Embryology Muscular And Skeletal Systems

Front 1

What are the 3 types of muscles derived from?

Back 1

► Skeletal muscle - derived from the myotome region of the somite
► Smooth muscle - differentiate from splanchnic mesoderm
► Cardiac muscle - develops from splanchnic mesoderm

Front 2

Smooth muscle

Back 2

► Differentiate mostly from splanchnic mesoderm
► Thin layers of elongated cells with single nuclei
► Found in the walls of the internal organs
► Vascular and lymphatic smooth muscle are
derived also from somatic mesoderm
► Muscles of the iris are derived from neuroectoderm

Front 3

Cardiac muscle

Back 3

► Develops from splanchnic mesoderm
► Fibers arise by differentiation and growth of single cells -- Once cell has differentiated it doesn’t divide anymore

► Myoblasts adhere to each other through intercalated disks -- myoblasts do not fuse, fiber composed of single cells connected by intercalated disc junction, enable them to behave like one cell, all of them are beating at the same time
- striated muscle
► Purkinje fibers - form the conducting system of the heart

Front 4

Absence of pectoralis major muscle

Back 4

► Congenital absence of the left pectoralis major muscle, associated with a low location of the left nipple
► Poland's Syndrome - absence or underdevelopment of the pectoralis muscle and webbing or fusion of the fingers (syndactyly) on the same side of the body, Poland's syndrome = rare
- helps move hand, absence = compromised hand movement

Front 5

Arthrogryposis multiplex congenita

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► Multiple joint immobility and stiffness due to hypoplasia of the associated muscles
► The affected muscles are replaced partially or completely by fat and fibrous tissue
- Sometimes very mild, not diagnosed
- If severe, surgery, physical therapy

Front 6

Development of the muscular system

Back 6

- develops from mesoderm, except muscles of the iris -- neuroectoderm
- mesoderm differentiates in 3rd week
- as notochord and neural tube form, mesoderm divided into 3 regions:
1. paraxial: segmented into somites
2. intermediate
3. lateral: divided by intraembryonic ceolum into splanchic mesoderm and somatic mesoderm

Front 7

Differentiation of somites of paraxial mesoderm

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1. Schlerotome--the medial region. Gives rise to the vertebrae.
2. Dermotome--medial to myotome layer. Gives rise to the dermis.
3. Myotome--the most external layer. Gives rise to the skeletal muscles.
The dermotome and myotome differentiate out of a single layer, called the dermomyotome.

Front 8

Myogenesis

Back 8

1. Elongation of the mesenchymal cell and cell nucleus, to form a myoblast.
2. Fusion of multiple myoblasts into a long, multinucleated cell, called a myotube.
3. The myotube starts to produce the proteins that compose the sarcomere, and differentiates into a muscle fiber. The nuclei in a muscle fiber migrate to the cell periphery, to stay out of the way of the sarcomere.

Front 9

Muscle growth during development vs. muscle growth after development

Back 9

During development, muscle growth results from the fusion of myoblasts and myotubes, to form muscle fibers. Skeletal muscle development mostly happens before birth, and is almost entirely completed by the first year. Increase in muscle size after the first year depends on increase in muscle fiber diameter due to the addition of myofilaments.
Populations of reserve cells, called satellite cells, sit between the sarcolemma and the basal lamina of the muscle fibers, and divide slowly throughout life. Satellite cells may fuse with the muscle fiber as needed, and contribute to the growth of the muscle.

Front 10

Myotome

Back 10

- group of tissues formed from somites that develop into body wall muscle
Each myotome region of a somite divides into two regions:
1. A dorsal, epaxial region
2. A ventral, hypaxial region
The epaxial region will form the muscles of the back, and the hypaxial region will form the muscles of the front. The developing mixed spinal nerve also splits into two, and sends a dorsal primary ramus to the epaxial region, and a ventral primary ramus to the hypaxial region.

Front 11

Limb development

Back 11

The limbs start to develop during the fourth week, as outgrowths from the ventrolateral body walls. The upper limbs begin to grow about 2-3 days before the lower limbs. The limbs begin as protrusions of mesenchyme, surrounded by ectoderm. Myoblasts migrate from the myotome region of the somites, circle the growing bones, and form the muscles of the limbs.

Front 12

Skeletal Muscles that Do Not Develop from The Somites

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1. The tongue muscles develop from the pharyngeal arches. The myoblasts that form the tongue muscles originate from the occipital myotomes, and migrate into the pharyngeal apparatus.
i. The muscles of the diaphragm also develop from the occipital myotomes, since the septum transversum (primitive diaphragm) is initially located at the cranial region of the embryo.
2. The ocular muscles develop from the preotic myotomes (three myotomes around the eye that are not derived from the somites
3. The muscles of mastication, facial expression, the pharynx, and the larynx are derived mostly from neural crest cells which migrate into the pharyngeal apparatus, and replace its mesenchymal cells.

