HUMAN EMBRYOLOGY

• Definition – Embryology is the study of the earlier stages

of development and, in human is restricted to the developmental processes occurring before birth

Value of embryology

The Value of the study embryology to the medical student is :

1. From the general biological aspect: It gives an understanding of how the different organs and tissues develop from a single cell (the fertilized ovum) into a complex multi cellular organism

The Value…

• 2. From the practical aspect the study of development, gives a rational explanation of relationships and position of many normal adult structures, e.g., the nerve supply to the tongue, the asymmetry of the veins in the abdominal and thoracic cavities.

The Value…• 3. Embryology includes not only the

development of the embryo but also the development of the membranes which connect the fetus to the mother, (i.e. placenta). A knowledge of the development, relations and properties of this placenta is essential in order to understand obstetrics and as a basis for advances of the human development, and is also obviously necessary for the understanding of the physiological relationship between the fetus and the mother

The Value…

• 4. Many pathological conditions can only be understood in the light of normal and abnormal development

• 5. As the student continues his studies through the basic medical sciences and into the clinical subjects embryology will be appreciated more and more as a great correlator of other morphological disciplines such as anatomy, pathology, physical diagnostic and surgery, and even of many physiological aspects of medicine

EMBRYOLOGICAL TERMINOLOGY

Most embryological terms have Latin or Greek origin. • Oocyte (L : ovum, egg)• Sperm (Gr : sperma, seed) or spermatozoon (G : spermatos , seed

+ zoon , animal )• Fertilization or conception• Zygote ( Gr: Zygotos, yoked)• Cleavage• Morula ( L: morus, mulberry• Blastocyt (Gr: blastos , germ + kystis , bladder)• Implantation• Gastrula (Gr. Gaster , stomach)• Neurula ( Gr : neurula, nerve)• Embryo (Gr : embryon )• Fetus (L: Unborn offspring) after the embryonic period (8 weeks)• Abortion ( L: aboriri, to miscarry)

Fertilization

Fertilization

Fertilization

Fertilization

Fertilization

Results of fertilization

• Stimulate the secondary oocyte to complete the second meiotic devision

• Restores the normal diploid number of the chromosome (46) in zygote

• Determines chromosomal sex of embryo: an X-bearing sperm produces a female embryo and a Y-bearing sperm produces a male embryo

• Causes metabolic activation of the oocyte and initiates cleavage (cell division of zygote)

The first week

cleavage

Cleavage

• The zygote undergoes cleavage (a series of mitotic cell divisions) into a number of smaller cells called blastomeres. Or more blastomeres, called a morulla.

• A. cavity soon forms in the morula, converting it into a blastocyt consisting of– The inner cell mass, or embryoblast,which gives rise

to the embryo and some extraembryonic tissues– A. blastocyst cavity, a fluid-filled space– The trophoblast, a thin outer layer of cell

Implantation

The Second week

Formation of amniotic cavity, embryonic disc, and yolk sac

• As implantation of the blastocyst progresses, a small cavity appears in the inner cell mass, which is the primordium of the amniotic cavity.

• Morphological changes occur in the embryoblast that result in the formation of the embryonic disc consisting of two layers: epiblast and hypoblast

Implantation

Amniotic cavity

The trophoblast

• The trophoblast later forms extraembryonic structures and embryonic part of the placenta.

• 4-5 days after fertilization, the zona pellucida is shed and the trophoblast adjacent to the inner cell mass attaches to the endometrial epithelium.

The trophoblast• As the blastocyst implants, more trophoblast

contacts the endometrium and differentiates into two layers

• The cytotrophoblast• The syncytiotrophoblast

• Small cavity appears in the inner cell mass, which is the primordium of the amniotic cavity, amnioblast forms the embryonic disc: consisting of two layers : epiblast (ectoderm) and hypoblast (endoderm)

The third weekFormation of Germ Layers

• Gastrulation : formation of germ layers, is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc. Gastrulation begins with formation of the primitive streak on the surface of the epibalst. During this period, the embryo is referred to as a gastrula.

Primitive Streak

Mesoderm

• Paraaxial mesoderm• Intermediate mesoderm• Lateral mesoderm: two layers

– somatic or parietal layer continous with the extraembryonic mesoderm covering the amnion

– splanchnic or visceral layer covering the yolk sac

Mesoderm

Somite

• The paraaxial mesoderm differentiates and begins to form the somite (Gr. Soma, body)

• By the end of the 5th week, 42 – 44 pairs somites are present

• The somites first appear in the future occipital region , they soon develop craniocaudally and give rise to most of the axial skeleton.

Neurulation : Formation of neural tube

• The process involved in the formation of the neural plate, and neural folds and closure of these folds to form the neural tube. These processes are complated by the end of the 4th week. During neurulation the embryo is referred to as a neurula

The fourth to eight weeksorganogenetic period

• All major external and internal strctures are established during organogenetic period, however, the functnion of most of , them minimal , except for the cardiovascular system.

