Basic Embryology

PowerPoint® Lecture Slides prepared by Leslie Hendon,

University of Alabama, Birmingham

HUMAN ANATOMY

fifth edition

MARIEB | MALLATT | WILHELM

Basic Embryology

PART 1

Embryology

Embryology – study of the origin and development of single individual

Prenatal period Embryonic period – first 8 weeks Fetal period – remaining 30 weeks

Embryonic Period

Figure 3.1 (1 of 2)

Fetal Period

Figure 3.1 (2 of 2)

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

The Basic Body Plan

Figure 3.2

The Embryonic Period

Week 1 – from zygote to blastocyst Conception – in lateral third of uterine tube

Zygote (fertilized oocyte) moves toward the uterusBlastomeres – daughter cells formed from zygote

Morula – solid cluster of 12–16 blastomeres “Mulberry”

Blastocyst – fluid-filled structure – ~ 60 cells

The Embryonic Period

Stages of first week Zygote 4-cell Morula Early blastocyst Late blastocyst (implants at this stage)

Fertilization and the Events of the First 6 Days of Development

Figure 3.3

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

Amniotic sac – formed by an extension of epiblast Outer membrane forms the amnion Inner membrane forms the amniotic sac cavity

Filled with amniotic fluid

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

Implantation of the Blastocyst

Figure 3.4 (1 of 3)

Figure 3.4 (2 of 3)

Figure 3.4 (3 of 3)

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

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!

The Primitive Streak

Figure 3.5e–h

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

Formation of the Mesoderm and Notochord

Figure 3.6

Neurulation

Neurulation – ectoderm starts forming brain and spinal cord Neural plate – ectoderm in the dorsal midline

thickens Neural groove – ectoderm folds inward

Neurulation

Neurulation (continued) Neural tube – a hollow tube pinches off into the

bodyCranial part of the neural tube becomes the brainMaternal folic acid deficiency causes neural tube

defects

Neurulation

Neural crest Cells originate from ectodermal cells Forms sensory nerve cells

Induction Ability of one group of cells to influence

developmental direction of other cells

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

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

HUMAN ANATOMY

fifth edition

Basic Embryology

PART 2

Changes in the Embryo

Figure 3.7a, b

Changes in the Embryo

Figure 3.7c, d

Week 4 – The Body Takes Shape

Folding of embryo laterally and at the head and tail Embryonic disc bulges; growing faster than yolk

sac “Tadpole shape” by day 24 after conception Primitive gut – encloses tubular part of the yolk

sac Site of future digestive tube and respiratory

structures

Figure 3.8

Derivatives of the germ layers Ectoderm forms

Brain, spinal cord, and epidermis

Endoderm forms Inner epithelial lining of the gut tube Respiratory tubes, digestive organs, and urinary

bladderNotochord – gives rise to nucleus pulposus within

intervertebral discs

Mesoderm – forms Muscle Bone Dermis Connective tissues (all)

Mesoderm differentiates further and is more complex than the other two layers

Derivatives of Germ Layers

Figure 3.10

Mesoderm (continued) Somites divides into

SclerotomeDermatomeMyotome

Intermediate mesoderm formsKidneys and gonads

Mesoderm (continued) Splanchnic mesoderm

Forms musculature, connective tissues, and serosa of the digestive and respiratory structures

Forms heart and most blood vessels Somatic mesoderm – forms

Dermis of skinBonesLigaments

HUMAN ANATOMY

fifth edition

Basic Embryology

PART 3

The Germ Layers in Week Four

Figure 3.9a–d

Week 5-8 – The Second Month of Embryonic Development

Limb buds form Embryo first looks recognizably human (week 8) Head is disproportionately large All major organs are in place

Figure 3.11

The Fetal Period

A time of maturation and rapid growth Cells are differentiating during the first half of the

fetal period Normal births occur 38 weeks after conception Premature birth is one that occurs before 38 weeks

PLAY Ultrasound of Fetus

Developmental Events of the Fetal Period

Table 3.1 (1 of 3)

Table 3.1 (2 of 3)

Table 3.1 (3 of 3)

Basic Embryology

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