PREEMBRYONIC PERIOD (FIRST TO THIRD WEEK)First week of life:• Day 1: Fertilization
Day 2 and 3: Cleavage• The original zygote divides about 30 hours after conception into two daughter cells called blastomeres.• Continued subdivisions of the original cell result in increasing numbers of blastomeres.• During cell division the dividing cells decrease in size. This type of cell division is called cleavage.• By the time the zygote is ready to enter the uterus, it contains a solid ball of 12 to 16 blastomeres called the morula (from the latin word for mulberry).
Day 4: Formation of the blastocyst• Fluid within the intercellular spaces of the morula gradually increases, and spaces on one side of the inner cell mass come together, forming a single cavity, the blastocele.• The outer layer organizes into the trophoblasts, which give rise to the placenta, and the inner layer of cells form the embryo.• The cavity of the blastocele fills with fluid, and the conceptus is now called the blastocyst.
Preparation of the endometrium• Resting phase• Proliferative phase• Secretory phase
Attachment of the blastocyst• The blastocyst attaches to the uterine lining in the V-shaped.• When the trophoblast (the outer cell layer) attaches to the endometrium, it proliferates and separates into an inner cytotrophoblastic layer (fetal side) and an outer syncytiotrophoblastic (placental side).• The outer layer develops finger like projections that proliferate and superficially attach the blastocyst to the endometrium within 6 days after conception.
MAJOR EVENTS OF FIRST WEEK POSSIBLE ABNORMALNORMAL EVENTS EVENTS Fertilization and Abnormal formation of the zygote implantation (30hours). Maternal infection Cleavage of the zygote or a genetic defect into 12 to 16 Hydatidiform mole blastomeres- the Abortion morula (day 2 and 3). Formation of the Ectopic implantation blastocyst ( day 5-8).
Second week of life• A slitlike amniotic cavity appears about day 8, and the yolk sac appears as a second cavity on day 12. Bilaminar embryonic disc is formed in between these two layers.• The endodermal disc becomes thicker at it’s cephalad end, forming the prochordal plate.• During early development of the nervous system, the function of the prochordal plate is to indicate the site of the mouth and to form the membranes of the mouth and throat.• The formation of the decidua, fetal membranes, and placenta extends beyond the second week, but their development begins at this point.
THIRD WEEK OF LIFE• During the third week of life, the conceptus develops rapidly. This period also coincides with the first missed menstrual cycle of the mother.• The primitive streak is formed during the third week, and three germ layers develop.• This periods from approximately day 15 to day 21, is called the “period of threes”; not only do the three germ layers develop, but the primitive streak, the notochord, and the neural tube are formed.
GASTRULATION• Gastrulation is the process by which the bilaminar embryo becomes a trilaminar embryo.• On about day 15, the cytotrophoblast cells proliferate into the blastocyst to form the extraembryonic mesoderm, which later become the extra-embryonic coelom.• The mesoderm lies between the ectoderm and the endoderm, completing the trilaminar disc of the primitive streak. All tissues and organs of the embryo are developed from these three layers.
Major derivatives of the embryonic germ layers
Notochord• Days 16-18• Primitive node epiblast cells invaginate and migrate anteriorly with some endoderm cells• Rod defining the body axis is formed• Future site of the vertebral column
Neurulation• Notochord signals overlying ectoderm• The neural tube is developed from the closure of the neural plate and the neural fold- a process called neurulation –at about 21 to 26 days.
• Closure of neural tube: begins at end of week 3; complete by end of week 4.• Extends cranially (eventually brain) and caudally (spinal cord)• Neural crest, lateral ectodermal cells, pulled along and form sensory nerve cells and other structures
DIVISON INTO SOMITES• About day 20, the mesoderm divides into paired bodies called somites.• Located on either side of the developing neural tube, these paired bodies give rise to the skeleton and muscle tissue.• During the somite period, day 20 to 30, 38 pairs of somites develop.• Their total number eventually reaches 42 to 44 pairs, of which 4 are occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8 to 10 coccygeal.• Some of the somites – first occipital and the fifth to seventh coccygeal –disappear, while the rest form the axial skeleton.• By the end of the third week of life, the conceptus is about 1.0 mm in length.
MAJOR EVENTS OF third WEEK POSSIBLENORMAL EVENTS ABNORMAL EVENTS• Formation of blood vessels within • Monozygous the chorionic villi ( day 13). twins result• Gastrulation or conversion of the bilaminar embryonic disc into the • Conjoined twins three- layered trilaminar disc (day are formed. 14). • The heart is most• Continued development of the susceptible to chorion with formation of tertiary teratogens chorionic villi( day 15-20). between the 19th• Development of the neural tube (day 18). and the 41st day.• Formation of somites (day 21).• Beginning of blood circulation (day 24).
fourth week of life• The embryo grows dramatically during the fourth week.• It more than triples its length to 3.5mm and its weight approximates 5 mg.
