Introduction to Embryology

• Embryology is the study of the development of the embryo.
• It is a science that studies the development of living beings after a series of transformations known as ontogenesis.
• The word "embryo" comes from the Greek word meaning "to swell" or "to take volume."
• The word "logos" means "study" or "science."

Stages of Embryology

• Embryology involves the mitotic division of a single cell with 2n chromosomes.
• This division results in multiple differentiated cells.
• These cells undergo differentiation and organogenesis.
• The final stage is the transformation from the cellular state to the adult state.

Types of Embryology

• Descriptive embryology is based on observation and follows a rigorous chronology from the fertilized egg to the complete organism, varying by species.
• In birds, it is done hourly, while in humans, follow-up occurs weekly.
• Experimental embryology studies the mechanisms of embryonic development and their effects on living beings, such as treating sterility with in vitro fertilization.
• Pathological embryology monitors embryonic development for anomalies at birth, resulting in one or more deformations.
• An example of an anomaly of endogenous origin is hereditary trisomy 21, occurring during the formation of male and female gametes.
• Anomaly of exogenous origin results from radiation-induced genetic mutations, leading to deformations.

Definitions

• A germ is the spherical form of an embryo from the zygote until the end of the 3rd week of embryonic development.
• An embryo is an organism in development from the first division of the fertilized egg (zygote) until the stage where the main organs are outlined (4th to 8th week).
• A fetus has bilateral symmetry and is an organism in development from the 8th week of pregnancy until birth.
• In humans, the egg (female gamete or oocyte II blocked in metaphase II) is alecithal (without reserves), unlike bird eggs that are rich in vitellus.

Stages of Embryology:

• An organism begins to exist via fertilization (fusion of male and female gametes).
• In synthesis, it is a series of transformations, and is undergone by a cell with 2n chromosomes, which continues through to the development of an organism.
• The stages are:
  • Gamete forms the zygote
  • The zygote becomes a blastula, this is the mass of cells
  • The blastula develops into a gastrula, the proper embryo.
  • Neurula leads to organogenesis
  • The organism then develops the foetus and becomes more specialized

Segmentation of Fertilized Egg

• Segmentation involves cell division without overall growth in size.
• The stages progresses like this:
  • Ovule
  • Two cells
  • Four cells
  • Eight cells
  • Several cells or blastomeres
  • Blastocyst

Key Concepts

• Stem cells are capable of self-renewal.
• Totipotent cells have the potential to differentiate into any cell lineage, embryonic or extra-embryonic, and are embryonic cells up to the morula stage.
• Pluripotent cells, found in the inner cell mass of the blastocyst, can differentiate into multiple cell lineages, including the three embryonic germ layers.

Phases of Embryonic Development

• Pre-morphogenesis occurs from the 1st to the 5th day.
• Primary morphogenesis occurs during the 2nd and 3rd weeks.
• Primary morphogenesis occurs in the 4th week of pregnancy (20th-29th day).
• Secondary and definitive morphogenesis occurs from the 5th week of pregnancy onward.

Embryogenesis Periods

• Pre-morphogenesis involves fecundation and segmentation, leading to the embryonic button.
• Primary morphogenesis involves gastrulation, resulting in a three-layered embryonic disc.
• Primary morphogenesis involves neurulation, leading to the neural streak.
• Secondary and definitive morphogenesis involves organogenesis, forming the embryo with recognizable organs.

Germ Layers

• M II continues during the fetal and postnatal periods, and up to puberty
  • Ectoderm
  • Endoderm
  • Mesoderm

Defining Embryonic Development

• During the first 8 weeks of pregnancy, organism is called embryo.
• The embryo acquires distinctive human characteristics.
• After the third month they are called a Foetus.
• Organ systems are already constituted, fetal period characterized by maturation and growth.

Anatomical Planes

• The planes of section used in the embryo:
  • Are similar to those in studying the germ.
  • Employ anteroposterior globe.
  • With the poles north and south (animal and vegetative).
• The equatorial plans under and over the equator

Coupes

• Longitudinal Coupes
  • There are two types
  • Sagittal, the medium part passing through the axe of symmetry, dividing the embryo equally
  • Parasagittal
  • Transversal Coupes
    • Planes that pass perpendicular, through the longitudinal plan of the germ