1. Lecture 10: Human Embryology - I
Faculty of Science, School of Sciences, Natabua Campus Lautoka
BIO706 Embryology
Lecture 10: Human Embryology - I

2. HUMAN EMBRYOLOGY
Development is the process that determines an organism’s form and function.

3. Gametes join in fertilization
We’ve already learned how gametes are produced (gametogenesis) through meiosis
What process produces every other cell in the human body?
MITOSIS

4. Fertilization
Are we there yet

5. Fertilization Divided into 4 steps:
1.Contact and recognition (“Casual Introductions”)
Sperm undergo capacitation (further maturation) within the female reproductive tract
Sperm were produced in the testes and matured in the epididymus until ejaculation
Sperm reach the egg in the oviduct where fertilization will occur

6. 2. Sperm Entry
Only ONE sperm is allowed to enter
Fast block - electrical charge in egg plasma membrane prevents polyspermy
Slow block - depolarization of egg plasma membrane due to Ca++ release

7. 3. Egg Activation
The release of calcium ions in egg plasma membrane also triggers protein synthesis
4. Fusion
The sperm nucleus is propelled to the egg nucleus by microtubules

8. Effects of sperm penetration
Following penetration, fusion of sperm with the egg membrane initiates a series of events including
egg activation
blocks to polyspermy
a sharp increase in protein synthesis and an increase in metabolic activity in general

9. Egg activation
After ovulation, the egg remains in a quiescent state until fusion of the sperm and egg membranes triggers reactivation of the egg’s metabolism.
There is a dramatic increase in the levels of free intracellular Ca2+ ions in the egg shortly after the sperm makes contact with the egg’s plasma membrane.

10. The released Ca 2+ act as second messengers in the cytoplasm of the egg, to initiate a host of changes in protein activity.
These many events initiated by membrane fusion are collectively called egg activation.

11. Blocks to Polyspermy
Membrane contact by the first sperm results in a rapid, transient change in membrane potential of the egg, which prevents other sperm from fusing to the egg’s plasma membrane.

12. In mammals, specialized vesicles called cortical granules, located just beneath the plasma membrane of the egg, release their contents by exocytosis into the space between the plasma membrane and the vitelline envelope or zona pellucida, respectively.
Additional sperm cannot penetrate through the hardened, elevated vitelline envelope, which is now called a fertilization envelope.

13. The nucleus of the unfertilized egg is not yet haploid because it had not entered or completed meiosis prior to ovulation.
Fusion of the sperm plasma membrane then triggers the eggs of human to complete meiosis
A single large egg with a haploid nucleus and one or more small polar bodies, which contain the other nuclei, are produced

14. The fusion of nuclei restores the diploid state
The haploid sperm nucleus fuses with the haploid egg nucleus to form the diploid nucleus of the zygote.
In mammals, including humans, the nuclei do not actually fuse. Instead sperm and egg nuclear membranes each break down prior to the formation of a new diploid nucleus.
A new nuclear membrane forms around the two sets of chromosomes

15. Cell Cleavage Patterns
Following fertilization, the second major event in vertebrate reproduction is the rapid division of the zygote into a larger and larger number of smaller and smaller cells.
This period of division, called cleavage, is not accompanied by an increase in the overall size of the embryo.

16. The resulting tightly packed mass of about 32 cells is called a morula, and each individual cell in the morula is referred to as a blastomere.
As the blastomeres continue to divide, they secrete a fluid into the center of the morula. Eventually, a hollow ball of to cells, the blastula, is formed. The fluid-filled cavity within the blastula is known as the blastocoel.

17. Cleavage follows fertilization
Cleavage is a series of rapid mitotic divisions (without cell growth)
The two-celled zygote divides repeatedly until a ball of cells is formed
This is the morula - 32 cells
Holoblastic cleavage: cell division occurs throughout the entire egg.
Morula

18. Forming a ball of cells surrounding a blastocoel, an inner cell mass is concentrated at one pole, it goes on to form the developing embryo.
The outer sphere of cells, called a trophoblast, These cells have changed during the course of mammalian evolution to carry out a very different function: part of the trophoblast enters the endometrium (the epithelial lining of the uterus) and contributes to the placenta, the organ that permits exchanges between the fetal and maternal bloods.
A mammalian blastula

19. These few cells are pluripotent (have the potential to become ANY of the 210 types of cells in the human body).
For a short period of time, just before they implant in the uterus, the cells of the mammalian blastocyst have the power to develop into many of the different types of cells in the body—and probably all of them.
These are embryonic stem cells

20. Gastrulation
At the end of the cleavage stage, cells making up the blastula move about and surface proteins help cells recognize each other
Certain groups of cells invaginate (dent inward) and involute (roll inward) from the surface of the blastula in a carefully orchestrated activity called gastrulation.

21. The events of gastrulation determine the basic developmental pattern of the vertebrate embryo.
By the end of gastrulation, the cells of the embryo have rearranged into three primary germ layers: ectoderm, mesoderm, and endoderm.

22. A primitive streak develops, through which cells destined to become mesoderm migrate into the interior, again reminiscent of gastrulation. The trophoblast has now moved further away from the embryo and begins to play a role in forming the placenta.

23. Developmental Processes during Neurulation
The process of tissue differentiation begins with the formation of two morphological features found only in chordates
the notochord
the hollow dorsal nerve cord.
This development of the dorsal nerve cord is called neurulation.

24. The notochord is first visible soon after gastrulation is complete, forming from mesoderm along the dorsal midline of the embryo.
It is a flexible rod located along the dorsal midline in the embryos of all chordates, although its function is replaced by the vertebral column when it develops from mesoderm in the vertebrates.

25. After the notochord has been laid down, a layer of ectodermal cells situated above the notochord invaginates, forming a long crease, the neural groove, down the long axis of the embryo.
The edges of the neural groove then move toward each other and fuse, creating a long hollow cylinder, the neural tube
which runs beneath the surface of the embryo’s back. The neural tube later differentiates into the spinal cord and brain.

26. The dorsal lip of the blastopore induces the formation of a notochord, and the presence of the notochord induces the overlying ectoderm to differentiate into the neural tube.
While the neural tube is forming from ectoderm, the rest of the basic architecture of the body is being determined rapidly by changes in the mesoderm.

27. On either side of the developing notochord, segmented blocks of mesoderm tissue called somites form; more somites are added as development continues.
Ultimately, the somites give rise to the muscles, vertebrae, and connective tissues.
The mesoderm in the head region does not separate into discrete somites but remains connected as somitomeres and form the striated muscles of the face, jaws, and throat.

28. The Neural Crest
Neurulation occurs in all chordates, in vertebrates, just before the neural groove closes to form the neural tube, its edges pinch off, forming a small strip of cells, the neural crest, which becomes incorporated into the roof of the neural tube. The cells of the neural crest later move to the sides of the developing embryo.
After migrating to different parts of the embryo, ultimately develop into the structures characteristic of the vertebrate body.

29. Derivation of the major tissue types
The three germ layers that form during gastrulation give rise to all organs and tissues in the body, but the neural crest cells that form from ectodermal tissue give rise to structures that are prevalent in the vertebrate animal such as gill arches and Schwann cells.

31. Summary

33. Questions are welcome