Germ Line Cells Determination

Herbert, B.Davao Medical School Foundation

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 .

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

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

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.

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

Germ Line Determination

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

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

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

Key Concept

• All sexually reproducing organisms arise from the fusion of gametes (eggs and sperm)

• All gametes arise from the primordial germ cells

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

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

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

Germ cell determination in Nematodes (Round worms)

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

The nematode

Remember cleavage and gastrulation:Asymmetrical divisions produce astem cell (P-lineage), “founder” cell.

Stem cell divisions are meridional

Founder cell divisions are equatorial

Germ Cell Migration

During cleavage the germ plasm rises up until it ends up in the endoderm at top and back near lip

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

Germ Cell Migration: Frogs

The endoderm belowmesdoderm are PGCs

Germ Cell Migration: Frogs

Migration anterior to gonads atendoderm-mesoderm boundary

~30 PGCs reach gonadsby fibronectin and Sdf-1

B. Vegetal Blastomere

• Chromosomes remain normal

Conditional Specification of mammalian PGCs

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

Picture the blastocyst full of yolk.....