BGDA Lecture - Development of the Embryo/Fetus 1

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Introduction

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In medicine foundations you were given a broad overview of human development. Now in BGDA we will be working through the human development process in more detail, focussing on key events.

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


Lecture Archive: 2014 Lecture Print Version | 2013 Lecture | 2012 | 2010 | 2008
Textbooks
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UNSW Embryology

Logo.png Hill, M.A. (2014). UNSW Embryology (14th ed.) Retrieved July 30, 2014, from http://php.med.unsw.edu.au/embryology
Links: More Embryology Textbooks

The Developing Human: Clinically Oriented Embryology

Citation: The Developing Human: Clinically Oriented Embryology 8th ed. Moore, Keith L; Persaud, T V N; Torchia, Mark G Philadelphia, PA : Saunders/Elsevier, c2011. (links only function with UNSW connection)

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Larsen's Human Hmbryology

Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009. (links only function with UNSW connection)

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More Textbooks?

BGDA Practical Classes

Practical 3 - Fertilization to Implantation Practical 6 - Implantation to 8 Weeks Practical 12 - Fetal Period
 
Practical 14 - Placenta and Fetal Membranes

Human Reproductive Cycle

  • Meiosis in gonad produces haploid gametes (egg and sperm)
Female Male
  • Menstrual Cycle a regular cycle of reproduction (28 days)
  • begins at puberty, release of 1 egg (oocyte) every cycle
  • Endocrine controlled (HPG axis) Hypothalamus - Pituitary - Gonad
  • continuous production of sperm (spermatozoa)
  • begins at puberty, release millions of spermatozoa
  • Endocrine controlled (HPG axis) Hypothalamus - Pituitary - Gonad

XXhpgaxis.jpg Menstrual cycle.png


Gametogenesis

Male

The testes have two functions.

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

Human spermatozoa take about 48 days from entering meiosis until morphologically mature spermatozoa.

Spermatozoa development: primordial germ cell - spermatogonia - primary spermatocyte - secondary spermatocytes - spermatid - spermatozoa

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

Seminiferous tubule cartoon.jpg

Female

The ovaries have two functions.

  1. produce the female gametes or oocytes
  2. produce female hormones, estrogen and progesterone (secondary sex characteristics, menstrual cycle)

In an adult human female the development of a primordial follicle containing an oocyte to a preovulatory follicle takes in excess of 120 days.

Human ovary follicle development.jpg

Human ovary follicle development

Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - tertiary follicle - preovulatory follicle


Follicle cells (support cells) Theca cells (produce hormone)


Links: Spermatozoa Development | Oocyte Development | MBoC - Figure 20-18. Influence of Sry on gonad development | Endocrinology - Comparative anatomy of male and female reproductive tracts

Fertilization

Early zygote showing polar bodies
  • Oogenesis - 1 gamete produced/meiosis + 3 polar bodies, meiosis is slow, 1 egg produced and released at ovulation
  • Spermatogenesis - 4 gametes produced/meiosis, meiosis is fast, 200-600 million sperm released at ejaculation


Ovulation icon.jpg Fertilization 001 icon.jpg

Fertilization Site

  • Fertilization usually occurs in first 1/3 of uterine tube (oviduct, Fallopian tube)
  • Fertilization can also occur outside uterine tube associated with Assisted Reproductive Technologies (IVF, GIFT, ZIFT...) and ectopic pregnancy
  • The majority of fertilized eggs do not go on to form an embryo

Fertilization - Spermatozoa

Fertilization - Oocyte

Embryo mitosis icon.jpg Week1 001 icon.jpg

Week 1 and 2

Human uterine tube ciliated epithelium (SEM)

Week1 summary.jpg

Human blastocyst day 1-6.jpg

Week 2 Implantation

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Normal Implantation

  • Uterine body
    • posterior, anterior, superior, lateral (most common posterior)
    • inferior implantation - placenta overlies internal os of uterus Placenta Previa


Abnormal Implantation

Tubal pregnancy.gif
  • Ectopic Sites
    • external surface of uterus, ovary, bowel, gastrointestinal tract, mesentery, peritoneal wall
    • If not spontaneous then, embryo has to be removed surgically
  • Uterine - tubal pregnancy (most common ectopic)


Early Placenta

  • interaction between implanting conceptus and uterine wall (endometrium)
  • The uterine lining following implantation (Decidua)
    • forms 3 distinct regions, at approx 3 weeks
    • Decidua Basalis - implantation site
    • Decidua Capsularis - enclosing the conceptus
    • Decidua Parietalis - remainder of uterus
  • uterine cavity is lost by 12 weeks
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Week 3 Gastrulation

Stage7 primitive streak
gastrulation
Trilaminar embryo SEM

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)

Trilaminar embryo

Mesoderm 001 icon.jpg

Notochord

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.

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  • 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).

stage 11 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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Week 4 Neuralation

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 ‎‎Neural Tube Close
Page | Play

Neuralplate 001 icon.jpg Neuraltube 001 icon.jpg

Ectoderm - 2 parts

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

Neural Plate

meningomyelocele

Neural plate movie icon.jpgNeuralplate cartoon.png


Links: Neural System - Abnormalities | Folic Acid and Neural Tube Defects

Cardiogenesis

Early Development of Heart Tube

Human heart SEM1.jpg

Heart Tube Fusion

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
Week3 folding icon.jpg

Blood Islands

fetal blood
  • 2 populations of cells
  • 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 (RBC).

These red blood cells differ from adult red blood cells in:

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


Links: Basic Cardiac Embryology

Early Placentation

Week2 001 icon.jpg

Placenta and placental membranes
Placenta anchoring villi

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

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:

  1. Primary Villi - cytotrophoblast
  2. Secondary Villi - cytotrophoblast + extraembryonic mesoderm
  3. 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.


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Cite this page: Hill, M.A. (2014) Embryology BGDA Lecture - Development of the Embryo/Fetus 1. Retrieved July 30, 2014, from //php.med.unsw.edu.au/embryology/index.php?title=BGDA_Lecture_-_Development_of_the_Embryo/Fetus_1

What Links Here?
Dr Mark Hill 2014, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G