Front 13

Smooth muscle

Back 13

Most smooth muscles are derived from the splanchnic mesoderm. The smooth muscle in the valls of many blood vessels and lymphatic vessels derives from the somatic mesoderm. Developing smooth muscle takes the form of thin layers of elongated, mononucleated cells, organized into sheet-like structures.
Smooth muscle is found in two major sites in the body:
1. Walls of internal organs
2. Vascular and lymphatic vessels
Unlike other smooth muscle, the muscles of the iris are derived from neural ectoderm

Front 14

Cardiac Muscle

Back 14

Cardiac muscle is a striated form of smooth muscle, derived from splanchnic mesoderm, like most smooth muscle. Unlike the myoblasts that form skeletal muscle, cardiac myocytes don't fuse, but are connected to each other via specialized junctions called intercalated discs, that allow all the cardiac cells to act as one unified cell.

Front 15

Heart development

Back 15

The heart begins to function by the beginning of the fourth week, at which point its structure is the shape of two tubes, fused together.
Dividing cardiac cells have the ability to disassemble their contractile elements, divide, and then reassemble them. This allows the heart to act as a functioning organ, even before it is fully developed.

Front 16

Purkinje fibers

Back 16

- conducting system of heart
Purkinje fibers are muscle cells with only a few contractile elements that can act like nerve cells and conduct electrical impulses. Purkinje cells are mostly found in two main centers:
1. Sinoatrial node (SA node)
2. Atrioventricular node (AV node)

Front 17

Absence of a muscle

Back 17

Ex: absence of the pectoralis major muscle. Characterized by compromised hand movement and lowered nipple.
1. Poland's Syndrome--absence of the pec major, associated with webbing or fusion of the fingers (syndactyly) on the ipsilateral side of the body

Front 18

Arthrogryposis Multiplex Congenita

Back 18

abnormal growth of muscles in many sites of the body. Characterized by immobility and stiffness in multiple joints, due to muscular hypoplasia (incomplete development). The affected muscles are partially or completely replaced by fat and fibrous tissue. This condition may be partially corrected by surgery and physical therapy. Etiology is multifactorial, although it has been associated with hypothermia, certain viruses, and some neurological causes.

Front 19

Muscular Dystrophy

Back 19

--characterized by muscle atrophy and weakness.
i. Duchenne MD occurs typically in young boys, due to a mutation in dystrophin, which is involved in preserving the membrane stability of muscle fibers. Absence of dystrophin leads to the death of many muscle cells. Because secretion of creatine phosphokinase (CPK)occurs as a result of muscle cell death, CPK levels are used to screen from MD babies.

Front 20

Congenital Torticollis

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shortening of the sternocleidomastoid results in tilting of the head. Birth trauma is the most common causes of this condition, leading to bleeding and necrosis of the muscle. May be treated by physical therapy.

Front 21

Development of the skeletal system

Back 21

The skeletal system develops from mesoderm and neural crest cells.
-the vertebrae and ribs develop from the sclerotome of the somites
-the limbs develop from the somatomesoderm, in the body wall
-the craniofacial structures develop from neural crest cells that replace the mesoderm in the pharyngeal apparatus

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2 different types of osteogenesis

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1. Intramembraneous ossification--bone formation occurs directly in the mesenchyme, without an intermediate cartilagenous stage, within sheaths of membrane. The flat bones of the calvaria and some facial structures develop this way.
2. Endochondral ossification--bones develop via ossification of a pre-existing cartilage model. The long bones develop this way.

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Intramembraneous Ossification

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Intramembraneous ossification starts during the embryonic period, continues throughout the fetal period, and is completed after birth.
1. Mesenchymal cells differentiate into bone-forming cells calls osteoblasts.
2. Osteoblasts begin to secrete a matrix called the osteoid.
i. Calcium phosphate gets deposited in the matrix, causing it to calcify and form bone.
3. Osteoblasts get trapped inside the bone, and are renamed osteocytes.
i. Some osteocytes are dead, some are alive
Growing bone forms concentric circular layers, called lamellae.
The osteoblasts are located on the periphery of the bone.
Capillaries are visible running through the bone, enclosed in circles of bone called osteons.

Front 24

Endochondral Ossification

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Mesenchyme develops into a cartilage model, from which the bone is formed. This is the way long bones form.
Ossification begins in the diaphysis, in a region called the primary ossification center.
1. Mesenchymal cells differentiate into cartilage progenitor cells called chondroblasts
2. Chondroblasts differentiate into cartilage.
3. Cartilage cells deposit a cartilagenous matrix (usually around week 6). Then, the cartilage matrix becomes vasculated, ossifies, and matures into bone.
Growth of peripheral cartilage and ossification of central cartilage happen simultaneously.
The middle of the bone is composed of spongy material, and contains the bone marrow (with all the cells of the hematopoetic system).
Secondary ossification begins after birth, and occurs in the epiphyses, in regions called secondary ossification centers. Secondary ossification ends at around 20-25 years of age. When secondary ossification is complete, the epiphysis fuses with the diaphysis, and there is no longer cartilage between the two layers of bone.

