05 Third Week of Development II.pptx embryology

Embryology: The Study of Development

Embryology is the branch of biology that studies the formation, growth, and development of an embryo from fertilization to birth. It helps us understand congenital abnormalities, organ development, and evolutionary biology.

1. Stages of Embryonic Development

Embryonic development occurs in sequential stages:

Fertilization: A sperm cell fertilizes an egg, forming a zygote.

Cleavage: The zygote undergoes rapid mitotic divisions, forming a morula (solid ball of cells).

Blastulation: The morula develops into a blastocyst, which implants in the uterus.

Gastrulation: Three germ layers form:

Ectoderm (skin, nervous system).

Mesoderm (muscles, bones, blood).

Endoderm (digestive and respiratory organs).


Neurulation: The neural tube forms, which later develops into the brain and spinal cord.

Organogenesis: Organs begin to develop, continuing into the fetal stage.


2. Importance of Embryology

Medical Applications: Understanding congenital defects like neural tube defects (e.g., spina bifida).

Evolutionary Biology: Compares embryonic stages across species to trace evolutionary links.

Reproductive Technology: Supports advancements in IVF and prenatal diagnostics.


3. Fetal Development

By the 8th week, the embryo becomes a fetus, with developing limbs, organs, and a beating heart. From this stage, growth and refinement continue until birth.

Embryology provides insights into human development, aiding in medicine, genetics, and evolutionary studies.

1. DERIVATIVES OF THE MESODERMAL GERM LAYER 1

2. Mesoderm: It differentiates into: 1-Axial mesoderm (primitive streak, primitive node, notochord) 2-Paraxial mesoderm gives final count of approximately 35 pairs of somites . These somites further differentiate into 3 components: a- Sclerotome (cartilage and bone) b- Myotome (muscles) c- Dermatome (dermis of skin) 3-Intermediate cell mass (gives kidneys& genital system) 2

3. 4-Lateral plate of mesoderm differentiates into: a- somatopleure (gives the voluntary muscles of chest and abdomen and the parietal layer of pleura and peritoneum. b- splanchnopleure (gives the involuntary muscles of heart, bronchial tree, and gut and the visceral layer of pleura and peritoneum). The cavity between these two layers is the intraembryonic coelom. 3

4. Derivatives of the Mesodermal Germ Layer 17th day- cells close to the midline proliferate to form paraxial mesoderm. More laterally, the mesoderm layer remains thin and is known as the lateral plate. intercellular cavities in the lateral plate , this tissue is divided into two layers: a layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer . a layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer. Together , these layers line a newly formed cavity , the intraembryonic cavity, which is continuous with the extraembryonic cavity on each side of the embryo. Intermediate mesoderm connects paraxial and lateral plate mesoderm. 4

5. 5 Transverse sections showing development of the mesodermal germ layer. A. Day 17. B. Day 19. C. Day 20. D. Day 21.

6. PARAXIAL MESODERM By the beginning of the third week, paraxial mesoderm is organized into segments. These segments, known as somitomeres First appear in the cephalic region, and their formation proceeds cephalocaudally . Each somitomere consists of mesodermal cells arranged in concentric whorls around the center of the unit. From the occipital region caudally, somitomeres further organize into somites . The first pair of somites arises in the occipital region of the embryo at approximately the 20th day of development . From here, new somites appear in craniocaudal sequence at a rate of approximately three pairs per day until , at the end of the fifth week, 42 to 44 pairs are present. 6

7. There are 4 occipital, 8 cervical , 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs . The first occipital and the last five to seven coccygeal somites later disappear, while the remaining somites form the axial skeleton . Because somites appear with a specified periodicity, the age of an embryo can be accurately determined during this early time period by counting somites . Somites give rise to Cells that form the vertebrae and ribs Dermis of the dorsal skin Skeletal muscles of the back and the skeletal muscles of the body wall and limbs. 7

