Human EmbryologyChapter 1. Introduction

SHAFIM AHMED

Embryology

What is Embryology?

1. Definition: the study of the origin and development

of an organism.

2. focuses on the development that occurs in the

prenatal period (gestational period)

Embryology Prenatal period: before birth

• 38 weeks(266days,9months) from conception to birth (average) “fetal” age

• Gynecologic timing has been from LMP (last menstrual

period) therefore refers to 40 weeks “gestational” age Date of conception has been difficult to time LMP is on average two weeks before ovulation

prenatal period

“Embryonic” period: ( embryo)

1.extends from fertilization through 8 weeks.

week 1 starts with fertilization and extends through the first cell divisions that produce an embryo with one germ layer.

week 2 produce two germ layers.

week 3 produce three germ layers and folds those a recognizable three dimensional body form.

week 4 through 8 (month 2) are the period of organogenesis.

2.Week 4 is particularly important, as the initial set-up of all

organ systems is completed in this week.

All the major events of organ formation and formation of body regions occurs during the period of organogenesis, which is completed in the embryonic period.

“Fetal” period: 9 weeks – birth (fetus)

• The major events that occur during the fetal period involve

growth in the size of organs by rapid increase in cell number, as

well as biochemical functional maturation.

• Most organs begin with a basic form of functioning during the

fetal period, but it is only for development of the organ itself

usually. The most important is exception is the cardiovascular system. Mature functional status is not achieved in most organ

untill after birth.

Prenatal Development

Embryonic development

fertilization - 8 weeksAll major organs formed

Fetal development

9 weeks – birthOrgans grow larger and

become more complex

Postnatal DevelopmentPostnatal Developmenttime period from fertilization to birth = gestationgestation

Relative Size of Human Conceptus

Figure 28.1

Four mammalian embryos at various Four mammalian embryos at various stages of development (HAECKEL):stages of development (HAECKEL):

• A PigA Pig• B CowB Cow• C RabbitC Rabbit• D HumanD Human

““Please, note the similarities in Please, note the similarities in the different developing the different developing mammalian species!”mammalian species!”

Embryonic body axis terminology

1.The first axis formed is the cranial-caudal axis

The cranial-caudal axis is established while the

embryo is still a flat disk or sheet of cells, long

before it fold into the three-dimensional structure.

This axis runs from the future head to the future

”tail” end.

2. The dorsal-ventral axis is established very shortly

thereafter.

The second major axis the dorsal-ventral, is

formed as the body folds. This axis defines,

respectively, the future” back” and “front” sides of

both embryonic and adult bodies.

Significance of Embryology in medicine

The knowledge of the basic principles of The knowledge of the basic principles of development will assist the student in understandingdevelopment will assist the student in understanding

relationships taught in gross anatomy, histology, and relationships taught in gross anatomy, histology, and pathology courses, and will later serve the clinician pathology courses, and will later serve the clinician in diagnosing certain conditions and in counseling in diagnosing certain conditions and in counseling congenital malformations (significance in congenital malformations (significance in gynecology/andrology and in molecular biology!).gynecology/andrology and in molecular biology!).

Content General Embryology 1.Fertilization 2.Cleavage 3.Implantation 4.Formation Of The Germ Layers 5.Folding Of The Embryo 6.Derivatives Of The Germ Layers 7.External Appearance Of The Embryo And Fetus 8.Placenta Special Embryology Systems development