PART 1 Basic Embryology

Embryology – study of the origin and development of single individual Prenatal period Embryonic period – first 8 weeks Fetal period – remaining 30 weeks Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Embryonic Period Figure 3.1 (1 of 2) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Fetal Period Figure 3.1 (2 of 2) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Skin – dermis and epidermis The Basic Body Plan Skin – dermis and epidermis Outer body wall – trunk muscles, ribs, vertebrae Body cavity and digestive tube (inner tube) Kidneys and gonads – deep to body wall Limbs Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Basic Body Plan Figure 3.2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 1 – from zygote to blastocyst The Embryonic Period Week 1 – from zygote to blastocyst Conception – in lateral third of uterine tube Zygote (fertilized oocyte) moves toward the uterus Blastomeres – daughter cells formed from zygote Morula – solid cluster of 12–16 blastomeres “Mulberry” Blastocyst – fluid-filled structure – ~ 60 cells Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Embryonic Period Stages of first week Zygote 4-cell Morula Early blastocyst Late blastocyst (implants at this stage) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Fertilization and the Events of the First 6 Days of Development Figure 3.3 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 2 – The Two-Layered Embryo Bilaminar embryonic disc – inner cell mass divided into two sheets Epiblast and the hypoblast Together they make up the bilaminar embryonic disc Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 2 – The Two-Layered Embryo Amniotic sac – formed by an extension of epiblast Outer membrane forms the amnion Inner membrane forms the amniotic sac cavity Filled with amniotic fluid Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 2 – The Two-Layered Embryo Yolk sac – formed by an extension of hypoblast Digestive tube forms from yolk sac NOT a major source of nutrients for embryo Tissues around yolk sac Gives rise to earliest blood cells and blood vessels Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Implantation of the Blastocyst Figure 3.4 (1 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Implantation of the Blastocyst Figure 3.4 (2 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Implantation of the Blastocyst Figure 3.4 (3 of 3) Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 3 – The Three-Layered Embryo Primitive streak – raised groove on the dorsal surface of the epiblast Gastrulation – a process of invagination of epiblast cells Begins at the primitive streak Forms the three primary germ layers Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Week 3 – The Three-Layered Embryo Three Germ Layers* Endoderm – formed from migrating cells that replace the hypoblast Mesoderm – formed between epiblast and endoderm Ectoderm – formed from epiblast cells that stay on dorsal surface *All layers derive from epiblast cells! Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Primitive Streak Figure 3.5e–h Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Primitive node – a swelling at one end of primitive streak The Notochord Primitive node – a swelling at one end of primitive streak Notochord forms from primitive node and endoderm Notochord – defines body axis Is the site of the future vertebral column Appears on day 16 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Formation of the Mesoderm and Notochord Figure 3.6 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal midline thickens Neural groove – ectoderm folds inward Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Neurulation (continued) Neural tube – a hollow tube pinches off into the body Cranial part of the neural tube becomes the brain Maternal folic acid deficiency causes neural tube defects Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Neurulation Neural crest Induction Cells originate from ectodermal cells Forms sensory nerve cells Induction Ability of one group of cells to influence developmental direction of other cells Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Mesoderm Begins to Differentiate Somites – our first body segments Paraxial mesoderm Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Mesoderm Begins to Differentiate Lateral plate – most lateral part of the mesoderm Coelom – becomes serous body cavities Somatic mesoderm – apposed to the ectoderm Splanchnic mesoderm – apposed to the endoderm Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings