- During the first trimester, the embryo forms three germ layers including the mesoderm which begins forming the foundation for bones. By week 9, bones start developing in the arms and legs. - In the second trimester, bone tissue develops around the head by week 13. By week 15, the skeleton rapidly develops and grows, with the skull becoming more prominent. - In the third trimester, most bones are developed although growth continues until birth, with the skeleton becoming harder in preparation for birth.

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬

2. Prenatal Growth &
Development of the
Skeletal System

3. Brief
Introduction

4. Introduction
• Prenatal development includes the development of the
embryo and of the fetus during Pregnancy.
• Prenatal development starts with fertilization, in the germinal stage
of embryonic development, and continues in fetal development
until birth.
• The development of the human embryo follows fertilization, and
continues as fetal development. By the end of the tenth week
of gestational age the embryo has acquired its basic form and is
referred to as a fetus.
• The next period is that of fetal development where many organs
become fully developed.
This fetal period is described both topically (by organ) and
chronologically (by time) with major occurrences being listed by
gestational age.

5. • Fertilization marks
the first germinal
stage of embryonic
development.
• When semen is
released into the
vagina, the
spermatozoa travel
through the cervix
and body of the
uterus and into the
fallopian tubes
where fertilization
usually takes place.
Fertilization

6. • Following fertilization
the embryonic stage
of development
continues until the
end of the 10th week
(gestational age) (8th
week fertilization
age).
• The first two weeks
from fertilization is also
referred to as the
germinal stage or
preembryonic stage.
(Fertilization – Cleavage
– Blastulation –
Implantation - Embryonic
disc )
The initial stages of Development of the embryo

7. Gastrulation
• It is the process during
embryonic development that
changes the embryo from a
blastula with a single layer of
cells to a gastrula containing
multiple layers of cells.
• Occur during week 2 - 3
following fertilization process.
• Transformation of bilaminar
disc (epiblast - hypoblast) into
trilaminar disc (ectoderm-
mesoderm-endoderm)
• So epiblast – hypoblast gives:
• Ectoderm
• Mesoderm
• Endoderm

8. Embryological source of skeletal
system
• The three germ layers are
the ectoderm, mesoderm
and endoderm, and are
formed as three
overlapping flat discs.
• It is from these three layers
that all the structures and
organs of the body will be
derived.
• Mesoderm and ectoderm
• Mesoderm
• paraxial and lateral
(somatic) plate
mesoderm.
• Ectoderm
◦ Neural crest

9. DEVELOPMENT
OF
THE SKELETAL
SYSTEM

10. • Intramembranous
ossification
➢ Bone formation in which the
mesenchyme differentiated
directly into the bone e.g. flat
bones of the skull.
• Endochondral
ossification
➢ The process of bone
formation in which the
mesenchymal cells give rise to
cartilaginous models first
which in turn become ossified
and form bone e.g. long bones
of the limb.
Ossification
Ossification: conversion of cartilage or other connective tissue into bone.

11. Development
of the skull

12. Development of the skull
Which consists of
A.The Neurocranium; a protective case for the brain
B.The Viscerocranium; the skeleton of the face

13. Neurorocranium
• Membranous neurocranium
➢ Formed by intramembranous
ossification.
➢Mesenchymal cells are derived
from neural crest and paraxial
mesoderm.
➢Cells then encircle the brain and
form most of the flat bones of the
skull

14. Neurocranium
• The cartilaginous neurocranium
(chondrocranium).
• Formed by a combination of mesodermal
sclerotome and neural crest cells.
• Cartilage are form around the brain
beginning at the notochord.
• Parachordal cartilage and the occipital
sclerotomes fused to form the base of
occipital bone.
• While the sphenoid and ethmoidal bones
are formed from the hypophysial cartilage
and the trabeculae cranii.
• All these pieces of bones fuse with each
other to form a strong base of the skull,
expect for the openings via which the
cranial nerves leaves the skull

15. Viscerocranium
1.Membranous Viscerocranium
• Dorsal portion
➢Undergoes intramembranous ossification and gives rise to the maxilla,
the zygomatic bone, the squamous temporal bones, the vomer and
the palatine bone
• Ventral portion
➢Contains the Meckel’s cartilage
➢This region become surrounded by mesenchymal cells that
condenses and ossifies by membranous ossification to form the
mandible

16. 2.Chondral Viscerocranium
• Dorsal portion
➢ Forms the malleus and incus (Meckel’s cartilage)
➢ Forms the stapes and the styloid process (Reichert’s cartilage)
• Ventral portion
➢Ossifies and forms the lesser cornu and the upper body of the
hyoid bone
➢Forms the greater cornu and lower body of the hyoid bone

17. Development of viscerocranium from
the neural crest and 1st & 2nd
Pharyngeal arches
Skull bones of a 3-month-old fetus show
the spread of bone spicules from
primary ossif cation centers in the
fl at bones of the skull

18. Development
of Vertebral
Column

19. Vertebral column
originate from the the sclerotomal cell.
A.During the fourth week of development
these sclerotomal cells from the somites
surround-:
➢ Ventromedial aspect of the notochord to
form the centrum and the intervertebral disc.
➢ Dorsal portion of the neural tube to form the
neural arch.
➢ Ventrolateral aspect of the body wall to
form the costal processes.
B. Chondrification begins in week six.
➢ Ossification begins before birth and
end during the 25th year
C. At birth, three primary ossification
centers are present in the centrum and in
each half of the vertebral (neural) arch
Formation of the vertebral column at
various stages of development.

20. Development
of Ribs &
sternum

21. Development of Ribs & sternum
Ribs
• Ribs are derived from the sclerotome portion of the paraxial
mesoderm which form the costal process of the vertebrae
• Costal process derived mainly from the thoracic vertebrae
• Primary ossification centers appear in the body of the ribs and
mostly become cartilaginous during weeks 13-14 of development.
• Secondary ossification centers appear for the head and tubercle of
the rib at puberty.

22. Sternum
• Develops from the somatic mesoderm in the ventral body wall
• Two sternal bars are formed on either side of the midline and these
later fuses to form
the cartilaginous model of the manubrium, sternabrae (body) and
the xiphoid process
• Ossification appear cephalo-caudally before birth except in the
xiphoid process which appears during childhood
• In neonate, the manubrium contains usually one main ossification
center. Ossification at the lowest segment begins shortly after birth
and that of the xiphoid process during the 3rd year of life

23. Development of Ribs &
sternum

24. Appendicular
Skeleton
• Limbs are derived from the somatic
layer of lateral mesoderm.
• Mesenchymal cells of this region
become activated and the limb
buds become visible as an
outpocketing
• Mesenchyme destined for the
limbs is covered by a layer of
ectoderm
• Ectoderm thickens and forms the
epical ectodermal ridge (AER)
which exerts an
inductive influence and initiates
growth

25. Appendicular
Skeleton
• Distal end of the limb buds become
flattened to form the handplates and
footplates.
• Fingers and toes are formed when the
mesenchyme of the handplates and the
footplates condensed to form digital rays
by apoptosis.
• Similarly, as the shape of the limbs is being
formed, mesenchyme in the buds
condenses and differentiates into
chondrocytes.
• Entire limb skeleton is cartilaginous by the
end of the sixth week of development.
• Joints are formed when chondrogenesis is
arrested and a joint interzone is induced.

26. Appendicular
Skeleton
• Development of the upper and
lower limbs is similar, except that,
the upper limb appeared
approximately 1 or 2 days ahead
of the lower limb.
• Upper limb buds develop opposite
the cervical segments.
• Lower limb buds form opposite the
lumbar and upper sacral
segments.
• End of the embryonic period,
primary ossification begins in the
diaphysis of the long bones.
• Endochondoral ossification
gradually progresses from
diaphysis of the bone toward the
end of the cartilaginous model.

27. Appendicular
Skeleton
• The (shaft) diaphysis of the long bone is
fully ossified at birth.
• The epiphysis is still cartilaginous and
secondary ossification centers appear in
the epiphyses of these bones.
• Persistence of the growth plates provide
for interstitial growth in the length of the
long bone.
• Periostuem provides for appositional
growth in the girth of these bones.
• Endochondral ossification advances on
both sides of the plate and finally the
plate disappear and the epiphysis unite
with the shaft of the bone when bone
has acquired
its full length.

29. Development of the Appendicular Skeleton

30. Factors affecting development
• Poverty
• Mother's age
• Maternal Nutrition
• Drug use
• Opioids
• Cocaine
• Methamphetamine
• Alcohol
• Tobacco use
• Diseases
• Mother's diet and physical
health
• Environmental toxins
• Genetics

31. Growth rate
• Growth rate of fetus is linear up to 37 weeks of gestation.
• The growth rate of an embryo and infant can be reflected as
the weight per gestational age, and is often given as the
weight put in relation to what would be expected by the
gestational age.
• A baby born within the normal range of weight for that
gestational age is known as appropriate for gestational age
(AGA).
• An abnormally slow growth rate results in the infant being small
for gestational age, and, on the other hand, an abnormally
large growth rate results in the infant being large for
gestational age.

32. Summary
• Prenatal Development is often divided into three parts,
called trimesters.
• First Trimester (Weeks 1-12)
The embryonic period starts the fifth week after conception. The
embryo of the baby forms into three layers, called the ectoderm,
mesoderm, and endoderm. The mesoderm is the middle layer
that begins to form, and is the foundation of the baby's bones.
• The baby continues to develop, and by week nine, bones
in the arms and legs begin to develop and grow.

33. Summary
• Second Trimester (Weeks 12-27)
• By week thirteen, tissue that will become bone develops around
the baby's head.
• By week fifteen, the baby's skeleton rapidly develops
bones and continues to grow. The skull becomes more
prominent around this time as well.

34. Summary
• Third Trimester (Weeks 28-40)
• By this point, most of the bones in the baby have been laid out
and continue to develop until they are full-term.
• By the end of the third trimester, babies have 300 bones, which
will eventually fuse to become 260 bones.
• These bones form from cartilage, a flexible substance, and
eventually turn into bone through the process
of ossification.

35. Reference
• langmans-medical-embryology-12th-ed