UNSW Embryology

Foundations - Lab 15 - Intro to Human Development

© Dr Mark Hill (2007)

Acknowledgements

Week 1 to 8 - The Embryonic Period

Development of Organs and Tissues

Introduction

You have now been through the earliest stages of embryonic development.

From the original simple trilaminar embryo all the organs and tissues will now begin to develop, details of these processes require more time than we currently have available.

What we will now look at are the changes in the external embryo appearnace during this period.

Key features to note are:

changes in overall size (CRL)
surface bulges that reflect organ development
development of the upper and lower limbs
development of the head

Comments

Dr Mark Hill

This page is about getting an overview of the external changes that occur during the embryonic period. In order to observe and understand the internal changes occuring in organ development is beyond the scope of this current practical.

These notes and linked materials have been prepared for Educational purposes only. Please email Dr Mark Hill if you wish to make a comment about this current project.

Changes in Overall Size

Actual Size

Enlarged Size

Crown to Rump Length (CRL)

In the first 8 weeks the embryo grows from 0.1 mm to about 3 cm in length.

During this time the embryo size is also measured from "crown" to "rump" or the Crown Rump Length (CRL).

Surface Bulges Reflect Organ Development

Surface of the Stage 13 Embryo showing surface features reflecting surface and internal development.

Carnegie Embryonic Stages

Now look through the embryonic stages of development.

Stage	Days (approx)	Size (mm)	Images (not to scale, click image to see original)	Events
1	1 (week 1)	0.1-0.15		fertilized oocyte, pronuclei
2	2 - 3	0.1-0.2		cell division with reduction in cytoplasmic volume, formation of inner and outer cell mass
3	4 - 5	0.1-0.2		loss of zona pellucida, free blastocyst
4	5 - 6	0.1-0.2		attaching blastocyst
5	7 - 12 (week2)	0.1-0.2		implantation
6	13 - 15	0.2		extraembryonic mesoderm, primitive streak
7	15 - 17 (week3)	0.4		gastrulation, notochordal process
8	17 - 19	1.0 - 1.5		primitive pit, notochordal canal
9	19 - 21	1.5 - 2.5		Somite Number 1 - 3 neural folds, cardiac primordium, head fold
10	22 - 23 (week4)	2 - 3.5		Somite Number 4 - 12 neural fold fuses
11	23 - 26	2.5 - 4.5		Somite Number 13 - 20 rostral neuropore closes
12	26 - 30	3 - 5		Somite Number 21 - 29 caudal neuropore closes
13	28 - 32 (week5)	4 - 6		Somite Number 30 leg buds, lens placode, pharyngeal arches
Stage 13/14 shown in serial embryo sections series of Embryology Program
14	31 - 35	5 - 7		lens pit, optic cup
15	35 - 38	7 - 9		lens vesicle, nasal pit, hand plate
16	37 - 42 (week6)	8 - 11		nasal pits moved ventrally, auricular hillocks, foot plate
17	42 - 44	11 - 14		finger rays
18	44 - 48 (week7)	13 - 17		ossification commences
19	48 - 51	16 - 18		straightening of trunk
20	51 - 53 (week8)	18 - 22		upper limbs longer and bent at elbow
21	53 - 54	22 - 24		hands and feet turned inward
Stage 22 shown in serial embryo sections series of Embryology Program
22	54 - 56	23 - 28		eyelids, external ears
23	56 - 60	27 - 31		rounded head, body and limbs
Following this stage Fetal Development occurs until birth (approx 40 weeks)