Extraembryonic Membranes

There is more to a developing reptile, bird, or mammal than the embryo itself. The embryos of these vertebrates are surrounded by several extraembryonic membranes, which originate from the embryo but are not part of it. The extraem-bryonic membranes function in nutrition, gas exchange, and waste removal.

Extraembryonic membranes form with contributions from all germ layers

We will use the chicken to demonstrate how the extraem-bryonic membranes form from the germ layers created during gastrulation. The yolk sac is the first extraembryonic membrane to form, and it does so by extension of the endodermal tissue of the hypoblast layer along with some adjacent mesoderm. The yolk sac grows to encloses the entire body of yolk in the egg (Figure 20.18). It constricts at the top to create a tube that is continuous with the gut of the embryo. However, yolk does not pass through this tube. Yolk is digested by the endodermal cells of the yolk sac, and the nutrients are then transported to the embryo through blood vessels that form from the mesoderm and line the outer surface of the yolk sac. The allantoic membrane is also an outgrowth of the extraembryonic endoderm plus adjacent mesoderm. It forms the allantois, a sac for storage of metabolic wastes.

Just as the endoderm and mesoderm of the hypoblast grow out from the embryo to form the yolk sac and the al-lantoic membrane, ectoderm and mesoderm combine and extend beyond the limits of the embryo to form the other ex-traembryonic membranes. Two layers of cells extend all along the inside of the eggshell, both over the embryo and below the yolk sac. Where they meet, they fuse, forming two membranes, the inner amnion and the outer chorion. The amnion surrounds the embryo, forming the amniotic cavity. The amnion secretes fluid into the cavity, providing a protective environment for the embryo. The outer membrane, the chorion, forms a continuous membrane just under the eggshell (Figure 20.18). It limits water loss from the egg and also works with the enlarged allantoic membrane to exchange respiratory gases between the embryo and the outside world.

Extraembryonic membranes in mammals form the placenta

In mammals, the first extraembryonic membrane to form is the trophoblast, which is already apparent by the fifth cell

5-Day chick embryo

Embryo Amnion

Gut Amnionic cavity Chorion Yolk

Allantoic membrane

| The first extraembryonic mem brane is the yolk sac, which is forming in the 5-day embryo.

5-Day chick embryo

Allantoic membrane

| The first extraembryonic mem brane is the yolk sac, which is forming in the 5-day embryo.

Human Extraembryonic Membranes

9-Day chick embryo

Embryo Gut Amnion Amnionic cavity Chorion—

Yolk sac

3| The mesodermal and ectodermal layers fuse below the yolk so that the chorion lines the shell.

9-Day chick embryo

Embryo Gut Amnion Amnionic cavity Chorion—

Yolk sac

Extraembryonic Membranes Birds

Allantois

Allantoic membrane

Allantois

Allantoic membrane

'1 Mesodermal and endodermal tissues form the allantois, a sac for metabolic wastes.

20.18 The Extraembryonic Membranes In birds, reptiles, and mammals, the embryo constructs four extraembryonic membranes. The yolk sac encloses the yolk, and the amnion and chorion enclose the embryo. Fluids secreted by the amnion fill the amniotic cavity, providing an aqueous environment for the embryo.The chorion, along with the allantois, mediates gas exchange between the embryo and its environment.The allantois stores the embryo's waste products.

20.19 The Mammalian Placenta In most mammals, nutrients and wastes are exchanged between maternal and fetal blood in the placenta, which forms from the chorion and tissues of the uterine wall.The embryo is attached to the placenta by the umbilical cord. Embryonic blood vessels invade the placental tissue to form fingerlike chorionic villi. Maternal blood flows into the spaces surrounding the villi.

Fetus

Placenta Fetal portion Maternal portion of placenta of placenta

Fetus

Extraembryonic Membranes

Uterus

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • huriyyah
    When does the extraembryonic membrane form in chick embryo?
    8 years ago
  • lloyd
    Which extraembryonic membrane of the amniote egg stores metabolic waste produced by the embryo?
    7 years ago
  • risto
    What are the functions of the four extraembryonic membranes of the chick egg?
    7 years ago
  • matilda
    Is extraembryonic membrane trophoblast?
    7 years ago
  • aleksander
    What are the function of extraemembryonic membranes in mammals,birds, and reptiles?
    6 years ago
  • Hiwet
    Which is NOT one of the four extraembryonic membranes of the amniote egg?
    6 years ago
  • carita tuikka
    Which is not one of the four extraembryonic membranes of the aminote egg?
    5 years ago
  • lachlan
    Is trophoblast an extra embryonic membrane in mammals?
    4 years ago
  • Louise Tate
    What is extra embryonic sac in chick?
    4 years ago
  • steven
    Which extraembryonic membeane forms blood?
    4 years ago
  • natalie
    What are the functions of the various embryonic membranes?
    3 years ago
  • agamennone
    What are the four extraembrayonic membranes that are formed from the three germs layer?
    3 years ago
  • Lola
    How are extraembryonic membranes formed?
    3 years ago
  • rhoda goodchild
    Which is first and last formed extra embryonic membranes in human?
    3 years ago
  • david
    What is the outermost extraembryonic membrane in humans?
    2 years ago
  • William
    When do the extraembryonic membranes form?
    2 years ago
  • iona
    Is the umbilical cord an extraembryonic membrane in humans?
    2 years ago
  • melody
    Which layer encloses extra embryonic endoderm in humans?
    1 year ago

Post a comment