Upload
others
View
26
Download
0
Embed Size (px)
Citation preview
1
Introduction to Human Development
1 Human development begins at fertilization when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male and becomes a single-celled zygote. Development involves many changes that transform the zygote into a multicellular human being. Embryology is concerned with the origin and development of a human being from a zygote to birth. The stages of development before birth are shown in Fig. 1.1.
IMPORTANCE OF AND ADVANCES IN EMBRYOLOGY
The study of prenatal stages and mechanisms of human development helps us understand the normal relationships of adult body structures and the causes of birth defects (congenital anomalies). Much of the modern practice of obstetrics involves applied or clinical embryology. Because some children have birth defects, such as spina bifida or congenital heart disease, the significance of embryology is readily apparent to pediatricians. Advances in surgery, especially in procedures involving the prenatal and pediatric age groups, have made knowledge of human development more clinically significant. In addition, as we discover new information about the development processes, we in turn have a better understanding of many diseases and their process, as well as their treatment.
Rapid advances in molecular biology have led to the use of sophisticated techniques (e.g., genomic technology, chimeric models, transgenics, and stem cell manipulation) in research laboratories to explore such diverse issues as the genetic regulation of morphogenesis, the temporal and regional expression of specific genes, and the mechanisms by which cells are committed or differentiate to form the various parts of the embryo. Researchers continue to learn how, when, and where selected genes are activated and expressed in the embryo during normal and abnormal development.
Development begins at fertilization (see Fig. 1.1, first week). The embryonic period covers the first 8 weeks of development of an embryo. The fetal period begins in the ninth week. Examination of the timetable shows that the most externally visible advances occur during the third to eighth weeks.
The critical role of genes, signaling molecules, receptors, and other molecular factors in regulating early embryonic development is rapidly being delineated. In 1995 Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric F. Wieschaus were awarded the Nobel Prize in Physiology or Medicine for their discovery of genes that control embryonic development. Such discoveries are contributing to a better understanding of the causes of spontaneous abortion and birth defects.
Robert G. Edwards (1925–2013) and Patrick Steptoe (1913–1988) pioneered one of the most revolutionary develop-ments in the history of human reproduction: the technique of in vitro fertilization. Their studies resulted in the birth of Louise Brown, the first “test tube baby,” in 1978. Edwards was awarded the Nobel Prize in 2010.
In 1997 Ian Wilmut and colleagues were the first to produce a mammal (a sheep dubbed Dolly) by cloning using the technique of somatic cell nuclear transfer. Since then, other animals have been cloned successfully from cultured differenti-ated adult cells. Interest in human cloning has generated considerable debate because of social, ethical, and legal implications. Moreover, there is concern that cloning may result in an increase in the number of neonates (newborns) with birth defects and serious diseases.
Human embryonic stem cells are pluripotential and capable of developing into diverse cell types. The isolation and culture of human embryonic and other stem cells may hold great promise for the development of molecular therapies.
DESCRIPTIVE TERMS
In anatomy and embryology, specific terms of position, direction, and various planes of the body are used. Descrip-tions of the adult are based on the anatomical position; the body is erect, the upper limbs are at the sides, and the palms are directed anteriorly (Fig. 1.2A). The descriptive terms of position, direction, and planes used for embryos are shown in Fig. 1.2B to E. In describing development, it is necessary to use words denoting the position of one part relative to another or to the body as a whole. For example, the vertebral column develops in the dorsal part of the embryo and the sternum is in the ventral part of the embryo.
2 BEFOREWEAREBORN
Day 1 of last normalmenstrual cycle
Antrum
Oocyte
Primary follicles
Oocyte
OvulationMaturefollicle
Oocyte
Ovary
Oocyte
1 Stage 1
Fertilization Zygote divides Morula Early blastocyst
Zona pellucida
Late blastocyst
Trophoblast
Embryoblast
8
Amniotic cavity
Bilaminar embryonicdisc
Primary umbilicalvesicle
9Lacunae appear insyncytiotrophoblast
10
Closing plug
AmnionCytotrophoblast 11
Eroded gland
Lacunarnetwork
Maternal blood
Embryonic disc Coelom
12 Extraembryonicmesoderm
13 Stage 6 begins
Primary villi
Prechordal plate
Connecting stalk
Embryonic disc
Amnion
14
2 Stage 2 begins 3 4 Stage 3 begins 5 6 Stage 4
Implantation begins
7 Stage 5 begins
EARLY DEVELOPMENT OF OVARIAN FOLLICLE
TIMETABLE OF HUMAN PRENATAL DEVELOPMENT1 TO 10 WEEKS
MENSTRUAL PHASE
AGE(weeks)
1
2
COMPLETION OF DEVELOPMENT OF FOLLICLE
CONTINUATION OF PROLIFERATIVE PHASE OF MENSTRUAL CYCLE
SECRETORY PHASE OF MENSTRUAL CYCLE
PROLIFERATIVE PHASE
DAYS
−1
−2
Fig. 1.1 Early stages of human development. An ovarian follicle containing an oocyte, ovulation, and phases of the menstrual cycle are shown.