• Elongation of the embryo has occurred, and it has become curved pon itself with the formation of a head and tail fold.• Lateral body folds develop making the embryo tubular rather than flat and disc shaped.• Closure of the neural tube begins in the area of the occiput and proceeds upward and downward from that point.• Somites formed in a craniocaudal sequence as the neural tube closes, can be observed through the ectoderm.
• The pericardial sac around the heart enlarges, causing the head region to elevate.• The larygotracheal groove and lung buds, which will become the respiratory system, are present.• The mandible and maxilla of the jaw become distinct, and rudimentary forms of the eyes, ears and nose are present.
• The intestinal system is formed from the yolk sac, and differentiation of the buds, which will become the oesophagus, stomach, liver and pancreas, is progressing.• The thyroid and thymus glands are also developing.• The primitive circulatory system is established, and the heart is beating.• The budlike projections on the surface of the embryo are the beginning of the limbs.
FIFTH WEEKS OF LIFE• As the embryo and then the fetus grow during the first half of pregnancy, it is measured by its crown –rump length (CRL).• The fetus is measured during the last half of pregnancy by its crown heel length (CHL), or standing height. The CRL grows from 4 to 8 mm in this week, and exceeds the growth.• The growth of the head is rapid and exceeds the growth of the body during this week. The embryo lengthens and bends into a C shape, while an additional 42 to 44 pairs of somites are added to its caudal end.
• The umbilical cord is formed from the union of the amnion, the yolk, and the connecting stalk. It now contains two umbilical arteries and one umbilical vein.• The doubling of the size of the heart makes it prominent, and its atria and ventricles are visible through the ectoderm. The embryo’s four limb buds are most vulnerable to teratogens at this time.
SIXTH WEEK OF LIFE• The head has become larger than the trunk and is bent over the heart prominence.• Elevations in the facial ectoderm are evident and the position of the eyes, nose, mouth, and the groove that is to become the external acoustic meatus is established.• In the upper limbs, the elbows and wrists are identifiable, and the hand plates develop ridges called finger rays.• The lower limbs are not developed to the same degree.• Changes are beginning to occur in the genital region. (The CRL is 8 to 14 mm, the weight 50 to 400mg).
SEVENTH WEEK OF LIFE• Cerebral hemispheres appear as the head enlarges rapidly.• The eyes move from a lateral to a more frontal position as the face elongates.• Prominences appear over the ventral body wall from early because their function is vital to the maintenance and survival of the embryo.• As the embryo continues to grow, the umbilical cord shrinks.• The arm and hand of the upper limbs and the thigh, leg, and foot segments of the lower limbs become apparent.• The fingers develop, the their growth is critical at this point (40 to 50 days).• The CRL is 14 to 20 mm; the weight is 400 to 1000 mg.
EIGHTH WEEK OF LIFE• During this final week of the embryonic period, the embryo exhibits definite human characteristics.• The cerebral hemispheres have grown so rapidly that the head now makes up 50% of the mass of the embryo.• The face occupies the lower half of the head, and the eyes continue to move to a more frontal plane.• Eyelids folds develop. These will become fused during the ninth week and remain so until the seventh month.
• The fingers lengthen, and the toes are distinct by the end of the eighth week.• The external ears are set low and are taking on their final shape.• Sexual differences in the external genitalia can now be seen by the trained eye.• (The CRL is 21 to 30mm; the weight 1000 to 3000 mg).
MAJOR EVENTS OF fourth to eighth WEEK POSSIBLE ABNORMAL NORMAL EVENTS EVENTS• Conversion of the flat trilaminar embryonic disc • Abnormalities of the into a c- shaped cylindrical embryo. genes and• Formation of the head, tail, and lateral folds. chromosomes.• Formation of the lateral and ventral body walls. • Alterations of• Acquisition of an epithelial covering by the maternal health, umbilicus through the expansion of the amnion. such as infection• Establishment of ventral position of the heart and from rubella or development of the brain in the cranial region of herpes. the embryo. • Ingestion of• Differentiation of the three germ layers into teratogenic various tissues and layers that will become substances. established as the major organ systems. • The risk of mortality• Appearance of the brain, limbs, ears, eyes, nose. is greater than at any other time of• Development of a human appearance by the life.