Phases of embryonic development

Human development may be divided into three phases, which to some extent are interrelated

1.The first phase of development is Growth (increase in size), which inlvolves cell division

2.The second phase is Morphogenesis which includes mass cell movement

Phases of embryonic development

3. The third pahse is Differentiation (maturation of physiological processes)

Completion of differentiation result ini the formation of tissues and organs that capable of performing specialized function.

Three germ layers

• Each of the three germ layers(ectoderm, mesoderm, and endoderm) gives rise to specific tissues and organs

• Ectoderm gives rise to : the epidermis, the central and peripheral nervous systems, the sensory ephithelia of the eye, ear, and nose, epidermis and its appandages (hair and nail), mammary gland, pituitary gland, and enamel of teeth

Neural creast cell

• Neural creast cells, derived from neuroectoderm, give rise to cells of the spinal, cranial nerves, and automatic ganglia, dermis, Pharyngeal arch origin (muscle, connective tissues, bone), suprarenal medulla, and meninges.

Three germ layers• Endoderm gives rise to the epithelial linings of the

respiratory and gastrointestinal (GI) tracts,parenchym of the tonsils, thyreoid and parathyroid glands, thymus, liver and pancreas, epithel of the bladder,urethra, cav. tympany.

• Mesoderm gives rise to connective tissue, cartilage, bone, striated and smooth muscles, heart, blood and lymphatic vessels, kidneys, ovaries and testes, genital ducts, serous membrane lining body cavities , spleen, and cortex suprarenal.

Fetal periodNinth week to birth

• The transformation of an embryo to a fetus is gradual, but the change is meaningful because it signifies that the embryo has developed into a recognizable human being and all major system have formed.

• A notable change occuring during the fetal period is the relative slow down in growth of the head compared with the rest of the body

Estimation of embryonic age

• Estimate of the age are determined from their external characteristics and measurements of their length.

• Because embryos of the third and early fourth weeks are straight , measurements of them indicate the greatest length (GL).

• The sitting height or Crown-Rump length (CRL) is most frequently used for older embryo..

• In embryos with greatly flexed neck, the CRL is actually a head-rump measurments.

• CHL (crown – Heel length) is sometimes measured for 8-week-old embryos

EXPECTED DATE of DELIVERY (EDD)

• The common delivery date rule (Nagele’s rule) is to count back 3 month from the first day of Last normal menstruation period (LNMP) , and add a year and 7 days.

• Example : • Fisrt day of LNMP: April. 4,2010• EDD : Jan 11, 2011 .

EXPECTED DATE of DELIVERY (EDD)

The EDD of fetus is 266 days or 38 weeks after fertilization, i.e. 280 days or 40 weeks after LNMP (last normal menstrual period)

Nagele’s rule: is to count back 3 month from the first day of LNMP, and add a year and 7 days.

• Example : • Fisrt day of LNMP: April. 4,2010• EDD : Jan 11, 20 .

• Ultrasound examinations of the fetus, in particular CRL measurments during the first trimester, are commonly used for a more reliable prediction of the EDD

The Pharyngeal Apparatus

• Pharyngeal arches• Pharyngeal Pouches• Pharyngeal grooves• Pharyneal membrane

The Pharyngeal Apparatus

• Development of the face• Development of the nasal cavities• Development of the tongue• Development of the palate• Development of the thyroid gland• Development of the salivary glands• Development of the

The respiratory system

• Development of the larynx• Development of the trachea• Development of the bronchi and lungs

The Digestive System

• Development of the foregut• Development of the spleen• Development of the midgsut• Development of the hindgut

The Uogenital System

• Development of the urinary system• Development of the supra renal glands• Development of the genital system• Development of the inguinal canal

The Cardiovascular system

• Development of the heart and vessles• Development of the lymphatic system• Aortic arch derivative• Anomaly of the heart and vessles• Fetal and neonatal cirulation

The Skeletal System

• Develoment of Bone and Cartilage• Development of the joint• Development of the axial skeleton• Development of the appendicular skeleton

The muscular system

• Develeopment of skeletal muscle• Development of the smooth muscle• Development of the cardiac muscle

The Limb

• Limb development• Dermatomes and ctaneus innervation of

limb• Blood supply of the limb

The Nervous system

• Origin of the nervous system• Development of the spinal cord• Congenital anomalies of the spinal cord• Development of the Brain• Congenital anomalies of the Brain• Congenital anomalies of the periversl NS• Congenital anomalies of the autonomic

The Eye and Ear

• Congenital anomalies of the aye• Congenital anomalies of the ear

Integumenta system

• Congenital anomalies of the skin• Congenital anomalies of the hair• Congenital anomalies of the nail• Congenital anomalies of the mammary

gld.• Congenital anomalies of the teeth