Front 25

What does fetal ossification depend on?

Back 25

Fetal ossification depends on the maternal supply of Ca and phosphorus. Vitamin D is needed for Ca absorption.
A deficiency in vitamin D may causes Rickets, which is characterized by shortened and deformed limbs in children.

Front 26

parts of the axial skeleton

Back 26

cranium, vertebral column, ribs, and sternum

Front 27

The Cranium

Back 27

The cranium is composed of two regions:
1. The neurocranium surrounds the brain
2. The viscerocranium forms the face
The flat bones of the calvaria (skull) in the neurocranium are formed via intramembraneous ossification. These bones are not fused at birth, but are connected by tough CT that forms the cranial sutures. Multiple sutures meet at six large, fibrous areas called fontanelles. The sutures provide some flexibility to the skull, allowing the head to pass through the birth canal in a process called molding. They also enable the growth of the brain, and persist for several years after birth (the sutures persist much longer than the fontanelles).
The viscerocranium (that forms the face) is formed by both intramembraneous and endochondral ossification.
-the little bones of the inner ear form by endochondral ossification
- the upper face bones (squamous temporal, maxilla, zygomatic) form by intramembranous ossification
-the mandible forms by intramembraneous ossification

Front 28

Acrania

Back 28

abnormal development of the calvaria (skullcap). May or may not be associated with development of the brain (anencephaly).

Front 29

Craniosynostosis

Back 29

premature closure of the cranial sutures. Has varying degrees of severity, depending on when the sutures closed. The etiology is unclear, although genetic factors are involved.
Because the brain cannot expand in the direction of the fused suture, it is forced to grow in the direction of the open sutures, often resulting in an abnormal head shape and facial features. Some cases of craniosynostosis may result in increased pressure on the brain and developmental delays.

Front 30

Development of the vertebral column

Back 30

Development of the vertebral column begins with migration of the sclerotome cells around the notochord and neural canal, towards the lateral body walls.
Each sclerotome has a cranial region, made of loosely arranged cells, and a caudal region, of densely arranged cells.
Two somites participate in the formation of each vertebra. The caudal part of a cranial sclerotome and the cranial part of a caudal sclerotome fuse to form the primordial vertebral body, called the centrum.
Some of the densely packed caudal cells migrate caudally, to form the intervertebral disk.
-the inner region of the disk, called the nucleosus pulposus, is formed from the notochord
-the outer region of the disk, called the annulosus fibrosis, is an elastic, fibrocartilagenous structure
The notochord degenerates, inside the vertebrae.
Arteries which supply the spinal cord enter the vertebral column through holes in the vertebrae bodies

Front 31

numbers of vertebrae

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-cervical: 7
-thoracic: 12
-lumbar: 5
-saccral: 5

Front 32

What happens if the notochord (inside the vertebrae) does not fully degenerate?

Back 32

a chordoma may form. A chordoma is a slow-growing, malignant tumor, formed out of remnants of the notochord that usually forms in the base of the cranium, or in the lumbrosaccral region. Chordomas usually infiltrate bone. Patients do not typically live past five years.

Front 33

Stages of vertebral development

Back 33

1. Cartilagenous stage, weeks 6-7, in which the mesenchymal cells form a cartilagenous model around the notochord. Centers of chondrification occur around the notochord, and in the costal processes that will give rise to the ribs.
2. Bony stage, weeks 7-8. Occurs in two stages, around the notochord, and in ossification centers around the vertebral arches;
i. Primary ossification, weeks 7-8
ii. Secondary ossification, begins after puberty and is completed around age 25

Front 34

The Ribs

Back 34

The ribs develop from mesenchymal processes called costal processes.
Cartilage forms during the embryonic period, bone during the fetal period.
There are 12 ribs:
-ribs 1-7 are true ribs
-ribs 8-10 are false ribs, attached to the sternum indirectly, through the cartilage of rib 7
-ribs 10-12 are floating ribs, and do not attach to the sternum

Front 35

Anomalies of the Ribs and Vertebrae

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1. Accessory ribs, may form in the lumbar or cervical regions, and are usually asymptomatic.
2. Forked ribs, form a fork and connect to the sternum on two sides
3. Hemivertebra, results from failure of half of a vertebra to form. May lead to lateral curvature of the vertebral column, called scoliosis.

Front 36

The Sternum

Back 36

The sternum develops from two bands of mesenchymal tissue that fuse in the ventral region of the body, beginning at the cranial end. It develops via endochondrial ossification.
Abnormal fusion of the two halves of the sternum results in a condition called cleft sternum, which can be corrected through surgery. Abnormal fusion of the lower region of the sternum results in a bifid (split) xiphoid process, which is usually asymptomatic.

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The Limbs

Back 37

The limbs are extensions of the somatic mesoderm, lined with the ectoderm.
The long bones of the limbs develop via endochondral ossification, around week 6. Primary ossification happens in weeks 7-12, secondary ossification begins in the late fetal period and continues after birth.