CHAPTER1—INTROduCTIONTOHumANdEvElOPmENT 3
7
AGE(weeks)
Head large but chinpoorly formed.
Grooves betweendigital rays
indicate fingers.
Upper limbslonger and bent
at elbows.
Fingers distinctbut webbed.
Beginning of
fetal period
Face has amore developed
profile.
Note growthof chin
comparedto day 44.
External genitaliahave begun
to differentiate.8
9
10
43 44 Stage 18 begins
CRL: 13.0 mm
Actual size
Eyelids forming
Ear
Large forehead
Eye
Nose
Fingers
Toes
50
57
64
58
65
59
66
60
Phallus
Genitalia
Perineum
Clitoris
Labiumminus
Labiummajus
Urogenitalgroove
Ears stillpositioned lower
Labioscrotalfold
Urogenital fold
67
61
68
62
69
63
70
51 52 Stage 21 beginsStage 20 begins 53 55 56 Stage 23
Eye Ear
Elbow
Wrist
Knee
Toes
Eye Ear
Placenta
Elbow
Wrist
Knee
Toes
CRL: 18 mm
Actual size
CRL: 30 mm
CRL: 50 mmCRL: 45 mm
CRL: 61 mm
Amniotic sacGenital tubercle
Urogenitalmembrane
Eyelid
Wrist, fingersfused
External earAnalmembrane
and
Wall of uterus
Uterinecavity
Smoothchorion
45 46 47 Stage 19 begins48 49
orGenitalia have
characteristicsbut still not
fully formed.
Stage 22 begins54
Genitaltubercle
Urethralgroove
Anus
Phallus
Genitalia
Perineum
Labioscrotalfold
Urogenital fold
Glans of penis
Scrotum
Urethralgroove
and
3
4
5
6
Primitive streak
15 16 Stage 7 begins
22 Stage 10 begins
29
36
30
37 Stage 16 begins
Oral and nasalcavities confluent
31
38
32 Stage 14 begins
39
33 Stage 15 begins
40
34
41
35
Cord
42
Eye
Eye
Ear
Stage 17 begins
23 24 Stage 11 begins 25 Stage 12 begins 27 28 Stage 13 begins
Stage 8 begins17 18 19 Stage 9 begins20 21First missed
menstrual period
Arrows indicate migration of mesenchymal cells
Migration of cells fromprimitive streak
Trilaminar embryo
AmnionNeural plate
Primitive streak
Length: 1.5 mm
Neural groove
Neural plate
Somite
Primitive node
Primitive streak
Brain
Neural groove
Somite
Thyroid gland begins to develop
Neural groove
First pairsof somites
Primitivestreak
Heart begins to beat
Neural folds fusing
Rostral neuropore
Primordia of eye and ear present
Caudal neuropore
Heart bulge
Rostral neuropore closes
2 pairs ofpharyngealarches
Otic pit
3 pairs of pharyngeal arches
Upperlimb bud
Indicates actual size
Fore- brain
Pharyngealarches
Site of otic (ear) pit
CRL = crown-rump length CRL: 5.0 mm
CRL: 8.5 mmCRL: 7.0 mm
CRL: 10.5 mm CRL: 12.5 mm
26
CRL: 5.5 mm
CRL: 9.5 mm
Upper lip and nose formed
Eye
Footplate
Digitalrays
Digitalrays
Ventral view
External acousticmeatus
Foot-plate
Eye
Ear
Lens pits, optic cups,nasal pits forming
Developing eye Upper limb bud
Lower limb bud
Heart
Eye
Hand- plate
Cerebral vesiclesdistinct
Foot- platepresent
Nasal pit
Primordial mouth
Large head
Cut edgeof amnion
PrimitiveCirculatorySystem
Fig. 1.1, cont’d
4 BEFOREWEAREBORN
Superior
Inferior
Sagittal plane
Median section Transverse section
Lateral
Anterior
A
C D E
B
Posterior
Cranial
Ventral
Dorsal
Caudal
Frontal (coronal) section
Fig. 1.2 Illustrations of descriptive terms of position, direction, and planes of the body. A, Lateral view of an adult in the anatomical position. B, Lateral view of a 5-week embryo. C and D, Ventral views of a 6-week embryo. The median plane is an imaginary vertical plane of section that passes longitudinally through the body, dividing it into right and left halves. A sagittal plane refers to any plane parallel to the median plane. A transverse plane refers to any plane that is at right angles to both the median and frontal planes. E, Lateral view of a 7-week embryo. A frontal (coronal) plane is any vertical plane that intersects the median plane at a right angle and divides the body into front (anterior or ventral) and back (posterior or dorsal) parts.
CLINICALLY ORIENTED QUESTIONS
1. Why do we study human embryology? Does it have any practical value in medicine and other health sciences?
2. Physicians date a pregnancy from the first day of the last normal menstrual period, but the embryo does not start
to develop until approximately 2 weeks later (see Fig. 1.1). Why do physicians use this method?
The answers to these questions are at the back of this book.
CHAPTER1—INTROduCTIONTOHumANdEvElOPmENT 4.e1
Answers to Chapter 1 Clinically Oriented Questions