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9. Intermediate Mesoderm connects paraxial mesoderm with the lateral plate differentiates into urogenital structures . In cervical and upper thoracic regions , it forms segmental cell clusters ( future nephrotomes ), whereas more caudally, it forms an unsegmented mass of tissue, the nephrogenic cord . Excretory units of the urinary system and the gonads develop from this partly segmented, partly unsegmented intermediate mesoderm. 10

10. Lateral plate mesoderm splits into parietal (somatic ) and visceral (splanchnic) layers. Mesoderm from the parietal layer, together with overlying ectoderm, forms the lateral body wall folds . These folds, together with the head (cephalic) and tail (caudal) folds, close the ventral body wall. The parietal layer of lateral plate mesoderm then forms the dermis of the skin in the body wall and limbs, the bones and connective tissue of the limbs, and the sternum. In addition, sclerotome and muscle precursor cells that migrate into the parietal layer of lateral plate mesoderm form the costal cartilages, limb muscles, and most of the body wall muscles. The visceral layer of lateral plate mesoderm, together with embryonic endoderm, forms the wall of the gut tube. 11

11. Blood and Blood Vessels Blood cells and blood vessels also arise from mesoderm . Blood vessels form in two ways: vasculogenesis , whereby vessels arise from blood islands and angiogenesis, which entails sprouting from existing vessels . The first blood islands appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions. 12

12. 13 Cross section through a 21-day embryo in the region of the mesonephros showing parietal and visceral mesoderm layers. The intraembryonic cavities communicate with the extraembryonic cavity (chorionic cavity). Section at the end of the fourth week. Parietal mesoderm and overlying ectoderm form the ventral and lateral body wall.

13. DERIVATIVES OF THE ENDODERMAL GERM LAYER The gastrointestinal tract is the main organ system derived from the endodermal germ layer. This germ layer covers the ventral surface of the embryo and forms the roof of the yolk sac. As a result of cephalocaudal growth and closure of the lateral body wall folds a continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube. The tube is divided into three regions: the foregut, midgut , and hindgut. 14

14. At its cephalic end, the foregut is temporarily bounded by an ectodermal–endodermal membrane called the oropharyngeal membrane. In the fourth week, the oropharngeal membrane ruptures , establishing an open connection between the oral cavity and the primitive gut. The hindgut also terminates temporarily at an ectodermal– endodermal membrane, the cloacal membrane. The membrane breaks down in the seventh week to create the opening for the anus. 15

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16. The role of the yolk sac is not clear. It may function as a nutritive organ during the earliest stages of development prior to the establishment of blood vessels . It also contributes some of the first blood cells , although this role is very transitory. One of its main functions is to provide germ cells that reside in its posterior wall and later migrate to the gonads to form eggs and sperm. 17

17. Hence , the endodermal germ layer initially forms the epithelial lining of the primitive gut and the intraembryonic portions of the allantois and vitelline duct. During further development , endoderm gives rise to: The epithelial lining of the respiratory tract; The parenchyma of the thyroid, parathyroids , liver , and pancreas The reticular stroma of the tonsils and the thymus ; The epithelial lining of the urinary bladder and the urethra The epithelial lining of the tympanic cavity and auditory tube. 18

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20. EXTERNAL APPEARANCE DURING THE SECOND MONTH At the end of the fourth week, when the embryo has approximately 28 somites , the main external features are the somites and pharyngeal arches . The age of the embryo is therefore usually expressed in somites . Because counting somites becomes difficult during the second month of development, the age of the embryo is then indicated as the crownrump length (CRL) and expressed in millimeters. CRL is the measurement from the vertex of the skull to the midpoint between the apices of the buttocks. By the beginning of the fifth week, forelimbs and hindlimbs appear as paddle-shaped buds . 21

21. 22 Human embryo (CRL 9.8 mm, fifth week) (×29.9). The forelimbs are paddle-shaped. Human embryo (CRL 13 mm, sixth week) showing the yolk sac in the chorionic cavity.

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