryonic period and parturition during which organs grow and begin to function.

Embryonic period in dog, cat, sheep

About 30 days.

Embryonic period in horse, cattle, and humans

About 56–60 days.

Clinical importance of embryology

Helps explain structural relationships, congenital defects, fertility challenges, prevention of birth defects, and some postnatal diseases.

Diploid (2n)

Contains maternal and paternal copies of each chromosome.

Haploid (1n)

Contains one chromosome from each homologous pair.

Somatic cells

Diploid cells containing homologous chromosome pairs.

Autosomes

Chromosomes other than the sex chromosomes.

Sex chromosomes

Chromosomes that determine sex; XX or XY.

Sister chromatids

Two copies of a chromosome produced after DNA replication.

1n, 1c

Haploid cell with one copy of each chromosome.

1n, 2c

Haploid cell with replicated chromosomes.

2n, 2c

Diploid cell with unreplicated chromosomes.

2n, 4c

Diploid cell after DNA replication.

Meiosis I

Separates homologous chromosomes.

Meiosis II

Separates sister chromatids.

Meiosis I daughter cells

1n, 2c.

Meiosis II starting state

1n, 2c.

Meiosis II ending state

1n, 1c.

Meiosis II

Splits dyads into individual chromosomes and produces haploid germ cells.

S phase before meiosis II

There is no S phase before meiosis II.

Metaphase II

Chromosomes line up at the equator.

Telophase II

Nuclear envelopes reform and cytokinesis occurs.

Products of meiosis II

Haploid germ cells with one copy of each gene.

Primordial germ cells

Specialized cells that transmit genetic information to the next generation and form gametes.

Origin of primordial germ cells

Epiblast cells.

Chromosome number of primordial germ cells

2n diploid.

Primordial germ cell migration

They migrate during gastrulation toward the developing gonads.

Genital ridge

Area where primordial germ cells eventually migrate during gonad development.

Primordial germ cell mitosis

Primordial germ cells can undergo mitosis while migrating.

Oogenesis

Development of ova in the ovary.

When does meiosis begin in females?

During fetal development.

Female meiotic arrest

Oocytes arrest in prophase I before birth and later arrest in metaphase II.

Oocyte arrest in dog and fox

Oocytes remain arrested until ovulation.

At puberty in females

Meiosis I is completed.

Products of female meiosis I

A larger secondary oocyte and a smaller first polar body.

Meiosis II in the oocyte

Begins and arrests at metaphase II.

When is meiosis II completed?

Only if a sperm enters the oocyte.

Oocyte activation

Activation caused by sperm entry that allows meiosis II to be completed.

Final products of oogenesis

A mature ovum and polar bodies; polar bodies eventually degenerate.

Spermatogenesis

Production of sperm in the testes.

When does male meiosis begin?

After puberty.

Male gametes

Sperm are haploid and have condensed chromatin.

Male meiotic arrest

There is no prolonged meiotic arrest like that seen in oogenesis.

Capacitation

Changes sperm undergo in the female reproductive tract that allow them to fertilize the oocyte.

Why is capacitation necessary?

Sperm cannot immediately fertilize the oocyte after entering the female reproductive tract.

Acrosomal reaction

A sperm reaction that allows penetration of the oocyte.

Site of fertilization

Typically the ampulla of the oviduct.

Zona pellucida

Protective glycoprotein layer surrounding the oocyte that sperm must penetrate.

First step of fertilization

Sperm attaches to and passes through the zona pellucida.

Cortical reaction

Exocytosis of cortical granules that prevents additional sperm from entering the oocyte.

Polyspermy

Entry of more than one sperm into the oocyte.

Zona block

Prevents additional sperm from entering after fertilization.

Male pronucleus

Forms from the sperm nucleus after sperm enters the oocyte.

Female pronucleus

Forms from the activated oocyte after completion of meiosis II.

Male pronucleus chromosome number

1n haploid.

Female pronucleus chromosome number

1n haploid.

Do male and female pronuclei fuse?

No. They move toward each other but remain separate.

What happens to pronuclei after fertilization?

Each pronucleus independently undergoes S phase.

Pronuclei after S phase

Each becomes 1n, 2c.

Pronuclear S phase

Chromosomes replicate without G1 or G2.

First mitotic division of the zygote

Dyads condense, nuclear membranes dissolve, and a mitotic spindle forms.

Zygote

The single-cell fertilized oocyte.

Zygote chromosome number

2n diploid.

Cleavage

Rapid mitotic divisions that increase cell number without increasing overall embryo size.

Blastomeres

Cells produced by cleavage.

Purpose of cleavage

Transforms the single-cell zygote into a multicellular embryo.

Early blastomeres

Genetically identical and initially totipotent.

Totipotent

Capable of producing all body cell types and the placenta.

Synchronous cleavage

Blastomeres divide at the same time, doubling cell number.

Asynchronous cleavage

Cells divide at slightly different times.

Fragmentation

Fragments produced during embryo cleavage.

High embryo fragmentation

Associated with lower embryo quality and reduced implantation chances.

Extensive fragmentation

More than 25–30% fragmentation is associated with lower implantation rates, higher abortion rates, and increased chromosomal anomalies.

8-cell stage

Stage associated with compaction and embryonic/zygotic genome activation.

Compaction

Increased adhesion between blastomeres beginning around the 8-cell stage.

E-cadherin

Associated with blastomere compaction.

Embryonic genome activation (EGA/ZGA)

The embryo begins controlling its own gene expression rather than relying primarily on maternal control.

Morula

Approximately the 16–32-cell stage.

Morula appearance

Resembles a mulberry.

Tight junctions

Form between cells of the morula.

Blastocyst

Stage beyond 32 cells containing a fluid-filled blastocoel.

Blastocoel

Fluid-filled cavity within the blastocyst.

Inner cell mass (ICM)

Cell layer that forms the embryo.

Embryonic pole

Region containing the inner cell mass.

Trophoblast

Outer cell layer that forms the placenta.

ICM components

Epiblast and hypoblast.

Epiblast

Gives rise to ectoderm, mesoderm, and endoderm through gastrulation.

Hypoblast

Gives rise to the yolk sac and lining of extraembryonic endoderm.

Three germ layers

Ectoderm, mesoderm, and endoderm.

Hatching

Escape of the blastocyst from the zona pellucida.

Why must the blastocyst hatch?

To allow continued growth and implantation.