When do germ layers form

The regulation occurs through signaling between cells and tissues and responses in the form of differential gene expression. Fertilization is the process in which gametes an egg and sperm fuse to form a zygote Figure To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, a layer called the zona pellucida protects the egg. At the tip of the head of a sperm cell is a structure like a lysosome called the acrosome, which contains enzymes.

When a sperm binds to the zona pellucida, a series of events, called the acrosomal reactions, take place. These reactions, involving enzymes from the acrosome, allow the sperm plasma membrane to fuse with the egg plasma membrane and permit the sperm nucleus to transfer into the ovum. The nuclear membranes of the egg and sperm break down and the two haploid nuclei fuse to form a diploid nucleus or genome.

To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg.

The development of multi-cellular organisms begins from this single-celled zygote, which undergoes rapid cell division, called cleavage Figure In mammals, the blastula forms the blastocyst in the next stage of development. All animals have germ layers in which the vertebrate germ layer structure is particularly pronounced, while the sponge animal's germ layer is the simplest and usually produces two to three major tissue layers sometimes referred to as primary germ layers.

Radiation-symmetric animals eg, coelenterates have two germ layer structures, including endoderm and ectoderm; bilaterally symmetrical animals have three germ layer structures, more than radiation-symmetric animals, endodermal and external mesoderm between germ layers.

All cells within the germ layer eventually develop into various tissues and organs of the animal. The inner, middle and outer blastoderm are named according to their position on the embryo at the end of the gastrula or at the beginning of the embryo. In fact, each layer of cells has its own characteristics in addition to the spatial position. For example, generally the endoderm cells are large, containing more yolk; ectodermal cells are small, and the division is fast.

By means of in vivo staining, the early distribution of the three germ layers can be determined in the gastrula or blastocyst stage, and the predicted fate of the embryo is depicted. This map essentially reflects that different germ layers may be associated with egg heterogeneity.

It has long been known that many animals have regional differences in egg quality. For example, eggs have movements, and the distribution of macronuclei such as egg yolk particles, pigment particles, organelles, and nucleic acid proteins in various regions of the egg is unequal. When the cleavage divides the egg into many small pieces, the uneven oocyst is distributed into different lobes. As the gut movement progresses, the substances originally assigned to different lobes are distributed into different germ layers.

These substances may affect further differentiation of each germ layer. For example, the lobes containing the yellow crescent region of the stalk will form muscle and interstitial cells in the future, and the lobes containing the gray crescent region will form the notochord and nerve tissue in the future, and the lobes containing the transparent material will form the epidermis in the future. The blastomeres of plant polar matter will form endoderm in the future.

In general, the ectoderm forms the epidermis and nerve tissue. The endoderm forms the intestinal epithelium and the digestive gland epithelium, which forms bone, muscle, blood, lymph and other connective tissues. Others are derived from the mesoderm. The germ layer differentiation is basically along the route on the drawing. It is a monocellular layer lining the yolk sac until cephalocaudal flexion of the embryo takes place Flexion takes the embryo from a flat disk to its basic embryonic body form.

The primitive gut originates from entoderm at the time of its flexion The yolk sac constricts, thus the intraembryonic entoderm future digestive tube and the extraembryonic entoderm forms the inner lining of the yolk sac are delineated Three major parts of the primitive gut are the foregut, the midgut, and the hindgut including the cloaca The oropharyngeal buccopharyngeal and cloacal membranes temporarily close the 2 ends of the primitive gut In humans, the buccopharyngeal membrane disappears at the beginning of week 4 The cloacal membrane lasts longer and at week 7, like the cloaca, it divides into an anterior urogenital membrane and posterior anal membrane , the latter being absorbed by week 9.

Related Organs. Prior to this the existing cells undergo rapid division and a mass of cells detach from the embryonic disc to form mesoderm. After the separation of mesoderm, the remaining cells of the embryonic disc form the ectoderm layer. In this manner the three germ layers such as ectoderm, mesoderm and endoderm are formed.

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