General Embryology Concised Notes PDF

**General Embryology -- Concised Notes** - **Embryology** is the study of the development of an individual before birth. - During the first two months the developing individual is called an ***embryo.*** After that it is called as ***fetus.*** - The ***testis*** is the male sex orga...

**General Embryology -- Concised Notes** - **Embryology** is the study of the development of an individual before birth. - During the first two months the developing individual is called an ***embryo.*** After that it is called as ***fetus.*** - The ***testis*** is the male sex organ or **male gonad**. The ***ovary*** is the female sex organ or **female gonad**. They produce ***gametes.*** - Male gametes produced by the testis are called ***spermatozoa.*** The process is called ***spermatogenesis.*** - Female gametes produced by the ovary are called ***ova.*** The process is called ***oogenesis.*** - Spermatogenesis and oogenesis are together called ***Gametogenesis.*** - ***Fertilization*** takes place when one spermatozoon enters an ovum. The fused ovum and sperm form the ***zygote.*** - Characters of parents are transmitted to offspring through codes borne on strands of - DNA. ***Genes*** are made of such strands of **DNA.** They are located on ***chromosomes.*** - A typical cell contains **46 chromosomes (*Diploid number).*** - A gamete contains **23 chromosomes (*Haploid number).*** - The diploid number of chromosomes is restored as a result of fertilization. - Multiplication of cells takes place by cell division. **The usual method of cell division**, seen in most tissues, is called ***mitosis.*** Daughter cells resulting from a mitotic division are similar to the parent cell, and have the same number of chromosomes (46). - **A special kind of cell division** takes place in the testis and ovary for formation of **gametes.** It is called **meiosis**. The gametes resulting from meiosis have the haploid number of chromosomes (23). **Stages in Spermatogenesis:** C:\\Users\\KARPAGAM\\Downloads\\IMG\_20150412\_213449.jpg **Stages in Oogenesis:**  - Fertilization of the ovum takes place in the ampulla of the uterine tube. The fertilized ovum is a large cell. It undergoes a series of divisions called - **clevage** - When there are 16 cells, the ovum is called a ***morula.*** It has an inner cell mass covered by an outer layer of cells, the ***trophoblast*.** - Fluid partially separates the inner cell mass from trophoblast. The morula now becomes a ***blastocyst*.** - The cells of the inner cell mass multiply, and are rearranged to form an ***embryonic disc**,* having **two *germ layers****(Bilaminar disc)* These layers are the *ectoderm* and *endoderm.* Later, a third germ layer, the *mesoderm,* forms between ectoderm and endoderm. - A cavity appears on the ectodermal side of the disc. This is the ***amniotic cavity.*** Another cavity appears on the endodermal side. This is the ***yolk sac.*** - At first the walls of the amniotic cavity and yolk sac are in contact with trophoblast. They are soon separated from the latter by **extra-embryonic mesoderm.** - A cavity, the extra-embryonic coelom appears and splits the extra-embryonic mesoderm into a **somatopleuric layer** (in contact with trophoblast) and a **splanchnopleuric layer** (in contact with yolk sac). Formation of germ layer in humans - The trophoblast and underlying somatopleuric mesoderm form a membrane called the **chorion.** - The cells forming the wall of the amniotic cavity form the **amnion.** - The amniotic cavity is now attached to trophoblast by some mesoderm into which the extra-embryonic coelom has not extended. This mesoderm forms the **connecting stalk.** - If we view the embryonic disc from the ectodermal side we see that near one edge it has a rounded area called the **prochordal plate.** Here ectoderm and endoderm are not separated by mesoderm. - An elevation, the **primitive streak**, is also seen on the embryonic disc. A line drawn through the prochordal plate and the primitive streak divides the embryonic disc into right and left halves. - Cells multiplying in the primitive streak move into the interval between ectoderm and endoderm and form the mesoderm (third germ layer). - Caudal to the primitive disc a round area called the **cloacal membrane** is seen. It is made up only of ectoderm and endoderm. - The cranial end of the primitive streak enlarges to form the **primitive knot.** - Cells of the primitive knot multiply and pass cranially to form a rod-like structure reaching up to the prochordal plate. This is the **notochordal process.** - The notochordal process undergoes change that convert it first into a canal and then into a plate, and finally back into a rod-like structure. This is the **notocbord.** - Most of the notochord disappears. Remnants remain as the nucleus pulposus of each intervertebral disc. - **Neurulation** is the process by which the neural plate forms the neural tube & neural crest. A wide strip of ectoderm overlying the notochord becomes thickened and forms the **neural plate**. - The ectodermal cells forming the plate are renamed as neurectodermal cells which give rise to brain & spinal cord. - The neural plate invaginates along its central axis and forms **neural groove.** The edges of the neural groove are called as neural folds.The neural folds begin to move closer & fuse converting the groove into neural tube. - The fusion begins at the cervical region & proceeds cranially & caudally. After fusion of neural grooves the cranial & caudal ends is still open called cranial and caudal neuropores. Cranial neuropore closes at 25th day and caudal neuropore closes at 27th day. Neurulation is complete by 4th week.  - Intra-embryonic mesoderm shows three subdivisions. The mesoderm next to the middle line is called the **paraxial mesoderm**. It undergoes segmentation to form somites. - The mesoderm in the lateral part of the embryonic disc is called the **lateral plate mesoderm.** A cavity called the intra-embryonic coelom appears in it and splits the mesoderm into a somatopleuric layer (in contact with - ectoderm) and a splanchnopleuric layer (in contact with endoderm). The intra-embryonic coelom later forms the pericardial, pleural and peritoneal cavities. - A strip of mesoderm between the lateral plate mesoderm and the paraxial mesoderm is called **the intermediate mesoderm.** - The embryonic disc, which is at first flat, undergoes folding at the cranial and caudal ends. These are the head and tail folds. Lateral folds also appear, as a result of these folds the endoderm is converted into a tube, the **gut**. It is divisible into foregut, midgut and hindgut. - After formation of the head fold the gut is closed cranially by the prochordal plate, which is now called the **buccopharyngeal membrane**. Caudally, the gut is closed by the **cloacal membrane.** - - In humans, **Six Pharyngeal arches are formed initially** of which five arches persist (1, 2, 3, 4 and 6) but the fifth arch will disappear. - The **umbilical cord** develops from the connecting stalk. It contains the right and left umbilical arteries, the left umbilical vein, and remnants of the vitello-intestinal duct and yolk sac. The ground substance of the umbilical cord is made up of Wharton\'s jelly derived from mesoderm. - In summary Neuroectoderm consists of : - Neural plate\> neural tube\>\>brain and spinal cord. - Neural crest\>\>consists of pluripotent cells which migrate to all areas and give rise to formation of many organs and tissues. **1.Derivatives of Neural crest cells:** 1. Spinal & sympathetic ganglia,ganglia of the cranial nerves. 2. Schwann cell. 3. Adrenal medulla. 4. Melanoblast. 5. Piamater & Arachnoid mater. 6. Odontoblast. 7. Chromaffin tissue. ------- ------------------------------ **2** **Derivatives of ectoderm:** ------- ------------------------------ **Ectoderm**- The ectoderm consists of columnar cells, which are, flattened or cubical toward the margin of the embryonic disk. Derivatives: Whole of the nervous system, Epidermis of the skin, Lining cells of the sebaceous, sudoriferous, and mammary glands, Hairs and nails, Epithelium of nose, air sinuses, cheeks and roof of mouth. Enamel of the teeth, Anterior lobe of hypophysis cerebri, Epithelium of cornea, conjunctiva, Lacrimal glands, Neuro epithelium of sense organs. ------- ------------------------------ **3** **Derivatives of mesoderm:** ------- ------------------------------ **Mesoderm**- Derivatives: Notocord- Nucleus pulposus of intervertebral disc (vertebral column). Somite- Sclerotome- Vertebrae and ribs. Dermatome-Dermis of dorsal body region Myotome- Trunk and limb muscles. Intermediate mesoderm- Kidney and gonads. Lateral plate mesoderm-Somatic- Parietal layer of pleura, pericardium and peritoneum. Splanchnic-Visceral layer of pleura, pericardium and peritoneum. Wall of respiratory and digestive tracts except epithelial lining. Heart and blood vessels. ------- ------------------------------ **4** **Derivatives of endoderm:** ------- ------------------------------ The endoderm consists at first of flattened cells, which subsequently become columnar. **Derivatives:** Epithelial lining of whole of digestive tract except part of mouth and pharynx and terminal part of rectum, Lining cells of all glands which open into digestive tract, including those of liver and pancreas, Epithelium of auditory tube and tympanic cavity, trachea, bronchi, alveoli,urinary bladder and part of urethra, follicles of the thyroid gland and thymus.. **Derivatives of All Three Germ Layers** Germ Layer Functions \| Muscular system, Excretory system, Respiratory system  -------- ------------------------------------------- **5.** **Derivatives of first pharyngeal arch:** -------- ------------------------------------------- **Skeletal elements:** From maxillary cartilage: Alispenoid , incus. From mandibular: Meckel's cartilage, malleus. Upper portion of external ear. Maxilla, zygomatic, squamous portion of temporal bone, mandible. **Muscles:** Muscles of mastication (temporalis, masseter, lateral and medial pterygoid), Mylohyoid, Anterior belly of digastric, Tensor tympani, Tensor veli palatini **Diagram:** ---- -------------------------------------------- 6. Derivatives of **second pharyngeal arch:** ---- -------------------------------------------- **Skeletal elements:** Stapes, Styloid process, Stylohyoid ligament, Lesser horns and upper rim of hyoid Lower portion of external ear. **Muscles:** 1\. Muscles of facial expression: Muscle of scalp - Occipitofrontalis Muscles of auricle - Auricularis anterior, auricularis posterior, auricularis superior. Muscles of the eyelid - Orbicularis occuli,corrugator supercilii. Muscles of the nose -- Procerus,compressor naris,dilator naris,depressor septi. Muscles around mouth - Orbicularis oris,levator labii superioris alaequae nasi,levator labii superioris,levator anguli oris,zygomaticus minor,zygomaticus major, depressor anguli oris, depressor labii inferioris, mentalis,buccinators. Muscle in the neck -- Plastyma. 2.Posterior belly of digastric. 3.Stylohyoid. 4.Stapedius. Untitled ---- ------------------------------------------- 7. Derivatives of **All pharyngeal arches:** ---- -------------------------------------------