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

Introduction&

Early Embryology

Human Structure & Development ANHB 2212

Semester 1 – 2010

Dr. Avinash Bharadwajavinash@anhb.uwa.edu.au

© 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…

© 2010 Avinash Bharadwaj

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!

© 2010 Avinash Bharadwaj

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!

© 2010 Avinash Bharadwaj

Embryology : Why and How

Embryology

Gross Form, Correlation

Errors in developmentStructuralMolecular

(Clinical aspects)

Real life aspects (“non medical”)

Evolutionary mirror

Cellular and Molecularphenomena

© 2010 Avinash Bharadwaj

Fertilisation Oöcyte at ovulation…

…Second meiotic divisionOne polar body

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

© 2010 Avinash Bharadwaj

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

© 2010 Avinash Bharadwaj

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)

© 2010 Avinash Bharadwaj

Cleavage

Nuclear-cytoplasmic ratio …Reduction of cell size

Journey through the uterine tube

Zona pellucida…retained until implantation

Morula Cell compaction

© 2010 Avinash Bharadwaj

Blastocyst Inner cell mass Embryo foetus Trophoblast mainly for nutrition

Invasive nature Zona pellucida disappears Implantation

© 2010 Avinash Bharadwaj

Implantation

Embryo within the endometrium

© 2010 Avinash Bharadwaj

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

Yolk sac.

A

Y

Embryo is “bilaminar”

Trophoblast

© 2010 Avinash Bharadwaj

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.

© 2010 Avinash Bharadwaj

Endoderm

Endoderm replaceshypoblast!

Epiblast

HypoblastEndoderm

H HEndoderm

© 2010 Avinash Bharadwaj

Remaining epiblast ectoderm

Mesoderm

Endoderm

Mesoderm1

2

© 2010 Avinash Bharadwaj

Mesoderm spreads…

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

© 2010 Avinash Bharadwaj

Notochord & Neural Plate Notochord

Cells from the primitive node Axis Induction…

Neural plate Ectoderm Neural groove

© 2010 Avinash Bharadwaj

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!

© 2010 Avinash Bharadwaj

Divisions of Mesoderm

Axis

ParaxialLateral plate Intermediate

© 2010 Avinash Bharadwaj

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 :

© 2010 Avinash Bharadwaj

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 ++

© 2010 Avinash Bharadwaj

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.

© 2010 Avinash Bharadwaj

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

© 2010 Avinash Bharadwaj

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