The cells of the theca interna produce oestrogens Fig. The follicle gradually increases in size and finally bursts and expels the ovum. This process of shedding of the ovum is called ovulation. The corpus luteum secretes progesterone, which is essential for maintenance of pregnancy.
The most obvious feature is a monthly flow of blood menstruation. The menstrual cycle is also divided into the follicular phase in which changes are produced mainly by oestrogens , and the luteal phase in which effects of progesterone predominate. Both phases are of roughly equal duration. Just before onset of menstruation, the blood supply to superficial parts of the endometrium is cut off Fig.
This part is shed off and there is bleeding. The menstrual cycle is influenced by oestrogens, by progesterone, by the follicle stimulating hormone FSH and by the Luteinizing hormone LH. The fertilized ovum is a large cell. It undergoes a series of divisions clevage. It has an inner cell mass covered by an outer layer of cells, the trophoblast.
The morula now becomes a blastocyst. These layers are the epiblast and the hypoblast. Later, the epiblast differentiates into three germ layers, the ectoderm outer , the endoderm inner , the mesoderm middle. Cells of the hypoblast become flattened and line the yolk sac. This is the amniotic cavity. Another cavity appears on the endodermal side. This is the yolk sac. They are soon separated from the latter by extraembryonic mesoderm. This mesoderm forms the connecting stalk. Here ectoderm and endoderm are not separated by mesoderm.
A line drawn through the prochordal plate and the primitive streak divides the embryonic disc into right and left halves. It is made up only of ectoderm and endoderm. This is the notochordalprocess. This is the notochord. Most of the notochord disappears. Remnants remain as the nucleus pulposus of each intervertebral disc. A wide strip of ectoderm overlying the notochord becomes thickened and forms the neural plate Fig. 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 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 Fig. 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. Caudally, the gut is closed by the cloacal membrane. The umbilical cord develops from the connecting stalk.
The ground substance of the umbilical cord is made up of Wharton's jelly derived from mesoderm. The cord is covered by amnion. The allantoic diverticulum arises from the yolk sac before formation of the gut Fig. After formation of the tail fold, it is seen as a diverticulum of the hindgut. The developing heart lies ventral to the cavity Fig. After formation of the head fold the pericardial cavity lies ventral to the foregut; and the developing heart is dorsal to the pericardial cavity Fig.
After formation of the head fold, it lies caudal to the pericardium and heart Fig. The liver and the diaphragm develop in relation to the septum transversum. This is called implantation. The villi are surrounded by maternal blood. Fetal blood circulates through capillaries in villi. All substances passing from mother to fetus and vice versa traverse this membrane.
They consist of a central core of cytotrophoblast covered by syncytiotrophoblast. As the placenta enlarges, septa grow into the intervillous space dividing the placenta into lobes.
The fully formed placenta is about six inches in diameter and about g in weight. A placenta attached lower down is called placenta praevia.
It can cause problems during child birth. Further enlargement of amniotic cavity obliterates the uterine cavity. Fused amnion and chorion called membranes bulge into the cervical canal during child birth and help to dilate it. Epithelia lining external surfaces of the body, and terminal parts of passages opening to the outside are derived from ectoderm.
Epithelium lining the gut, and of organs that develop as diverticula of the gut, is endodermal in origin. Epithelium lining most of the urogenital tract is derived from mesoderm. In some parts, it is endodermal in origin.
Mesenchyme is made up of cells that can give rise to cartilage, bone, muscle, blood and connective tissues. Blood cells are derived from mesenchyme in bone marrow, liver, and spleen. Lymphocytes are formed mainly in lymphoid tissues. Most bones are formed by endochondral ossification, in which a cartilaginous model is first formed and is later replaced by bone.
Some bones are formed by direct ossification of membrane intramembranous ossification. An area where ossification starts is called a centre of ossification. In the case of long bones the shaft or diaphysis is formed by extension of ossification from the primary centre of ossification. Secondary centres of variable number appear for bone ends. The part of bone ossified from a secondary centre is called an epiphysis. In growing bone the diaphysis and epiphysis are separated by the epiphyseal plate which is made up of cartilage.
Growth in length of a bone takes place mainly at the epiphyseal plate. Somites undergo division into three parts. These are: a the dermatome which forms the dermis of the skin; b myotome which forms skeletal muscle; and c sclerotome which helps to form the vertebral column and ribs. Skeletal muscle is derived partly from somites and partly from mesenchyme of the region.
Most smooth muscle is formed from mesenchyme related to viscera, and blood vessels. Cardiac muscle is formed from mesoderm related to the developing heart. Anemia: Understanding The Concepts. Goodman and Gilman Pharmacology Pdf Download. Friday, January 14, Forgot your password? Get help. Create an account. Contents hide. Please enter your comment! Please enter your name here. You have entered an incorrect email address!
Follow us on:. Editor's Pick. Clinical pictures and case descriptions are used to provide this information, and this material has been increased and updated in this edition.
Genetics: Because ofthe increasingly important role ofgenetics and molecular biology in embryology and the study ofbirth defects, basic genetic and molecular principles are discussed. Extensive Art Program: The artwork has always been designed to enhance understanding ofthe text and includes four-color line drawings, scanning electron micrographs, and clinical pictures. Once again, artwork has been added, especially to Chapter 18, to illustrate new concepts in development ofthe central nervous system, diaphragm, ear, and other structures.
Summary: At the end ofeach chapter is a summary that serves as a concise review ofthe key points described in detail throughout the chapter. Problems to Solve: Problems related to the key elements ofeach chapter are provided to assist the students in assessing their understanding ofthe material. Detailed answers are provided in an appendix at the back ofthe book.
Glossary: A glossary ofkey terms has been expanded and is located in the back ofthe book. Medical Embryology is the study of the human development. The study of human development is really complex and exhausting given the amount of detail the medical students have to absorb. From the molecular basis of cardiac development to assuming its nal gross anatomical structure — this entire process is a true marvel of mother nature.
But learning everything in detail can be a stressful and daunting task because everything that you will have to learn is in the form of a step-by-step process i. It oers the medical students, nursing undergraduates, and healthcare professionals a very comprehensive text on embryology with a strong clinical emphasis.
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