How are germ cells not reduced in number?

Question

If germ cells produce haploid daughter cells by meiosis and are thereby "consumed" (where there was a germ cell there are then 4 daughter cells), where do the germ cells come from? I asked my biology teacher if they undergo mitosis prior to meiosis, but she said they don't and proceeded to give an unclear answer. Is this true? If so, are they regenerated from somatic cells elsewhere in the body?

My question basically is - if the germ cells of, say, the testes are dividing by meiosis to produce sperm, where are the germ cells coming from so that the testes don't shrink and ultimately disappear as they're converted completely into sperm?

Answer 1

In case of gametogenesis (let us talk about spermatogenesis) gametes are formed from meiotic division of Primary spermatocytes.

In Primates Primary spermatocytes are cells that that are formed from mitotic division of B spermatogonia (which is another class of germ cells) which inturn are formed from mitosis of A_p spermatogonia which arise from mitotic division of A_d spermatogonia. A_d spermatogonia are inturn formed from Primordial germ cells (see below for details).

A_d spermatogonia are dark A type spermatogonia, reserve stem cells which occasionally divde to renew itself and A_p spermatogonia are pale A type spermatogonia, renewing stem cells.

Why the testis doesn't run out of germ cells? _{(adaptation of your key question)}

The reserved A_d spermatogonia and renewal of both A_d and A_p spermatogonia are responsible for the continuous cycles of spermatogenesis.

This process starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. (wikipedia)

The process of spermatogenesis in human:

In non-primates like mice it is little different, the following diagram should explain it:

Note:

• Germ cells are cells that give rise to gametes ( present at any level of gametogenesis).
• Primordial germ cells are embryonic cells that originate from a different location (outside the gonads) and migrate into the gonads during the course of embryonic development to divide into germ cells.
• In females, oogenesis also involve production of germ cells (cells of primary follicles) from PGCs through mitosis but the primary follicles produced is fixed, doesn't increase after birth so at a definite age of 40-45 gametogenesis stops.

Further reading:

• Stem Cells in Reproductive Medicine by Carlos Simón, Antonio Pellicer, Renee Reijo Pera
• A paper

Answer 2

"How are germ cells not reduced in number?"

It does happen. Germ cells do eventually run out. It is called menopause in women. And age related infertility in men.

As for your question of where do germ cell come from? Much like were do muscles cells come from?, you have to be specific. Germ cells comes from the gonads (ovaries and testis) would be a simple answer.

EDIT:

They are coming from Spermatogonial stem cells (SSCs), cells that for the most part are immortal, and can replicate endlessly (well ~60 years anyway).

https://www.ncbi.nlm.nih.gov/pubmed/23784829