2010 BGD Lecture - Development of the Embryo/Fetus 1

Introduction

--Mark Hill 20:53, 3 May 2010 (EST) I am currently updating the notes that will appear here before the lecture on Thursday this week for 2010. Previous 2008 Lecture

• Begin by reviewing the recent Foundations Lecture and Practical.
• This lecture covers conceptus development from fertilization to implantation to trilaminar embryo formation.
• The lecture will also introduce early fetal membranes and placentation.

For those that cannot wait, here are some images to get you started......

Gametogenesis

Male

The testes have two functions.

1. produce the male gametes or spermatozoa
2. produce male sexual hormone, testosterone (external genitalia, sex characteristics)

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  Historic testis drawing

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  Adult Seminiferous tubule showing spermatozoa developmental stages

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  Seminiferous tubule cross-section and supporting cells

• Spermatogonia - are the first cells of spermatogenesis
• Primary spermatocytes - large, enter the prophase of the first meiotic division
• Secondary spermatocytes - small, complete the second meiotic division
• Spermatids - immature spermatozoa
• Spermatozoa - differentiated gamete

Sertoli cells (support cells) Interstitial cells or Leydig cells (produce hormone)

Female

The ovaries have two functions.

1. produce the female gametes or oocytes
2. produce female hormones, estrogen and progesterone

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  overview of ovary

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  three stages of follicle development

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  primary follicle

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  secondary follicle

Follicle cell (support cell) Theca cell (hormone secreting)

MBoC - Figure 20-18. Influence of Sry on gonad development

Week 1 and 2

Week 2 Implantation

Week 3 Gastrulation

gastrulation

Trilaminar embryo SEM

Trilaminar embryo

Gastrulation, (Greek = belly) means the formation of gut, but has been used in a more looser sense to to describe the formation of the trilaminar embryo. The epiblast layer, consisting of totipotential cells, derives all 3 embryo layers: endoderm, mesoderm and ectoderm. The primitive streak is the visible feature which represents the site of cell migration to form the additional layers. Historically, gastrulation was one of the earliest observable morphological event occurring in the frog embryo.

• primitive node - region in the middle of the early embryonic disc epiblast from which the primitive streak extends caudally (tail)
  • nodal cilia establish the embryo left/right axis
  • axial process extends from the nodal epiblast
• primitive streak - region of cell migration from the epiblast layer forming sequentially the two germ cell layers (endoderm and mesoderm)

Notochord

Movie - notochord 1 | - notochord 2

Stage7 axial process

The notochord is a structure which has an early mechanical role in embryonic disc folding and a major signaling role in patterning surrounding embryonic tissue development. This signaling role patterns many different tissues (neural plate, neural tube, somites, endodermal organs). It has its own sequence of development from a primitive axial process and is a developmental feature not present in the adult anatomy.

• axial process an initial epiblast hollow epithelial tube which extends in the midline from the primitive pit, cranially in the embryonic disc (toward the oral membrane).
  • neuroenteric canal is a transient communication between the amnionic cavity and the yolk sac cavity formed by the axial process.
• notochordal plate forms from the axial process merging with the endoderm layer.
• notochord forms from the notochordal plate which then separates back into the mesoderm layer as a solid column of cells lying in the midline of the embryonic disc and running rostro-caudally (head to tail).
  • An alternate name for the notochord is "axial mesoderm".

Somitogenesis

stage 9 Embryo

stage 10 Embryo

Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.

Coelom, meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The intraembryonic coelom is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities (pericardial, pleural, peritoneal).

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  trilaminar embryo

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  mesoderm regions

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  somite coelom

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  neural tube and neural crest

stage 9 Embryo

Somite initially forms 2 main components

• ventromedial- sclerotome forms vertebral body and intervertebral disc
• dorsolateral - dermomyotome forms dermis and skeletal muscle

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  paraxial mesoderm

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  early somite

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  sclerotome and dermomyotome

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  dermatome and myotome

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  epaxial and hypaxial muscles

Week 4 Neuralation

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  Stage 10 Embryo (Week 4, 22 - 23 days)

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  Stage 10 Embryo (Week 4, 22 - 23 days)

• Stage11 SEM1.jpg

  Stage 11 Embryo (Week 4, 23 - 26 days)

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  Stage 11 Embryo (Week 4, 26 - 30 days)

Ectoderm - 2 parts

• midline neural plate
  • columnar
• lateral surface ectoderm
  • cuboidal
  • sensory placodes
  • epidermis of skin, hair, glands, anterior pituitary, teeth enamel

Neural Plate

• extends from buccopharyngeal membrane to primitive node
• forms above notochord and paraxial mesoderm
• neuroectodermal cells
  • broad brain plate
  • narrower spinal cord
• 3 components form: floor plate, neural plate, neural crest

Cardiogenesis

The Human Heart from day 10 to 25 (scanning electron micrograph)

• forms initially in splanchnic mesoderm of prechordal plate region - cardiogenic region
  • growth and folding of the embryo moves heart ventrallly and downward into anatomical position
• week 3 begins as paired heart tubes that fuse to form single heart tube
• begins to beat in Humans- day 22-23

Blood Islands

fetal blood

• 2 populations of cells
  • peripheral- form endothelial cells
  • core- form blood cells (haemocytoblasts)
• all vessels (arteries and veins) appear initially the same

Blood Formation

Mouse hematopoietic stem cell location

• blood formation from stem cells occurs initially in the extraembryonic mesoderm of the yolk sac
• then later (week 5) throughout embryonic mesenchyme
• blood stem cells then migrate into the liver
  • then spleen, bone marrow, lymph nodes

Red Blood Cells

The only cells in the blood are nearly entirely fetal red blood cells. These cells differ from adult red blood cells in:

• often remaining nucleated
• contain fetal haemoglobin - which has different oxygen and carbon dioxide binding characteristics

Early Placentation

Placenta and placental membranes

Placenta anchoring villi

The trophoblast layer has now differentiated into two morphologically distinct cellular layers.

• Syncitiotrophoblasts - form a multinucleated cytoplasmic mass by cytotrophoblast cell fusion and both invade the decidua and secrete hCG
• Cytotrophoblasts - form a cellular layer around the blastocyst, proliferates and extends behind syncitiotrophoblasts

Early Utero-Placental exchange - transfer of nutrition from maternal lacunae filled with secretions from uterine glands and maternal blood from blood vessels. The development of trophoblast villi extending into the uterine decidua.

There are three stages of villi development:

• Primary Villi - cytotrophoblast
• Secondary Villi - cytotrophoblast + extraembryonic mesoderm
• Tertiary Villi - cytotrophoblast + extraembryonic mesoderm+ blood vessels

There are two main types of early villi:

• Anchoring villi - attached to decidua
• Floating villi - not attached to decidua, floating in maternal lacunae.

Glossary Links

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link

2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12

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Cite this page: Hill, M.A. (2024, May 1) Embryology 2010 BGD Lecture - Development of the Embryo/Fetus 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Lecture_-_Development_of_the_Embryo/Fetus_1

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