introduction early embryology...the developing human – three phases early development embryonic...

Introduction&

Early Embryology

Human Structure & Development ANHB 2212

Semester 1 – 2010

Dr. Avinash [email protected]

© 2010 Avinash Bharadwaj

Welcome! The Promise :

An exciting unit! All possible help during the course of the unit

About the unit : All information on the unit website…

http://www.lab.anhb.uwa.edu.au/hsd212 … and on WebCTIt is meant to be read!

A few points and thoughts…


Some Key Issues Progress from the ‘transition year’… Analytical, conceptual thinking Integrative unit.

Gross anatomy, histology, embryology Being an educated lay person Anything that is here in the unit…

…is here for a reason!


UWA student email account must be active Check your email regularly Class List emails for important announcements No auto redirect to other accounts!

All mails to be sent from UWA student account.

Course Reader : first three chapters ‘must-read’especially if you have not done ANHB 1101.

Important!


Embryology : Why and How

Embryology

Gross Form, Correlation

Errors in developmentStructuralMolecular

(Clinical aspects)

Real life aspects (“non medical”)

Evolutionary mirror

Cellular and Molecularphenomena


Fertilisation Oöcyte at ovulation…

…Second meiotic divisionOne polar body

Site of fertilisation : Ampulla Sperm : Acrosomal reaction Zona pellucida Single sperm…


Single sperm enters Meiosis complete (two polar bodies).

1

♀

♂

Female and male pronuclei2

First mitotic division begins.(Cleavage).Note mitotic spindle.

3Two smaller cells.Diploid

4


Fertilisation

Completion of meiosis II Polar bodies (Also dealt with in Week 11)

Restoration of diploidy Initiation of cleavage Chromosomal sex determination Genetic and evolutionary implications

Paternal and maternal chromosomes Further mixing during next gametogenesis (crossing

over)


Cleavage

Nuclear-cytoplasmic ratio …Reduction of cell size

Journey through the uterine tube

Zona pellucida…retained until implantation

Morula Cell compaction


Blastocyst Inner cell mass Embryo foetus Trophoblast mainly for nutrition

Invasive nature Zona pellucida disappears Implantation


Implantation

Embryo within the endometrium


Embryo : Two layers – Epiblast and hypoblastAlso note : Amnion + Amniotic cavity.

Yolk sac.

A

Y

Embryo is “bilaminar”

Trophoblast


A B

A

B

Cut edge of amnion

Primitive streak

Germ Layers…

Flat, bilaminar embryo Epiblast and hypoblast Epiblast endoderm, mesoderm, ectoderm

The beginning : Primitive streak.


Endoderm

Endoderm replaceshypoblast!

Epiblast

HypoblastEndoderm

H HEndoderm


Remaining epiblast ectoderm

Mesoderm

Endoderm

Mesoderm1

2


Mesoderm spreads…

A small area at the head end…no mesoderm! (More about this later).


Notochord & Neural Plate Notochord

Cells from the primitive node Axis Induction…

Neural plate Ectoderm Neural groove


Neural Tube and Crest Plate groove tube “Lips” of the groove : crest Surface ectoderm continuous Crest buried dorsal to the tube.

Nervous system is ectodermal!


Divisions of Mesoderm

Axis

ParaxialLateral plate Intermediate


But we are not ‘flat’!

Ectoderm : Interface with the exterior Endoderm : Mainly concerned with

food intake Mesoderm : “everything in between”,

including coelomic cavity

“Folding” : a complex process

Key point at this stage :


Fate of Germ Layers

Ectoderm Epidermis (not skin!), nervous system

Mesoderm Muscle, Connective tissues : “Form” of the body Some epithelial structures

Parts of urogenital systems Endothelium of heart and blood vessels “Serous membranes”

Endoderm Lining of the digestive system ++


The Link with Histology – an Example

Mesoderm muscle (3 types), different types of connective tissue…

Each has specialised cells Cell differentiation : commitment to form

specialised cells Full differentiation implies structural and

functional specialisation Some undifferentiated cells persist even in

adult life.


The Developing Human – Three Phases

Early development Embryonic phase – organogenesis

Most sensitive phase Potential for maldevelopment Effects of ‘teratogens’ Each system has its particular sensitive periods

The foetal period Mostly growth, but… Differentiation, Histogenesis and Functional

development continue


Summary The beginning :

Fertilisation, cleavage, implantation

Concepts introduced in this lecture, to be developed in later weeks :

Three germ layers and their importance Embryology as the key to :

Anatomical form and relationships Developmental defects (elementary!) Immense variety of specialised cells

introduction early embryology...the developing human – three phases early development embryonic...

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