UNSW Embryology

Foundations

Practical - Introduction to Human Development

© Dr Mark Hill (2010)

Acknowledgements

Week 1 to 8 - The Embryonic Period

Development of Organs and Tissues

Introduction

You have now been through the earliest stages of conceptus or 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 appearance during this embryonic period, also called the organogenic period as most organs have begun to form by the end of 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

This page is about getting an overview of the external changes that occur during the embryonic period. Understanding 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
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)

For more details see Carnegie Stages or Development Week by Week

Carnegie Stage Movie

Now watch the changes in actual size of the embryo during this period. (scale bar is 5 mm)

Stages Movie

(More? human embryology movies)

Next

Link to next page in this Practical - Week 9 to 36 This is all about the Fetal Period of development.

Glossary

Use the alphabetical list below to find definitions of terms that are new to you.

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

Terms

Carnegie stages - Carnegie stages are a system of classifying embryonic development based on the external features and related internal changes that affect appearance and growth of the embryo. Note that the stages are not directly dependent on either age or size, but upon the appearance of specific embryonic features. Early human and other species embryos can be classified by these stages. The term "carnegie stages" are named after the famous USA Institute which began collecting and classifying embryos in the early 1900's. (More? Carnegie Stages | About the Carnegie Institute | Carnegie Stages - Scanning Electron Micrography | Carnegie Stage Comparision)

conceptus - The entire product of conception, that is all the structures derived from the zygote and includes not only the embryo, but also the placenta and membrane components. (More? Week 1 Notes)

embryonic period - (embryonic stage, organogenic period) In humans, the first 8 weeks of development is considered the embryonic stage and is divided into 23 Carnegie stages based upon developmental milestones. This has also been described as the "organogenic period". The following time (week 9 to 36) is considered the fetal period. (More? Embryo Stages | Fetal)

extraembryonic mesoderm - Cells from the conceptus that contribute to placenta and fetal membranes. Described as "extraembryonic" because it is tissue lying outside the embryonic trilaminar disc (ectoderm, mesoderm and endoderm) and "mesoderm", because of the connective tissue cellular organization. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)

gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes. (More? Gastrulation)

neural folds - The central region of the trilaminar embryo ectoderm called the neural plate region folds dorsally, generating two neural folds, which later fuse to form the neural tube. The mid-line depression between the two folds is described as the neural groove. In humans at approximately day 18-19 post-fertilization to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate ->(day 18-19) neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)

neural plate - The first stage in early development of the central nervous system. In the trilaminar embryo (ectoderm, mesoderm, endoderm) the central region of the ectoderm (in the midline above the mesodermal notochord) initially forms a columnar epithelium described as the neural plate. This epithelium will fold dorsally to form the neural groove, which then fuses to form an initially open at either end hollow neural tube. The neural tube forms the central nervous system (brain and spinal cord). Developmental sequence: neural plate -> neural groove -> neural tube -> Central Nervous System -> brain and spinal cord. (More? Neural Notes)

paraxial mesoderm - The two lateral strips of mesoderm lying beside the axial mesoderm (notochord). This mesoderm at the body level will segment into somites, at the head level it remains unsegmented. (More? Musculoskeletal Notes | Week 3 Notes)

primitive streak - Region visible on the surface of the early epiblast embryonic disc showing the region where gastrulation (cell migration to form endoderm) is occurring. In the human embryo this process occurs from week 3 through to week 4. (More? Carnegie Stage 7 | Week 3 Gastrulation)

somite - Segmental block (ball) of mesoderm formed from paraxial mesoderm adjacent to notochord (axial mesoderm) forming muscle and connective tissues of the body. Differentiates to form two intermediate components called the sclerotome and dermamyotome (then dermatome and myotome). Note the paraxial mesodernm of the head region does not segment. (More? Musculoskeletal Notes | Week 3 Notes)

somitocoel - A transient cavity that appears within each of the early forming somites which is then lost as cells proliferate within the somite. (More? Musculoskeletal Notes | Week 3 Notes)

somitogenesis - The process of segmentation of the paraxial mesoderm to form pairs of somites, or balls of mesoderm . The process begin cranially (humans day 20) and extending caudally at 1 somite/90 minutes until on average 44 pairs eventually form. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. (More? Musculoskeletal Notes | Week 3 Notes)

Quick Movie Links

Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)

Movie of Mouse Embryo Growth (this shows a mouse embryo growing)

Quick Links

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Genes/DNA Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Urogenital Musculoskeletal Integumentary

Movies ----------------- Embryo Growth Ultrasound Week 1 Week 2 Week 3 Week 4 Stage 13/14 Stage 22 Neural Head and Neck Heart Vasculature Gastrointestinal Urogenital Other Embryos Larsen Internet

Other Embryos ----------------- Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G