4. Germ Line Cells Determination
Herbert, B.
Davao Medical School Foundation

5. Student Learning Objectives
1. You should understand that sexual reproduction requiring the fusion of
gametes from male and female gonads occurs in specific organisms.
2. You should understand that the primordial germ cells that give rise to
gametes arise outside of the gonads and must migrate to them.
3. You should understand that in most organisms the primordial germ cells
are specified conditionally, while in some they are specified
autonomously by cytoplasmic determinants in the egg.
4. You should understand that migration of the germ cells from their site of
origin to the gonads is an essential part of reproductive success .

6. Introduction
• In all plants and some animals, somatic
cells can readily form new organisms
– Cnidarians, flatworms, tunicates
• In many animals, there is an early division
between somatic and germ cells
– Insects, roundworms, vertebrates

7. Definition of Terms
Fertilization
•Process of union of two gametes whereby the somatic
chromosome number is restored and the development of a
new individual is initiated. cf. fecundation, insemination,
pollination
Gonad
•A reproductive gland (as an ovary or testis) that produces
gametes
Gametes
•Mature male or female germ cell usually possessing a
haploid chromosome set and capable of initiating formation
of a new diploid individual by fusion with a gamete of the
opposite sex

8. Definition of Terms
Gametogenesis
•Processes by which the sperm and the egg are formed
•Involves forming the PGCs and getting them into the
genital as the gonad is forming
Germ Cells
•Provide the continuity of life between generations
•Mitotic ancestors of our own germ cells once resided in
the gonads of reptiles, fishes, and invertebrates.
Primordial Germ Cells
•Precursors of germ cells.

9. Definition of Terms
Germ line
•The cellular lineage of a sexually reproducing
organism from which eggs and sperm are
derived
•Contains the genetic material in a cellular
lineage which can be passed to the next
generation

10. Germ Line Determination

11. Insects, Roundworms and Vertebrates
• Possesses a clear and early separation of
germ cells from somatic cell types
• Established germ line separates from the
somatic cells early in development
• Germ cells do not arise from within the
gonad itself but from Primordial Germ Cells
(PGCs), their precursors and then migrate
into the developing gonads

12. Several other animal phyla: cnidarians, flatworms, tunicates
• Include the entire plant kingdom and some
invertebrates
• Division not well established
• Somatic cells readily become germ cells
(even in adult organisms) and give rise to
new individuals

13. The Formation of the Germ Plasm and
the Determination of the PGCS

14. Key Concept
• All sexually reproducing organisms arise
from the fusion of gametes (eggs and
sperm)
• All gametes arise from the primordial germ
cells

15. Two step process
– Primordial germ cells (PGCs) are determined
in a specific location in the embryo
– PGCs migrate to the gonad and become the
progenitor population for eggs and sperm

16. Two Methods of Germ Cell Determination
• Autonomous Specification
– Egg cytoplasmic determinants
– Called ‘Germ Plasm’
– Nematodes, flies, frogs
• Conditional Specification
– Signals from surrounding cells
– Majority of sexually reproducing organisms
– Including mammals

17. The Germ Plasm
• The determination of the PGCs is brought
about by the cytoplasmic localization of
specific proteins and mRNAs in certain
cells of the early embryo (true for most
animals except mammals)
• These cytoplasmic components are
referred to as the germ plasm

18. Germ cell determination in Nematodes
(Round worms)

19. Parascaris aequorum (AKA Ascaris megalocephala)
•Has only two chromosomes per haploid cell
•Cleavage plane of the first embryonic
division to form an animal derived
blastomere is equatorial, separating the
animal half from the vegetal half of the
zygote

20. The nematode
Remember cleavage and gastrulation:
Asymmetrical divisions produce a
stem cell (P-lineage), “founder” cell.
Stem cell divisions are meridional
Founder cell divisions are equatorial

21. Germ Cell Migration
During cleavage the germ plasm rises up until it
ends up in the endoderm at top and back near lip

22. A. Animal Half of the Blastomere
• Before the cells divide, there is a
fragmentation of the ends of the
chromosomes into dozens of pieces
(Chromosome diminution)
• Only a portion of the original chromosome
survives

23. Germ Cell Migration: Frogs
The endoderm below
mesdoderm are PGCs

24. Germ Cell Migration: Frogs
Migration anterior to gonads at
endoderm-mesoderm boundary
~30 PGCs reach gonads
by fibronectin and Sdf-1

25. B. Vegetal Blastomere
• Chromosomes remain normal

26. Conditional Specification of
mammalian PGCs

27. Conditional Specification of mammalian PGCs
• Posterior of epiblast at the junction of the
primitive streak and extraembryonic
ectoderm
– Cells are no different from other epiblast
– Specified in gastrulation before 3 layers form
– Wnts from endoderm make them competent
– BMPs from extraembryo ectoderm finish it

28. Picture the blastocyst full of yolk.....